VSEBINA – CONTENTS PREGLEDNI ^LANKI – REVIEW ARTICLES On the determination of safety factors for machines using finite element computations O dolo~itvi faktorjev varnosti za naprave pri izra~unu z metodo kon~nih elementov L. B. Getsov, B. Z. Margolin, D. G. Fedorchenko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Polimeri v beli tehniki Polymer materials in white goods industry V. Vasi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 IZVIRNI ZNANSTVENI ^LANKI – ORIGINAL SCIENTIFIC ARTICLES Characterization of multilayer PACVD TiN/Ti(B-N)/TiB2 coatings for hot-worked tool steels using electron spectroscopy techniques Karakterizacija ve~plastne PACVD TiN/Ti(B-N)/TiB2 prevleke za orodna jekla za delo v vro~em s tehnikami elektronske spektroskopije M. Jenko, D. Mandrino, M. Godec, J. T. Grant, V. Leskov{ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 An investigation of the stretch reducing of welded tubes Raziskava raztezne redukcije varjenih cevi S. Re{kovi}, F. Vodopivec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Experimental analysis of crack initiation and growth in welded joint of steel for elevated temperature Eksperimentalna analiza nastanka in rasti razpoke v zvaru jekla za povi{ano temperaturo M. Burzi}, @. Adamovi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 The role of chloride salts on high temperature corrosion of 321 stainless steel Vloga kloridnih soli pri visokotemperaturni koroziji nerjavnega jekla 321 N. Amin, M. M. Amin, S. B. Jamaludin, K. Hussin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Raztapljanje CO2 v embalirani vodi ali brezalkoholni pija~i in s tem povezane mo`ne po{kodbe Problems associated with the dissolution of CO2 in the case of bottled water and non-alcoholic beverages D. Drev, M. Pe~ek, J. Panjan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Priprava Co-feritnih nanodelcev z ozko porazdelitvijo velikosti z metodo termi~nega razpada oleatov Preparation of Co-ferrite nanoparticles with a narrow size distribution by the thermal decomposition of oleates S. Gyergyek, D. Makovec, M. Drofenik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 STROKOVNI ^LANKI – PROFESSIONAL ARTICLES Centreline formation of the Nb(C,N) eutectic in 0.15 % C; 0.0071 % N; 0.022 % Nb; 0.033 % Al and 0.003 % S structural steel Sredinsko izcejanje in nastanek evtektika Nb(C,N) v konstrukcijskem jeklu z 0,15 % C; 0,0071 % N; 0,022 % Nb; 0,033 % Al in 0,003 % S J. Berneti~, B. Brada{kja, G. Kosec, B. Kosec, E. Bricelj . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 LETNO KAZALO, LETNIK 42, 2008 – INDEX, VOLUME 42, 20087 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 ISSN 1580-2949 UDK 669+666+678+53 MTAEC9, 42(6)235–309(2008) MATER. TEHNOL. LETNIK VOLUME 42 [TEV. NO. 6 STR. P. 235–309 LJUBLJANA SLOVENIJA NOV.–DEC. 2008 L. B. GETSOV ET AL.: ON THE DETERMINATION OF SAFETY FACTORS FOR MACHINES ... ON THE DETERMINATION OF SAFETY FACTORS FOR MACHINES USING FINITE ELEMENT COMPUTATIONS O DOLO^ITVI FAKTORJEV VARNOSTI ZA NAPRAVE PRI IZRA^UNU Z METODO KON^NIH ELEMENTOV Leonid B. Getsov, B. Z. Margolin, D. G. Fedorchenko SPbSPU (St.Petersburg), CRISM "Prometey" (St.Petersburg), SNTK (Samara), Russia getsovonline.ru Prejem rokopisa – received: 2007-04-06; sprejem za objavo – accepted for publication: 2008-03-25 Principles for the selection of modern methods for the determination of local strength safety factors in design computations of GTE parts for static and cyclic loading are suggested. It is shown that the selection of methods for the evaluation of local strength safety factors should be carried out applying special criteria and computations including adequate models of visco-elasto-plasticity. On the basis of the analysis of computational practice the minimum values of local strength safety factors for static and cyclic loading, which may be recommended for FEM computations, have been proposed. Key words: safety factor, finite element computation, creep, loading, cycle, fatigue Predlo`eni so principi za izbiro metod za dolo~itev lokalnih faktorjev varnosti za GTE-dele in za stati~no ter cikli~no obremenitev. Izbira metod za oceno lokalnega faktorja varnosti za trdnost je mogo~a z uporabo primernih meril in primernih modelov visko-elasto-plasti~nosti. Na podlagi analize prakse izra~unavanja so predlo`ene najni`je vrednosti za faktorje varnosti za lokalno trdnost za stati~no in cikli~no obremenitev pri FEM-izra~unih. Klju~ne besede: faktor varnosti, izra~uni z metodo kon~nih elementov, lezenje, obremenitev, cikel 1 INTRODUCTION The wide and universal propagation of commercial finite element packages (ANSYS, ABAQUS, MARC, LS DYNA, NASTRAN etc) for computations in design of machines and civil structures made possible to define more accurately the stress-strain state (SSS) and opened the way to solve some problems connected with the normalization of safety factors. One of these problems is the determination of the possibility of defining more reliable values for safety factors values based on the more accurate knowledge of the SSS of the construction. The more reliable reduction of safety factors would allow to decrease the weight of material for the con- struction, however, it may also increase the risk of flaws arising during the exploitation. The application of the finite element methods (FEM) for computing SSS in the locations of stress concentration makes it possible to design more accurately the configuration of these details of components, to obtain minimal stresses, increase the life time of parts supporting static stresses, and, more importantly, also the lifetime of parts submitted to cyclic loading. FEM is indispensable for prognosticating the crack propagation rate in parts with geometry, tempera- ture and stress gradients where conventional computa- tion schemes cannot be applied with sufficient reliability. In the process of normalization of strength values for parts, it should be provided for the introduction of safety factors, both for material properties and for loading parameters of a construction. In both cases the risk may arise of use of non verified data. It is known that some cases damages of parts during the turbine exploitation were caused by the improper evaluation of local strength in the stage of design. As examples of such events may be mentioned in particular cracks networks revealed at the inspection of gas turbine rotors after a determined operational time; cracks on gas turbine disk rims; thermal fatigue cracks on the border and the back of cooled working and regulation blades and cracks on components of combustion chambers of gas turbine engines (GTE). The basic principles for the normalizing of safety factors considering the local static and low-cycle fatigue strength in this paper were experimentally verified in an independent report. The experience of normalizing safety factors, both in gas turbine and reactor design is widely used 1,2,3. The application of FEM requires a high qualification designer skilled in computational mechanics, inasmuch as the computation results depend significantly on the methods of partitioning an analyzed part with a finite element (FE) network and the selection of FE types. With the aim to describe the material properties in determining the SSS of constructions applying FEM, the method of "average values minus two or three average quadratic deviations" is used. In some cases, for exam- ple, for creep strain, it is necessary to apply "average values plus two or three average quadratic deviations". However, the insufficient quantity of experimental data for new materials, insufficient information’s on the dependence of properties on operating conditions as well Materiali in tehnologije / Materials and technology 42 (2008) 6, 237–242 237 UDK 539.3:519.61 ISSN 1580-2949 Review article/Pregledni ~lanek MTAEC9, 42(6)237(2008) as data accounting for the influence of environment, restricts the application of this method. In this situation, to make easier the proper application of available experi- mental data, is expedient to use a sufficiently widespread concept of the "upper and lower envelop curves"4. Generally, the process of rupture at static loading may be of three types: a) exhaustion of short-time plasticity, b) creep, c) brittle. It is evident that the differentiation of safety factors depends on the type of rupture and should be considered in the normalization of local stresses. It is clear that the greatest safety factor value should be considered in the case of brittle fracture that may occur in the range of maximal scatter of material parameters. 2 STATIC STRENGTH 2.1 Static strength of deformable materials The presence of stress concentration does not lead to a decrease of the bearing capacity of deformable materials in case of short-time or long-time static loading. From here on, the term "bearing capacity of plastic materials" should be understood as the conditions in which the ultimate load causing the rupture of a construction is determined with the loss of bearing capacity according to the "plastic hinge mechanism". If the value of long-time plasticity of a deformed material exceeds 4–5 %, it is not sensitive to the notch effect in long-time strength tests. Also heterogeneous cast alloys are not notch-sensitive. The temperature dependence of the plasticity of materials is not monotonous. Thus, analyzing a material state with consideration of the exploitation parameters, it is necessary to have on disposal the data of material deformability as function of the temperature and the strain rate (creep rate). The analysis of experimental and calculated data indicates that the value of the ratio Kσ = σBn/σBs may be taken as a criterion for the material plasticity (σBn and σBs are ultimate strength values determined by testing notched and smooth specimens). Alloys with Kσ ≥ 1.3 obtained at appropriate temperatures for specimens with ασ = 3.5–4.5 (ασ is coefficient concentration of stress), submitted to short and long-time tensile tests, are not propensive to brittle rupture. The use of alloys with Kσ < 1.3 is permitted only on the base of results of appropriate tests that include the statistical evaluation of results of tests of specimens with initial cracks (Sharpy impact tests) and low-cycle fatigue characteristics obtained from tests with notched specimens. We may assert, for this reason, that the introduction of FEM-computations for parts from plastic materials and the more precise determination of SSS at stress concentration locations should not be the base for the correction of safety factors related to the bearing capacity of constructions. At the same time, if the bearing capacity of constructions is ensured, the assumption of the needlessness of eva- luation of safety factors related to the static strength and based on local stress values, is justified. The analysis of the criteria defined in the strength standards 1,2, as well as the suggested approaches to the evaluation of the static strength and the experience of exploitation of various parts show that all attempts to restrict the value of yield strength are senseless. On the other hand, it became generally accepted that in case of appropriate ultimate strain exceeding 4–6 %, it is not necessary to take into account the residual stresses in the computations of static strength. The same is valid also for the thermal stress σT, if σT = 2α∆T < 2E % (α – coefficient of linear expansion; E – Young elastic modulus; ∆T – range of temperature variation). In such approach to the normalization of the static strength of constructions, it is necessary to verify the respect of the condition that the value of J-integral is below its critical value Jc. Thus, for example, according to 1, in this case the maximal nominal static stresses (without accounting for concentrators) for pressure vessels are permitted to be below of 1/1.5 for yield strength and below of 1/2.6 for tensile strength. The following approaches are expedient to apply for the evaluation of safety factors related to local stresses: 1. The application of the proper model of kinematical hardening is justified for solving many practical problems. However, the optimal is the SSS compu- tation and the choice of a plasticity model depend on the material analyzed and of the loading in accor- dance with the conception of multimodel approach8. 2. The static strength of deformable materials should be evaluated on the base of exhaustion of the ultimate material plasticity ε*, which, in turn, depends on the loading rate or time. Incidentally, one should differ ultimate states for intragranular and intergranular rupture. Intragranular rupture is characterized by the absence of dependence of ultimate strains on loading rate, at the same time, for intragranular rupture, the ultimate strain diminishes with the decrease of loading rate. 3. If the local strength is evaluated with respect to the short-time plastic strain, the safety factor on strains ε*/εp (εp – plastic strain) should not be lower than 2.0, with ε* defined with regard to the stress state triaxility by the following equations ε* = εp ult 1.7exp(–1.5σ/σi) (1a) ε* = εp np Keσi 2/3(σiσcp) (1b) which give a conservative estimation of the plasticity. Here εpult – ultimate strain (deformability) at short time ten- sion; Ke – characteristic of material state (at brittle state – Ke = 1, at plastic state – Ke = 1.2); σ – mean stress. 238 Materiali in tehnologije / Materials and technology 42 (2008) 6, 237–242 L. B. GETSOV ET AL.: ON THE DETERMINATION OF SAFETY FACTORS FOR MACHINES ... The value of εp is defined with elastoplastic com- putation using an appropriate plasticity model and the lower strain envelop curve. In this case safety factor on stresses shall not be lower than 1.2–1.4. It should be noted also, that the problem of nor- malizing of the static strength needs further development on the base of comprehensive investigations of material properties aimed to the improve the plasticity models for computing three-dimensional SSS and to further develop the rupture criteria. It should be noted that it is necessary to adapt effectively, after comprehensive testing, new plasticity models to commercial FE packages. 2.2 Safety factors for local strength for creep loading By considering the safety factors for local strength, it is expedient to proceed from the following considera- tions: 1. The evaluation of rupture situation of parts operating at creep deformation can be implemented with applying the ultimate strain value, which depends on temperature, time and of the stress state rigidity. Therefore, as in the case of normalizing, the safety factors for static strength of parts from deformable materials, the use of FEM computations and the more exact knowledge of SSS for stress concentration locations cannot be the base for correcting the values of creep safety factors. In this case, there is no need to use of modern methods for stress computation. The safety factors for creep should be defined with applying the crack initiation criteria. 2. Correction of modern safety factors for creep should be based on the improvement of creep models, especially applied to parts operating in three-dimen- sional stress state and submitted to multifactor and nonstationary loading, as well as on the results of the analysis of creep characteristics and long-time strength of materials. 3. For the description of the influence of material properties and stress complexity in a part on its deformability, it is expedient to apply the following equations that are analogous to (1a) and (1b) p* = 1.7 εc exp(–1.5σ/σi) (2a) p* = εnKeσi 2/3(σiσcp) (2b) where: εc – critical creep strain at uniaxial loading; p* – ultimate creep strain (deformability) at the complex stress state; Ke – characteristic of material state (Ke = 1 – for brittle state and Ke = 1.2 – for plastic state). These, as well as equations (1a) and (1b), give a conservative evaluation of the ultimate strain. In this case and considering the values of accumulated creep strains along the upper envelop curve, the minimal strain safety factor value should not be below 2. For the determination of the safety factors on life time (Kτ,N) and on stresses (Kσ), it is recommended to use the life-time lower envelop curves obtained with the probability of 99 %. It is expedient to apply safety factor values not lower than Kσ = 1.2 and Kτ,N = 1.5. In this range the minimal value of the safety factor should be selected. In some situations the values of safety factor may be determined with the use of the average curves and depending on the scatter of material properties, the safety factors should be not less than Kσ = 2 and Kτ,N = 10. 4. It has been shown in several investigations (STP ASTM No 165, 1954 11) that for the accounting of a nonstationary situation in computations using the formulas of linear summation of damages (in deformation or time interpretation), a conservative estimation is obtained on condition that the sum of damages is taken equal to 0.87. For the constructions submitted to a large number of launchings and stops it is necessary to take into account the effect of cyclic loading on the parameters of creep and life-time strength. 2.3 Safety factors in conditions of brittle fracture The criterion characterizing the brittle fracture is the value of plain strain stress intensity factor K1. For constructions with flaws, the computed values of stress intensity factor K1 should be compared with its critical K1c value. The brittle strength is assumed to be ensuring if the following condition is observed: K1 ≤ K1c (3) It is recommended to calculate the value of K1 accor- ding the following equation (1): K1 = [ ] η σ σ ⋅ + ⋅ + ( ) ( ) . ( ) . . . p p q qM M a/ a/ c π 10 1 46 2 3 0 5 1 65 0 5 (4) where: η – coefficient accounting for the influence of stress concentrations; σp – tension component of stress intensity; σq – bending component of stress intensity; Mp = 1 + 0.12(1 – a/c); Mq = 1 – 0.64 a/h; a – crack depth, generally assumed to be elliptical; c – crack half length; h – area within which bending stress component remains positive (the value of h for the formula (4) is permit- ted to be taken equal to half wall thickness). For constructions with non detected flaws, the value of K1 (according1) should be computed assuming the presence of defects of size comparable with the sensitivity of the inspection apparatus. Here, it is also necessary to account for the dimensions of a "shaded" zone where it is impossible to check up the presence of flaws in exploitation. In design, it is generally assumed that the construction should ensure the safety for the crack of size equal to 1/4 thickness of the part bearing Materiali in tehnologije / Materials and technology 42 (2008) 6, 237–242 239 L. B. GETSOV ET AL.: ON THE DETERMINATION OF SAFETY FACTORS FOR MACHINES ... section (wall), that is considered as defect size in the computation3. In case of sufficiently careful inspection during the exploitation, the crack size may be taken as equal to the sensitivity parameter of the inspection apparatus, or in case of detected crack, to the size of the crack. As a rule, it is assumed that the safety factor on K1c shall be not less than 2 and a lower safety factor may be adopted in case of availability of sufficient statistical data. 3 CYCLIC STRENGTH In the case of evaluation of cyclic strength, various methods have been suggested for determining the local strength safety factors. These may be conditionally divided in five groups: computational for a rigid cycle, computational for a general situation, computational- experimental, based on the theory of adaptability and based on deformation criteriia. 2.1 For cyclic loading and rigid cycle (case of uni- axial loading with cycle asymmetry coefficient r ≈ −1), it is expedient to use the values of amplitude intensity of conditionally elastic full strains as parameters of loading: ∆ ∆ε ε ε= 2 3/ ij ij (5) The resistance to fatigue for elastic cyclic deforma- tion is evaluated applying the Goodman’s equation: σ σ σ σmax ( )= −−1 1 / B (6) with: σmax – maximum cycle local stress with account of stress concentration; σ – average cycle stress; σ–1 – endurance limit of a material for symmetrical cycle with account of stress concentration. The determination of the resistance to elasto-plastic deformation at cyclic loading is possible using of cyclic strain curves. In this case the conditions for the rupture at elasto-plastic cyclic deformation is obtained applying the Coffin’s deformation criterion: ( *)( )∑ =∆ε εpk m/ Nd 1 (7) where: ∆ε p k( ) – plastic strain amplitude in K-cycle; m – constant; ε* – ultimate strain deduced applying the equations (1a) and (1b). In connection with the generally insufficiency of experimental data for the statistical analysis, as in case of creep static loading, it is expedient to evaluate the value of local strength safety factor with the use of the average curve and assuming the value of 2 in case of evaluation based on stress (or strain) amplitude, or equal to 10 if the evaluation is based on life time parameters. However, if the quantity of experimental data is sufficient and the lower envelop curve is reliable, the values of safety factors defined by stresses and by life time parameters may be taken equal to 1.2 and 1.5, correspondingly. It should be noted that the values of deformability ε* and coefficient m in equation (6) should be determined expe- rimentally for every material. In the case of using the universal value of coefficient m in Coffin’s equation (7), the values of safety factors should be increased. 2.2 For cyclic loading in a general case, when the unilateral accumulation of strain (characteristic for a mild cycle and generally called "ratcheting") and stress variation (characteristic for a rigid cycle) takes place, different approaches to the evaluation of cyclic strength safety factors may be applied. Among all the known strength characteristics of a material, the life time under cyclic loading is depends mostly on the influence of factors related to the construction, technology, metal- lurgy and operation. Therefore, the evaluation of the life time under cyclic loading for constructions is possible considering the results of test specimens and construc- tion components with accounting of all above mentioned factors. The main operational factors affecting the life time of a part under cyclic loading, are temperature and holding time at maximal loads and temperature, cyclic asymmetry, superposition of high-frequency component upon the low-frequency variation of loading. The reali- zation of tests within all the range of operational loading is a rather labor consuming task. Therefore, is quite urgent to develope methods based on conventional tests of specimens for the evaluation of life time of construc- tions submitted in operation to complex loading. For low-cyclic loading, material damages may be computed applying the deformation or energy criteria of rupture. Here, for computing the kinetics of stress-strain state, both for complex noncyclic loading and for cyclic loading with altering loading parameters, instead of a number of cycles n (or number of semi cycles k) it is expedient to use the relations of Odquist’s type as parameters of of the actual state of the material. These relations are expressed by the following formulas 12,13: λ ε ε1 = −∫ d p p ; d d dp p pε ε ε= ( ) .2 3 0 5/ ij ij ; ε ε εp p pd d= ( ) .2 3 0 5/ ij ij (8a) λ2 = −∫ dp p; dp / p pij ij= ( ) .2 3 0 5 ; p / p pij ij= ( ) .2 3 0 5p p (8b) ∆λ λ λ1 1 0= − ≥−( ) ( )k k k – the ordinal number of a semi cycle. The increment of nonelastic strains (dεne) and the value of nonelastic strain intensity (εne) are defined with the equations: d d dne p pε εij ij ijp= ε ε εne ne ned d= ( ) .2 3 0 5/ ij (9) For the case of creep for known stress, the accu- mulated creep strains should be distinguished from the nonelastic strains. In the particular cases of cyclic loading, instead of the mentioned parameters, by simple transformation the L. B. GETSOV ET AL.: ON THE DETERMINATION OF SAFETY FACTORS FOR MACHINES ... 240 Materiali in tehnologije / Materials and technology 42 (2008) 6, 237–242 computation of damages is replaced with the traditional applying the values of cycles and semi cycles. Then, for the evaluation of the life time under cyclic loading, Coffin’s type formulas may be used: ( )∆ε p 2 1N C= ; ∆ε 2 pi C=∑ 1 (10a) ( )∆p N Cn = 2 ; ( )∆p Ci n =∑ 2 (10b) For the evaluation of the static component of life time, the value of εp (or Odquist’s parameter) is com- pared with the ultimate strain ε* of a material. The described approach probably has one only deficiency, it is unsuitable for characterizing damages in conditions of neutral loading ways (neutral load path). This deficiency in the describing of the nonstationary cyclic loading can be avoided by applying the Coffin’s formula (10a) and V. V. Novozhilov’s suggestion16 on the dependence of the accumulation rate of micro damages p: D k A= =∫ ρ λd ; p = Gεp (11) where G = dσ/dεp is deformation hardening parameter. 2.3 The methods of adaptability computation allow to determine the cyclic strength safety factors for the general case for sign-variable flow and increasing defor- mation10,11. The ultimate material characteristics for the sign-variable flow are: σs – half value of the cyclic yield strength S0.4 in a stable cycle with the tolerance of plastic strain amplitude within the cycle of 0.4 %. In the case of presence of stress concentrators: σs = E N Nε σ ε( ) ( ) , with ε(N) – semi amplitude of the full strain corresponding to the appearance of low-cycle fatigue macro crack in N cycles and σε(N) – in accordance with ε(N) on the isochronous cyclic strain curve. For creep in one semi cycles: σs =S0.4c – 0.5S0.4, with S0.4c – cyclic yield strength by presence of creep. For the progressing deformation, as ultimate charac- teristics σs = σB – for transitional modes and σs = σLTS (t,Σ∆τ) – for stationary modes are taken with σLTS – long-time strength in accordance with the all life time length of loading. In 16,17,18 the results of the analyses of stress-strain state and strength of the disks and rims of regulating apparatus (two- and three-dimensional computations) are discussed. The following values of safety factors may be recommended for the strength computations of GTE disks : KSVF = 1.2–1.5 – for sign-variable flow and KPD = 1.9–2.2 – for progressing deformation. For the central part of disks is preferable to specify higher values of safety factor KSVF and lower KSVF values for not central stress concentrators. 4 CONCLUSIONS 1. Principles for the selection of methods for the determination of local strength safety factors in design computations of GTE parts for static and cyclic loading have been proposed. 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Kabelevskiy: The questions of theories of plasticity and creep at cyclic nonisothermal loading; Strength of materials, (1978) 6, 660–665 14 V. V. Novozilov, Yu. I. Kadashevitch: Microstresses in engeneering materials. L: Mechanical Engineering, 1990, 223 L. B. GETSOV ET AL.: ON THE DETERMINATION OF SAFETY FACTORS FOR MACHINES ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 237–242 241 15 V. I. Tsejtlin, D. G. Fedorchenko: Estimation of safety factors at multicomponent loading in view of scatter of materials properties; Strength of materials (1979) 9, 31–33 16 A. R. Beljakov, L. B. Getsov, A. E. Ginzburg et al: Strength of guide vane components of gas turbines. Strength of materials (1989) 11, 119–129 17 Beljakov, L. B. Getsov, V. K. Dondoshanskiy, J. B. Shneerson in A. R. Beljakov, L. B. Getsov, V. K. Dondoshanskiy, J. B. Shneerson: Use of the adaptability theory in calculation for the strength of gas turbine discs, Strength of materials (1988) 11, 100–106 18 L. B. Getsov, A. A. Nigin, M. G. Kabelevskiy. Use of finite-element method for numerical evaluation of thermal cycling endurance of disks, Strength of materials (1979) 4, 69–72 L. B. GETSOV ET AL.: ON THE DETERMINATION OF SAFETY FACTORS FOR MACHINES ... 242 Materiali in tehnologije / Materials and technology 42 (2008) 6, 237–242 V. VASI]: POLIMERI V BELI TEHNIKI POLIMERI V BELI TEHNIKI POLYMER MATERIALS IN WHITE GOODS INDUSTRY Vasilije Vasi} Razvoj PPA, Gorenje, d. d., Partizanska 12, 3503 Velenje, Slovenija vaso.vasicgorenje.si Prejem rokopisa – received: 2007-10-04; sprejem za objavo – accepted for publication: 2008-06-16 Velik prispevek k tehni~no-tehnolo{kem razvoju izdelkov bele tehnike so prispevali prav polimerni materiali in njim pripadajo~e tehnologije. Razvoj je opazen ne samo pri izbolj{anju primarnih funkcionalnih lastnosti, temve~ je tudi vse bolj uveljavljen okoljevarstveni vidik razvoja izdelka (t. i. "eco design"). Za izbrane primere so prikazane tehni~na in ekonomska upravi~enost rabe polimernih materialov in pripadajo~ih tehnologij ter specifi~nost snovanja tak{nih izdelkov. Prikazane so tudi nekatere usmeritve v sedanji rabi polimernih materialov in v bli`nji prihodnosti ob uporabi nanotehnologije. Klju~ne besede: polimerni materiali, bela tehnika, "eko-design", nanotehnologija Significant contribution to technical and technological development of white goods products is due to polymers and related technologies. Development reflects not only the improvement of primary functional features, but also the ecology aspect of product design (eco-design) and pleasurable industrial design. The shown examples demonstrate the technical and economical feasibility of polymer application and related technologies as well as the design particularity of such products. The polymer application is growing and in the near future we could expect even bigger presence of nanotechnology. Key words: polymer materials, white goods, eco design, nanotechnology 1 UVOD Industrija bele tehnike je zrela industrijska panoga, ki sledi usmeritvam tehnolo{kega razvoja na razli~nih pod- ro~jih in socio-ekonomskim zahtevam trga. Med najpo- membnej{a podro~ja tehnolo{kega razvoja bele tehnike se uvr{~ata razvoj novih materialov in elektronika, s katerimi se dosegajo nove ali izbolj{ane funkcionalne lastnosti in ergonomsko v{e~ne oblike. Med izdelke bele tehnike pri{tevamo velike in majh- ne gospodinjske aparate – npr. hladilnike, zamrzoval- nike, pralne stroje, su{ilnike perila, me{alnike itd. Zaradi velikih svetovnih kulturolo{kih in socio-ekonomskih razlik velja tudi razli~en na~in pri potrebah ter s tem tudi pri snovanju gospodinjskih aparatov. Na evropskem trgu bele tehnike so najuspe{nej{i proizvajalci izdelkov bele tehnike iz [vedske, Nem~ije in Italije ter iz nekaterih drugih de`el – npr. [panije, Tur~ije in Francije. Razli~na podjetja si na razli~ne na~i- ne ustvarjajo konkuren~no prednost, ki temelji bodisi na tehnolo{ki prednosti (superiornosti), v{e~nosti izdelkov (industrijsko oblikovanje) ali pa na prepoznavnosti in ugledu lastne blagovne znamke. Pri razvoju vsake naslednje generacije izdelkov bele tehnike prevladujejo klju~ne tr`ne usmeritve – pove~ana konkuren~nost na trgu, ob~utljivost cen, nenehne zahteve kupcev po pove~ani funkcionalnosti in v{e~nosti ter ergonomija. Osnovni razlogi za uvedbo polimernih materialov in zamenjavo kovinskih elementov so predvsem manj{a masa polimernega izdelka, velika korozijska in okoljska odpornost ter velika fleksibilnost pri izdelkih zapletenih oblik. Razen ekonomike in tehni~nih razlogov je v ospredju vse bolj stroga okoljevarstvena zakonodaja (npr. WEEE, RoHS, IEC 60335), ki jo predpisujejo po- samezne dr`ave ali pa posamezna svetovna (pano`na ali strokovna) zdu`enja (npr. CECED – Evropsko zdru`enje proizvajalcev bele tehnike) – npr. obveznost za recikla`o lastnih izdelkov in okolju prijazna proizvodnja. V podjetju Gorenje, d. d., namenjamo veliko pozor- nost razvoju polimernih materialov in pripadajo~ih tehnologij. V zadnjih letih se je opravilo kar nekaj raziskav na podro~ju polimernih materialov in tehnolo- gij, ter ocenil potencial uvedbe nanotehnologij v indu- striji bele tehnike.1,2,3,4 Preliminarne {tudije izvedljivosti uporabe so bile del razvojno-raziskovalnih aktivnosti v okviru tehnolo{ke mre`e Inteligentni polimerni materiali in tehnologije, kjer smo strokovno utemeljili in dokazali pomen investicije za modernizacijo razvoja polimernih produktov na primeru polimerne pralne kadi pralnega stroja.5 V prispevku bodo prikazane specifi~nosti in prak- ti~ne projektne izku{nje v Gorenje, d. d., pri razvoju plasti~ne kadi za sodobne pralne stroje in antibakterijske za{~ite pri hladilno-zamrzovalnih aparatih. 2 PREGLED UPORABE POLIMERNIH MATERIALOV IN PRIPADAJO^IH TEHNOLOGIJ V INDUSTRIJI BELE TEHNIKE Polimerni materiali so multifunkcijski, saj lahko nasprotno od drugih materialov (npr. kovine) izkazujejo hkrati ve~ dobrih, a med seboj fizikalno razli~nih last- nosti. Vzporedno z razvojem polimernih materialov in Materiali in tehnologije / Materials and technology 42 (2008) 6, 243–249 243 UDK 620.1/.2:678:66.017 ISSN 1580-2949 Pregledni ~lanek/Review article MTAEC9, 42(6)243(2008) pripadajo~ih tehnologij se ve~a tudi masni dele` poli- mernih materialov v izdelkih bele tehnike. Od leta 1960 pa do 2000 se je namre~ ute`ni dele` polimernih mate- rialov v produktih bele tehnike pove~al na 30% 6. Razloge za uporabo polimernih materialov v indu- striji bele tehnike in nadome{~anje kovinskih elementov lahko strnemo v naslednjih to~kah:7 – doseganje novih in uporabniku prilagojenih speci- fi~nih lastnosti izdelka (npr.du{enje vibracij, hrupa); – izbolj{anje sedanjih lastnosti izdelka, kar lahko po- meni zmanj{ano porabo energije do 30 %, – enostavno doseganje kompleksnih oblik izdelka s konvencionalnimi postopki predelave (npr. injekcij- sko stiskanje); – znatno zmanj{anje proizvodnih stro{kov (tudi do 20 %); – zmanj{anje nujnih investicij v proizvodno linijo (tudi do 50 %); – odprava problemov s korozijo; – pove~anje mo`nosti reciklabilnosti izdelka in njego- vih komponent ter – ni`ja cena aparatov z uporabo polimernih materialov (vsaj 25 %). V beli tehniki so prevladujo~i polimeri plastomeri: PS, ABS in PP ter duromer PUR, kot je razvidno iz dia- grama o porabi polimernih materialov v izdelkih bele tehnike (Slika 1 8). Vendar se danes polimerni materiali zelo redko uporabljajo v kon~nih izdelkih brez dodatka aditivov ali polnil, s katerimi sku{amo izbolj{ati sedanje, `elene lastnosti kon~nega izdelka (npr. zmanj{anje skr~kov – dimenzijska stabilnost, udarna `ilavost, togost, mo~) in hkrati predelovalnost polimernega materiala. To sicer pogosto pomeni pozitiven finan~ni u~inek, vendar pa moramo le najti kompromis med `elenimi lastnostmi in predelovalnostjo polimernega materiala. Z dinami~nim razvojem aditivov in polnil so postali polimerni materiali v industriji bele tehnike {e bolj konkuren~ni kovinam, kar nam tudi prikazujejo lastnosti nekaterih materialov, podane na sliki 2.9 Zelo podoben primer koristnosti uporabe polimernih materialov v evoluciji izdelka bele tehnike se lahko poka`e pri sorodnem gospodinjskem aparatu, sesalnika za prah (Tabela 1,10 Slika 310). Za ~as iznajdbe sesalnika se smatra leto 1908 in od takrat do danes se je njegova masa zmanj{ala skoraj na pol, isto~asno pa se je mo~ pove~ala za 40-krat. Uvedba polimernih materialov je omogo~ila razen v{e~nej{e oblike tudi manj sestavnih delov. Dana{nje ohi{je sesalnika je sestavljeno iz 4 delov, ki so zdru`eni z enim samim veznim elementom, v primerjavi z ohi{jem iz leta 1950 iz 11 delov in 8 veznimi elementi. Mo`ne so tudi izbolj{ave materiala in njegove vrste (npr. kompozit ali nano-polnila), vendar niso tr`no konkuren~na s sedanji- mi tehni~nimi re{itvami pri sesalnikih.10 Nasprotno od kovin velja za polimerne materiale, da imajo zelo dobre lastnosti du{enja in da so njihove materialne lastnosti ~asovno spremenljive ter zelo odvis- ne od vpliva temperature in/ali vlage. Prav zato je treba pri konstruiranju izdelkov iz polimernih materialov upo- {tevati fizikalni pojav lezenja (sprememba dimenzij) polimernega izdelka pod vplivom (konstantne) obreme- nitve ali relaksacije (sprememba obremenitve, npr. nosil- nosti) pod vplivom (konstantne) deformacije.9 V. VASI]: POLIMERI V BELI TEHNIKI 244 Materiali in tehnologije / Materials and technology 42 (2008) 6, 243–249 Slika 2: Primerjava natezne trdnosti in nateznih modulov za kovine in polimere Figure 2: Strenght and flexural modulus for different polymer mate- rials and metals Slika 1: Poraba polimernih materialov v industriji bele tehnike v Evropi v letu 2000 Figure 1: Consumption of polymer materials in white goods industry for year 2000 Tabela 1: Prikaz evolucije sesalnika za prah od iznajdbe do danes Table 1: Evolution of vacuum cleaner from the invention to the present moment Sesalnik Pogon/Proizvajalec Leto Prevladujo~i material Mo~ (W) Masa (kg) Cena Ro~ni pogon 1900 les, jadrovina, usnje 50 10 £ 240 / $ 380 / 304 Elektromotor/Cylinder 1950 lahko jeklo 300 6 £ 96 / $ 150 / 120 Elektromotor/Cylinder 1985 brizgan ABS in PP 800 4 £ 60 / $ 95 / 76 Elektromotor/Dyson 1995 PP, PC in ABS 1200 6,3 £ 193 / $ 300 / 240 Kot primer razvoja navajamo multifunkcionalni polimerni material za potrebe hladilno-zamrzovalne tehnike (MABS – BASF Terlur). Razen navadnih last- nosti material omogo~a bolj{o prozornost, odpornost proti razpokam, povzro~enim s ~istili in olji, visoko udarno `ilavost in dobre mehanske lastnosti. Prav tako ima izbolj{ano zvo~no izolativnost v primerjavi s seda- njim SAN materialom, in s tem se lahko bistveno pri- speva k zmanj{anju hrupnosti hladilno-zamrzovalnega aparata.11 V primerjavi z drugimi materiali, npr. kovinami, je toplotna prevodnost polimerov 100- do 1000-krat manj- {a, z nekaterimi polnili pa se prevodnost lahko pove~a za 3- do 4-krat. Ta lastnost se izka`e za zelo uporabno pri hladilno-zamrzovalnih aparatih (izolacija) in pri pralnih strojih (plasti~na kad pralnega stroja). Vendar pa so prav zaradi dobre toplotne izolativnosti (slabe toplotne pre- vodnosti) polimerni materiali zelo zahtevni za predelavo (Op.a. Slaba toplotna prevodnost upo~asnjuje cikel predelave zaradi potrebe po enakomernem ohlajanju produkta) in zato za la`jo predelavo potrebujejo dodatek aditivov ali polnil.12 Zato moramo za doseganje kon~nih funkcionalnih lastnosti upo{tevati tudi zna~ilnosti postopkov predelave. V industriji bele tehnike so najpogosteje zastopane enostavnej{e oblike polimernih tehnologij: – injekcijsko stiskanje – notranje komponente hla- dilno-zamrzovalnih aparatov (police- ABS) in pralni stroj (pralna kad – PP); – ekstrudiranje – celice in protivrata hladilno- zamrzovalnih aparatov (PS); – toplotno preoblikovanje (vakuumiranje) – celice in protivrata hladilno-zamrzovalnih aparatov (PS); – tehnologija formiranja poliuretanskih pen – toplotna izolacija hladilno-zamrzovalnih aparatov. Pri snovanju polimernih komponent v beli tehniki si med drugim pomagamo tudi z ra~unalni{kimi orodji za modeliranje predelovalnih procesov – npr. MoldFlow©, UGS NASTRAN, kjer sku{amo predvideti vedenje poli- mernega materiala med predelovanjem in pri kasnej{i rabi izdelka pri predvidenih pogojih delovanja. 3 VLOGA NAPREDNIH POLIMERNIH MATERIALOV PRI RAZVOJU INDUSTRIJSKIH IZDELKOV Osnovni namen uvedbe naprednih materialov je dose~i izbolj{avo sedanjih in dodatnih funkcionalnosti izdelka, ki jih sicer z navadnimi materiali in postopki ne bi bilo mogo~e dose~i. Pri navadnih polimernih me{anicah (blendih) in polimerih s polnili moramo namre~ za doseganje `elene multifunkcionalnosti narediti kompromis med izbolj{avo `elene lastnosti in drugimi lastnostmi materiala ter stro{ki in procesibilnostjo (predelovalnostjo). Omenjene omejitve brez te`av premagujemo s polimeri na osnovi mikro- in nanokompozitov. V nadaljevanju so predstavljeni trije zgledi sedanje in mo`ne uporabe naprednih materialov v industriji bele tehnike, in sicer: – prepre~evanje prask na povr{ini aparatov; – antibakterijska/antimikrobna za{~ita; – odpravljanje razpok v materialu in – uvajanje mikro- in nanopolnil v polimerne materiale. Prepre~evanje prask na povr{ini aparatov je lahko zgled, zna~ilen za avtomobilsko industrijo. Na avtomo- bilih se zaradi vse pogostej{ega pranja v avtopralnicah uni~uje lak, ker se na {~etkah avtopralnic nahajajo majhni del~ki. Prepre~evanje abrazije laka so se pri Mercedes Benzu lotili z uporabo nanodelcev pri novem laku. Za premaz uporabljajo nanokerami~ne polimerne kompozite, ki tvorijo zelo gosto mre`o v strukturi premaza. Le-ta prepre~uje mikropraske, ki s~asoma nastanejo in so pogosto posledica abrazije. Proizvajalci zagotavljajo, da nano-premaz omogo~a trojno pove~anje odpornosti proti praskam v primerjavi z navadnim premazom.1,2 V svetu velja vse ve~ja zdravstvena ozave{~enost kupcev in mednje spada tudi nevarnost oku`b, ki so posledica ~loveku nevarnih mikroorganizmov. Znano je namre~, da so dolo~eni tipi plastike ali s plastiko pre- vle~eni materiali zelo ugodni za rast mikroorganizmov. Kontaminacija se lahko poka`e v obliki vidne rasti V. VASI]: POLIMERI V BELI TEHNIKI Materiali in tehnologije / Materials and technology 42 (2008) 6, 243–249 245 Slika 3: Prvi ro~ni sesalnik za prah (1908) in sodobna izvedba (2005) Figure 3: First vacuum cleaner (1908) and contemporary vacuum cleaner (2005) mikroorganizmov na povr{ini materiala, razbarvanja ali smradu, v najslab{em primeru pa lahko celo pripelje do poslab{anja vizualnih in funkcionalnih lastnosti plastike. Med zadnje pri{tevamo mo~ in upogljivost, elektri~no izolativnost in prozornost. Na Japonskem so se zaradi gostote populacije in specifi~ne klime kar nekajkrat soo~ili z mno`i~nimi zastrupitvami s hrano, ki je priha- jala v stik s polimernimi izdelki. Zaradi tega se je v industriji bele tehnike `e pred leti pri~ela intenzivno uporabljati antibakterijska/anti- mikrobna za{~ita, ki se navadno izvede s t. i. anti- mikrobno (antibakterijska) plastjo, naneseno na osnovni (nosilni) material (Slika 4a). Tak{en pristop je mo`en tako pri polimernih materialih (npr. koekstruzija vrhnje plasti na nosilni (polimerni) material) ali pa pri drugih materialih (npr. kovinah, keramiki, lesu). Obstaja pestra ponudba antibakterijskih sredstev, ki so najpogosteje anorganskega izvora (srebrovi nanodelci) in se med seboj razlikujejo po stopnji delovanja, u~inkovitosti glede na dolo~en mikroorganizem, zahtevano stopnjo dodajanja, termi~no stabilnost in obstojnost proti spi- ranju. Princip delovanja temelji na kovinskih ionih, ki so tako stabilizirani v vrhnji (koekstrudirani) povr{ini, da se aktivirajo v stiku z drugim agentom, kot je npr. ma{~oba (Slika 4b). Kovinski ioni v vsakem primeru vzajemno delujejo s celi~nimi membranami mikrobov, predvsem pa na osnovi delovanja encimov, in prepre~ujejo njihovo rast. Celice bakterij absorbirajo srebrove ione, ki prepre~ijo delitev RNA/DNA in tako zavirajo rast (t. i. biostati~no delovanje).4,13 Polimer, namenjen za strukturne elemente, je izpo- stavljen po{kodbam v obliki razpok, ki se lahko raz{irijo globoko v strukturo, kar je zelo te`ko zaznati ter skoraj nemogo~e popraviti. Razpoke povzro~ijo mehansko degradacijo, v elektronskih komponentah pa so vzrok za kasnej{e funkcionalne napake. Nastanek (mikro)razpok je posledica termi~nega in mehanskega staranja, kar je dolgoro~en problem pri polimernih materialih. Re{itev omenjenega problema nam lahko ponudijo polimeri z vgrajenimi mikrokapsulami t. i. celilnega sredstva, ki se sprosti ob pojavu razpoke. Polimerizacija celilnega sred- stva se spro`i v stiku z vgrajenim katalizatorjem kateri obkro`a robove razpoke (Slika 4b 13). Na osnovi eksperi- mentalnih rezultatov nam ponujena tehni~na re{itev lahko povrne po razpoki kar 75 % togosti materiala. V redno industrijsko uporabo v beli tehniki so pri{li tudi polimerni materiali z mikropolnili. Zgled tak{nega materiala je mikropolnila v PP (Borealis-Borcom), ki je omogo~il med 8 % in 24 % manj uporabljenega materi- ala ob isto~asnem pove~anju udarne `ilavosti za 30 %.14 Zaradi zelo podobnih dimenzijskih sprememb (skr~kov) izdelka po obdelavi ob zamenjavi obstoje~ega materiala z mikrokopozitom ni potrebna draga spremem- ba orodja. Zelo va`no pa je vedeti, da se zaradi zmanj- {anja mase izdelka in na splo{no manj uporabljenega materiala zmanj{ajo logisti~ni stro{ki. Za enako koli~ino uporabljenega polipropilena z vsebnostjo talka (20 %) ali kalcijevega karbonata (CaCO3; 40 %) se lahko dose`ejo tudi do 15 % prihranki na materialu.14 Mikrokompoziti so dejansko osnova za razvoj nano- strukturnih materialov in nanokompozitov, ki kljub obetajo~im napovedim {e niso popolnoma za`iveli v vsakdanji industrijski praksi in izdelkih bele tehnike. 4 EKOTEHNOLO[KO SNOVANJE IZDELKA BELE TEHNIKE Med prvine trajnostnega razvoja se uvr{~a na~in snovanja izdelka, ki upo{teva vse faze razvoja – na~rto- vanje izdelka, proizvodnjo, uporabo in recikliranje. Tak{en na~in snovanja se imenuje angle{ko Life Cycle Management (LCM) in obravnava celoten trajnostni cikel produkta kot celoto ter optimizira interakcijo med na~rtovanjem izdelka, proizvodnjo ter aktivnostmi v in po eksploataciji (uporabi). V. VASI]: POLIMERI V BELI TEHNIKI 246 Materiali in tehnologije / Materials and technology 42 (2008) 6, 243–249 Slika 4: Prikaz delovanja antibakterijske za{~ite in odprave razpok v polimernem materialu: a) Antibakterijska povr{inska za{~ita polimera, b) Princip celjenja razpoke v polimernem materialu Figure 4: Demonstration of antimicrobial protection and crack healing in the polymer materials: a) antimicrobial surface of polymer materials, b) principle of crakc healing in the polymer material Osnovni cilj je racionalna raba razpolo`ljivih virov in maksimalna u~inkovitost trajnostnega cikla, upravljanja s podatki o izdelku, tehni~ni podpori in seveda s celot- nimi stro{ki (Slika 5 15,16). V mnogih industrijskih dr`avah, v{tev{i Evropsko unijo, se je predlagalo ali celo `e uveljavilo kar nekaj okoljevarstvenih zahtev za produkte, s katerimi `elijo zmanj{ati vpliv na okolje z razli~nimi ukrepi: – eko-oznake ali okoljske deklaracije – s katerimi se ozna~uje ta okoljsko ovrednoten izdelek in njegov mo`en vpliv na okolje; – okoljsko ozave{~anje javnosti in nakup okoljsko prijaznej{ih izdelkov; – razvr{~anje izdelkov glede na njihov okoljski vpliv in na razpolo`ljive (naravne) vire ter – obvezujo~e recikliranje lastnih izdelkov po koncu uporabe (eksploatacije). Za vsak izdelek veljajo razli~ne faze trajnostnega cikla, ki jih lahko povzamemo kot in`eniring in proiz- vodnjo na eni strani ter ~as uporabe na drugi. Vsak proizvajalec mora zagotoviti zanesljiv in predvsem ne- {kodljiv nastanek (proizvodnjo) in delovanje izdelka ter mo`nost kasnej{e ponovne uporabe ali okolju prijazne razgraditve. Za primer pralnega stroja in v primerjavi z ostalimi produkti je pokazan primer trajanja posameznih traj- nostnih faz (Slika 6 16). Proizvajalci polimernih materialov in pripadajo~ih tehnologij v beli tehniki imajo zato s stali{~a omenjene okoljske regulative zelo to~no predpisane mejne vred- nosti za uporabo v sodobnem gospodinjskem aparatu (npr. toplotna izolativnost, reciklabilnost, hrupnost), ki so iz leta v leto vse bolj stroga in zahtevna. 5 PRIMER IZ PRAKSE – RAZVOJ POLIMERNEGA MATERIALA ZA BELO TEHNIKO V GORENJU V podjetju Gorenje, d. d., je `e dolgo poznan in uveljevljen najzahtevnej{i sistem okoljskega (eko- lo{kega) mened`menta po prvinah standarda ISO 14001, WEEE direktive (2002/96/EC), RoHS-direktive (2002/95/EC). V letu 2003 se je sistem okoljskega mened`menta poenotil in nadgradil s sistemom EMAS – Energy Management and Audit Scheme (EC/761/2001). Podjetje tako pri izbiri ne samo polimernih materialov, temve~ vseh komponent upo{teva Evropsko direktivo o okoljskem na~rtovanju izdelkov, ki porabljajo energijo (EuP – Eco-design Requirements for Energy-Using Products).17,18 Najbolj izrazita zgleda uporabe polimernega materiala v proizvodnem programu bele tehnike skupine Gorenje, d. d., sta plasti~na kad pralnega stroja (kompo- zit PP×40 CaCO3) in antibakterijska za{~ita hladilno zamrzovalnega aparata (PS).4,5 Uporaba polimerne pralne kadi v proizvodnji ima pred podobno iz nerjave~e plo~evine kar nekaj pred- nosti:5,18,20 – manj{e {tevilo sestavnih delov in s tem posledi~no tudi pralnega stroja; – izbolj{anje nekaterih funkcionalnih lastnosti pral- nega stroja (npr. energijska u~inkovitost, hrup, vibracije…); – avtomatiziranost in poenostavljenost proizvodnega procesa, kar posledi~no pomeni hitro, ceneno in fleksibilno proizvodnjo z racionalnej{im {tevilom delavcev; V. VASI]: POLIMERI V BELI TEHNIKI Materiali in tehnologije / Materials and technology 42 (2008) 6, 243–249 247 Slika 5: Primer trajnostne dobe gospodinjskega aparata – pralnega stroja Figure 5: Appliance life cycle – washing machine Slika 6: Prikaz posameznih faz trajnostnega cikla za nekatere izdelke Figure 6: Phases of life cycle of some typical products – manj{e {tevilo delavcev, potrebnih za sestavo pralne skupine in posledi~no ve~ja produktivnost; – zaradi ni`jih stro{kov proizvodnje in manj kompo- nent pralnega stroja se s tem ve~a konkuren~nost podjetja na trgu. Primerjava pralnega stroja s polimerno pralno kadjo in pralnim strojem s kovinsko kadjo nam nazorno prika- zuje spodnja shema (Slika 7 20). Pri pralnih strojih so nas predvsem zanimale termo-mehanske lastnosti polimerne kadi na dinami~ne obremenitve (npr. vibracije, hladna in topla voda) in zagotavljanje lastnosti v trajnostni dobi izdelka (t. i. ~asovna odvisnost materialnih lastnosti). Zamenjava kovinske s polimerno kadjo je zahtevala precej{njo spre- membo sestavnih komponent (tudi do 30 komponent 18) in uvedbo novih programskih orodij za snovanje izdelka (MoldFlow©) ter upo{tevanje zakonitosti mehanike polimerov in kompozitov.20,21 Razvoj hladilno-zamrzovalnih aparatov se je usmeril v pove~anje toplotne izolativnosti polimernih materialov ter hkrati v du{enje vibracij in hrupa celotnega sistema. Pri teh aparatih smo se sre~ali tudi s tr`no zahtevo za pove~anje stopnje zadravstvene in higienske za{~ite upo- rabnikov, ki smo jo dosegli z antimikrobno (antibakte- rijsko) za{~ito.4,22 Projekt je zajemal uvedbo izbolj{anega tehnolo{kega postopka (koekstruzije) in poznavanje zakonitosti delovanja aditivov ter postopkov preizku- {anja u~inka aditivov polimera. Antimikrobna za{~ita je bila uvedena pri veliki ve~ini hladilno-zamrzovalnih aparatov, in s tem se je potrdila skrb podjetja za kon~nega kupca gospodinjskih aparatov Gorenje. Razen izbolj{anja funkcionalnih lastnosti izdelka je uporaba polimernih materialov znatno pove~ala v{e~nost izdelka in s tem t. i. emocionalno komponento, ki jo lahko dosegajo oblikovalci z drznimi oblikami in barv- nimi odtenki.23 6 SKLEPI V beli tehniki imajo polimerni materiali in njim pripadajo~e tehnologije velik pomen pri razvoju za okolje in uporabnika racionalnega izdelka. Razen znatne izbolj{ave klju~nih funkcionalnih lastnosti (npr. ener- gijska u~inkovitost, zdravstvena za{~ita, manj{i ~as pranja, manj vibracij in hrupa) so polimerni materiali izdelkom bele tehnike pove~ali v{~enost in ergono- mi~ost. Zaradi velike vsebnosti polimernih komponent v industriji bele tehnike se je tudi bistveno spremenil koncept snovanja izdelkov, spodbudile pa so tudi aktivnosti na za to vejo industrije pomembnih razisko- valnih podro~jih v Sloveniji (npr. mehanika polimerov in kompozitov, brizganje polimerov, orodja za simulacijo). V bli`nji prihodnosti tako lahko pri~akujemo ve~jo vsebnost mikro- in nanopolnil v polimernih materialih oz. ve~jo vsebnost mikro- in nanokompozitov na osnovi polimernih materialov. V. VASI]: POLIMERI V BELI TEHNIKI 248 Materiali in tehnologije / Materials and technology 42 (2008) 6, 243–249 Slika 7: Shematski prikaz sestave pralnega stroja s kovinsko in polimerno pralno kadjo; a) Pralni stroj s kovinsko kadjo, b) Pralni stroj s polimerno kadjo Figure 7: Description of washing machine with metal and polymer tub; a) washing machinme with metal tub, b) washing machine with polymer tub 7 LITERATURA 1 Heath, D., Vasi}, V.: Advanced polymer and smart polymer mate- rials in major domestic appliance design, Ljubljana: Institut "Jo`ef Stefan" – International Centre for Sustainable Development, 2004, 49 2 Heath, D., Vasi}, V.: Nanotehnologija, Gorenje, Informacijski bilten – GiB, Velenje – Gorenje, 1 (2004), 1–9 3 Vasi}, V., Umek, P.: Potencials of nanomaterials and nanotechno- logy in the white goods industry, SLONANO 2004 / 3rd Slovenian workshop on nanoscience and nanotechnology, Ljubljana, 21–22 October, 2004. – Ljubljana: Institut "Jo`ef Stefan", 2004, 20 4 Vasi}, V, Me`a, M., Dov{ak, T.: NGC 600 ABP – Uvajanje in analiza antibakterijske za{~ite v hladilno zamrzovalnih aparatih nove generacije, project documentation, Velenje: Gorenje, 2005, 200 5 Vasi}, V., Dimitrievski, I., Cvelbar, R., Emri, I.: Intelegentni poli- merni materiali in pripadajo~e tehnologije, ocena tehnolo{kega potenciala Slovenije na podro~ju okoljskih tehnologij in materialov, elaborat, Center za eksperimentalno mehaniko, Katedra za mehaniko polimerov in kompozitov, Univerza v Ljubljani, Fakulteta za stroj- ni{tvo, april 2003 6 Posch, W.: Plastics for improved major appliances, International Appliance Technology Conference 2005, 9 7 Hagan, R. S., Keelan, W. R.: Plastics – Key materials for innovation and producitivity in major appliances, American Plastics Council, 1994, 21 8 Posch, W.: Borealis polypropylene – helping to shape the future of the white goods industry, International Appliance Manufacturing (2002), 184–187 9 Desai, K. C.: Structural plastics – better performance and low cost, International Appliance Conference, 2005 10 Ashby, M: Material selection in material design, Butterworth- Heinemann, 2nd ed, 1999 11 BASF – Terlur MABS: Transparent, stress cracking resistant and sound dampening material, 2005 12 Van der Vegt, A. K.: From polymers to plastics, Delft University Press, 2002, 149 13 White, S. R., co: Autonomic healing of polymer composites, Nature, 409 (2001), 794–817 14 Gubo, R.: Slim down for best performance – Borcom Microcom- posites: A new product generation to minimize weight and costs, International Appliance Manufacturing, (2005), 124–128 15 The European Eco-label: The European ecological label for washing machine – product fact sheet, Comission Decission 2000/45/EC, ec.europa.eu/environment/ecolabel/pdf/infokit/washmach_en.pdf, 2 16 Westkämper, E., Alting, L., Arndt, G.: Life cycle management and assessment: approaches and visions towards sustainable manu- facturing, Proceedings of the Institution of Mechanical Engineers. Part B. Journal of engineering manufacture, 215 (2001) B, 599–626 17 Gorenje – Corporate Social Responsibility Report and EMAS – Environmental Statement, Velenje: Gorenje, 2006 18 Vasi}, V., Vaupot, J.: Necessary energy regulations for achieving rational energy consumption – Gorenje case, Klimatizacija, grejanje i hla|enje / zbornik radova pisanih za 35. kongres o grejanju, hla|enju i klimatizaciji. – Beograd: Savez ma{inskih in elektrotehni~kih in`enjera i tehni~ara Srbije (SMEITS), 2004, 120–129 19 Sovi~, B.: Polimerna pralna kad prina{a {tevilne prednosti, Pika na G, Velenje – Gorenje, 1 (2007), 20–21 20 Vasi}, V.: Vloga polimerov in pripadajo~ih tehnologij v industriji bele tehnike, TM IPMT @ivljenjski cikel polimernega produkta, Vse`ivljensko izobra`evanje : `ivljenski cikel polimernega produkta, Ljubljana, 21 junij 2006, Gospodarska zbornica Slovenije 21 Fr`ovi}, M.: Gorenje razvija materiale za pralne stroje in hladilnike, Finance 52 (2006), 2 22 Heath, D., Vasi}, V.: Antimikrobna za{~ita pri gospodinjskih apa- ratih. GiB, Velenje – Gorenje, 9 (2005), 12–22 23 DuPont: Elegant design details, DuPont Engineering Design 1–2 (2005), 1–2 KRATICE ABS – Akrilonitril-butadien-stiren kopolimer ALUM – Aluminij CECED – Conseil Européen de la Construction d’appareils Dome- stiques – Zdru`enje proizvajalcev aparatov bele tehnike EMAS – ECO – Management and Audit Scheme / Evropska uredba o orodju za sistematizirano ravnanje z okoljem EuP – Eco-design Requirements for Energy-Using Products GF – Glass Fibres – Steklena vlakna LCM – Life Cycle Management / Menad`ment trajnostnega cikla produkta MABS – Metilmetakrilat-akrilonitril-butadien-stiren kopolimer PA – Poliamid POM – Polioksimetilen PS – Polistiren PUR – Poliuretan PVC – Polivinilklorid RNA/DNA – Ribonucleic acid / Deoxyribonucleic acid – Ribonu- kleinska kislina / Deoksinukleinska kislina RoHS – Restriction of the use of certain hazardous substances in electrical and electronic equipment / Omejitev uporabe nevarnih materialov v elektri~nih in elektronskih napravah SAN – Stiren-akrilonitril WEEE – Waste Electrical and Electronic Equipment Directive / Di- rektiva o odpadnem materialu elektri~nih in elektronskih naprav ZAMAK – Cinkova zlitina (Zink – Aluminium – Magnesium – Kupfer), New_Jersey_Zinc_Company V. VASI]: POLIMERI V BELI TEHNIKI Materiali in tehnologije / Materials and technology 42 (2008) 6, 243–249 249 M. JENKO ET AL.: CHARACTERIZATION OF MULTILAYER PACVD TiN/Ti(B-N)/TiB2 COATINGS ... CHARACTERIZATION OF MULTILAYER PACVD TiN/Ti(B-N)/TiB2 COATINGS FOR HOT-WORKED TOOL STEELS USING ELECTRON SPECTROSCOPY TECHNIQUES KARAKTERIZACIJA VE^PLASTNE PACVD TiN/Ti(B-N)/TiB2 PREVLEKE ZA ORODNA JEKLA ZA DELO V VRO^EM S TEHNIKAMI ELEKTRONSKE SPEKTROSKOPIJE Monika Jenko1, Djordje Mandrino1, Matja` Godec1, John T. Grant2, Vojteh Leskov{ek1 1Institute of Metals and Technology, Lepi pot 11, Ljubljana, Slovenia 2University of Dayton, 300 College Park, Dayton, OH 45469, USA monika.jenkoimt.si Prejem rokopisa – received: 2008-10-13; sprejem za objavo – accepted for publication: 2008-11-18 Multilayer Ti(B-N) layers have been sandwiched between a TiN coating on treated AISI H11 steel and an outermost TiB2 coating. The films were deposited with plasma-assisted chemical vapour deposition (PACVD) and have been characterized using electron spectroscopy techniques. The thickness of the total coating is 1.6 µm and comprised 21 layers. Earlier studies of such coatings using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS) suffer from their relatively large analysis depths. In this work, Field-emission Auger electron spectroscopy (FE-AES) was used to examine the composition of the multilayered films since it has a smaller analysis depth. AES line-scans across cross-sectioned samples and AES depth profiling were used and are shown to be well suited for characterizing these multilayered coatings. These results are compared with combined Field-emission scanning electron microscopy (FE-SEM) and wavelength dispersive spectroscopy (WDS) measurements of the cross-sectioned samples. Key words: plasma-assisted chemical vapour deposition (PACVD), hard TiN/Ti(B-N) coating, AISI H11 tool steel, Auger electron spectroscopy (AES), Field-emission SEM, wavelength-dispersive spectroscopy (WDS) Ve~plastne Ti(B-N) plasti so vrinjene med TiN prevleko jekla AISI H11 in zunanjo TiB2 plast. Plasti so bile nanesene po postopku kemijske parne faze ob pomo~i plazme (PACVD), raziskali smo jih z razli~nimi tehnikami osnovanimi na elektronski spektroskopiji. Debelina celotne prevleke iz 21 plasti je 1.6 µm. Prej{nje raziskave tovrstnih prevlek z rentgensko difrakcijo (XRD), energijsko disperzijsko spektroskopijo (EDS), valovno disperzijsko spektroskopijo (WDS) niso dovolj natan~ne zaradi relativno velike analizne globine. V ~lanku predstavljamo uporabo Augerjeve elektronske spektroskopije s FEG izvorom elektronov (FE-AES) za raziskavo sestave posameznih plasti v ve~plastni prevleki. AES linijska analiza preko preseka vzorca in AES globinski profil se je pokazala za zelo primerno za karakterizacijo tovrstnih ve~plastnih prevlek. Rezultate smo primerjali z meritvami z vrsti~no elektronsko mikroskopijo s FEG izvorom elektronov (FE-SEM) in valovno disperzijsko spektroskopijo (WDS) na pre~no prerezanih vzorcih Klju~ne besede: s plazmo podprt nanos preko kemijske parne faze (PACVD), trde TiN/Ti(B-N) prevleke, AISI H11 orodno jeklo, Augerjeva elektronska spektroskopija (AES) vrsti~no elektronska mikroskopija s FEG izvorom elektronov (FE-SEM), valovno disperzijska spektroskopija (WDS) 1 INTRODUCTION Hard thin films, such as Ti(B-N), are well known for providing surfaces with a high hardness, and good corrosion and wear resistance, giving them a wide range of industrial applications 1–15. Duplex treatments con- sisting of plasma nitriding the steel surface first and then using plasma-assisted chemical vapour deposition (PACVD) to deposit the hard coating has proven successful in improving the wear, fatigue and corrosion resistance, as well as the load-carrying capability, of steel substrates 16–21. The increasing industrial demand for improved hard coatings with tailored properties for die casting and forging tools requires the development of multi-element and/or multi-phase hard films, as well as a better understanding of their composition. Most of the published studies of Ti(B-N) film com- positions use scanning electron microscopy (SEM), wavelength-dispersive electron-probe microanalysis (EPMA) 7,8,10,12, Rutherford backscattering spectroscopy (RBS) 5, Bragg-Brentano X-ray diffraction (XRD) 2,4,7,8,9,10,12 and in some cases transmission electron microscopy (TEM) 4,6. These techniques suffer from their relatively large analysis depths or lack of depth re- solution. In this investigation we have studied PACVD- deposited thin films using Auger electron spectroscopy, which is better suited for Ti(B-N) multilayer characte- rization We demonstrate that a combination of tech- niques such as AES depth profiling and FE-SEM give a better insight into the chemistry and structure of multilayered Ti(B-N) thin films. WDS measurements were made on the same multilayered structure for com- parison. Materiali in tehnologije / Materials and technology 42 (2008) 6, 251–255 251 UDK 669.14.018.252:542.428.2 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 42(6)251(2008) 2 EXPERIMENTAL 2.1 Deposition of the multilayered coating The base material used for the substrate was AISI H11 tool steel with the chemical composition (all in mas %): Fe, 0.39 C, 0.34 Mn, 1.07 Si, 4.93 Cr, 1.26 Mo, 0.35 V, 0.011 Ti, 0.013 P and 0.0004 S. The samples were vacuum heat treated with the same procedure used for forging dies and then plasma nitrided in a Metaplas Ionon HZIW system. The conventional and plasma heating to the working temperature took 3 hours and the nitriding lasted for 24 hours. After nitriding, the samples were polished and sputter cleaned before the multi- layered PACVD films were deposited. The PACVD deposition on the prepared steel was carried out using the standard TiN/Ti(B-N) process developed by Rübig GMBH 22 in a bipolar-pulsed glow discharge at a constant temperature of 530 °C, a pressure of 200 Pa and a bias voltage of –500 V. The deposition process was as follows: 1 hour of TiN, 8 hours of alter- nating low and high boron-content Ti(B-N) multilayers with the high boron content continuously increasing and, finally, 1 hour of TiB2. The TiN films were grown using N2 and TiCl4, Ar and H2 gasses, and for the Ti(B-N) layers, BCl3 was also added to the deposition chamber. The purity of all the gasses used was 99.9%. The coating had a total of 19 alternating Ti(B-N) layers between TiN and TiB2 resulting in a total layer thickness of 1.6 µm. The multilayer arrangement started with a TiN layer on the steel sample, and finished with a TiB2 layer as the surface layer. The boron-containing coating was chosen due to its small grain size, typically 5–7 nm, resulting in an increased resistance to plastic deformation and abrasion when compared to TiN 16,20,21. 2.2 AES line scan and AES depth-profile analysis The AES instrument used was a Thermo Scientific VG Microlab 310-F composed of two ultra-high-vacuum chambers, one for sample insertion and one for the analysis. The electron gun has a thermally assisted Schottky field-emission source that provides a stable electron beam in the range of 0.5 to 25 keV. The electron energy analyzer is of the double-focusing spherical sector type with an electrostatic input lens and can provide an energy resolution of between 0.02% and 2%. The spectrometer has five electron detectors and spectra were acquired with a constant retard ratio (CRR) of 4, which provides an energy resolution that is 0.5% of the pass energy. For the cross-section studies, samples were cross-sectioned using a JEOL Cross Section Polisher, Model SM-09010, and analyzed using an AES linescan at 10 keV beam energy. AES depth profiles of the hard coatings were also measured with a 10 keV electron beam and the sample was sputtered with a 1.2 nA current of Ar ions at 3 keV. The AES data were acquired using Eclipse V2.1 ver07 software and processed using CasaXPS software. 2.3 WDS analysis A metallographic cross-section of the multilayered TiN/Ti(B-N)/TiB2 sample was prepared using a classic metallographic procedure with a Gatan PECS 682 ion polisher. Secondary-electron images and back-scattered electron images were obtained with a JEOL JSM 6500F FE-SEM, with a working distance of 7.1 mm, an accele- rating voltage of 8 kV and a beam current of 0.08 nA. By reducing the accelerating voltage to 8 kV, the area of the emitted backscattered electrons was reduced to a level that enabled the separate layers to be imaged. The WDS line scans were obtained in the FE-SEM using the following standards: pure boron for boron analysis, stoichiometric TiN for nitrogen analysis and pure titanium for titanium analysis. The TiN standard was made with thin film deposition, using a Balzers Plasma Sputron; the TiN stoichiometry was confirmed by XRD 15. The WDS analysis was performed at 5 kV and 10 kV accelerating voltages at currents of 5.0 nA and 9.2 nA, respectively, using an Oxford Instruments INCA WAVE 700. 3 RESULTS AND DISCUSSION 3.1 AES line-scan analysis Figure 1(a) shows an SEM image of the cross- sectioned, multilayered TiN/Ti(B-N)/TiB2 hard coating, together with the position where the AES line-scan was made across the sample. Figure 1(b) shows the elemen- tal concentrations determined from the AES line scan. The N Auger peak overlaps a Ti Auger peak at 420 eV and the N concentration was calculated by comparing the overlapping peak with the 420 eV Ti Auger peak and applying the following relationships: I S Sc c385 385 385≈ +α αN TiN Ti (1) I Sc420 4≈ α Ti Ti 20 (2) giving c c S S I I S S N Ti Ti 20 N Ti 85 N ≈ ⋅ −4 385 385 420 3 385 (3) where I385 and I420 are the peak-to-peak intensities of the overlapping Ti and N peaks at approximately 385 eV and the Ti peak at 420 eV respectively, the S are sensitivity factors for N and Ti peaks at the indicated energies, and the c are the indicated N and Ti concen- trations. The combined N and Ti profile were calculated first, then decoupled using the Ti profile from the Ti peak at 420 eV. Note that B decreases while the N increases with distance from the surface, although the undulations of eight of the boron-depleted regions and eight of the boron-rich regions can be seen in the Auger linescan. There also appears to be a relatively flat region in N concentration near the interface with the steel substrate (corresponding to the TiN layer). M. JENKO ET AL.: CHARACTERIZATION OF MULTILAYER PACVD TiN/Ti(B-N)/TiB2 COATINGS ... 252 Materiali in tehnologije / Materials and technology 42 (2008) 6, 251–255                                                                                                             !    "#                                                               $ %&'() & *+ ,-.,&./0/)' )! $1*/*2&. 3,45 '6'6 ,)/'78  $     9 6 $      : ;;# < =">== =   ?      &/                     &/            @58        &/  ?   ?  A    ? B   ?   &/  ?   B  ? ?B9  A9 @58 B    8&$        '6'66            &8       ?  <;;   C         '   !      &8                8&$  ? ? B  '6'669?  ?B9  A9 &8 9? B $ ? 9 <;; C   ?  9  '   ! BA B &8 9? B B D 9 ?B     &8         6'6'            E  "=          &8  ?   6'6' A      ? ?B9 D "=   ?  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( % 4 8    " ''  ";#: "J $ 8  , $   8  &   7 !    . ( % 4 8    " ''  ";#: ; 3 - $  $ - 8  , $  8  $  = '' :" " $ 8  &   , *  , $  8  ,     "< ''  :="  $ 33  . 7$-L,   +66  $ %&'() & *+ ,-.,&./0/)' )! $1*/*2&. 3,45 '6'6 ,)/'78  $     9 6 $      : ;;# < =">== ==   ['M]6  '6                  &8          ,                      '             ['M]6  '6 B9 ? B ?   ? B 9 B &8  ?    (  4 D H     9 9 B  A  9 B '  9 H  ?   S. RE[KOVI], F. VODOPIVEC: AN INVESTIGATION OF THE STRETCH REDUCING OF WELDED TUBES AN INVESTIGATION OF THE STRETCH REDUCING OF WELDED TUBES RAZISKAVA IZTEZNE REDUKCIJE VARJENIH CEVI S. Re{kovi}1, Franc Vodopivec2 1Faculty of Metallurgy, University of Zagreb, Sisak, Croatia 2Institute of Materials and Technology, Ljubljana, Slovenia reskovicsimet.hr Prejem rokopisa – received: 2008-10-20; sprejem za objavo – accepted for publication: 2008-11-12 The results of an investigation into the hot stretch reducing of high-frequency welded steel tubes are presented. The internal stresses were determined after every processing pass. The selected processing parameters ensured that the coefficient of plastic extension was maintained in the range that prevents the tearing of the tube wall and achieves the required geometrical shape as well as the planned properties of the finished tube. Key words: welded tube, hot stretch reducing, micro-alloyed steel, internal stresses, coefficient of plastic extension Predstavljeni so rezultati raziskave iztezne redukcije visokofrekven~no varjenih jeklenih cevi. Po vsakem prehodu so bile dolo~ene notranje napetosti. Izbrani procesni parametri zagotavljajo, da se koeficient plasti~nega podalj{ka ohranja v razponu, ki prepre~uje trganje cevne stene in zagotavlja, da se dose`ejo predpisane mere in mehanske lastnosti cevi. Klju~ne besede: varjene cevi, vro~e iztezna redukcija, mikrolegirano jeklo, notranje napetosti, koeficient plasti~nega podalj{ka 1 INTRODUCTION With proper hot working small additions of micro-alloying elements can improve the properties of hot-rolled sheets produced from structural steels 1,2. Hot-rolled welded tubes are manufactured from hot-rolled sheets with carbide precipitates formed by deformation-induced precipitation during the final stages of hot rolling. In the technology of the stretch reducing of welded tubes, the initial tube blank is processed at an appropriate temperature and without internal tool (mandrel) to a different diameter and wall thickness 3. The calculation of the per-pass reductions of the tube diameter and the wall thickness is relatively complex 3,4,5, and their proper sequence depends on the type of steel, the rolling temperature and the rate and the extent of reduction of the tube’s diameter and the wall thickness. In this study the results of an experimental investigation on the evolution of the microstructure and internal stresses for a sequence of stretch-reducing passes for a micro-alloyed steel are presented. The findings in this investigation were used in the selection of the optimal parameters for industrial processing. In the process of manufacturing hot-rolled welded tubes, the sheet is formed at room temperature in a tube pre-form, high-frequency welded, heated first to the normalising temperature, to homogenise the micro- structure in the weld, and then heated to the hot-rolling temperature, processed with stretch-reducing passes to the required size and air cooled 5. The processing mill consists of several three-rolls high stands 4,5. For proper processing, a balancing of the maximum allowed changes to the tube diameter and the wall thickness is required. Achieving the final tube size depends on the maximum allowed deformation and stressing of the steel at the temperature of every processing pass. The reduction of the diameter occurs in several passes and depends on the total reduction and the number as well as the size and design of passes. In the initial processing stands of the investigated mill, the tube diameter decreased quickly to a constant value, then it decreased more slowly towards the end pass to ensure to obtain the required tube diameter and wall thickness. The deformation was 3–5 % per pass and stand 4,5. The maximum extent of the wall reduction depends on the steel’s plastic elongation, the total and the per-pass. To prevent hot tearing of the tube wall it is necessary to know for every stand, the maximum coefficient of the steel’s plastic extension (stretching), which is given as a ratio of the axial stressing and the steel’s elongation 4,5,and can achieve a maximum value of Zt = 1. For Zt = 0.5 the wall thickness is increased, and for a reduction of the tube diameter by 3–5%, the tube length and the wall thickness are increased. Experience shows that up to Zt = 0.55, the wall thickness remains un- changed. However, for Zt = 1 the tube diameter and the tube-wall thickness are reduced simultaneously and tube-wall ruptures occur frequently. For this reason, the maximum value of the coefficient of plastic extension is Zt =0.7–0.8 4,5. 2 FRAMEWORK AND SCOPE OF THE INVESTIGATION The fundamental processes and mechanisms of austenite hot deformation, carbide precipitation and Materiali in tehnologije / Materials and technology 42 (2008) 6, 257–262 257 UDK 669.14.018.298:621.791 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 42(6)257(2008) austenite grain growth are involved in the processing of micro-alloyed steels 6. After proper thermomechanical processing, the hot-rolled sheet has a fine-grained and homogenous microstructure and good mechanical properties 1. With higher temperatures, coarsening of the precipitates occurs, with the kinetics depending on the temperature, the solid solubility and the diffusivity 6,7. With a smaller solid solubility, the coarsening rate of the precipitates is reduced, and it is also less for nitride than for carbide particles 6. Precipitates with a diameter over a critical value and at large mutual distance accelerate the static recrystallisation of austenite, while for a small mutual distance the small precipitates may even hinder the static recrystallisation of austenite 8,9,10,11. In niobium micro-alloyed structural steels the static recrystallisation of austenite occurs at a sufficient temperature if the per-pass rolling deformation is above approximately 12 % 12,13. In the investigated processing the per-pass reduction was below this level, and therefore the deformation energy was released only with recovery. The large number of point and line defects introduced into the steel by the plastic deformation produces strain hardening and softening processes with a mutual relation depending on the steel’s chemical composition, the initial microstructure and the extent of deformation, the rate of deformation and the deformation temperature. Hardening is the result of an increase in the density of the deformation defects and softening corresponds to a decrease. The rates of diffusion, precipitation and precipitate coarsening can be increased in non-recrystallised austenite by up to two orders of magnitude 10,11,14 when compared to that in the recrystallised austenite, and this affects significantly the density and the mobility of the vacancies and dislo- cations. The precipitation rate depends on the temperature, the degree of deformation and the content of elements affecting the recrystallisation, especially niobium. In the hot-rolled sheet used in this investigation, we found mostly particles formed by deformation- induced precipitation 1. With high-frequency welding the steel is locally heated up to 1400 °C; however, the heating time is very short and it does not produce a significant change in the size and distribution of the precipitates. With the subsequent reheating of the tube blanks to 850 °C, the microstructure in the weld area is homogenized, while the size and the distribution of the precipitates are not affected. The initial temperature of the stretch reducing of the welded tubes depends on the number of passes and should be sufficiently high to ensure the finishing temperature is above the austenite-to-ferrite transformation 4,5. For low-carbon steel it is in the range 1100–950 °C. During soaking, coarsening of the austenite grains occurs and part of the niobium carbonitride is dissolved in austenite, as the solid solubility is attained only at a higher temperature. Parallel coarsening of the non-dissolved precipitates could also occur. In the process of mastering the technology of stretch reducing high-frequency welded tubes from niobium micro-alloyed steels, in the central passes of the processing line tearing of the tube wall occurred frequently, especially in the weld area. The aim of this work was to investigate the microstructure processes that may be related to the tearing. In the frame of the investigation the microstructure, the mechanical properties and the evolution of the internal stresses generated by the deformation were investigated. 3 EXPERIMENTAL WORK In Tables 1 and 2 the chemical composition of the steel and the mechanical properties of the sheet determined from specimens cut out from the initial, centre and end of the coil, are shown. The microstructure of the steel sheet consisted of fine polygonal ferrite and pearlite grains (Figure 1). Table 1: Composition of the steel, w Tabela 1: Sestava jekla, w C Mn Si P S Al Nb O2 N2 0.14 0.8 0.12 0.011 0.018 0.005 0.049 0.005 0.009 The 370 mm × 3.6 mm steel band of the required length was cut out from the coil, then it was shaped to a tube blank with a diameter of 117 mm, which was high-frequency welded 15, heated to 850 °C for the homogenisation of the microstructure and the relaxation of the internal stresses, then heated to the hot-working temperature and processed within the temperature range 960–830 °C. In Figure 2 the microstructure is shown for different parts of the welded tube: the section of the weld, the heat-affected zone, the base material and the weld. The weld is narrow and the microstructure of the S. RE[KOVI], F. VODOPIVEC: AN INVESTIGATION OF THE STRETCH REDUCING OF WELDED TUBES 258 Materiali in tehnologije / Materials and technology 42 (2008) 6, 257–262 Table 2: Mechanical properties of the sheet Tabela 2: Mehanske lastnosti traka Re/MPa Rm/MPa A5/% Kcv/(J/mm2) Direction Axial Transv. Axial Transv. Axial Transv. Axial Transv. Coil onset 500 492 599 590 32.5 29.8 123 147 Coil centre 496 490 592 586 30.5 30.3 120 138 Coil end 495 495 590 576 30.1 31.9 118 127 characteristic heat-affected zones is similar to that in the standard welds of structural steels 15. The reduction to the final size of d 48.3 mm × 3.2 mm is achieved in 12 passes, applying a per-pass deformation, preventing the tearing of the tube wall, the achieving of the final diameter and ensuring that the final pass temperature is just above the austenite-to- ferrite transformation point, which strongly affects the properties of the steel in the finished tube. The tests and examinations were carried out on samples of A, a hot rolled sheet; B, a welded tube; C, a tube after heating to the rolling temperature and water quenching, and D, a finished tube. At 12 selected points of the processing, the mill was stopped and the samples of the so-far processed tube 1 to 12 were cut out and air cooled or water quenched. Using these samples the diameter of the tube and the thickness of the tube wall were checked and the specimens for mechanical tests and optical microstructure examinations were prepared. Carbide and carbonitride particles were extracted from the steel with electrolytic dissolution and identified with X- ray diffraction analysis. For the identification of the phases found in isolates the data in 16,20 were used. The internal stresses were determined on specimens with a finely ground and etched surface. The Debye diffraction lines were checked for the wavelength and the (310) peak of -iron, the widths of the (110), (200) and 211) lines for iron were assessed at half intensity and the internal stresses were deduced quantitatively using the method proposed in 17,18,19. 4 RESULTS AND DISCUSSION 4.1 Processing parameters From the data obtained on samples cut out from the tube after all stretch reducing passes the partial deformations shown in Figures 3 and 4 were deduced. The pass temperature is also given in both figures. The per-pass decrease of the tube diameter is large and virtually constant in the initial 7 passes, after this it decreases. The decrease of the tube diameter depends on the processing procedure. As a rule, the stretch reducing in the first passes results in the maximum decrease of the tube. In Figure 3 a decrease of 57 % of the tube diameter was obtained in the first four passes. The wall thickness is achieved by stretching, and it depends strongly on the steel extension in every pass that is lower in the previous passes 4,5. On the investigated mill the drawing reduction of the wall thickness of the structural steel occurs in the S. RE[KOVI], F. VODOPIVEC: AN INVESTIGATION OF THE STRETCH REDUCING OF WELDED TUBES Materiali in tehnologije / Materials and technology 42 (2008) 6, 257–262 259 Figure 3: The per-pass () and total () reduction of the diameter Slika 3: Redukcija premera cevi, na vtik () in skupna () Figure 2: Tube-weld area a) macrography of the weld section, b) micrography of the welding area: A- base steel, B – heat-affected zone, C – weld Slika 2: Podro~je zvara cevi a) makrografija prereza zvara, b) mikrografija podro~ja zvara: A – osnovno jeklo; B – toplotma zona, C – var Figure 1: Microstructure of the sheet Slika 1: Mikrostruktura traku temperature range from 1000 °C to 800 °C by the value Z = 0.6–0.72, and a maximum per-pass wall reduction of 2 %. For the processing of niobium micro-alloyed steel, the maximum coefficient of plastic extension is lower, i.e., Z = 0.65 % (Figure 5). The required tube diameter and tube-wall thickness can be achieved only with smaller per-pass reductions and, accordingly, the value of the coefficient of plastic extension is lower also. The curve of total deformation in Figure 3 was found to be virtually ideal for the processing of the investigated tube on the used stretch-reducing line, as the defor- mation parameters in Figure 5 ensured stable processing and the required size and properties of the tube. In the range of the analytical accuracy, the content of niobium carbo-nitride was equal for all the specimens 20. This confirms the assumption that the content of niobium carbide is not affected by the processing parameters. It is interesting that in the weld a small quantity of niobium nitride was also detected. 4.2 Internal stresses The difference in the shapes of the X-ray diffraction spectra for the base material and for the weld is very clear (Figure 6). Since ferrite is the matrix in both cases, the absence of the Kα doublet confirms the presence of internal stresses in all the specimens water quenched from the processing temperature and used for the X-ray examination. After hot plastic deformation, the profile of the diffraction line is similar, and this indicates a partially homogenised microstructure (Figure 7); however, the internal stresses are still slightly greater in the weld. The diffraction line for the base material is virtually equal for the blank and the processed tube, although it is very different for the weld. The presence of the Kα doublet after stretch reduction indicates that the hot deformation had a favourable effect on the microstructure and on the internal stresses in the weld. The intensity of internal stresses after the following processing passes is shown in Figure 8. The stresses remain constant in the specimens up to the third pass, then increase quickly in the following six passes, with a constant coefficient of plastic extension, and then gradually decrease in subsequent passes, parallel to a decrease in the coefficient of plastic extension. The evolution of stresses is virtually equal for the base material and the weld, and this indicates an identical reaction of both to the deformation and the equal extent of the interpass softening processes. It also confirms that the applied thermal regime of the tube blank before the start of the stretch reducing helped to avoid a greater intensity of internal stresses and a greater propensity for tearing of the tube wall in the weld area. S. RE[KOVI], F. VODOPIVEC: AN INVESTIGATION OF THE STRETCH REDUCING OF WELDED TUBES 260 Materiali in tehnologije / Materials and technology 42 (2008) 6, 257–262 Figure 6: Welded tube. Profile of the diffraction lines (220) for the weld (1) and the base steel (2). Slika 6: Zvarjena cev. Profil uklonskih ~rt (220) za zvar (1) in za osnovno jeklo (2) Figure 5: Change of Z() and the total reduction of tube εt () in stretch reducing Slika 5: Sprememba Z() in skupna redukcija cevi εt () Figure 4: The per-pass () and total () reduction of the thickness of the tube wall Slika 4: Redukcija debeline stene cevi, na vtik () in skupna () Figure 7: Finished tube. Profile of the diffraction lines (220) for the weld 1) and the base steel (2). Slika 7: Izdelana cev. Profil difrakcijskih ~rt (220) za zvar (1) in za osnovno jeklo (2) The faster cooling of the specimens for the X-ray examination explains why the internal stresses are even greater than the yield stress determined for the air-cooled specimens. The internal stresses are greater for a greater density of lattice defects generated by the plastic deformation of austenite and the decreasing extent of the recovery due to the lowering of the processing temperature. It is logical to assume that with an increased density of lattice defects the steel’s hot workability is lower and that the relation between the stresses in different specimens is preserved after quenching. The first assumption is confirmed by the fact that the tube-wall tearings were more frequent for the passes with greater internal stresses and the second is the difference in the level of internal stresses for the specimens quenched after a different total deformation. In the range of the increase of the internal stresses the per-pass deformation was constant. The stresses were determined from X-ray spectra at room temperature and these are not equal to the stresses at the processing temperature. In reality, the stresses are a relative measure of the extent of the release of deformation hardening and of the residual deformation capacity of the steel, which are of essential importance for the smooth operation of the stretch-reducing line. On the basis of the assessment of the intensity of internal stresses and of the processing experience it can be concluded that the increased internal stresses due to the incomplete release of the deformation energy with interpass recovery and the lowering of the workability can be balanced with the selection of the proper value of the coefficient of plastic extension. 4.3 Properties of the finished tube The mechanical properties determined for the sections of the tube with a weld and without a weld and are shown in Table 3. The very fine grain size (Figure 9) ensures that the tube has excellent mechanical properties, in accord with the standard requirements. The mechanical and technological properties are virtually equal for the weld and the base material and confirm that the temperature-deformation regime made it possible to achieve a sufficient degree of homo- genisation of the microstructure of the weld and the base material, the processing without tube-wall tearings, and the good mechanical properties of the finished tube. S. RE[KOVI], F. VODOPIVEC: AN INVESTIGATION OF THE STRETCH REDUCING OF WELDED TUBES Materiali in tehnologije / Materials and technology 42 (2008) 6, 257–262 261 Figure 8: Evolution of internal stresses (weld, base material ) and coefficient of plastic extension Z () with respect to the processing temperature. Slika 8: Evolucija notranjih napetosti (var , osnovni material ) in koeficient plasti~nega podaj{ka Z () v odvisnosti od temperature predelave Figure 9: Section and microstructure of the finished tube wall in the weld area (A – weld zone) Slika 9: Prerez in mikrostruktura stene cevi v podro~ju zvara (A – podro~je zvara) Table 3: Mechanical and technological properties of the finished tube Tabela 3: Mehanske lastnosti izdelane cevi Spec. Re/MPa Rm/MPa A5/% Kcv/(J/mm2) Weld B.mat. weld B.mat. Weld B.mat. weld B.mat. 1 502 497 590 596 32.5 31.7 104 110 2 495 492 592 609 32.5 32.7 101 122 3 496 500 592 603 31.9 32.01 106 112 5 CONCLUSIONS The quality and reliability of the hot stretch reducing of high-frequency welded tubes depend strongly on the understanding of the processes in the steel at the operating temperature and the mastering of these processes in individual passes as well as during the entire processing line. With the initial heating of the welded blank a sufficient homogenisation of the weld and base material’s microstructure and the internal stresses were achieved. The initial hot-working temperature ensured the steel had sufficient ductility on the processing line and a sufficient finishing temperature above the austenite-to-ferrite transformation. The per-pass changes of the tube diameter and the wall thickness ensured a smooth processing without any tube-wall tearings. Of great importance to mastering the hot stretch reducing of tubes was understanding the per-pass evolution of the internal stresses and the choice of the per-pass deformation parameters, ensuring that we maintain the optimum value of the coefficient of plastic extension. 6 REFERENCES 1 F. Vodopivec, S Re{kovi}, I. Mamuzi}. Evolution of substructure during the continuous rolling a microalloyed steel strip. Mat. Sci. Techn.15 (1999), 1293–1299 2 S. Re{kovi}. Studij mehanizama precipitacije i rekristalizacije u podru~ju zavr{nog oblikovanja mikrolegiranog ~elika (Investigation of the mechanisms of precipitation and recrystallisation by finishing of a microalloyed steel), Dr. Diss., University of Zagreb. 1997 3 K. H. Staat. Streckreduzierwalzwerk, Metalurgija. 10 (1971), 13–22 4 R. Kri`ani}. Raspodjela brzina u zoni deformacije i prora~un broja okretaja pri kontinuiranom toplom valjanju cijevi na izvla~no-redu- cirnom stanu (Deformation rate and number of revolutions by the continuous hot rolling of tubes on the stretch reducing mill). Metalurgija. 26 (1987), 109–116 5 R. Kri`ani}. Prora~un povr{inskog presjeka stijenke cijevi pri valjanju na izvla~no-reducirnom stanu (Calculation of tube wall cross –section at rolling on the stretch reducing mill). Metalurgija. 27(1988), 3–11 6 R. W. K. Honeycombe. Fundamental aspects of precipitation in microalloyed steels, HSLA Steels Technology & Applications, Proceedings of the International Conference on Technology and Applications of HSLA steels, Philadelphia, USA, (1983), 243–261 7 T. Tanaka, N. Tabata, T. Hatamura. Three Stages of the Controlled- Rolling Process. Proceedings “Micro Alloying 75”, Union Carbide Corporation, New York, 1977, 107–120 8 T. M. Hoogendorn, M. J.Spanrant. Quantifying the Effect of Microalloying Elements on Structures during Processing , Ibiden, (1977), 57–63 9 W. Roberts. Recent Innovations in Alloy Designing and Processing of Microalloyed Steels. HSLA Steels – Technology and Appli- cations. Proceedings of the International Conference on Technology and Applications of HSLA STEELS, Philadelphia, Pennsylvania, USA, (1983), 243–251 10 J. J. Jonas, I. Weiss. Dynamic precipitation and coarsening of niobium carbonitrides during the hot rolling of HSLA steel, Metall. Trans., 11A (1980), 403–410 11 J. Jonas, I. Weiss. Effect of Precipitation on Recrystallization in Microalloyed steel. Met. Sci. 65 (1979), 238–245 12 T. Tanaka, N. Tabata, T. Hatamura, C. Shiga. Three Stages of the Controlled-Rolling Process, Microalloying75, Ed. U. C. Corporation, N. York, 1977, 107–120 13 I. Kozasu, C. Ouchi, T. Sampei, T. Okita. Hot Rolling as a High- Temperature Thermo-Mechanical Process. Ibidem, 1977, 120–129 14 T. Gladman. Recrystallization and Grain Growth of Multi-Phase and Particle Containing Materials. Ed. N. Hansen, A. R. Jones and T. Leffers, Risd National Laboratory, Denmark, 1980, 19 15 S. Re{kovi}, M. Prelo{}an. Zavarivost niobijem mikrolegiranog ~elika visokofrekventnim postupkom (High frequency welding of a niobium microalloyed steel). Zavarivanje 35, 1992, 191–199 16 J. T. Norton, R. E. Hiller. Structure of Cold Drawn Tubing. Trans. AIME, 99 (1993), 190 17 C. N. Wagner, A. S. Tetelman. Diffraction from Layer Faults in bcc and fcc Structures, J.Appl. Phys. 33 (1992), 3080 18 R. E. Smallman, K. H. Westmacott. Stacking faults in face-centred cubic metals and alloys, Phil. Mag. 2 (1957), 669 19 B. Warren. X-Ray Studies of Deformed Metals. Prog. Metal Phys. 8 (1959), 147–202 20 Vj. Novosel-Radovi}. Rentgenska difrakcija u laboratoriju @eljezare Sisak 8X rays diffraction in the laboratory of Steelwork Sisak). Strojarstvo. 36 (1994), 9–13 S. RE[KOVI], F. VODOPIVEC: AN INVESTIGATION OF THE STRETCH REDUCING OF WELDED TUBES 262 Materiali in tehnologije / Materials and technology 42 (2008) 6, 257–262 M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH IN WELDED JOINT OF STEEL FOR ELEVATED TEMPERATURE EKSPERIMENTALNA ANALIZA NASTANKA IN RASTI RAZPOKE V ZVARU JEKLA ZA POVI[ANO TEMPERATURO Meri Burzi}1, @ivoslav Adamovi}2 1 Institute "GO[A", d. o. o., Milana Raki}a 35, 11000 Belgrade, Serbia 2 University of Novi Sad, "Mihajlo Pupin" Faculty of Technical Engineering, \ure \akovi}a bb, 23000 Zrenjanin, Serbia meribneobee.net Prejem rokopisa – received: 2008-07-10; sprejem za objavo – accepted for publication: 2008-07-22 This paper presents results of experimental investigation of crack resistance by static and variable loading of alloyed steel A-387 Gr. 11 for elevated temperature application and its welded joint. Using SEN-B, CT and Charpy pre-cracked specimens, the significance of heterogeneity of microstructure and mechanical properties of welded joints on fracture toughness and fatigue crack initiation and propagation, at room and working temperatures, is evaluated. Keywords: Alloyed steel, welded joint, fracture toughness, crack propagation rate, fatigue crack, fatigue threshold V ~lanku je predstavljena eksperimentalna raziskava odpornosti razpoke pri stati~ni in izmeni~ni obremenitvi pri jeklu A- 387Gr 11 za uporabo pri povi{ani temperaturi in zvarih tega jekla. Z uporabo SEN-B-, CT- in Charpy-preizku{ancev z razpoko smo ocenili pomen heterogenosti mikrostrukture in mehanskih lastnosti zvara za `ilavost loma in za za~etek ter za napredovanje utrujenostne razpoke pri sobni in pri delovni temperaturi. Klju~ne besede: legirano jeklo, zvar, `ilavost loma, hitrost napredovanja razpoke, prag utrujenosti 1 INTRODUCTION The in-service behaviour of alloyed steel A-387 Gr. 11 Class 1, for pressure vessels, used for high tempera- ture applications, depends on the properties of its welded joint, with parent metal (BM), heat-affected-zone (HAZ) and weld metal (WM) as constituents. Critical locations, regarding integrity of welded joint can be formed in HAZ and WM 1. Qualification of specified welding technology of plates, 96 mm thick, of steel A-387 is performed according to standard EN 288-3 2. While determining the plane strain fracture tough- ness, KIc, of welded joint constituents with hetero- geneous microstructure, one must bear in mind, in order to hold the validity of theoretical assumptions and meanings of fracture toughness as measured property, that fracture mechanics is based on material homo- geneity, including the region of crack tip. A characteristical property of the welded joint is the heterogeneity of microstructure and mechanical proper- ties, together with, irregular internal stress distribution with residual stresses and stress concentration. These important problems do not exclude experimental determination of plane strain fracture toughness, KIc, of welded joint and its constituents, although they present difficulties in the interpretation of measured values and obtained results 3–5. For better understanding of crack occurrence and its growth effect in welded joints of steel for elevated temperatures, applied in equipment for high pressure, it is necessary to quantify the parameters controlling the strain behaviour in crack tip vicinity and crack resis- tance. Therefore, in this paper the effect of heterogeneity of microstructure and mechanical properties on fracture toughness, KIc, fatigue crack growth rate, da/dN, and fatigue threshold stress intensity factor range, Kth, of A-387 steel and its welded joint constituents is experi- mentally investigated at room temperature (20 °C) and at working temperature (540 °C) 6. 2 MATERIAL FOR TESTING The welded joint sample (350 × 500 × 96) mm with double "U" weld metal in the middle of the steel A-387 was used for this investigation 6. Two welding proce- dures were applied 6: • shielded metal manual arc welding (SMAW) with coated electrode LINCOLN Sl 19G (AWS: E8018-B2 for root weld passes; • Submerged arc welding (SAW), applying as consumable wire LINCOLN LNS 150 and flux LINCOLN P230, for filler passes. The welded sample and scheme of cutting out of specimens from welded joint (OM, HAZ and WM) are shown in Figure 1. The chemical composition and mechanical properties of A-387 1 steel are given in Tables 1 and 2. The chemical composition of electrode LINCOLN Sl 19G and wire LINCOLN LNS 150 Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 263 UDK 669.14.018:621.791.05:539.42 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 42(6)263(2008) according to certificates is given in Table 3, and the mechanical properties according to certificates are given in Table 4. Table 1: Chemical composition of tested steel A-387 Tabela 1: Kemi~na sestava jekla A-387, ki je bilo uporabljeno za preizkuse Chemical composition, w/% C Si Mn P S Cr Mo 0.15 0.29 0.54 0.022 0.011 0.93 0.47 Table 2: Required mechanical properties of tested steel A-387 Tabela 2: Predpisane mehanske lastnosti za jeklo A-387 Yield stress, min. Tensile strength Elongation Impact energy Rp0,2/MPa Rm/MPa A/% KV/J 315 490-620 25 > 85 Table 3: Chemical composition of filler metal 6 Tabela 3: Kemi~na sestava deponiranega materiala 6 Filler material Chemical composition, w/% C Si Mn P S Cr Mo LINCOLN Sl 19G 0.08 0.045 0.35 0.025 0.025 1.10 0.50 LINCOLN LNS 150 0.11 0.18 0.37 0.020 0.020 1.04 0.47 Table 4: Mechanical properties of filler metal 6 Tabela 4: Mehanske lastnosti deponiranega materiala 6 Filler material Yield stress Rp0,2/MPa Tensile strength Rm/MPa Elongation A/% Impact energy KV/J LINCOLN Sl 19G 505 640 23 > 95 LINCOLN LNS 150 490 610 26 > 100 3 TENSILE PROPERTIES Tensile testing of specimens taken from parent metal, from weld metal, and from butt welded joint, were performed on an machine in displacement control, at room and at working temperature. The specimen from WM for testing at room temperature, was machined from the available material, according to standard EN 895 7. For easier comparison of results the specimen from BM is of the same dimensions according the standard EN 10002-1 were used. Specimen from welded joint was made according EN 895. For testing at the temperature of 540 °C the same specimen design according to standard ASTM E1475-00 8 was used for all welded joint consituents with dimensions adopted to available equipment. Typical stress – strain curves for specimens from BM, WM and from welded joint, tested at room and M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... 264 Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 Figure 1: Scheme of testing sample of double “U” weld metal and specimens sampling 6 Slika 1: Shema preizku{anca z dvojnim U-zvarom in odvzem vzor- cev 6 Figure 2: Diagrams stress – elongation: a) BM, b) WM, c) welded joint Slika 2: Odvisnosti napetost–podalj{ek: a) BM, b) WM, c) zvarni spoj working temperature, are given in Figure 2. The testing results at room and at working temperatures are given in Table 5 for BM, in Table 6 for WM and in Table 7 for the specimens of welded joint. The effect of testing temperature on tensile properties is clear. At higher temperature the values of yield stress and tensile strength are smaller, and elongation values are increased, as seen in Figure 2 and in Tables 5–7 7. However, this conclusion is very simplified and apparent, as it will be discussed. Table 5: Results of tensile testing of BM specimens Tabela 5: Rezultati nateznih preizkusov BM-preizku{ancev Specimen Testing temperature C Yield stress Rp0,2/MPa Tensile strength Rm/MPa Elongation A/% OM-1-1N 20 330 495 37.6 OM-1-2N 318 479 36.1 OM-1-3N 324 488 38.7 OM-2-1N 540 219 284 40.1 OM-2-2N 212 279 39.6 OM-2-3N 226 303 39.9 Table 6: Results of tensile testing of WM specimens Tabela 6: Rezultati nateznih preizkusov WM-preizku{ancev Specimen Testing temperature C Yield stress Rp0,2/MPa Tensile strength Rm/MPa Elongation A/% OM-1-1N 20 491 576 32.7 OM-1-2N 504 592 31.6 OM-1-3N 496 585 33.9 OM-2-1N 540 338 401 36.9 OM-2-2N 331 396 36.2 OM-2-3N 345 409 37.8 Table 7: Results of tensile testing of welded joint specimens Tabela 7: Rezultati nateznih preizkusov WM-preizku{ancev Specimen Testing tempe- rature C Yield stress Rp0,2/MP a Tensile strength Rm/MPa Elongatio n* A/% Location of fracture ZS – 1 – 1 20 322 488 33.5 BM ZS – 1 – 2 319 497 32.2 BM ZS – 1 – 3 315 491 31.9 BM ZS – 2 – 1 540 221 278 35.8 BM ZS – 2 – 2 224 285 34.6 BM ZS – 2 – 3 217 277 37.9 BM *measured at L0 = 100 mm, as comparative value (is not material property) The basic requirement in welded structures design is to assure the required strength. In most welded structures this is achieved with superior strength of WM compared to BM (overmatching effect). In tested case this is achieved at room and at working temperatures (Figure 2 a, b, Tables 5, 6). An additional proof of ovetmatching is the fracture of specimens from welded joint in BM and that the difference of values of yield stress and tensile strength in Tables 5 and 7 is minor, at the level of measurement error. It is to notice the good agreement between yield stress and tensile strength values from test (Table 5) and specified values (Table 2). The obtained results of tensile properties of WM, Table 6 and Figure 4b, confirmed that welding technology was properly specified (welding procedure specification – WPS – is a separate document), including preheating and post-weld heat treatment. Special attention in tensile properties should be paid to elongation. When material is homogenious, as here BM and WM should be considered, elongation is usefull for comparison. For welded joint, the elongation value is meaningless, since in measuring length of 100 mm enter BM i WM, of different tensile properties, but also a part of HAZ is included, in which tensile properties are unknown. Nevertheless, the character of obtained tensile curves shows that material is ductile and it has an approximate ratio of unifirom and non-uniform elon- gation 1 : 2 (Figure 2). From the aspect of in-service behaviour of the welded structure, it is to underline that for real values elongation is elastic, and only locally and in limited amount also plastic, so the elongation values from Figure 2 and Tables 5–7 can serve only for com- parison and can not be the base for material behaviour assessment, especially HAZ, occurrence and crack propagation. In the performed test, of special importance is that the obtained strength values at the working temperature are within specified levels. This will significantly contribute to crack resistance evaluation of the statically and variably loaded heterogenious structure, such as welded joint and heat-affected-zone are. 4 EXAMINATION OF MICROSTRUCTURE A macrograph of butt welded joint of A-387 steel is given in Figure 3. Clearly recognized are: parent metal (BM) and weld metal (WM), and also heat-affected-zone (HAZ) in between 6. M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 265 Figure 3: Macrograph of welded joint Slika 3: Makroposnetek zvarnega spoja The uniform microstructure of parent metal, in addition to light polygonal crystals of ferrite, contains of pearlite as polygonal dark micro-constituent. The BM microstructure is presented in Figure 4 with solidifi- cation grain size 5 according to ASTM 6. The weld metal microstructure is bainite and grain boundary ferrite, Figure 5 6. HAZ microstructure comprises from coarse grained bainite which is located next to the WM and fine grained bainite, next to the BM, Figure 6 6. One has to have in mind that this local microstructure can signifi- cantly differ from microstructures at other locations in HAZ. 5 FRACTURE TOUGHNESS TESTING The effect of microstructure and mechanical pro- perties heterogeneity of welded joint constituents on the plane-strain fracture toughness, KIc, can be assessed locating a fatigue pre-crack tip on the specimen in different regions and following the regions of fracture growth. 5.1 Procedure and testing results Fracture toughness testing were performed using three-points bend, 17.5 mm thick specimens (SEN-B), Figure 7a, and 8 mm thick, compact tension specimens (CT), Figure 7b to according the standard ASTM E1820 9. Three-point bend (SEN-B) specimens were tested at room temperature. Only CT specimens were tested at working temperature. Fracture toughness, KIc, a measure of fracture toughness, JIc, is determined based on J-integral critical value, by testing according to ASTM E813-89 standard 10: K J EIc Ic = ⋅ −1 2ν (1) where: E – elasticity modulus, and  – Poisson’s ratio. For the determination of the J-integral a single specimen testing method by successive partial unloading was applied. By data pairs applied force, F, – crack opening displacement, , the points of basic relationship curve were obtained (Figure 8, left). The procedure for the determination of critical value, as measure of the fracture toughness, JIc, requires the design of resistance curve (J-R curve), shown in Figure 8, right, in which M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... 266 Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 Figure 7: Specimen for fracture mechanic testing: a) SEN-B speci- men, b) CT specimen Slika 7: Preizku{anci za preizkuse mehanike loma: a) SEN-B, b) CT Figure 6: Microstrukture of the heat-affected-zone (HAZ) Slika 6: Mikrostruktura toplotne zone Figure 5: Microstrukture of weld metal (WM) Slika 5: Mikrostruktura vara Figure 4: Microstrukture of parent metal (BM) Slika 4: Mikrostruktura osnovnega materiala M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 267 Figure 11: Diagrams F – δ (a) and J – ∆a (b) for the specimen with a notch in WM for operating temperature Slika 11: Odvisnosti F – δ (a) in J – ∆a (b) za preizku{anec z razpoko v WM pri delovni temperaturi Figure 9: Diagrams F – δ (a) and J – ∆a (b) for the specimen with a notch in BM for operating temperature Slika 9: Odvisnosti F – δ (a) in J – ∆a (b) za preizku{anec z razpoko v BM pri delovni temperaturi Figure 10: Diagrams F – δ (a) and J – ∆a (b) for the specimen with a notch in WM for room temperature Slika 10: Odvisnosti F – δ (a) in J – ∆a (b) za preizku{anec z razpoko v WM pri sobni temperaturi Figure 8: Diagrams F – δ (a) and J – ∆a (b) for the specimen with a notch in BM for room temperature Slika 8: Odvisnosti F – δ (a) in J – ∆a (b) za preizku{anec z razpoko v BM pri sobni temperaturi crack increase is determined based on compliance change. Basic, but more expensive, is the procedure in ASTM E813 standard with the multi specimens (of the same size) method with different length of fatigue pre-crack, and different compliance. In a single specimen test, the specimen is unloaded in intervals to about 30 % of the actually attained level of force chosen by experience with the type of material. Based on the change of line slope of the compliance, C, with crack extension, the crack increase, a, between two successive unloadings, corresponding to the attained value of force, is determined as: ∆ ∆a a b c c ci i i i i i i = + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⋅ −⎛ ⎝ ⎜ ⎞ ⎠ ⎟− − − − − 1 1 1 1 1η (2) The next steps are the determination of critical value, JIc, and use of this value in Eq. (1) for the calculation of the fracture toughness, KIc, according the single specimen compliance method. 5.2 Discussion of fracture toughness testing results The obtained diagrams are presented in Figure 8 for specimens of BM tested at room temperature and in Figure 9 for specimens tested at 540 °C. The correspon- ding curves for WM are given in Figures 10 and 11, and for HAZ in Figures 12 and 13 6,11. The calculated values of fracture toughness, KIc, are given in Table 8 for the specimens notched in BM, WM and HAZ. The microstructural and mechanical heterogeneities of a welded joint affect its resistance to crack propa- gation. Therefore, in specification for fracture mechanics testing conditions should prescribe not only the test procedure and location of a fatigue crack, but also the method of interpretation and meaning of the obtained results 6. The character of curves varies depending on the notch i.e. fatigue crack, tip location and testing tempe- rature. It is possible to observe an almost identical character of individual curves in each group, the difference between the diagrams for individual specimens lies exclusively in the maximal force value, Fmax, which is directly dependent on the fatigue crack length, a, and on testing temperature 6. The maximal value of KIc at room temperature was obtained for specimens notched in WM (mean KIc value ≈145 MPa m1/2). Somewhat lower KIc values exhibited the specimens notched in BM (mean value KIc ≈130 MPa m1/2). The scatter of results is small, 10–15 MPa m1/2 in terms of minimum and maximum values. Lower KIc values belong to specimens notched in HAZ. The differences do not indicate an important reduction of properties 6. The close KIc values for BM and HAZ are related to the microstructure. Namely, both constituents have ferrite-pearlite microstructures of similar crack resistance at static loading. It should be next in mind that in performed testing the location of fatigue crack tip is a M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... 268 Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 Figure 13: Diagrams F – δ (a) and J – ∆a (b) for the specimen with a notch in HAZ for operating temperature Slika 13: Odvisnosti F – δ (a) in J – ∆a (b) za preizku{anec z razpoko v HAZ pri delovni temperaturi Figure 12: Diagrams F – δ (a) and J – ∆a (b) for the specimen with a notch in HAZ for room temperature Slika 12: Odvisnosti F – δ (a) in J – ∆a (b) za preizku{anec z razpoko v HAZ pri sobni temperaturi random one, and that in HAZ can exist the regions of different microstructure and lower fracture toughness. Table 8: Results of testing the critical J-integral, JIc, and the critical stress intensity factor, KIc Tabela 8: Rezultati preizkusa kriti~nega J-integrala, J1c in kriti~nega faktorja intenzitete napetosti KIC Desig- nition Testing tempera- ure C Critical J-integral JIc/(kJ/m2) Critical stress intensity factor, KIc/ (MPa m1/2) Critical crack length ac/mm BM-1s 20 77.8 132.4 52.8 BM-2s 75.2 130.3 51.1 BM-3s 73.2 128.4 49.7 BM-1p 540 53.9 90.2 46.1 BM-2p 49.7 87.4 43.4 BM-3p 55.1 92.1 48.1 WM-1s 20 97.2 148.0 66.0 WM -2s 93.7 145.3 63.6 WM -3s 92.2 143.1 61.7 WM -1p 540 62.2 97.8 54.3 WM -2p 60.3 96.3 52.6 WM -3p 55.6 92.5 48.5 HAZ-1s 20 65.1 121.1 44.2 HAZ -2s 70.2 125.2 47.4 HAZ -3s 71.3 125.6 47.9 HAZ -1p 540 48.7 86.6 42.5 HAZ -2p 46.8 85.2 41.6 HAZ -3p 47.3 85.9 42.2 By applying the fundamental formula of fracture mechanics: K aIc c= ⋅σ π (3) and introducing the value of allowable stress σdoz = σ, for the shape factor equals to unity, approximate values of critical crack length, ac, can be calculated, (Table 8). Largest crack length, ac, can occur under static load in WM, but without brittle fracture occurrence. For static loading, the given differences in KIc value should not have significant effect on structural safety. It is obvious that allowable stress, lower than yield stress, will produce higher values for critical crack length and if in the tested material the crack of length less than critical, there is no danger of brittle fracture. Such a crack has to be detected and its length assessed by convenient non-destructive testing method. After the integrity analysis it is possible, under defined conditions, allow for structure service even in crack growth period. Important data for a decision about the extended service of cracked component are crack growth rate and its dependance on applied load. The chages of KIc value are then important, since critical crack length, ac, is directly depended on KIc value. The effect of temperature on fracture toughness KIc, is given in Table 8. The reduction of 35–45 % in fracture toughness at working temperature compared to room temperature depends on fatigue crack tip location (BM, WM, HAZ), with maximum value of KIc in the specimen notched in WM. Obtained J – a curves are of almost identical character, only the value of maximum force Fmax, is different, and it is directly related to the fatigue crack length a. 5.3 Fatigue analysis by fracture mechanics If a structural component is continuously exposed to variable loads, fatigue crack may initiate and propagate from severe stress raisers if the stress intensity factor range at fatigue threshold, Kth, is exceeded. A basic contribution of fracture mechanics in fatigue analysis is the division of fracture process to crack initiation period and the growth period to critical size for fast fracture. The total number of cycles to fracture, Nu, is divided into number of cycles for fatigue crack initiation, Ni, and for its growth to the value critical for fracture, Np: (Nu = Ni + Np) The development in the research of material behaviour for variable loading is achieved applying experimental and theoretical approaches. The analysis of stress and strain state at growing fatigue crack tip by applying linear elastic fracture mechanics (LEFM) enabled to develop the Paris equation for metals and alloys, which relates fatigue crack growth rate da/dN to stress intensity factor range K through coefficient C and exponent m 12: d d a N C K m= ( )∆ (4) The standard ASTM E647 13 defines the testing of pre-cracked specimen for fatigue crack growth rate measurement, da/dN, and for the calculation of the stress intensity factor range, K. Two basic requirements in ASTM E647 are: the crack growth rate should be above 10–8 m/cycle to avoid fatigue threshold region and load should be of constant amplitude. Standard Charpy size specimen, fatigue pre-cracked in different welded joint constituents, and instrumented by foil RUMUL RMF A-5, of measuring length 5 mm (Figure 14), for continuous monitoring of crack length, were tested at room temperature under variable loading for the determination of fatigue crack growth rate, da/dN, and stress-intensity factor range at fatigue thres- hold, Kth. The testing was performed in load control, by M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 269 Figure 14: Charpy specimen instrumented by foil RUMUL RMF A-5 for continuous monitoring of crack length Slika 14: Charpy preizku{anec s merilno folijo RUMUL RMF A-5 za zvezno merjenje dol`ine razpoke three-points bending on the FRACTOMAT high- frequency resonant pulsator. CT specimens were tested on working temperature, since at 540 °C the measuring foils can not be used, and load line displacement is measured instead. The relations da/dN – K are presented in Figure 15 for the specimens pre-cracked in the parent metal (PM), in Figure 16 for specimens pre-cracked in the weld metal (WM) and in Figure 17 for specimens pre-cracked in the heat-affected-zone (HAZ). The values of coeffi- cient C and exponent m, with the values of stress-intensity factor range at fatigue threshold, Kth, are given in Table 9. The dominant almost linear middle part of curve in Figures 15–17 is covered by Paris law and is practically most important, since it allows to define the difference between fatigue crack low growth rates (initiation) close to fatigue threshold, and high rates (KIc), when fracture occurs. The application of Paris equation is very convenient for fatigue of structures produced of materials of elevated and high strength. As it can be seen from Table 9, the position of the fatigue crack-tip and the testing temperature significantly affect the Kth values and the fatigue-crack growth 6. For comparison of the properties of welded joint constituents the crack growth rates are calculated for different values of stress-intensity factor range K. As a referent value K M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... 270 Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 Table 9: Parameters of Paris equation Tabela 9: Parametri Parisove ena~be Specimen designation Test temperature Stress-intensity factor range at fatigue threshold Coefficient Exponent Crack growth rate da/dN at ∆K = 10 MPa m1/2 °C ∆Kth/(MPa m1/2) C m nm/cycle BM–1s 20 6.8 2.98 ⋅ 10–13 3.62 1.24 ⋅ 10–09 WM–1s 6.8 3.88 ⋅ 10–13 3.82 2.56 ⋅ 10–09 HAZ–1s 6.7 3.05 ⋅ 10–13 4.01 3.12 ⋅ 10–09 BM–1p 540 5.9 3.11 ⋅ 10–13 4.08 3.74 ⋅ 10–09 WM–1p 6.2 3.27 ⋅ 10–13 4.14 4.51 ⋅ 10–09 HAZ–1p 6.1 3.38 ⋅ 10–12 3.17 5.00 ⋅ 10–09 Figure 16: Fatigue crack growth rate per cycle, da/dN, vs. stress intensity factor range, ∆K, specimens pre-cracked in weld metal, tested at room temperature (left) and at 540 °C (right) Slika 16: Rast utrujenostne razpoke na cikel da/dN v odvisnosti od faktorja intenzitete napetosti ∆K, preizku{anec z razpoko v varku, preizkus pri sobni temperaturi (levo) in pri 540 °C (desno) Figure 15: Fatigue crack growth rate per cycle, da/dN, vs. stress intensity factor range, ∆K, specimens pre-cracked in parent metal, tested at room temperature (left) and at 540 °C (right) Slika 15: Rast utrujenostne razpoke na cikel da/dN v odvisnosti od faktorja intenzitete napetosti ∆K, preizku{anec z razpoko v osnovnem materialu; preizkus pri sobni temperaturi (levo) in pri 540 °C (desno) Figure 17: Fatigue crack growth rate per cycle, da/dN, vs. stress intensity factor range, ∆K, specimens pre-cracked in heat-affected- zone, tested at room temperature (left) and at 540 °C (right) Slika 17: Rast utrujenostne razpoke na cikel da/dN v odvisnosti od faktorja intenzitete napetosti ∆K, preizku{anec z razpoko v toplotni zoni, preizkus pri sobni temperaturi (levo) in pri 540 °C (desno) = 10 MPam is accepted, which is within a middle part of the diagram, where Paris law is valid, Figures 15–17. The fatigue crack-growth rate at room temperature, da/dN, is 1.24·10–09 µm/cycle for the specimen of BM, 2.56·10–09 µm/cycle for specimen of WM and 3.12·10–09 µm/cycle for specimen of HAZ At the temperature of 540 °C, corresponding values are higher: (3.74·10–09; 4.51·10–09; 5.00·10–09 for BM, WM and HAZ, in respect 6. The behaviour of welded joint and its constituents should affect the change of curve slope in validity part of Paris law. Materials of lower fatigue-crack growth rate have lower slope in the diagram da/dN vs. K 6. Slow growth is confirmed for specimens cracked in BM and WM, since for the same growth rate, greater factor intensity range is required. The maximum fatigue crack growth rate is expected when stress intensity factor range approaches to plane strain fracture toughness, when brittle fracture is possible 14. In spite of significant differences in fatigue-crack growth rate, the obtained values are still low and acceptable. That means that tested steel and its welded joint exhibited acceptable level of fatigue-crack growth resistance and can be successfully applied for variable loading in case of detected crack-like defects, primarily for low-cycle fatigue. 6 CONCLUSIONS The following conclusions were derived: • The resistance to crack growth and obtained values of KIc and ac of the welded joint are affected by its microstructural and mechanical heterogeneity and by the testing temperature. Feritte-lamelar pearlite microstructure of the WM has a better resistance to crack growth in static loading condition than the feritte-pearlite microstructure of BM of uniform grain size, and the feritte-pearlite microstructure of HAZ of different grain size. Obtained close KIc values of BM and HAZ are explained by the position of fatigue crack tip in HAZ region of the micro- structure similar to that in BM. • The testing temperature influences the fracture toughness KIc and the crack critical length acr values. The reduction of fracture toughness is of 35–45%, depending on fatigue crack tip location (BM, WM i HAZ). Specimen notched in WM has the highest value KIc, whereas for BM and HAZ obtained KIc values are lower. Results obtained at working temperature are proportionally lower compared to results at room temperature, and are a consequence of lower material properties at elevated temperature. • Notch location and crack initiation, as well as testing temperature affect values of fatigue threshold Kth and fatigue crack growth parameters. • The minimum fatigue-crack growth rate exhibited the specimens pre-cracked in BM, and the maximum fatigue crack-growth rate in specimens pre-cracked in HAZ. This is directly connected to the effects that microstructural heterogeneity in HAZ regions has on fatigue-crack growth rate, da/dN. • Specimens of welded joint constituents at working temperature (540 °C) exhibited two to four-fold higher crack-growth rates when compared to room temperature under variable loads in tests of the fatigue threshold and fatigue crack growth para- meters that this is explained by reduced material properties at elevated temperature. 7 REFERENCES 1 S. Sedmak, A. Sedmak, Integrity of Penstcok of Hydroelectric Powerplant, Structural Integrity and Life, 5 (2005) 2, 59–70 2 JUS EN 288-3:1992, Specification and approval of welding procedures for metallic materials – Part 3: Welding procedure tests for arc welding of steels, Slu`beni list SRJ, (1995), 25 3 J. Vojvodic Tuma, A. Sedmak, Analysis of the unstable fracture behaviour of a high strength low alloy steel welment, Engineering Fracture Mechanics, 71 (2004), 1435–1451 4 E. O. Argoub, A. Sedmak, M. A. Esasamei, Structural Integrity Assessment of Welded Plate with a Crack, Structural Integrity and Life, 4 (2004) 1, 39–46 5 K. Geri}, PhD Thesis, University of Belgrade, Faculty of Tehnology and Metallurgy, Belgrade, 1997 6 M. Burzi}, PhD Thesis, University of Novi Sad, Technical faculty, 2008 7 EN 895, Welded butt joints in metallic materials – Transverse tensile test, 1995 8 ASTM E 1475-00, Standard Test Method for Measurement of Creep Crack Growth Rates in Metal, Annual Book of ASTM Standards 03.01.2000, 936–950 9 ASTM E 1820-99a, Standard Test Method for Measurement of Fracture Toughness, Annual Book of ASTM Standards, 03.01, 1999 10 ASTM E813-89, Standard Test Method for JIc, A Measure of Fracture Toughness, Annual Book of ASTM Standards, 03.01. 1993, 651 11 M. Burzi}, Z. Burzi}, J. Kurai, The prediction of residual life of reactors in RNP, CertLab Pan~evo, 2006 12 P. C. Paris, F. Erdogan, A Critical Analysis of Crack Propagation Laws, Trans. ASME, Journal Basic Eng., 85, 4, 528 13 ASTM E647, Standard Test Method for Constant-Load-Amplitude Fatigue Crack Growth Rates Above 10-8 m/cycle, Annual Book of ASTM Standards, 03.01. 1995, 714 14 M. Burzi}, Z. Burzi}, J. Kurai, D`. Ga~o, Fatigue Behaviour of Alloyed Steel for High Temperature, First Serbian (26th YU) Congress on Theoretical and Applied Mechanics, Kopaonik, Serbia, 2007, 1085–1090 M. BURZI], @. ADAMOVI]: EXPERIMENTAL ANALYSIS OF CRACK INITIATION AND GROWTH ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 263–271 271 N. AMIN ET AL.: THE ROLE OF CHLORIDE SALTS ON HIGH TEMPERATURE CORROSION ... THE ROLE OF CHLORIDE SALTS ON HIGH TEMPERATURE CORROSION OF 321 STAINLESS STEEL VLOGA KLORIDNIH SOLI PRI VISOKOTEMPERATURNI KOROZIJI NERJAVNEGA JEKLA 321 Neelofar Amin, Mohammed Misbahul Amin, Shamsul Baharin Jamaludin, Kamarudin Hussin School of Materials Engineering, PPK Bahan Taman Muhibah UniMAP, University Malaysia Perlis, 02600 Jejawi, Perlis, MALAYSIA neelofaraminyahoo.com Prejem rokopisa – received: 2008-04-20; sprejem za objavo – accepted for publication: 2008-07-07 The effect of CaCl2 and BaCl2 salt coatings on the high temperature corrosion of 321 stainless steel at 950 °C in a slow current of air for the period of 72 hours were studied. The 321 alloy was severely attacked by calcium- and barium-chlorides due to formation of volatile chlorides. The data have been complemented by oxidation kinetics measurements and morphological structures were analyzed using scanning electron microscope (SEM). The elemental distribution on the alloy surface deposits were characterized by using energy dispersive X-ray (EDAX) analysis. The alkaline earth metal chloride salts have deleterious effect on the protectivity of the scale and rapid degradation of the alloy is noted. Key words: 321 stainless steel, Hot Corrosion, CaCl2, BaCl2, Scale Raziskan je bil vpliv prekritij s solmi CaCl2 in BaCl2 na visokotemperaturno oksidacijo nerjavnega jekla 321 v po~asnem toku zraka pri 950 °C v trajanju do 72 h. Zaradi nastanka volatilnih kloridov sta oba klorida zlitino mo~no napadla. Dolo~ena je bila kinetika oksidacije, morfologija pa je bila dolo~ena z opazovanjem v vrsti~nem mikroskopu. Porazdelitev elementov v depozitu na povr{ini je bila dolo~ena z energijsko disperzivno spektrometrijo (EDAX). Kloridi alkalnih kovin mo~no zmanj{ajo varovalnost {kaje in povzro~ijo hitro degradacijo zlitine. Klju~ne besede: nerjavno jeklo 321, vro~a korozija, CaCl2, BaCl2, {kaja 1 INTRODUCTION The intensification of process engineering in almost every branch of modern technology, and development of new technologies make increasingly higher requirements for metallic construction materials, especially for their heat and scaling resistance. The increase in operating efficiency of certain installations or plants is generally achieved by the application of higher temperatures and pressures and higher flow velocities of gases and vapours which creates gas corrosion hazard for the construction materials1–7. The corrosion process under hot gases or vapours being a mixture of many aggressive components, proceeds usually many times faster and is characteristic by a non-uniform attack of metal surface8-12. It became obvious that chloride, always present in such industrial gases, is one of the most dangerous aggressive compo- nents of the above mentioned atmospheres13,14. In this study, the 321 stainless specimens were subjected to treatments of CaCl2 and BaCl2, oxidised at 950 °C for times ranging 12 h to 72 h was chosen in order to highlight the corrosion phenomena. Finally the results of morphologies were carried out by using SEM (model Jeol 6460-LA) as well as energy- dispersed X-ray spectroscopy profiles of elements of deposits on the surface of corroded 321 alloy were presented. 2 EXPERIMENTAL The present study has been carried out using commercially available 321 stainless steel. Its chemical composition was as follows: C–0.08 %, Mn–2 %, Si–1 %, Cr–17.5 %, Ni–10.5 %, P–0.045 %, S–0.03 % and Fe-balance. The 321 stainless steel sheets were cut into small pieces of size (20 × 12 × 3) mm. The surface of each specimens was polished mechanically with (180, 320, 600) grades of silicon carbide paper. The specimens were coated with CaCl2 and BaCl2 in a preheated condition to obtain a layer of salt deposition 13. The coated specimens were dried and weighed, followed transferred into a crucible. The salt coated alloy were oxidised at 950 °C for the periods of 72 h in slow current of air and mass changes were recorded at every 12 h of interval. For each condition, two series of specimens were corroded in accordance with reference 14. The microstructural and microchemical characte- rization were performed using Jeol 6460-LA scanning electron microscope equiped with a energy dispersive X-ray spectrometer and analyzer. 3 RESULTS Figure 1 shows the curve of mass change versus exposure time of 321 stainless steel coated with CaCl2 and BaCl2, oxidized at 950 °C in slow blowing air. The Materiali in tehnologije / Materials and technology 42 (2008) 6, 273–276 273 UDK 669.14.018.8:620.193 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 42(6)273(2008) graph of the mass change increases by increasing the exposure times shows for the uncoated alloy, a parabolic curve. The specimens coated with calcium and barium chloride exhibited the weight gain upto 24 h and mass loss with further increase of annealing time. The conclusion is that chlorides are more reactive in presence of metal because of the formation of volatile metallic chloride. The greater activity of calcium chloride is explained by the weaker bonding of calcium chloride than that of barium chloride. Morphological Studies of 321 stainless steels In Figure 2 (a, b) the 321 alloy coated with CaCl2 is shown, the scales formed are rough exhibiting a ten- dency to deform, wrinke and microcrack. The formation of metallic chlotide may have proceeded through the formation of intermediate volatile species, f.i. CrO2Cl2. some of which evaporate and some of decompose and accumulate at the alloy/salt interface in the form of Cr2O3. The SEM micrograph of specimen coated by BaCl2 exposed at 950 °C for 72 h (Figure 3 a, b) showing the presence of internal and pitting corrosion. The specimens exposed at 950 °C were badly deteriorated by layers of oxides and metallic chloride. The deposit film shows the presence of voids and pores especially in the outer layers of the scale. Energy Dispersive X-Ray Analysis (EDAX) Figures 4 and 5, the EDAX spectra, elements and compounds had been verified the content of the elements of the specimen coated by calcium chloride and barium chloride exposed at 950 °C for 72 h. The results of spectra and elements showed that the surface was mainly N. AMIN ET AL.: THE ROLE OF CHLORIDE SALTS ON HIGH TEMPERATURE CORROSION ... 274 Materiali in tehnologije / Materials and technology 42 (2008) 6, 273–276 Figure 3: Outer surface of scales formed on 321 stainless steel coated with BaCl2, oxidised at 950 °C for 72 h Slika 3: Zunanja povr{ina {kaje, ki je nastala pri 72-urni oksidaciji nerjavnega jekla 321, prekritega z BaCl2 Figure 2: Outer surface of the scale formed on 321 stainless steel coated with CaCl2, oxidised at 950 °C for 72 h Slika 2: Zunanja povr{ina {kaje, ki je nastala pri 72-urni oksidaciji nerjavnega jekla 321, prekritega s CaCl2 Figure 1: Oxidation behavior of 321 stainless steel without coated, coated with CaCl2 & BaCl2 as shown by a plot of mass change vs time, oxidized at 950 °C for 72 h Slika 1: Oksidacija nerjavnega jekla 321 brez prekritja in z njim s CaCl2 in BaCl2. Sprememba mase v odvisnosti od ~asa pri oksidaciji do 72 hr pri 950 °C composed of nickel (Ni), chromium (Cr), iron (Fe) and oxygen (O). The results were logically acceptable because 321 stainless steel containing 17.5 % of chro- mium, 10.5 % of nickel and iron as balance. Tables 1, 2 show the elemental analysis results of specimen coated with calcium- and barium- chloride that exposed at 950 °C for 72 h. According to the results in Tables 1, 2, iron oxide, FeO, was the main oxide product, it appearance as the site product while corrosion occur. Iron oxide formed when iron (Fe), reacting with oxygen in air, the equation shown as below: 2Fe + O2 → 2FeO Besides that, Cr2O3 also occur in great quantities as the protective films for the stainless steel. In addition, the surface also accompanied with other oxides such as NiO, Cr2O3, SiO2 and Chloride with small amount. Table 1: Composition of the scale on the specimen coated with calcium chloride, oxidized at 950 °C for 72 h in blowing air from EDAX. Deduced from the EDAX analysis. Tabela 1: Sestava {kaje na vzorcu, prekritem s kalcijevom kloridom, ki je bil oksidiran v zra~nem toku 72 h pri 950 °C. Prera~unano iz EDAX-analize. Element E/keV Mass, w/% Compound Mass, w/% O 30.63 Si 1.739 5.46 SiO2 11.68 S 2.307 4.79 SO3 0.96 Cl 2.621 0.36 Cl 0.36 Cr 5.411 3.63 Cr2O3 5.31 Fe 6.398 39.03 FeO 50.20 Ni 0.851 16.10 NiO 31.49 Total 100.00 100.00 Table 2: Composition of the scale on the specimen coated with barium chloride, oxidized at 950 °C for 72 h in blowing air from EDAX. Deduced from the EDAX analysis. Tabela 2: Sestava {kaje na vzorcu, prekritem z barijevom kloridom, ki je bil oksidiran v zra~nem toku 72 h pri 950 °C. Prera~unano iz EDAX-analize. Element E/keV Mass, w/% Compound Mass, w/% O 24.78 Si 1.739 2.24 SiO2 4.80 S 2.307 1.20 SO3 0.29 Cl 2.621 1.12 Cl 1.29 Cr 5.411 10.18 Cr2O3 13.60 Fe 6.398 37.29 FeO 47.98 Ni 7.471 23.19 NiO 32.04 Total 100.00 100.00 4 DISCUSSION The corrosive environment of calcium chloride and barium chloride effect onto the hot corrosion behavior of 321 stainless steel at 950 °C, exhibiting the breakdown of protection film on stainless steel caused the formation of a volatile, CrO2Cl2. The reaction can be described as follows 12,13: CaCl2 + Cr2O3 + 3/2O2 → CrO2Cl2 + CaCrO4 N. AMIN ET AL.: THE ROLE OF CHLORIDE SALTS ON HIGH TEMPERATURE CORROSION ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 273–276 275 Figure 5: EDAX spectra of the elements and compounds of the specimen coated with BaCl2, oxidized at 950 °C for 72 h in blowing air Slika 5: Posnetek analizirane povr{ine in EDAX-spekter elementov v vzorcu, prekritem z BaCl2, ki je bil oksidiran v zra~nem toku 72 h pri 950 °C Figure 4: EDAX spectra of the elements on the specimen coated with CaCl2, oxidized at 950 °C for 72 h in blowing air Slika 4: Posnetek analizirane povr{ine in EDAX-spekter elementov v vzorcu, prekritem s CaCl2, ki je bil oksidiran v zra~nem toku 72 h pri 950 °C The formation of volatile products such as CrO2Cl2 and CaCrO4, exerts sufficient vapour pressure to break the passivation of oxides on 321 stainless steel. Once the passive film breaksdown, the molten CaCl2 further attacks the stainless steel and lead to corrosion. The equation: 2Fe + 2CaCl2 + O2 → 2CaO + 2FeCl2 2Cr + 3CaCl2 + 3/2O2 → 2CrCl3 + 3CaO Cr + CaCl2 + 3/2O2 → CrO2Cl2 + CaO The chlorides formed such as CrO2Cl2 and CrCl3 were released at the salt interface and get oxidized to release chlorine gas: 4CrCl3 + 6O2 → 4CrO3 + 6Cl2 4CrO2Cl2 + 2O2 → 4CrO3 + 4Cl2 The chloride might be entrapped between inner oxide layers of the alloy and get condensed on cooling and appear as distinct and discrete phase in the scales 13. BaCl2 has undergone the same reaction as CaCl2. Since calcium is more electronegative than barium, the bonding of calcium chloride is weaker than that of the barium chloride. All common metals are very soluble in chloride form and so the reaction rate is increased. The presence of chloride ions in the electrolytic solution affects nearly every aspect of the corrosion behavior. The oxide layer that protects the steel breaks down in the presence of chloride, causing pits to form. This type of corrosion can lead to structural failures 3. Temperature is a factor in activation controlled corrosion. Raising the temperature will also increase the corrosion rate as the activation energy decreases with temperature. Increasing solution temperature increased the susceptibility to both pitting and active dissolution 1. The corrosion rate of each test decreases with the increasing of exposure times. It is because of the decrease of the concentration of chloride ions with time, most of the chloride ions that leads to corrosion has reacted with the chromium oxyde film protecting the surface of the steel and activate the unprotected surface 12. This determined the concentrations of chloride solution will affect the corrosion rate. The chemical reactions of chloride ions are: Fe2+ + 2Cl– → FeCl2 At high temperatures in chloride salts increased the susceptibility to both pitting and active dissolution, resulted in increased corrosion current densities and peak current densities 4. 5 CONCLUSION The results of the study shown the increased chloride content, the easily the passive region shrinks and simultaneously with the formation of corrosion of metallic chloride and fluxing products exhibiting the profuse deteriotion of the surface of the 321 stainless steel. The patterns of the graphs follow the kinetic theory of reaction where the reactions rates were directly proportional to the increasing of temperature. It means as the temperature increases in the time for the initiating corrosion attack is decreased. The CaCl2 coated alloy shown higher weight loss than the coated with BaCl2. Pitting corrosion, internal corrosion and stress corrosion cracking were the commonest corrosion found in 321 stainless steel in presence of chloride. From the results of EDAX, iron oxide, FeO, Cr2O3 and NiO occur in great quantities on the corroded stainless steel surface in addition to other oxides such as SO3, SiO2 and chloride with small composition. ACKNOWLEDGEMENT The authors would like to thankfully acknowledged for the financial assistance in the form of Fundamental Research Grant Scheme: Vote-9003-00144 from Mini- stry of Higher Education, Malaysia. 6 REFERENCES 1 Mrowec S, Werber T., Gas corrosion of metals, National Bureau of Standards, Washington D.C. (1978) 2 Rahmel A., Schwenck W., Korrosion und Korrosionschudz von Stahlen, Verlag Chemie Weinheim, 1977, Chapter 6 3 Stringer J., Hot corrosion of high temperature alloys, Proc. Intnl. Symp., High Temperature Alloys, Electrochemical Society Inc., Princton, N. J., (1976), 513 4 Hart A. B., Cutler A. J. B., Deposition and corrosion in gas turbine, Appl. Sci. Publishers, London 1973, 371 5 Attia A. A., Salih S. A., Baraka A. M., J Electrochem. Acta, (2002), 48:113 6 Chester T. Sims T. Hagel W. C., The Superalloy, John Wiley & Sons, New York, 1972 7 Badawy W. A. Alkharafi F. M. Al-Hassan E. Y., Corros. Prevention & Control, (1999) 46, 51 8 Kolman, D. G., Ford, D. K., Butt, D. P. Nelson, T. O., Corrosion of 304 stainless steel exposed to nitric acid-chloride environments. Materials Corosion and Environment Effects, Laboratory Los Alamos, National Laboratory, (2005), 209 9 Nishimura R., Maeda Y., Strees corrosion cracking of sensitized type 316 austenitic stainless steel in hydrochloric acid solution – effect of sensitizing time. Corrosion Science. 45 (2003) 8, 1847–1862 10 Abdallah M., Rhodanine Azosulpha. Drugs as corrosion inhibitors for corrosion of 304 stainless steel in hydrochloric acid solution. Journal of Corrosion Science. 44 (2001) 4, 717–728 11 Huntz A. M., Lefevre B., Cassino F., Mat. Sci. Eng. A290 (2000), 190–197 12 M. Misbahul Amin, Hot corrosion behavior of inconel-600 alloy in presence of NaCl and Na2CO3 at 850 °C. Prakt. Metallogr (1993), 30:5 13 M. Misbahul Amin, The CsCl- and CsNO3-induced high temperature oxidation of Nimonic-90 alloy at 1123K. Applied Surface Science. 115 (1996) 355–360 N. AMIN ET AL.: THE ROLE OF CHLORIDE SALTS ON HIGH TEMPERATURE CORROSION ... 276 Materiali in tehnologije / Materials and technology 42 (2008) 6, 273–276 D. DREV ET AL.: RAZTAPLJANJE CO2 V EMBALIRANI VODI ALI BREZALKOHOLNI PIJA^I ... RAZTAPLJANJE CO2 V EMBALIRANI VODI ALI BREZALKOHOLNI PIJA^I IN S TEM POVEZANE MO@NE PO[KODBE PROBLEMS ASSOCIATED WITH THE DISSOLUTION OF CO2 IN THE CASE OF BOTTLED WATER AND NON-ALCOHOLIC BEVERAGES Darko Drev1,2, Mitja Pe~ek1,2, Jo`e Panjan2 1In{titut za vode Republike Slovenije, Univerza v Ljubljani, Hajdrihova 28c, 1000 Ljubljana, Slovenija 2Fakulteta za gradbeni{tvo in geodezijo, Jamova 2, 1000 Ljubljana, Slovenija Prejem rokopisa – received: 2008-06-05; sprejem za objavo – accepted for publication: 2008-08-13 Pri vodi in brezalkoholni pija~i, ki ima raztopljen ogljikov dioksid, lahko nastane pri odpiranju plastenke ali steklenice po{kodba. Ta po{kodba je lahko posledica izmeta zama{ka in pija~e v obraz, pa tudi eksplozije steklenice. To se lahko zgodi, ~e je v vodi prevelik tlak raztopljenega CO2 in je tudi zama{ek pokvarjen. Eksplozija steklenice lahko nastane pri nekvalitetnem materialu. Pravilno izdelan zama{ek bi moral med odpiranjem v steklenici postopno zmanj{evati tlak. ^e se to ne zgodi in je v plastenki velik tlak CO2, lahko pride do izmeta zama{ka v obraz ter tudi burnega iztoka teko~ine iz plastenke. Tak{ni primeri niso samo teoreti~ni, temve~ se dogajajo tudi v praksi. Eksplozija steklenice nastane predvsem takrat, kadar so v steklu prekomerne napetosti kot posledica nehomogenosti materiala in neustrezne izdelave. Klju~ne besede: brezalkoholne pija~e, ogljikov dioksid, CO2, plastenke, tlak, po{kodbe In the case of bottled water or bottled non-alcoholic beverages that contain dissolved carbon dioxide, opening the (glass or plastic) bottle can prove dangerous to the person opening it. Unscrewing the bottle cap can cause the cap or the contained liquid to be ejected away from the bottle with sufficiently high speed to cause physical harm to a person, or the entire bottle can explode. A properly functioning cap reduces bottle pressure slowly and continuously during the bottle opening process. The ejection of the cap and contained liquid is caused by exceedingly high pressure in the bottle, while both the ejection effect or the explosion of the bottle is caused by a malfunction of the bottle cap due to insufficient homogeneity of the materials used or inadequate processes applied in the production of the cap. Key words: non-alcoholic beverages, carbon dioxide, CO2, plastic bottle, problems 1 UVOD Pri izbiri materiala za embaliranje vode in brez- alkoholnih pija~ niso pomembne samo tiste njegove lastnosti, ki zagotavljajo higiensko neopore~nost, temve~ tudi druge lastnosti, kot so na primer: mo`nost enostavne manipulacije, nizka cena, mo`nost recikla`e itd. Spellman, 1999, Havelaer, 2003. Glede higienske neopore~nosti lahko pri{tevamo steklo med kemijsko in fizikalno najprimernej{e materiale za embaliranje pija~. Glavna pomanjkljivost steklene embala`e je lomljivost in s tem povezane te`ave pri transportu in ravnanju z njo Nölle,1997. V zadnjih dvajsetih letih se zato embalira vedno ve~ vode in brezalkoholnih pija~ v plastenke Drev, 2005. Med polimernimi materiali se za naj- manj{e plastenke najve~ uporablja od 250 ml do 2,5 l polietilentereftalat (PET), pri ve~jih posodah pa tudi polietilen (PE), polivinilklorid (PVC), polistiren (PS), polipropilen (PP) itd. Razlog za to je enostavno delo in nizka cena embala`e. Pri embalirani vodi in brez- alkoholnih pija~ah pa ne smemo pozabiti na zama{ke. Ti so pogosto izdelani iz druga~nega materiala kot steklenice oziroma plastenke. Pri steklenicah so navadno kovinski zama{ki s polimernimi ali plutovinastimi tesnili, pri plastenkah pa gre navadno za podoben material kot pri plastenki ali pa za katerega izmed standardnih termoplastov (PE, PP, PVC). V tem ~lanku se omejujemo le na problematiko mo`nih po{kodb pri odpiranju steklenic ali plastenk. Velika te`ava pa nastane, ~e steklenico ali plastenko raznese v navzo~nosti ljudi. Pri tem so lahko posledice eksplozije steklenice mnogo huj{e kot pri plastenki. Drobci stekla lahko po{kodujejo ljudi na ve~ji razdalji in mnogo huje, kot je to mo`no pri plastenki. Eksplozije steklenic ali plastenk, v katerih je embalirana gazirana pija~a, so posledica prevelikega tlaka CO2 v posodi ter tudi napak in s tem povezanih stri`nih napetosti v materialu. Pri steklu, ki je krhko, so lahko v~asih te stri`ne napetosti tako velike, da se lahko razleti steklenica tudi brez pove~anega tlaka CO2 v notranjosti. Pri plasti~ni embala`i pa to ni mogo~e. Stri`ne napetosti so posledica razli~nih raztezkov in skr~kov v materialu, ki nastanejo zaradi nehomogenosti materiala, temperaturne razlike, migracijskih procesov itd. 2 TEORETI^NI DEL 2.1 Raztapljanje ogljikovega dioksida v vodi Maksimalna koli~ina CO2, ki se lahko raztopi v koka- koli ali vodi, je dolo~ena s Henryjevim zakonom (slika 1). Ta zakon pravi, da je pri dani temperaturi koli~ina Materiali in tehnologije / Materials and technology 42 (2008) 6, 277–283 277 UDK 641.87:543.272.61 ISSN 1580-2949 Izvirni znanstveni ~lanek/Original scientific article MTAEC9, 42(6)277(2008) plina, ki se raztopi v teko~ini, premo sorazmerna njego- vemu parcialnemu tlaku (slika 1). ni = Ki pi (1) Tu pomenijo: ni mol/m3 – koli~ina plina, ki se raztopi v teko~ini Ki mol Pa–1m–3 – konstanta Henryjevega zakona pi Pa – parcialni tlak plina V tabelah 1 in 2 so prikazane Henryjeve konstante in konstante C za glavne sestavine zraka. Tabela 1: Henryjeve konstante za vodo in pline pri 298 K (de.wiki- pedia.org/wiki) Table 1: Henry’s constants for the solubility of some gases in water at 298 K Ena~ba: k C p H, cp voda plin = k p C H, cp plin voda = k p x H, px plin voda = k C p H, cc voda plin = Enota: mol l bar plin ⋅ l bar molplin ⋅ bar mol mol plin plin ⋅ brez dimenzije O2 1,3·10–3 769,23 4,259·104 3,180·10–2 H2 7,8·10–4 1282,05 7,099·104 1,907·10–2 CO2 3,4·10–2 29,41 0,163·104 0,8317 N2 6,1·10–4 1639,34 9,077·104 1,492·10–2 He 3,7·10–4 2702,7 14,97·104 9,051·10–3 Ne 4,5·10–4 2222,22 12,30·104 1,101·10–2 Ar 1,4·10–3 714,28 3,955·104 3,425·10–2 CO 9,5·10–4 1052,63 5,828·104 2,324·10–2 Henryjeva konstanta je odvisna od temperature: C H R K /T i= = −∆ solv d d ln( ) ( )1 (2) Tu pomenijo: ∆solvH J/mol  entalpija R J/molK – plinska konstanta C K – konstanta T K – temperatura Tabela 2: Konstante C za razli~ne pline Table 2: Constants C for different gases plin O2 H2 CO2 N2 He Ne Ar C /K 1700 500 2400 1300 230 490 1300 Ugotavljamo, da se pri dvakrat pove~anem tlaku pri isti temperaturi dvakrat pove~a koli~ina raztopljenega plina. Seveda pa velja tudi nasprotno: ~e se tlak zmanj{a, se zmanj{a koli~ina plina, ki je lahko raztopljena v teko~ini in ta plin se iz teko~ine izlo~i. Plinska ena~ba za parcialni tlak posameznega plina: pi = (mi/Mi) RV/T (3) ^e plinsko ena~bo za parcialni tlak zdru`imo z Dal- tonovim zakonom, dobimo: p = p1 + p2 + ... + pn = = (m1/M1 + m2/M2 + ... + mn/Mn)·RV/T (4) xi = mi/m (5) Iz Daltonovega zakona tako dobimo ena~bo za par- cialni tlak posameznega plina: pi = (xi/Mi) Mzmesi p (6) Tu pomenijo: p Pa = tlak zmesi plinov v celoti pi Pa = parcialni tlak posameznega plina xi mol/mol = masni dele` posameznega plina v zmesi Mi g/mol= molska masa plina (masa enega kilo mola plina) Mzmesi g/mol = molska masa zmesi V termodinamiki ne govorimo o koncentracijah, temve~ o aktivnostih dolo~ene komponente. Tudi v na{em primeru je aktivnost CO2 nekoliko druga~na od koncentracije in je podana z naslednjo ena~bo: a p p A A A = 0 (7) Tu pomenijo: aA = aktivnost plina pA = parcialni tlak realneg plina pA0 = parcialni tlak realnega plina V idealni raztopini velja, da je parcialni tlak linearno sorazmeren dele`u komponente v raztopini. ^im ve~ji je njen dele`, tem vi{ji je parcialni tlak. Ogljikov dioksid se raztaplja v vodi po Henryjevem zakonu v odvisnosti od temperature in tlaka (tabeli 3, 4). S slike 2 je razvidno, da se pri enakem tlaku polnjenja raztopi razli~na koli~ina CO2 pri razli~nih temperaturah. ^e je bila na primer pozimi v polnilnici temperatura samo 10 °C, poleti pa 30 °C, je lahko nastala pri pritisku polnjenja 3 bar pribli`no 100-odstotna razlika v koli~ini raztopljenega CO2. Zato je pomembno, da so polnilnice tudi zaradi teh razlogov klimatizirane. D. DREV ET AL.: RAZTAPLJANJE CO2 V EMBALIRANI VODI ALI BREZALKOHOLNI PIJA^I ... 278 Materiali in tehnologije / Materials and technology 42 (2008) 6, 277–283 Slika 1: Prikaz topnosti plina CO2 v vodi po Henryjevem zakonu in primerjava topnosti za idealne raztopine po Raoulovem zakonu 2 Figure 1: Solubility of the gas CO2 in water according to the Henry’s law and comparison of the solubility to ideal solutions according to the Raoult’s law 2 V primeru, da je bila gazirana brezalkoholna pija~a polnjena pozimi pri relativno nizki temperaturi (na primer 10 °C), brezalkoholno pija~o pa odpira oseba poleti v naravi, kjer nima hladilnika ali hladilne torbe, bo lahko temperaturna razlika vsaj 20 °C. Ta razlika pa povzro~a spro{~anje CO2 in s tem znatno pove~an tlak. Tabela 3: Topnost CO2 v odvisnosti od parcialnega pritiska CO2 pri 1 bar 16 Table 3: Solubility of CO2 at a partial pressure for CO2 of 1 bar abs 16 T/°C 0 10 20 30 40 50 80 100 Topnost CO2 v vodi s/cm3 /g  1,8 1,3 0,88 0,65 0,52 0,43 0,29 0,26 Disociacijska konstanta ogljikove kisline je odvisna od temperature (Lide, 1991). Tabela 4: Odvisnost disociacijske konstante ogljikove kisline (K1A) od temperature Table 4: Dissociation constant (K1A) of carbonic acid at various temperatures Temperatura T/°C 0 5 10 15 20 25 30 35 40 45 50 K1A.107 2,643,04 3,4 3,1 4,6 4,5 4,1 4,0 5,4 5,3 5,9 CO2 (l) + H2O (l) ↔ H2CO3 (l) [ ] [ ] [ ] K c c c l l l = H CO CO H O 2 3 2 2 = [ ] [ ] c c l l H CO CO 2 3 2 55 5, Tu pomenijo: (l) = teko~ina (liquid) K = ravnote`na konstanta [ ] [ ] K K c c r l l = ⋅ = ≈55 5 0 0017, , H CO CO 2 3 2 (8) 2.2 Migracijski procesi voda-plastenka-okolica Plastenke so prakti~no neprepustne za migracijske procese snovi iz okolice v vodo in iz vode v okolico. Pri steklenicah so ti migracijski procesi {e mnogo manj{i. Za plo~evinke pa pogosto to ne velja v celoti. Procesi raztapljanja kovinskih ionov v vodi so znatni, posebno {e, ~e kakovost plo~evine ni najbolj{a. ^e so v vodi prisotni razni dodatki (brezalkoholne pija~e), se lahko ti procesi {e pospe{ijo. Migracijski procesi so odvisni od: – lastnosti materiala, – lastnosti permeatov, – tlaka in koncentracije permeata, – naknadne oksidacije v vodi, – sestave atmosfere, – drugih dejavnikov. Pri plastenkah se pojavljajo zelo minimalni procesi prodiranja plinastih produktov iz atmosfere v teko~ino (vodo, kokakolo) in iz teko~ine v atmosfero (slika 3). Nobena plastika ni povsem neprepustna za pline, kot sta na primer kisik (O2) in ogljikov dioksid (CO2). Poleg migracijskih procesov plinastih produktov pa se lahko raztapljajo dolo~ene druge snovi iz embala`e v teko~ino in obratno Gächter,1989. Vsi ti procesi morajo biti zelo minimalni, kar se dose`e z ustrezno izbiro plastike in dovolj tesnim zama{kom. Materiali za izdelavo plastenk za `ivila morajo biti iz ustreznih materialov ter tudi atestirani. PET – polietilentereftalat je v osnovi zelo primeren za embaliranje `ivil, vendar pa ne vsak, temve~ le tisti, ki je bil izdelan na ustrezen na~in in tudi atestiran. Na primer, PET- regenerat se ne sme ve~ uporabljati za embaliranje `ivil, ker ni dovolj kemijsko stabilen. Prepustnost CO2 skozi stene plastenke je odvisna od difuzijskega koeficienta in koncentracij CO2 v teko~ini in v zraku: F D c x D c c l x = − = − −d d 1 2 (9) D. DREV ET AL.: RAZTAPLJANJE CO2 V EMBALIRANI VODI ALI BREZALKOHOLNI PIJA^I ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 277–283 279 Slika 3: Prikaz difuzijskega toka CO2 skozi steno plastenke Figure 3: Sheme of the diffusion flow of CO2 through the plastic bottle wall Slika 2: Odvisnost topnosti CO2 v vodi od tlaka in temperature v okolici Figure 2: Dependence of the CO2 solubility in water on pressure and temperature Tu pomenijo: Fx m s–1 – tok p1 Pa – parcialni pritisk D m2 s–1 – difuzijski koeficient c mg/m3 – koncentracija l m – debelina folije Prepustnost PET-embala`e je odvisna od »topnosti« CO2 v polimeru (slika 4). "Topnost" je odvisna od temperature, kristalini~nosti, molske mase PET ter dodatkov v plastiki. Prepustnost pove~a dele` amorfne oblike, nizka molekulska masa ter velika koli~ina dodatkov. Zato je pri embaliranju vode in brezalkoholnih pija~ zelo pomembno, da je material relativno ~ist in ima ustrezno molekulsko maso. Na sliki 5 je prikazan primer vpliva temperature na topnost CO2 pri dolo~eni sestavi PET. Na sliki 6 pa je prikazana prepustnost O2 za polimerne materiale PET za plastenke. Tabela 5: Prepustnost PET plastike za pline 15 Tabele 5: Permeability of a PET layer plastics for gases 15 Material µCO2 cm3mm/(m2d bar) µO2 cm3mm/(m2d bar) PET 16 4 OPET 8 2 PEN 2 0,5 PVDE oslojen 0,05 0,03 EVOH 0,05 0,01 SiO2 0,01 0,002 V tabeli 3 je prikazana prepustnost plastenke za CO2 in O2 in nekaterih drugih materialov, ki se uporabljajo za embaliranje pija~. Iz vseh navedenih podatkov je razvidno, da so PET-plastenke prakti~no neprepustne za CO2 in druge pline. Zato ostanejo v plastenki {e dolgo ~asa prevelike koncentracije CO2, ki so bile vnesene pri polnjenju. To povzro~a potencialno nevarnost po{kodb. 2.3 Vpliv pH vrednosti na koli~ino plinastega ogljiko- vodika in s tem tudi na pritisk v plastenki V kisli vodi in gaziranih brezalkoholnih pija~ah sta v ravnote`ju raztopljeni in plinasti CO2 : H2O + CO2 H2CO3 ↔ H + + HCO3 – ↔ 2H+ + CO3 – H2CO3 â HCO3– + H+ [ ] [ ] [ ] K c c c a1 + 3 – 2 3 H HCO H CO = ⋅ = 4,3 · 10–7 (10) HCO3 – â CO3 2– + H+ [ ] [ ] [ ] K c c c a2 + 3 2 – 3 – H CO HCO = ⋅ = 6 · 10–11 (11) [ ] [ ] K x c c a1 3 – 2 3 HCO H CO = ⋅ ( [ ] [ ] ≈ ⋅x c c HCO CO +H CO 3 – 2 2 3 [ ] [ ] ≈ ⋅x c c l HCO CO 3 – 2 ) (12) [ ] [ ] K x c c a2 3 2 – 3 – CO HCO = ⋅ (13) Tu pomenijo: x mol/L= cH+ Ka1mol/L= konstanta razpada H2CO3 Ka2mol/L= konstanta razpada HCO3- D. DREV ET AL.: RAZTAPLJANJE CO2 V EMBALIRANI VODI ALI BREZALKOHOLNI PIJA^I ... 280 Materiali in tehnologije / Materials and technology 42 (2008) 6, 277–283 Slika 6: Prikaz migracijskih procesov v plastenki z brezalkoholno pija~o 9 Figure 6: Migration processes in soft drink plastic bottle 9 Slika 4: Topnost CO2 v PET (59 % amorfnega dela) v odvisnosti od nadtlaka pn in zunanje temperature 9 Figure 4: Solubility of CO2 in PET (59% of amorphous faze) in dependence of the pressure and temperature 9 Slika 5: Prikaz prepustnosti O2 za PET plastenke 12 Figure 5: Permeability of PET plastic bottels for oxygen 12 Kot je razvidno iz ravnote`ne reakcije, je vsebnost plinastega CO2 odvisna delno tudi od kemije in ne samo od Henryjevega zakona o topnosti CO2 v vodi. Dolo~ene raztopljene snovi vplivajo na pH-vrednost, ta pa na ravnote`je plinastega in raztopljenega CO2. Ker se lahko s ~asom spreminja sestava raztopljenih snovi v vodi, se spreminja tudi pH-vrednost. S tem se spreminja tudi razmerje med topnih in plinastim CO2. Koncentraciji vodikovih cH+ ali hidroksilnih ionov cOH– sta povezani preko konstante disociacije vode, zato je pH-vrednost merilo za koncentracijo vodikovih cH+ in hidroksilnih ionov cOH–. H2O H + + OH– Za konstanto disociacije velja naslednja formula: [ ] [ ] [ ] c c c H OH H O + – 2 ⋅ = KH2O = 1,8 · 10 –16 (14) [ ]c / / H O L M 0 g L g L2 H O2 = =1 100 18 = 55,5 mol/L [ ]c H O2 ·1,8·10–16 = 55,5·1,8·10–16 = 1·10–16 = Kw (15) lg cH+ + lg cOH– = –14 (16) pH = – lg cH+ (17) Tu pomenijo: KH2Omol/L= konstanta razpada vode Kw (mol/L)2 = ionski produkt vode Na podlagi zgoraj navedenih reakcij in ena~b lahko izra~unamo ravnote`je med CO2 in pH vrednostjo kot je prikazano na sliki 7. Iz navedenega je razvidno, da se z zmanj{anjem pH-vrednosti pove~a koli~ina plinastega CO2. To pa se lahko zgodi zaradi kemi~nih reakcij raztopljenih substanc v brezalkoholni pija~i. Te reakcije so navadno zanemarljive, saj mora ostati nespremenjena kakovost ustekleni~enih brezalkoholnih pija~. CO2 pa nastaja tudi pri biokemijskih procesih razgradnje raztopljenih organskih snovi v vodi oziroma brezalkoholni pija~i. Za to so potrebne bakterije in ustrezni pogoji. Pri embalirani vodi in brezalkoholnih pija~ah mora biti zagotovljena sterilnost embala`e in pija~e, zato so tak{ne reakcije malo verjetne. Vendar pa jih ne moremo v celoti izklju~iti, posebno {e pri sadnih sokovih, kjer je velika koli~ina hraniva za razvoj bakterij. 3 PRAKTI^NI DEL 3.1 Primer spro{~anja CO2 zaradi dviga temperatur in dodajanja topila v gazirano brezalkoholno pija~o a) Pove~anje tlaka zaradi dviga temperature – Pri preiskavi gazirane mineralne vode je bila iz- merjena koli~ina 3,8 mg CO2/l. – Iz grafikona na sliki 2 je razvidno, da se z dvigom temperature od 15 °C na 40 °C pove~a tlak CO2 iz 1 bar na pribli`no 4 bar. – 3 bar nadtlaka lahko povzro~i burno sprostitev brez- alkoholne pija~e tako kot je prikazano na sliki 8. – Pri steklenicah z velikimi napetostmi v materialu pa lahko nastane celo eksplozija steklenice. Steklenice za brezalkoholne pija~e so navadno preizku{ene na tlak 7 bar. b) Spro{~anje CO2 zaradi raztapljanja drugih snovi, ki dvigujejo pH vrednost 3.2 Izra~un pH vrednosti za gazirano mineralno vodo pred dodatkom NaHCO3 in po njem: – raztopili smo 1 g NaHCO3 v 1 L gazirane mineralne vode; – v gazirani mineralni vodi je bila izmerjena vsebnost CO2; 3,8 mg/L = 0,086 mol/L in 7,7 mg HCO3–/L = 0,126 mol HCO3–/L; – pufer H2CO3 in NaHCO3 ima pH = 6,4; – izra~un pH-vrednosti pred dodatkom NaHCO3: pH = 6,4 + lg [ ] [ ] c c HCO CO 3 2 – = 6,4 + lg 0 0 ,126 mol L ,086 mol L / / = 6,4 + 0,16 = 6,56 – izmerjena pH vrednost gazirane mineralne vode je bila pribli`no 6,5; 1 mol H2CO3 = 62 g 1 mol CO2 = 44 g 1 mol HCO3 – = 61 g 1 mol NaHCO3 = 84 g – v 1 L gazirane mineralne vode smo dodali 10 g NaHCO3 (0,12 mol); – sprostila se je znatna koli~ina CO2, pri ~emer se pH-vrednost ni opazno spremenila; izra~un pH vrednosti po dodatku NaHCO3: pH = 6,4 + lg [ ] [ ] c c HCO CO 3 2 – = 6,4 + + lg (0 0 ,126 + 0,12) mol L ,086 mol L / / = 6,4 + 0,45 = 6,85 D. DREV ET AL.: RAZTAPLJANJE CO2 V EMBALIRANI VODI ALI BREZALKOHOLNI PIJA^I ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 277–283 281 Slika 7: Ravnote`je CO2 – pH-vrednost 7 Figure 7: pH-CO2 equilibra 7 – Izmerjena pH-vrednost po dodatku NaHCO3 je bila pribli`no 6,5, kar ni bistvena sprememba glede na prvotno stanje. 3.3 Spro{~anje CO2 zaradi dodatka NaHCO3 v koka- kolo: – pred dodatkom NaHCO3 je bila izmerjena vrednost pH = 3; – po dodatku 1 g NaHCO3 v 1 L Coka cole je nastala burna reakcija spro{~anja CO2, pribli`no tako kot je prikazano na sliki 8; – po dodatku NaHCO3 je imela Coka cola pH = 6; – pri Coka coli je poleg ogljikove tudi znatna koli~ina fosforne kisline. Zato je treba upo{tevati poleg ravnote`nih reakcij H3CO3 tudi ravnote`ne reakcije H3PO4 H3PO4 â H2PO4- + H+ [ ] [ ] [ ] K c c c1 = ⋅H H PO H PO + 2 4 – 3 4 = 1·10–2 H2PO4 – â HPO4–2 + H+ [ ] [ ] [ ] K c c c 2 = ⋅H H PO H PO + 2 4 2 – 2 4 – = 1·10–7 HPO4 –2 â PO4–3 + H+ [ ] [ ] [ ]K c c c 3 = ⋅H PO HPO + 4 3 – 4 2 – = 1·10–12 pH = – lg cH+ = 3 cH+ = –lg cH+ = 10–3 mol/l Izra~un spro{~ene koli~ine CO2 po dodatku NaHCO3: [ ] [ ] [ ]c c cH CO HCO H2 3 3 – + = ⋅ ⋅ −4 3 10 7, = [ ] [ ]0,09 + 0,01 0,001⋅ ⋅ −4 3 10 7, = 10 4 3 10 4 7 − −⋅, = 232 mol/L 232 mol/L CO2 = 4 g/L CO2 3.4 Po{kodbe pri odpiranju plastenk Oseba iz manj{ega kraja na Dolenjskem, ki je odpirala dvolitrsko plastenko kokakole, je dobila zaradi tega po{kodbe. Zama{ek ji je vrglo v levo li~nico, hlape in teko~ino pa v levo oko. V trenutku jo je mo~no zapeklo in na to oko ni videla ni~ ve~. Zaradi tega je morala poiskati zdravni{ko pomo~ v bolni{nici. Na okulisti~nem oddelku je ostala 13 dni. Delne posledice po{kodbe pa so ostale. Od{kodnino za nastalo po{kodbo posku{a izto`iti na sodi{~u. V navedenem primeru je bil v plastenki povi{an tlak CO2 ter tudi pokvarjen zama{ek. Tega ni bilo mo`no enostavno odpreti, temve~ so bili za to potrebni dodatni napori. Zaradi tega se je v kokakolo vna{ala dodatna kineti~na energija, ki je spro{~ala vsebnost neraztop- ljenega CO2. Poleg tega pa odpiranje pokvarjenega zama{ka ni povzro~alo postopnega spro{~anja tlaka v plastenki. Poznan je primer iz Nem~ije, ko je plastenka s koka- kolo eksplodirala v rokah devetletnega otroka. Zardi po{kodb, ki jih je pri tem dobil otok, je bila izpla~ana od{kodnina 10.000 DM. Podobno kot za navedena primera kokakole je poznano {e ve~ primerov po{kodb z drugimi gaziranimi brezalkoholnimi pija~ami in ustekleni~eno kislo vodo. V fazi polnjenja se je lahko raztopila dvakrat ve~ja koli~ina CO2 kot pri nekoliko vi{ji temperaturi pri enakem tlaku. ^e se steklenica oziroma plastenka gazirane pija~e pred odpiranjem {e precej obra~a, se znatni del raztopljenega CO2 sprosti. Razlog za to je vnos kineti~ne energije v vodo, kar vpliva na dodatno spro{~anje CO2. Koli~ina neraztopljenega CO2 se tako nekajkrat pove~a, kar po Henryjrevem zakonu pomeni tudi zvi{anje tlaka v plastenki. V takem primeru lahko pride pri odpiranju plastenke do pribli`no tak{nega pojava, kot je prikazan na sliki 8. 4 SKLEPI Koli~ina raztopljenega CO2 v vodi ali brezalkoholni pija~i ni pomembna samo zaradi zdravstvenih in kulinari~nih zahtev, temve~ lahko vpliva tudi na po{kodbe uporabnikov. Te so sicer zelo redke, vendar pa jih ne smemo zanemariti. V ~lanku smo analizirali D. DREV ET AL.: RAZTAPLJANJE CO2 V EMBALIRANI VODI ALI BREZALKOHOLNI PIJA^I ... 282 Materiali in tehnologije / Materials and technology 42 (2008) 6, 277–283 Slika 8: Prikaz burne ekspanzije Coka cole po odstranitvi zama{ka Figure 8: Ejection of liquid after removal of the Coca-cola bottle cap vzroke po{kodb in se v konkretnih primerih omejili le na po{kodbe, ki lahko nastanejo zaradi prevelike koli~ine CO2 pri odpiranju plastenk. Obravnavali pa smo problem bistveno {ir{e, tj. z vidika nastajanja prekomerne koli~ine plinastega CO2 v ustekleni~eni pija~i. Koli~ina raztopljenega in plinastega ogljikovega dioksida je v glavnem definirana z osnovnimi plinskimi zakoni (Henryjev zakon), kar se pogosto pozablja. Tudi vrsta in koli~ina raztopljenih snovi imata dolo~en vpliv na topnost oziroma spro{~anje CO2. Veliko bolj problema- ti~no pa je nastajanje CO2 pri biokemijskih procesih (alkoholno vretje, itd.), kar povzro~a {e ve~jo nevarnost po{kodb pri odpiranju steklenic. Prekomerna koli~ina CO2 pa ni nevarna le pri odpiranju steklenic, temve~ tudi med hranjenjem. ^e je tlak tako velik, da ga embala`a ve~ ne zdr`i, nastane eksplozija. Eksplozije plastenk niso tako problemati~ne kot eksplozije steklenic, saj je steklo krhko in trdo. Drobci stekla lahko zletijo v zrak ter povzro~ijo precej{nje po{kodbe navzo~ih ljudi. Varnej{e odpiranje steklenic in plastenk morajo omogo~iti tudi zama{ki. Neustrezni zama{ki so ena izmed velikih hib, ki jih lahko ugotovimo potro{niki v vsakdanjem `ivljenju. Pri plastenki mora biti izdelan zama{ek tako, da zagotavlja popolno zaprtje vsebine, dokler ga ne za~nemo odvijati. Ko za~nemo odpirati plastenko, mora priti do enostavnega razdvajanja fiksnega dela (~e obstaja) od zama{ka z navojem. Navojni del bi moral biti narejen tako, da se pri odvijanju postopno spro{~a tlak. Odpiranje steklenic s kovinskim pokrovom ni problemati~no, ~e uporabljamo ustrezno odpiralo. Pri dvigovanju pokrov~ka se tlak postopno izena~uje. Bistveno ve~ji problem pa so ustekleni~ene pija~e z zama{ki. 5 LITERATURA 1 Brydson, J. A., Plastics Materials, Butterworth Heinemann, 1999 2 Dean, J. A., Lange's Handbook of Chemistry, McGraw, Inc., 1992 3 Drev, D., Problematika embalirane vode. V: Ro{, Milenko (ur.). Zbornik referatov. Ljubljana: Slovensko dru{tvo za za{~ito voda, 2005, 128–138 4 Frimmel, F. H., Wasser und Gewasser, Ein Handbuch (Gebundene Ausgabe), Spektrum Akademischer Verlag, 1999 5 Gächter, R., Müller, H., Taschenbuch der Kunststoff – Additive, Hanser Verlag, Wien, 1989 6 Havelaer, A. H., Melse, J. M., Quantifying public health risk in the WHO Guidalines for Drinking – Water Quality, RIVM report 73401022/2003, 2003 7 Lide, D. R., CRC Handbook of Chemistry and Physics, 71 ed. Boca Raton, Ann Arbor, Boston: CRC Press, 1991 8 Jolly, W. L. Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill, 1991 9 Müller, K., O2 – Durchlässigkeit von Kunststoffflaschen und Ver- schlüssen – Messung und Modellierung der Stofftransportvorgänge, PhD Thesis, Technische Universität München, 2003 10 Mette, M., Ein Beitrag zur Gasdurchlässigkeit Permeabler Geträn- kenflaschen unter dem Aspekt der Haltbarkeit des Füllgutes-Teil 1, Brauindustrie, 3 (2003), 150–153 11 Nölle, G., Technick der Glasherstellung, Wiley VCH Verlag, (1997) 12 Orzinski, M., Untersuchung der Permeation von anorganische Gasen und organische Verbindungen durch barriereverbesere Kunststoff- flaschen und ihre messtechnische Erfassung, PhD Thesis, Technische Universität Berlin, D83, 2007 13 Preeti, C., Multi-component transport of gases and vapors in poly(ethylene terephtalate), PhD Thesis, Georgia Institute of Tech- nology, 2006 14 Pravilnik o presku{anju izdelkov in snovi, ki prihajajo v stik z `ivili, (Uradni list RS, 131/039) 15 Palzer, G., Establishment of a standard test precedure for PET bottle materials with respect to chemical inertness behavior including the preparation of a certified PET reference material, PhD Thesis, Technische Universität München, 2001 16 Physical and engineering data, January 1978 ed. The Hague: Shell Internationale Petroleum Maatschappij BV, 1978 17 Spellman, F. R. The drinking water handbok, CRC PRESS, 1999 18 Uredba Evropskega parlamenta in Sveta, 27. oktober 2004 o mate- rialih in izdelkih, namenjenih za stik z `ivili, in o razveljavitvi direktiv 80/590/EGS in 89/109/EGS 19 Uredba o izvajanju Uredbe Evropskega parlamenta in Sveta ES o materialih in izdelkih, namenjenih za stik z `ivili in o razveljavitvi direktiv 80/590/EGS in 89/109/EGS, (Uradni list RS, 53/05, 66/06) 20 Witt G. Taschenbuch der Fertigungstechnik, Hanser, 2005 D. DREV ET AL.: RAZTAPLJANJE CO2 V EMBALIRANI VODI ALI BREZALKOHOLNI PIJA^I ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 277–283 283 S. GYERGYEK ET AL.: PRIPRAVA Co-FERITNIH NANODELCEV Z OZKO PORAZDELITVIJO VELIKOSTI ... PRIPRAVA Co-FERITNIH NANODELCEV Z OZKO PORAZDELITVIJO VELIKOSTI Z METODO TERMI^NEGA RAZPADA OLEATOV PREPARATION OF Co-FERRITE NANOPARTICLES WITH A NARROW SIZE DISTRIBUTION BY THE THERMAL DECOMPOSITION OF OLEATES Sa{o Gyergyek1, Darko Makovec1, Mihael Drofenik1,2 1Odsek za sintezo materialov, Institut »Jo`ef Stefan«, Jamova 39, 1000 Ljubljana, Slovenija 2Fakulteta za kemijo in kemijsko tehnologijo, Univerza v Mariboru, Smetanova 17, 2000 Maribor, Slovenija saso.gyergyekijs.si Prejem rokopisa – received: 2007-10-30; sprejem za objavo – accepted for publication: 2008-07-23 V prispevku opisujemo sintezo nanodelcev kobaltovega ferita z ozko porazdelitvijo velikosti z metodo termi~nega razpada organskega kompleksa. Sinteza nanodelcev je potekala v dveh stopnjah. V prvi smo sintetizirali `elezov in kobaltov oleat z reakcijo kobaltovega (II) in `elezovega (III) klorida z natrijevim oleatom v me{anici topil. V drugi stopnji smo raztopino oleatov, ki smo ji dodali razli~ne koli~ine oleinske kisline, segreli do vreli{~a topila (heksadeken 282 °C ali oktadeken 316 °C). Na povi{ani temperaturi oleati razpadejo in tvorijo oksidne nanodelce. Na nanodelce je vezan monomolekulski sloj oleinske kisline, ki omogo~a dispergiranje nanodelcev v nepolarnih topilih. Povpre~na velikost nanodelcev kobaltovega ferita je odvisna od temperature, ~asa siteze in koli~ine dodane oleinske kisline. Sintetizirani nanodelci v obmo~ju velikosti med 9 nm in 20 nm izkazujejo ferimagnetno vedenje ter magnetne lastnosti, ki se spreminjajo s povpre~no velikostjo nanodelcev. Predpostavili smo mehanizem nastanka nanodelcev kobaltovega ferita, ki vklju~uje koalescenco manj{ih nanodelcev in njihovo rekristalizacijo. Klju~ne besede: kobaltov ferit, nanodelci, magnetni nanodelci A synthesis method for the preparation of narrow-size-distribution Co-ferrite nanoparticles by thermal decomposition of oleates is presented. A two-step method was used to produce the nanoparticles. In the first step cobalt and iron oleates were synthesized by reacting iron (III) and cobalt (II) chlorides with sodium oleate in a mixture of solvents. In the second step the oleates solution, to which different amounts of oleic acid were added, was heated to the solvents’ boiling point (hexadecene 282 °C or oktadecene 316 °C). At elevated temperatures oleates decompose and oxide nanoparticles are formed. The nanoparticles are than coated with a mono-molecular layer of oleic acid, are hydrophobic and can be dispersed in non-polar organic solvents. The average size of the cobalt ferrite nanoparticles depends on the temperature, time of the synthesis and the concentration of oleic acid. In the size range between 9 nm and 20 nm the synthesized nanoparticles exhibited ferromagnetic behavior and size-dependent magnetic properties. A mechanism for the formation of cobalt ferrite nanoparticles with re-crystallization of nanoparticles composed of smaller nanoparticles is proposed. Key words: cobalt ferrite, nanoparticles, magnetic nanoparticles 1 UVOD Magnetni nanodelci, kot so npr. feritni, so pomemben material zaradi zna~ilnih magnetnih, magnetorezistivnih in magnetoopti~nih lastnosti,1 ki jih v grobozrnatem materialu ne opazimo.2-4 Njihova uporaba se razteza od tehnolo{ke, kot so magnetne teko~ine5 in magnetno hranjenje informacij,6 do biomedicinske, kot sta na primer ciljna dostava zdravil7 ali pove~anje kontrasta pri slikanju z NMR-tehniko.8 Kobaltov ferit je tehnolo{ko zanimiv predstavnik skupine feritnih materialov zaradi velike energije mag- netne anizotropije in velikih magnetoopti~nih koeficien- tov.9 Tako je bilo razvitih veliko razli~nih sinteznih metod za pripravo nanodelcev kobaltovega ferita, kot so sol-gel,10 hidrotermalna sinteza,11 koprecipitacija,12 koprecipitacija v mikroemulzijah13 in koprecipitacija s segrevanjem z mikrovalovi,14 v zadnjem ~asu pa tudi termi~ni razpadi organskih kompleksov, kot je npr. razpad oleatov.15 Zadnja omenjena metoda omogo~a sintezo nanodelcev z ozko porazdelitvijo velikosti in enostavno prilagajanje velikosti nanodelcev s spremi- njanjem sinteznih pogojev. Pri tej metodi raztopimo predhodno sintetizirane oleate v nepolarni teko~ini z visokim vreli{~em. Pri povi{ani temperaturi vreli{~a oleati razpadajo in nastanejo nanodelci ferita, ki so prekriti z monomolekulskim slojem oleinske kisline. Za oleinsko kislino je znano, da dobro stabilizira suspenzije nanodelcev v nepolarnih topilih.16 Tako lahko sinteti- zirane nanodelce dispergiramo v nepolarnih topilih, kot so npr. dekan, heksan ali toluen, in pripravimo magnetne teko~ine ali pa z reakcijami zamenjave ligandov nano- delce ustrezno funkcionaliziramo.17 Nastanek monodisperznih delcev je odvisen od lo~it- ve nukleacije delcev od njihove rasti.18 Zagotovitev tak{nih pogojev je pri ve~ini sinteznih metod prakti~no nemogo~a. Pri segrevanju oleatov v topilu z visokim vreli{~em pa lahko tak{ne pogoje do neke mere izpolnimo. Poglaviten vzrok je stopenjski razpad oleatov pri razli~nih temperaturah. V primeru sinteze maghemita s termi~nim razpadom `elezovega oleata Fe(ol)3 je bilo ugotovljeno, da ena molekula oleinske kisline disociira Materiali in tehnologije / Materials and technology 42 (2008) 6, 285–289 285 UDK 543.428.3:669.25 ISSN 1580-2949 Izvirni znanstveni ~lanek/Original scientific article MTAEC9, 42(6)285(2008) pri temperaturi 220 °C – 240 °C, kar povzro~i nukle- acijo, drugi dve pa pri temperaturi okoli 300 °C. Disociacija drugih dveh oleinskih kislin povzro~i intenzivno rast jeder, ki so nastala pri ni`ji temperaturi.15 Tako sta nukleacija in rast lo~eni za pribli`no 60 °C. Pri tem delu smo sintetizirali Co-feritne nanodelce s termi~nim razpadom oleatov. Poudarek je bil na razis- kavah vpliva sinteznih pogojev na povpre~no velikost nanodelcev in na njihove magnetne lastnosti. S sprem- ljanjem ~asovnega poteka pa smo posku{ali priti do podrobnej{ih informacij o mehanizmu nastanka nano- delcev kobaltovega ferita, saj prisotnost oleatov z razli~nima kationoma in s tem z razli~nim tempera- turnim vedenjem, najverjetneje modificira predpostav- ljen mehanizem nastanka, opa`en pri nanodelcih maghemita (gama Fe2O3). 2 EKSPERIMENTALNI DEL Nanodelce kobaltovega ferita smo pripravili po modificirani dvostopenjski metodi termi~nega razpada `elezovega (III) in kobaltovega (II) oleata.15 V prvi stopnji smo sintetizirali kobaltov (II) in `elezov (III) oleat. V bu~ki z obrusom smo raztopili 20 mmol `ele- zovega (III) klorida, 10 mmol kobaltovega (II) klorida in 80 mmol natrijevega oleata v topilu s sestavo: 30 mL vode, 40 mL etanola in 70 mL heksana. Reakcijsko me{anico smo refluktirali 4 h pri vreli{~u zmesi topil. Med refluktiranjem nastaneta `elezov (III) in kobaltov (II) oleat, ki sta netopna v vodni fazi in se sproti ekstrahirata v heksansko fazo. Po 4 h refluksa lo~imo oleate od vodne faze v liju lo~niku. Heksansko fazo, ki vsebuje raztopljene oleate, smo sprali z destilirano vodo. Oleate smo izolirali z odparevanjem heksana pri 60 °C. 24 g oleatov smo raztopili v 133 g heksadekena ali oktadekena. V drugi stopnji smo k raztopinam oleatov dodali razli~ne koli~ine oleinske kisline (OA), raztopine segreli s hitrostjo segrevanja ≈3 K/min do vreli{~a topila (heksadeken 282 °C in oktadeken 316 °C) in refluktirali kraj{i ~as 0,5 h ali dalj{i ~as 3 h. Tabela 1 prikazuje sintezne pogoje. Nad 250 °C je mogo~e opaziti burno reakcijo, ki je posledica termi~nega razpada oleatov. Nanodelce smo izolirali s flokulacijo, ki jo povzro~i dodatek acetona v velikem prebitku, in s centrifugi- ranjem pri 5000 min–1, 10 min. Dodatek acetona mo~no spremeni dielektri~no konstanto medija, kar povzro~i tudi zmanj{anje topnosti stranskih produktov in nezreagiranih reaktantov. Te smo odstranili z ve~kratnim intenzivnim spiranjem oborine s heksanolom. Heksanol se je izkazal kot primeren medij za ~i{~enje produktov, saj zaradi delno polarnega zna~aja ne omogo~a nastanka stabilnih suspenzij hidrofobnih nanodelcev, hkrati pa je dobro topilo za oleate. Po spiranju smo ~iste delce dispergirali v heksanu, s centrifugiranjem na 5000 min–1, 10 min pa odstranili aglomerate. Nanodelce smo karak- terizirali z rentgensko pra{kovno difrakcijo (XRD) (Bruker AXS D4 ENDEAVOR), presevno elektronsko mikroskopijo (TEM) (Jeol 2010F) in z meritvami magnetnih lastnosti z magnetometrom z vibrirajo~im vzorcem (VSM) (Lake Shore 7307 VSM). Vzorce za TEM smo pripravili s su{enjem razred~ene stabilne suspenzije nanodelcev v heksanu na mre`ici za TEM, vzorce za druge raziskave pa smo pripravili z izolacijo nanodelcev iz heksanske suspenzije z acetonom. Povpre~no velikost nanodelcev smo ugotovili iz {iritve uklonov XRD-spektrov (dXRD) z ra~unalni{kim progra- mom Topaz™, povpre~no velikost in standardni odklon pa iz merjenja velikosti vsaj 100 nanodelcev na TEM-posnetku (dTEM). 3 REZULTATI IN DISKUSIJA Pri povi{ani temperaturi oleati razpadajo in tvorijo oksidne nanodelce. Tak{ni nanodelci so prevle~eni z monomolekularskim slojem oleinske kisline tudi v pri- meru, ko v reakcijsko zmes oleinska kislina (OA) ni bila dodana. Med sintezo oleatov nastaja OA s hidrolizo natrijevega oleata. Zaradi nepolarnega zna~aja se ekstrahira v heksansko fazo. Nizka temperatura odpa- revanja heksana in visoka tempratura vreli{~a OA je razlog, da je dele` OA o~itno dovolj velik, da stabilizira suspenzijo nanodelcev tudi v primeru, ko OA nismo dodali k raztopinam oleatov. Sloj OA, ki je vezan na povr{ino, prepre~uje njihovo aglomeracijo in omogo~a dispergiranje nanodelcev v nepolarnih topilih. Produkt sinteze so kristalini~ni spinelni nanodelci, kar je razvidno iz XRD- spektrov na sliki 1. Izjema je vzorec, pripravljen s segrevanjem kraj{i ~as (0,5 h) pri kon~ni temperaturi (vzorec CF2). Difraktogram tega vzorca prikazuje razen uklonov, zna~ilnih za spinelno strukturo, tudi {iroke uklone, ki se skladajo s strukturo CoO. Delci v vseh vzorcih so sferi~ne oblike z ozko 286 Materiali in tehnologije / Materials and technology 42 (2008) 6, 285–289 S. GYERGYEK ET AL.: PRIPRAVA Co-FERITNIH NANODELCEV Z OZKO PORAZDELITVIJO VELIKOSTI ... Slika 1: XRD-spektri nanodelcev Co-ferita. Spektri so indeksirani v skladu s spinelno strukturo. Pri vzorcu CF2 so z * ozna~eni refleksi CoO. Figure 1: XRD-spectra of Co-ferrite nanoparticles. Spectra are in- dexed according to spinel structure. CoO reflections are marked by * in the spectrum of the sample CF2. porazdelitvijo velikosti (Slika 2). Meritev velikosti nanodelcev na TEM-posnetkih (dTEM, tabela 1) je poka- zala relativno ozko porazdelitev velikosti s standardnim odklonom od povpre~ne velikosti 10 % – 20 %. Veli- kost, ugotovljena na TEM-slikah, se dobro sklada z velikostjo, dolo~eno iz XRD- spektrov (dXRD, tabela 1), kar dokazuje dobro kristalini~nost nanodelecv. Izjema je vzorec, pripravljen s segrevanjem kraj{i ~as (vzorec CF2), kjer je velikost, dolo~ena iz XRD-spektrov (10 nm za spinelne nanodelce), precej manj{a od velikosti, ugotovljene na TEM slikah. Ve~anje velikosti nano- delcev z vi{anjem temperature sinteze (vzorca CF1 in CF5) je verjetno povezano predvsem s hitrej{im razpa- danjem oleatnih kompleksov, kar pospe{uje rast kristalitov.15 Na velikost nanodelcev vpliva tudi dodatek OA, ki se ve`e na povr{ino nanodelcev, in s tem verjetno ote`uje prenos snovi do rasto~ega nanodelca kar povzro~i zmanj{anje velikosti nanodelcev z nara{~anjem dodane koli~ine OA (vzorci CF1, CF3 in CF4). Povpre~na velikost nanodelcev ima klju~en vpliv na magnetne lastnosti materiala (tabela 1 in slika 3). Z manj{anjem povpre~ne velikosti nanodelcev se zmanj- {ujejo tudi magnetizacija, remanenca in koercitivnost. Zaradi velikega razmerja med povr{ino in volumnom nanodelcev je znaten dele` atomov na povr{ini nanodelca. Ker je sloj povr{inskih atomov neurejen, ne prispeva k magnetnemu momentu nanodelca oz. zmanj{a magnetizacijo nanodelcev v primerjavi z grobozrnatim materialom. Dele` povr{inskih atomov raste z manj- {anjem povpre~ne velikosti nanodelcev, kar se izra`a z manj{anjem nasi~ene magnetizacije z manj{anjem povpre~ne velikosti nanodelecev. Prav tako se z zmanj- {evanjem povpre~ne velikosti nanodelcev ve~a dele` OA, ki je vezana na povr{ini in red~i magnetno fazo. Energija magnetne anizotropije se manj{a z volumnom nanodelca, kar se izra`a z manj{anjem remanence in koercitivnosti. Zopet je izjema vzorec po kraj{em ~asu sinteze (vzorec CF2), pri katerem magnetizacija ne dose`e nasi~enja niti pri relativno visokem polju 796 kA m–1. Slabe magnetne lastnosti se ne skladajo z veli- kostjo delcev, opa`enih s TEM. Analiza XRD-spektra S. GYERGYEK ET AL.: PRIPRAVA Co-FERITNIH NANODELCEV Z OZKO PORAZDELITVIJO VELIKOSTI ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 285–289 287 Slika 3: Krivulje odvisnosti magnetizacije od magnetnega polja, izmerjene pri sobni temperaturi Figure 3: Room temperature magnetisation curves as a function of magnetic field Slika 2: TEM-posnetek nanodelcev CF1 Figure 2: TEM-image of nanoparticels CF1 Tabela 1: Vpliv temperature (T), ~asa sinteze (t) in koli~ine dodane OA izra`ene z masnim razmerjem med maso oleatov in maso OA (mOA/mOL) na povpre~no velikost nanodelcev (dXRD) in (dTEM), in na magnetne lastnosti nanodelcev (Ms-nasi~ena magentizacija, Mr-remanentna magnetizacija in Hci-koercitivnost) Table1: Effect of temperature (T), synthesis time (t) and amount of added OA expressed by mass ratio between mass of OA and mass of oleates (mOA/mOL) on average nanoparaticles size (dXRD) and (dTEM), and magnetic properties of nanoparticles (Ms-saturation magnetization, Mr-remanent magnetization and Hci-coercitivity) Oznaka vzorca T/°C t/h mOA/mOL dXRD/nm dTEM/nm Ms/10 –4 T/g Mr/10–4 T/g Hci/ (79,6 A m–1) CF0 316 0,5 0 15 – – – – CF1 316 3 0 21 20 ± 2 57 28 1873 CF2 316 0,5 0,3 10* 16 ± 2 10 0,4 79 CF3 316 3 0,3 14 11 ± 2 49 14 720 CF4 316 3 0,6 13 14 ± 1 35 11 402 CF5 282 3 0 10 9 ± 2 49 4 41 *samo velikost spinelnih nanodelce vzorca sicer ka`e na prisotnost dveh faz, vendar tega na TEM-posnetku ni opaziti. EDS-analiza na posameznega nanodelca poka`e skoraj identi~no sestavo kot pri vzorcu, ki je bil sintetiziran dalj{i ~as (vzorec CF1). Nizke vrednosti za magnetne lastnosti tega vzorca (vzorec CF2), ki je bil sintetiziran kraj{i ~as, so verjetno posledica slab{e kristalini~nosti, kar lahko opazimo z visokolo~ljivostno TEM (HRTEM). HRTEM nano- delcev po dalj{em ~asu sinteze (vzorec CF1) ka`e periodi~no mre`no sliko, ki se sklada z njihovo dobro kristalini~nostjo (slika 4a). Pripadajo~a elektronska difrakcija prikazuje ostre obro~e refleksov, katerih oddaljenost od centralnega pramena ustreza spinelni strukturi (slika 4b). Na HRTEM-sliki nanodelcev je po kraj{em ~asu sinteze (vzorec CF2) jasno viden neenakomeren kontrast – manj{a podro~ja z urejeno periodi~nostjo se menjajo z neurejenimi podro~ji (slika 5a). S slab{o urejenostjo kristalne strukture nanodelcev CF2 se sklada pripadajo~a elektronska difrakcija (slika 5b), ki ka`e raz{irjene obro~e refleksov. [irina refleksov se ne sklada z velikostjo nanodelcev. Na osnovi podrobne analize HRTEM lahko ugotovimo, da je vsak nanodelec sestavljen iz domen urejenega materiala, ki se nekoliko razlikujejo v svoji orientaciji. Bistveno slab{e magnetne lastnosti nanodelcev CF2, sintetiziranih kraj{i ~as, je torej o~itno posledica njihove slabe kristali- ni~nosti. Posebna notranja nanostrukturiranost, ki jo ka`ejo nanodelci po kraj{em ~asu sinteze, je verjetno posledica kompleksnega, relativno zapletenega mehanizma njiho- vega nastanka. Pri povi{ani temperaturi nastanejo nanodelci CoO in nanodelci `elezovega oksida spinelne strukture (magnetit ali maghemit), ki se nato koalescirajo in tvorijo ve~je sferi~ne nanodelece. Po dalj{em ~asu pri temperaturi sinteze kompozitni delci rekristalizirajo v nanodelce kobaltovega ferita, ki imajo urejeno notranjo strukturo. Podoben mehanizem nastanka nanodelcev, ki vklju~uje rekristalizacijo aglomeratov manj{ih nano- delcev, je bil opa`en tudi pri sintezi nanodelcev Fe3O4 s podobnim postopkom.19 Na kinetiko nastanka nano- delcev kobaltovega ferita ima mo~an vpliv tudi prisotnost OA. Vzorec CF0, ki smo ga pripravili brez dodane OA, je vseboval le spinelne nanodelce (slika 1), ~eprav je bil pripravljen pod enakimi pogoji kot kompozitni nanodelci CF2. Oleinska kislina torej o~itno zavre hitrost procesov med sintezo. 4 SKLEP Pri tem delu smo raziskovali sintezo nanodelcev kobaltovega ferita z metodo termi~nega razpada oleatov. Metoda omogo~a pripravo nanodelcev z ozko poraz- delitvijo velikosti, povpre~na velikost nastalih nano- delcev pa je odvisna od temperature, ~asa sinteze in koncentracije OA. Sintetizirani nanodelci v obmo~ju velikosti med 9 nm in 20 nm izkazujejo ferimagnetno vedenje ter magnetne lastnosti, ki se spreminjajo s povpre~no velikostjo nanodelcev. Predpostavili smo mehanizem nastanka nanodelcev kobaltovega ferita, ki vklju~uje koalescenco manj{ih nanodelcev in njihovo rekristalizacijo. 5 LITERATURA 1 E. Tirosh, G. Shemer, G. Markovich, Chem. Mater. 18 (2006), 465–470 2 R. C. Ashoori, Nature, 379 (1996), 413–419 3 I. M. L. Billas, A. Chatelain, W. A. de Heer, Science, 265 (1994), 1682–1684 4 T. Hyeon, S. L. Seung, J. Park, C. Yunhee, N. B. Hyon, J. Am. Chem. Soc., 123 (2001), 12798–12801 5 R. E. Rosenweg, Ferrohydrodynamics, Dover Publications, New York, 1985 6 D. H. Han, H. L. Luo, Z. Yang, J. Magn. Magn. Mater., 161 (1996), 376–378 7 U. Häfeli, W. Schüt, J. Teller, M. Zborowski, Scientific and Clinical Applications of Magnetic Carriers, Plenum, New York, 1997 8 Q. A. Pankhurst, J. Connolly, S. K. Jones, J. Dobson, Applications of magnetic nanoparticles in biomdecine, J. Phys. D:Appl. Phys., 36 (2003), R167–R181 9 W. F. J. Fontijn, P. J. van der Zaag, L. F. Feiner, R. Metselaar, M. A. C. Devillers, J. Appl. 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Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang, T. Hyeon, Nature Materials, 3 (2004), 891–895 16 A. Ko{ak, D. Makovec, A. @nidar{i~, M. Drofenik, Mater. Tehnol., 39 (2005), 37–41 17 M. Lattuada, T. A. Hatton, Langmuir, 23 (2007), 2158–2168 18 T. Sugimoto, Monodispersed Particles, Elsevier, Amsterdam, 2001 19 D. Caruntu, G. Caruntu, Y. Chen, C. J. O’Connor, G. Goloverda, V. L. Kolesnichenko, Cehm. Mater., 16 (2004), 5527–5534 20 X. Batlle, A. Labarta, J. Phys. D: Appl. Phys., 35 (2002) R15–R42 S. GYERGYEK ET AL.: PRIPRAVA Co-FERITNIH NANODELCEV Z OZKO PORAZDELITVIJO VELIKOSTI ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 285–289 289 J. BERNETI^ ET AL.: CENTRELINE FORMATION OF THE Nb(C,N) EUTECTIC ... CENTRELINE FORMATION OF THE Nb(C,N) EUTECTIC IN 0.15 % C; 0.0071 % N; 0.022 % Nb; 0.033 % Al AND 0.003 % S STRUCTURAL STEEL SREDINSKO IZCEJANJE IN NASTANEK EVTEKTIKA Nb(C,N) V KONSTRUKCIJSKEM JEKLU Z 0,15 % C; 0,0071 % N; 0,022 % Nb; 0,033 % Al IN 0,003 % S Jure Berneti~1,2, Bo{tjan Brada{kja1,2, Gorazd Kosec1, Borut Kosec2, Erika Bricelj1 1ACRONI, d. o. o., Cesta Borisa Kidri~a 44, SI-4270 Jesenice, Slovenia 2Faculty of Natural Science and Engineering, Department of Materials and Metallurgy, University of Ljubljana, A{ker~eva 12, SI-1000 Ljubljana, Slovenia jure.berneticacroni.si Prejem rokopisa – received: 2008-09-23; sprejem za objavo – accepted for publication: 2008-10-22 During a routine control, a very small through thickness reduction of area was found for one tensile specimen of a 90-mm plate. Careful investigations of the fracture and the section of specimens cut from the as-solidified continuously cast 250-mm slab showed that the cause was the presence of coarse particles of niobium carbo-nitride as a constituent of the quasi-eutectic Fe-Nb(C,N) that forms because of the centerline segregation of niobium. Key words: structural steel, heavy plates, reduction of area, eutectic niobium carbo-nitride Pri rutinski kontroli lastnosti jekla je imel raztr`ni preizku{anec 90-milimetrske mm plo{~e zelo majhno kontrakcijo v smeri debeline. Preiskava prelomne povr{ine in prereza preizku{ancev, izrezanih iz kontinurino litega 250-milimetrskega slaba, je pokazala, da je vzrok zanjo prisotnost velikih zrn niobijevega karbonitrida v spa~enem evtektiku Fe-Nb(C,N), ki je nastal zaradi sredinske segregacije niobija. Klju~ne besede: konstrukcijsko jeklo, debele plo{~e, kontrakcija v smeri debeline, evtektik niobijevega karbonitrida 1 INTRODUCTION The reduction of area in the through thickness direction is an essential mechanical property of thick steel heavy plates intended for fillet welds. In the standard EN 101641 three quality classes, Z15, Z25 and Z35, with minimal average values for the through thickness reduction of area of three tests, 15 %, 25 % and 35 %, and minimal individual values, 10 %, 15 % and 25 %, respectively, are specified. During routine testing in a mechanical laboratory for one specimen only 9.5 % of the through thickness reduction of area was found, although the declared plate class was Z35. The sample is shown in Figure 1. According to Vodopivec et al. 2 the content of sulphur is the primary reason for the low ductility in the through thickness direction because of the lamelar tearing with fracture propagation also along the interface between the sulphide inclusion and the ferrite matrix. The mass fraction of sulphur in the tested steel, which was only 0.003 %, excludes the possibility of a low reduction of area due to sulphide inclusions. A small amount of niobium was added to the investigated structural steel to achieve the required mechanical properties. The addition of Nb could also affect the through thickness ductility of heavy plates because of the formation of coarse niobium carbo-nitride particles as constituents of the degenerated eutectic Fe-Nb(C,N), which may form with a high content of niobium or because of defective solidification of the steel3. To identify the cause of the low reduction of area, detailed investigations of specimens cut at different distances from the surface of the as-solidified slab were carried out. 2 EXPERIMENTAL The structural steel (S 355 J2+N) was melted in an EAF (electric arc furnace), VD (vacuum degassing) treated, continuously cast and cut into slabs of dimen- sions (250 × 1085 × 4770) mm. The slabs were cooled to Materiali in tehnologije / Materials and technology 42 (2008) 6, 291–294 291 UDK 669.14.018.298:669.18 ISSN 1580-2949 Professional article/Strokovni ~lanek MTAEC9, 42(6)291(2008) Figure 1: Macroscopic image of S 355 J2+N structural steel, showing the reduction of area specimen taken from a 90-mm heavy plate in the thickness direction Slika 1: Makroskopski posnetek kontrakcijskega preizku{anca kon- strukcijskega jekla S 355 J2+N. Preizku{anec je vzet po debelini iz 90-milimetrske debele plo{~e room temperature and after surface grinding reheated in a pusher-type furnace to a temperature of 1250 °C and hot rolled to 90-mm-thick plates. The chemical compo- sition of the heat is listed in Table 1. First, samples perpendicular to the slab casting direction were examined after grinding and deep-etching for 40 min in 25 % H2SO4 at 70 °C, which revealed the as-cast macro- structure. From this specimen, samples 1, 2 and 3 in the thickness direction were cut out for metallographic examination, as shown in Figure 2. From the 90-mm heavy plate, specimens were cut out in the thickness direction and submitted for tensile testing and exami- nations with optical and scanning electron microscopes (SEM) as well as energy-dispersive X-ray spectroscopy (EDXS). 3 RESULTS AND DISCUSSION Figure 3 shows a secondary-electron image of a fracture surface of one specimen with coarse niobium carbo-nitride inclusions and small MnS inclusions. The spots of the EDXS analyses of both inclusions are marked with arrows in Figure 3 and the results are given in Table 2. In the mapping micrographs in Figure 4 the bright areas represent the element in the particles and show the morphology of the particles and the main elements in large inclusions. Most of the particles observed on the fracture surface showed a large content of niobium. On the basis of fractographs it was con- cluded that niobium-containing particles (Nb,Ti)(C,N) were the main cause for the poor through thickness reduction of area of the steel plate. From the location of the fracture of the tensile specimen shown in Figure 1, we assumed that the source of the coarse precipitates was a very strong centreline segregation during the solidification of the steel slab. J. BERNETI^ ET AL.: CENTRELINE FORMATION OF THE Nb(C,N) EUTECTIC ... 292 Materiali in tehnologije / Materials and technology 42 (2008) 6, 291–294 Table 1: Chemical composition of the S 355 J2+N steel grade in mass fractions w/% Tabela 1: Kemijska sestava jekla S 355 J2+N v masnih dele`ih w/% Element C Si Mn P S Cr Cu Ni Al Nb Ti N w/% 0.15 0.49 1.10 0.018 0.003 0.14 0.29 0.12 0.033 0.022 0.005 0.0071 Figure 3: SEM fractograph (secondary electron image) of the reduc- tion of area specimen. The analysed particles are marked with arrows, and the EDXS analyses are presented in Table 2 Slika 3: SEM-slika (sekundarni elektroni) prelomne povr{ine kontrak- cijskega preizku{anca. To~ke opravljene EDXS analize prikazujeta pu{~ici, rezultati so podani v tabeli 2 Figure 2: As-cast sample taken from slab perpendicular to the casting direction Slika 2: Vzorec iz slaba pravokotno na smer ulivanja jekla Table 2: Results of the spot EDXS analyses. The place of the analysis is marked with an arrow in Figure 3. Tabela 2: Rezultati to~kovne EDXS analize. Mesto analize na sliki 3 je ozna~eno s pu{~ico. Element Fe Mn S Ti Nb conc. w/% (Nb,Ti)(C,N) 28.661 0.728 – 3.853 66.758 MnS 3.747 70.306 25.861 – – Tabela 3: Results of the spot EDXS analyses. The place of the ana- lysis is marked with an arrow in Figure 5. Table 3: Rezultati to~kovne EDXS analize. Mesto analize na sliki 5 je ozna~eno s pu{~ico. Element C Fe Mn S Ti Nb Al Pb conc. w/% (Nb,Ti) (C,N) 3.18 33.09 – – 3.76 59.97 – – MnS – 3.56 61.8933.09 – – – – Pb 2.01 9.16 – – – – 1.62 84.46 This conclusion is confirmed by the fact that from three samples, as shown in Figure 2, the niobium-rich precipi- tates were only found in the specimen cut from the slab centre in sample number 1. The precipitates are very similar to the Fe-Nb(C,N) eutectic known as "Chinese script"4. Besides the niobium-rich particles, a minor number of very small manganese sulphide inclusions and lead droplets were found. All these phases were only found in the centreline of the cast slab. The results of the spot EDXS analyses from the cast slab are presented in Table 3 and the spots of the analyses are marked in Figure 5. The analyses show that the niobium carbo-nitride particles also contain the mass fraction of Ti approximately 3.7 %, despite there being only 0.005 % of titanium in the steel originating from the steel scrap used. A similar composition of niobium carbo-nitride was reported for Nb-Ti micro-alloyed steels5. The solubility of the niobium carbo-nitride with the approximate composition Nb(C0.9N0.1) in structural steel is given by the equation6,7: lg Nb C N ( ( ) ( ) ( ) w w w+⎡ ⎣⎢ ⎤ ⎦⎥ 12 14 = 2,26 – 6770 T with w(Nb), w(C), and w(N) being the mass fractions of the elements in the steel and T being the temperature in K. Considering the actual contents of niobium, carbon and nitrogen, a solution temperature of 1140 °C was deduced, indicating that the slab soaking temperature was sufficient for a complete solution in austenite of the niobium carbo-nitride with the approximate composition Nb(C0.9N0.1). The fact that coarse niobium-rich precipi- tates were also found in the hot-rolled plate after heating the slabs to 1250 °C indicates that their composition differs from that of the soluble niobium carbo-nitride. The solubility of niobium carbide in austenite is greater than the solubility of niobium nitride8,9. It is assumed that the stability of particles in the investigated steel is due to their high content of nitrogen. The shape and size of the coarse carbo-nitride par- ticles suggest that they are constituents of a degenerated quasi-eutectic Fe-Nb(C,N). The location of the eutectic in the centre of the slab and the composition of the steel suggest that its formation is an improper solidification process related to a high casting temperature, a high slab solidification rate or a deficiency in the secondary slab cooling. 4 CONCLUSIONS When considering the contents of carbon, nitrogen and niobium in a steel, all the carbo-nitride phase with the approximate composition Nb(C0.9N0.1) is in a solid solution in austenite at 1140 °C. Since the slab soaking temperature was 1250 °C, it is evident that the carbo- nitride found in the examined steel does not have the quoted composition and that it has a higher content of nitrogen and correspondingly a higher solution tempera- J. BERNETI^ ET AL.: CENTRELINE FORMATION OF THE Nb(C,N) EUTECTIC ... Materiali in tehnologije / Materials and technology 42 (2008) 6, 291–294 293 Figure 5: SEM picture of degenerated eutectic in form of "Chinese script" from the sample number 1 of the as-cast slab. The analysed particles are marked with arrows, and the EDXS analyses are pre- sented in Table 3 Slika 5: SEM-posnetek spa~enega evtektika z obliko "kitajske pisave" iz vzorca {tevilka 1 kontinuirno ulitega slaba. To~ke opravljene EDXS-analize prikazujejo pu{~ice, rezultati so podani v tabeli 3 Figure 4: EPMA mapping of Nb(C,N) and MnS particles Slika 4: Ploskovna mikroanaliza delcev Nb(C,N) in MnS ture in austenite. The shape and the size of the niobium- rich particles suggest that they are the constituents of a degenerated eutectic Fe-Nb(CN) that formed because of the improper solidification process of continuous cast slabs. Acknowledgement The authors wish to thank Prof. Ladislav Kosec and Mrs. Nika Breskvar (University of Ljubljana) for the SEM and EDXS analyses. 5 REFERENCES 1 SIST EN 10164:2005: Steel products with improved deformation properties perpendicular to the surface of the product – Technical delivery conditions 2 F. Vodopivec, M. Gabrov{ek, I. Rak, B. Rali}, J. @vokelj, @elezarski zbornik, 12 (1978) 1, 1–16 3 V. K. Heikkinen, R. H. Packwood, Scand. J. Metallurgy, 6 (1977), 170-175 4 F. Haddad, S. E. Amara, R. Kesri, S. Hamar-Thibault, Journal de Physique IV, 122 (2004), 35–39 5 Dae-Hee Woo, Sang-Min Lee, Henri Gaye, Hae-Geon Lee, The formation behaviour of large Nb-Ti carbonitride precipitates during undirectional solidification of Nb-Ti microalloyed steel, Interna- tional Conference on clean steel 7, Balatonfüred, Hungary, 4-6 June 2007 6 K. J. Irvine, F. B. Pickering, T. Gladman, Journal of The Iron and Steel Institute, (1967), 161–182 7 A. M. Elwazri, A. Fatehi, J. Calvo, D. Bai, S. Yue, ISIJ International, 48 (2008) 1, 107–113 8 F. Vodopivec, M. Gabrov{ek, B. Rali}, Metal Science, 9 (1975), 324–326 9 F. Vodopivec, M. Gabrov{ek, B. Rali}, @elez. zbor., 4 (1976), 193–198 J. BERNETI^ ET AL.: CENTRELINE FORMATION OF THE Nb(C,N) EUTECTIC ... 294 Materiali in tehnologije / Materials and technology 42 (2008) 6, 291–294 LETNO KAZALO – INDEX Letnik / Volume 42 2008 ISSN 1580-2949 © Materiali in tehnologije IMT Ljubljana, Lepi pot 11, 1000 Ljubljana, Slovenija M EHNOLOGIJEIN ATER IALI M A T E R I A L S A N D T E C H N O L O G Y MATERIALI IN TEHNOLOGIJE / MATERIALS AND TECHNOLOGY VSEBINA / CONTENTS LETNIK / VOLUME 42, 2008/1, 2, 3, 4, 5, 6 2008/1 A failure criterion for single-crystal superalloys during thermocyclic loading Merilo za prelom monokristala superzlitine pri termocikli~ni obremenitvi L. Getsov, A. Semenov, A. Staroselsky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Accelerated creep testing of new creep resisting weld metals Preizkusi pospe{enega lezenja zvarov novega jekla, odpornega proti lezenju S. T. Mandziej, A. Výrostková, M. [olar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Zveza med analiznimi rezultati – karbonatna bomba in termi~na analiza Connection between analysis results – carbonate bomb and thermal analysis @. Poga~nik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Developing and testing a new type-8K mould for tool-steel ingot casting Razvoj in preizkus nove kokile vrste 8K za ulivanje ingotov iz orodnega jekla M. Balcar, L. Sochor, R. @elezný, P. Fila, L. Martínek, L. Kraus, D. Ke{ner, J. Ba`an . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 An AES investigation of brushed AISI 304 stainless steel after corrosion testing AES-preiskave krta~enega nerjavnega jekla AISI 304 po korozijskem preskusu M. Torkar, D. Mandrino, M. Lamut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Using a FIB to prepare Al(OH)3 samples for the TEM Uporaba FIB za pripravo vzorcev Al(OH)3 za TEM I. Nikolic, V. Radmilovic, T. Z. Sholklapper, D. Blecic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 2008/2 The interaction of SOFC anode materials with carbon monoxide Reakcije med anodnimi materiali SOFC in ogljikovim monoksidom B. Novosel, M. Avsec, J. Ma~ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Lubrication flow during the rolling of seamless tubes Tok maziva pri valjanju brez{ivnih cevi D. ]ur~ija, I. Mamuzi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 The influence of cooling rate on the microstructure of an Al-Mn-Be alloy Vpliv ohlajevalne hitrosti na mikrostrukturo zlitine Al-Mn-Be N. Rozman, T. Bon~ina, I. An`el, F. Zupani~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Tailoring the microstructure of ZnO-based ceramics Kontrola razvoja mikrostrukture v ZnO keramiki S. Bernik, M. Podlogar, N. Daneu, A. Re~nik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 A mechanism for the adsorption of carboxylic acids onto the surface of magnetic nanoparticles Mehanizem adsorbcije karboksilnih kislin na povr{ino magnetnih nanodelcev A. Drmota, A. Ko{ak, A. @nidar{i~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Analysis of the temperature profiles during the combustion synthesis of doped lanthanum gallate Analiza temperaturnih profilov med zgorevalno sintezo dopiranega lantanovega galata M. Marin{ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 A metallographic examination of a fractured connecting rod Metalografska preiskava preloma ojnice R. Celin, B. Arzen{ek, D. Kmeti~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 2008/3 On the different nature of time-dependent and time-independent irreversible deformation O razli~ni naravi ~asovno odvisne in ~asovno neodvisne ireverzibilne deformacije L.B. Getsov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Low-temperature transport properties of the ε-phases Al-Pd-(Mn, Fe, Co, Rh) Nizkotemperaturne transportne lastnosti ε-FAZ Al-Pd-(Mn, Fe, Co, Rh, …) D. Stani}, I. Smiljani}, N. Bari{i}, J. Dolin{ek, A. Bilu{i}, J. Lukatela, B. Leonti}, A. Smontara . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 296 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 LETNO KAZALO – INDEX The action of a laser on an aluminium target Obsevanje aluminijaste tar~e z laserjem V. Hen~-Bartoli}, T. Bon~ina, S. Jakovljevi}, D. Pipi}, F. Zupani~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Changes in the microstructure of Fe-doped Gd5Si2Ge2 Spremebe v mikrostrukturi zlitine Gd5Si2Ge2, dopirane z Fe I. [kulj, P. McGuiness, B. Podmilj{ak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Development of microstructure during the hot plastic deformation of high clean steels for power plants Razvoj mikrostrukture med vro~o plasti~no deformacijo visoko ~istega jekla za energetske naprave Kuskulic, T., Kvackaj, T., Fujda, M., Pokorny, I., Bacsó J., Molnarova, M., Kocisko, R., Weiss, M., Bevilaqua, T. . . . . . . . . . . . . . . . . 121 The off-axis behavior of a unidirectional fiber-reinforced plastic composite Zunajosno obna{anje enosmernih z vlakni oja~enih plasti~nih kompozitov T. Kroupa, V. La{ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 The influence of carbon content on the corrosion of MGO-C refractory material caused by acid and alkaline ladle slag Vpliv vsebnosti ogljika na korozijo ognjevzdr`nega materiala MGO-C v kisli in bazi~ni pe~ni `lindri Z. Adolf, P. Suchánek, I. Husar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 An evaluation of the properties of rotor forgings made from 26NiCrMoV115 steel Ocena lastnosti izkovkov za rotorje iz jekla 26NiCrMoV115 M. Balcar, V. Turecký, L. Sochor, P. Fila, L. Martínek, J. Ba`an, S. Nme~ek, D. Ke{ner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 2008/4 Applications of focused ion beam in material science Uporaba fokusiranega ionskega curka v znanosti materialov L. Repetto, G. Firpo, U. Valbusa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Materials and Technology: historical overview Materiali in tehnologije: zgodovinski pregled N. Jamar, J. Jamar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Low energy-high flux nitridation of metal alloys: mechanisms, microstructures and high temperature oxidation behaviour Nitriranje kovinskih zlitin s fluksom z majhno energijo in veliko gostoto: mehanizmi, mikrostrukture in visokotemperaturno oksidacijsko vedenje F. Pedraza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Numerical and experimental analyses of the delamination of cross-ply laminates Numeri~na in eksperimentalna analiza delaminacije v kri`nih plo{~atih laminatih R. Zem~ík, V. La{ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Application of the theory of physical similarity for the filtration of metallic melts Uporaba teorije fizikalne podobnosti za opis filtriranja kovinske taline K. Stránský, J. Ba`an, J. Dobrovská, M. Balcar, P. Fila, L. Martínek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 Priprava nanokompozita za biomedicinske aplikacije Preparation of nano-composites for biomedical applications S. ^ampelj, D. Makovec, L. [krlep, M. Drofenik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 The development of a chill mould for tool steels using numerical modelling Razvoj kokile za orodna jekla z uporabo numeri~nega modeliranja M. Balcar, R. @elezný, L. Sochor, P. Fila, L. Martínek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 2008/5 The effect of compositional variations on the fracture toughness of 7000 Al-alloys Vpliv sprememb v sestavi na `ilavost loma aluminijeve zlitine vrste 7000 M. Vratnica, Z. Cvijovi}, N. Radovi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Wear mechanism of duplex-coated P/M Vanadis 6 ledeburitic steel Mehanizem obrabe ledeburitnega jekla P/M Vanadis 6 z dupleksno prevleko P. Jur~i, M. Hudáková . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 Analiza toplotnih razpok na orodjih za tla~no litje aluminija Analysis of thermal cracks on die casting dies D. Klob~ar, J. Tu{ek, M. Pleterski, L. Kosec, M. Muhi~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Lasersko reparaturno varjenje termorazpok na orodjih za tla~no litje aluminija Laser repair welding of thermal cracks on aluminium die casting dies M. Pleterski, J. Tu{ek, L. Kosec, D. Klob~ar, M. Muhi~, T. Muhi~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 297 LETNO KAZALO – INDEX Use of artificial neural networks in ball burnishing process for the prediction of surface roughness of AA 7075 aluminum alloy Uporaba umetnih nevronskih mre` za napoved hrapavosti povr{ine pri krogelnem glajenju aluminijeve zlitine AA 7075 U. Esme, A. Sagbas, F. Kahraman, M. K. Kulekci . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Merjenje obrabne obstojnosti strukturne keramike Al2O3 Wear-resistance measurement of structural Al2O3 ceramics M. Ambro`i~, S. Veskovi~ Bukudur, T. Kosma~, K. Krnel, D. Eterovi~, N. Petkovi~ Habe, I. Pribo{i~ . . . . . . . . . . . . . . . . . . . . . . . . . 221 2008/6 On the determination of safety factors for machines using finite element computations O dolo~itvi faktorjev varnosti za naprave pri izra~unu z metodo kon~nih elementov L. B. Getsov, B. Z. Margolin, D. G. Fedorchenko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Polimeri v beli tehniki Polymer materials in white goods industry V. Vasi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Characterization of multilayer PACVD TiN/Ti(B-N)/TiB2 coatings for hot-worked tool steels using electron spectroscopy techniques Karakterizacija ve~plastne PACVD TiN/Ti(B-N)/TiB2 prevleke za orodna jekla za delo v vro~em s tehnikami elektronske spektroskopije M. Jenko, D. Mandrino, M. Godec, J. T. Grant, V. Leskov{ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 An investigation of the stretch reducing of welded tubes Raziskava iztezne redukcije varjenih cevi S. Re{kovi}, F. Vodopivec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Experimental analysis of crack initiation and growth in welded joint of steel for elevated temperature Eksperimentalna analiza nastanka in rasti razpoke v zvaru jekla za povi{ano temperaturo M. Burzi}, @. Adamovi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 The role of chloride salts on high temperature corrosion of 321 stainless steel Vloga kloridnih soli pri visokotemperaturni koroziji nerjavnega jekla 321 N. Amin, M. M. Amin, S. B. Jamaludin, K. Hussin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Raztapljanje CO2 v embalirani vodi ali brezalkoholni pija~i in s tem povezane mo`ne po{kodbe Problems associated with the dissolution of CO2 in the case of bottled water and non-alcoholic beverages D. Drev, M. Pe~ek, J. Panjan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Priprava Co-feritnih nanodelcev z ozko porazdelitvijo velikosti z metodo termi~nega razpada oleatov Preparation of Co-ferrite nanoparticles with a narrow size distribution by the thermal decomposition of oleates S. Gyergyek, D. Makovec, M. Drofenik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Centreline formation of the Nb(C,N) eutectic in 0.15 % C; 0.0071 % N; 0.022 % Nb; 0.033 % Al and 0.003 % S structural steel Sredinsko izcejanje in nastanek evtektika Nb(C,N) v konstrukcijskem jeklu z 0,15 % C; 0,0071 % N; 0,022 % Nb; 0,033 % Al in 0,003 % S J. Berneti~, B. Brada{kja, G. Kosec, B. Kosec, E. Bricelj . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 2008/Posebna {tevilka HEAT TREATMENT Possibilities of heat transfer control during quenching B. Li{~i} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 The influence of cooling rate and austenitization temperature on the microstructure and properties of a medium carbon microalloy forging steel M. Abed, A. Zabett . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 The use of new types of large and middle size vacuum batch furnace for the heat treatment of moulds and dies J. Ben-Hamida, M. Rink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Two-component diffusive steel saturation R. Ivanov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Heat treatment and welding effects on mechanical properties and microstructure evolution of 2024 and 7075 aluminium alloys H. Maamar, K. Mohamed, R. Otmani Rafik, F. Toufik, D. Nabil, A. Djilali . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Affect of vibratory weld conditioning on impact toughness of weld B. Pu~ko. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 298 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 LETNO KAZALO – INDEX The effect of aging parameters on properties of maraging steel I. Kladari}, D. Kozak, D. Krumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Austempering heat treatment effect on mechanical properties of aisi o1 steel J. Vatavuk, L.C.F. Canale, George E. Totten . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Cooling aspects of vacuum furnaces R. Stein, B. Zieger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 In-situ monitoring of vacuum carburizing M. Bruncko, A. C. Kneissl, Ivan Anzel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Heat treatment of corrosion resistant tool steels for plastic moulding R. Schneider, J. Perko, G. Reithofer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Heat treatment of hot work tool steels – size matters! G. Reithofer, T. Collins, R. Schneider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Direct method of tracing of oxidation in metals and alloys I. An`el . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Heat treatment of tool steels A. Molinari, M. Pellizzari . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 CRYOGENIC TECHNOLOGY The effect of some heat treatment parameters on the properties of AISI D2 C. Henrik Surberg, P. Stratton, K. Lingenhöle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Effect of deep-cryogenic treatment on high speed steel properties F. Cajner, V. Leskov{ek, D. Landek, H. Cajner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Deep sub zero processing of metals and alloys – Part I K. M. Iyer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Deep sub zero processing of metals and alloys – Part II C. L. Gogte, Kumar M. Iyer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 SURFACE ENGINEERING Correlation between sputtering conditions and growing properties of (TiAl)N/AlN multilayer coatings E. Altuncu, F. Üstel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Examination on surface properties of modified aisi 1090 steel by pulse plasma technique A. Ayday, A. Özel, C. Kurnaz, M. Durman . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Impact wear resistance of laser-clad valve seats with stellite 6 alloy S. S. Chang, H. C. Wu, Chun Chen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Observation mullite structure depending on spraying parameters G. Erdogan, F. Ustel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Characteristics of electrocodeposited Ni–Al2O3 nano particle reinforced metal matrix composite (MMC) coatings H. Gül, F. Kiliç, S. Aslan, A. Alp, H. Akbulut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Fabrication and characterization of Ni–SiC Metal matrix composite (MMC) nano-coatings by electrodeposition F. Kiliç, H. Gül, S. Aslan, A. Alp, H. Akbulut. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Surface modification of low and medium carbon steel by using electrolytic plasma thermocyclic treatment L. C. Kumruo lu, A. Ayday, A. Özel, A. Mýmaro lu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Magnetic-assistance in cylinder-surfaces finish P.S. Pa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Structural characteristics of plasma nitrided 32CrMoV33 hot working die steel A. Turk, C. Býndal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 The effect of bias voltage on oxidation behavior of monolayer TiAlN and multilayer TiAlN/AlN coatings F. Üstel* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Properties of hard Ni-P-Al2O3 and Ni-P-SiC coatings on Al-based casting alloys D. Vojtìch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 299 LETNO KAZALO – INDEX Investigation of sputter craters after GDOES analysis of TiCN coatings P. Panjan, Ð. Gor{}ak, M. ^ekada, L. ]urkovi}*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Previous or subsequent electron beam hardening of thermochemical treated and PVD hard coated steels for tools and components G. Sacher, R. Zenker, H.-J. Spies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Microstructural and tribological investigation of AlTiN-coated conventional and powder metallurgy cold work tool steel substrates P. Panjan, Ð. Gor{}ak, L. ]urkovi}, M. ^ekada. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Plasma nitrocarburizing of AISI H-13 steel for improved abrasion resistance G. E. Totten, L.C. Cassteletti, R.M. Muñoz Riofano, A. Lombardi Neto. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Contemporary industrial application of nitriding and its modifications J. Michalski, P. Wach, J. Tacikowski, M. Betiuk, K. Burdyñski, S. Kowalski, A. Nakonieczny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Influence of ion etching in low pressure arc discharge in plasma on duplex coat adhesion produced by gas nitriding and PA PVD-arc processes M. Betiuk, J. Michalski, K. Burdynski, P. Wach, A. Nakonieczny . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 State of the art deposition technologies and coatings for tool and die applications F. Papa, T. Krug, R. Tietema. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Development of optimal PVD nano-composite coatings for aluminium alloy die casting applications D. Ugues, E. Torres, M. Perucca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Enhancements of thermal fatigue resistance of hot working tooling: the role of materials, heat treatments and coatings M. Rosso . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Novelity in diffusion coating technology B. Matijevi}, M. Stupni{ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Thermal stability and age hardening of metastable hard coatings P. H. Mayrhofer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Hard coatings for dies and moulds J. Kiefer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Possibilities of strengthening of diffusion nitrided layers by shot-peening A. Nakonieczny, I. Pokorska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Nitrooxidation of tools manufactured from high-speed steel T. Babul, Z. Obuchowicz, W. Grzelecki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 MECHANICAL AND PHYSICAL PROPERTIES OF TOOL AND DIE MATERIALS Tools material behavior at elevated temperatures J. Brni}, M. ^anaðija, G. Turkalj, D. Lanc, T. Pepelnjak, B. Bari{i}, G. Vukeli}, M. Br~i} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Behaviour at elevated temperature of 55NiCrMoV7 tool steel M. G. De Flora, M. Pellizzari . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Modern pre-hardened tool steels in die-casting applications P. Hansson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Innovative testing method for the evaluation of thermal shock and mechanical wear S. Harksen, W. Bleck . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Heat-resistant castings in carburising furnace B. Piekarski . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Fractographic evaluation of gigacycle fatigue failure of a high Cr alloyed cold work tool steel C. R. Sohar, A. Betzwar-Kotas, C. Gierl, B. Weiss, H. Danninger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Anisotropy effects on gigacycle fatigue behavior of 12%chromium alloyed cold work tool steel C. R. Sohar, A. Betzwar-Kotas, C. Gierl, B. Weiss, H. Danninger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Load on punch during fineblanking inconel 718 D. ^esnik, M. Bizjak, J. Rozman. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 The effect of service conditions on crack initiation and propagation in welded joint of high-alloy steel X20 M. Burzi}, D. Ga~o, D. Burzi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 300 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 LETNO KAZALO – INDEX Mechanical properties of boronizing steels as repercussion of boron phases D. Krumes, I. Kladari}, I. Vitez. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Effect of heat treatment on the mechanical properties of tool steels R. Ebner, S. Marsoner, W. Ecker, M. Leindl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Determination of lower bound of fracture toughness of suspension spring material N. Gubeljak, J. Predan, B. Sen~i~, J. Vojvodi~ Tuma, M. Jenko. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 ADVANCED TOOL MATERIALS Fatigue resistant PM tool steels Z. Devrim Caliskanoglu, J. Perko, H. Lenger. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Development of a hybrid tool steel produced by spark plasma sintering M. Pellizzari, M. Zadra, A. Fedrizzi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Stress state of 12% Ni maraging steel after a modified procedure of precipitation hardening J. Grum, M. Zupan~i~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 TRIBOLOGY Wear behaviour of plasma – sprayed Al-12Si/SiC composite coatings under dry and water – lubricated sliding S. Akgün, S. ahin, F. Üstel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Analysis of abrasive wear resistance of the D2 tool steel in relation to heat treatment D. Gorscak, T. Filetin, K. Grilec, M. Godec, D. Kapudija . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Influence of deep-cryogenic treatment on tribological properties of P/M high-speed steel B. Podgornik, V. Leskov{ek, J. Vi`intin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Wear resistance of thin protective layers in abrasion conditions under high-pressures V. Maru{i}, G. Mari} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Evaluation methods and surface engineering techniques for improved galling properties of forming tools J. Vi`intin, B. Podgornik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 Influence from tool roughness on the risk of work material adhesion and transfer M. Hanson, S. Hogmark, S. Jacobson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 APPLICATIONS OF NANOTECHNOLOGY Characterization of multilayer PACVD coatings for hot-worked tool steels using electron spectroscopy techniques M. Jenko, V. Leskov{ek, J. T. Grant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Design of recycle process of color filter using arc-form tool P.S. Pa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 MATHEMATICAL MODELLING AND PROCESS SIMULATION Genetic programming and Jominy test modelling M. Kova~i~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Prediction of hardness distribution within axially symmetrical workpieces thereupon high pressure gas quenching B. Li{~i}, T. Filetin, T. Lübben, D. Landek, D. Lisjak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 Utilization of the Kuyucak method to simulate laboratory and commercial quenching processes G. E. Totten, G. Sánchez Sarmiento, R. M. Muñoz Riofano, L. C.F. Canale. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 Computer simulation of mechanical properties of steel dies B. Smoljan, S. Smokvina Hanza, D. Iljki}, G.E. Totten, I. Felde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 RAPID PROTOTYPING OF TOOLS AND DIES Potentials of lens technology I. Pal~i~, M. Bala`ic, M. Milfelner, B. Semoli~, B. Buchmeister . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 Microstructure and mechanical characteristics of DMLS TOOL-INSERTS B. [u{tar{i~, S. Dolin{ek, M. Godec, M. Jenko, V. Leskov{ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 APPLICATIONS AND MATERIAL SELECTION FOR TOOLS AND DIES Importance of selection of the tool steel grades and PVD coatings in cold work tools D. Gor{~ak, T. Filetin, D. Cackovic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Experimental analyses of the influence on corrosive environment in seawater of vlore bay to the centre on fatigue of steel A -3 V. Kasemi, A. Haxhiraj . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 301 LETNO KAZALO – INDEX Selection of tool materials for cold forming operations using a computerized decision support system I. Czinege, T. Réti, I. Felde . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 An analysis of relationships between behaviour and microstructure constitution of hot-work tool steel B. Smoljan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Heat treating of H13 dies according to the NADCA and GM powertrain specification T. Wingens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Bodycote, global leader in thermal processing T. Wingens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 LETNO KAZALO – INDEX 302 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 MATERIALI IN TEHNOLOGIJE / MATERIALS AND TECHNOLOGY AVTORSKO KAZALO / AUTHOR INDEX LETNIK / VOLUME 42, 2008, A–@ A Adamovi} @. 263 Adolf Z. 131 Ambro`i~ M. 221 Amin M. M. 273 Amin N. 273 An`el I. 65 Arzen{ek B. 93 Avsec M. 51 B Ba`an J. 33, 135, 175 Bacsó J. 121 Balcar M. 33, 135, 175, 183 Bari{i} N. 105 Berneti~ J. 291 Bernik S. 69 Bevilaqua T. 121 Bilu{i} A. 105 Blecic D. 45 Bon~ina T. 65, 111 Brada{kja B. 291 Bricelj E. 291 Burzi} M. 263 C Celin R. 93 Cvijovi} Z. 191 ^ ^ampelj S. 179 ] ]ur~ija D. 59 D Daneu N. 69 Dobrovská J. 175 Dolin{ek J. 105 Drev D. 277 Drmota A. 79 Drofenik M. 179, 285 E Esme U. 215 Eterovi~ D. 221 F Fedorchenko D. G. 237 Fila P. 33, 135, 175, 183 Firpo G. 143 Fujda, M. 121 G Getsov L. B. 3, 99, 237 Godec M. 251 Grant J. T. 251 Gyergyek S. 285 H Hen~-Bartoli} V. 111 Hudáková M. 197 Husar I. 131 Hussin K. 273 J Jakovljevi} S. 111 Jamaludin S. B. 273 Jamar J. 151 Jamar N. 151 Jenko M. 251 Jur~i P. 197 K Kahraman F. 215 Ke{ner D. 33, 135 Klob~ar D. 203, 211 Kmeti~ D. 93 Ko{ak A. 79 Kocisko R. 121 Kosec B. 291 Kosec G. 291 Kosec L. 203, 211 Kosma~ T. 221 Kraus L. 33 Krnel K. 221 Kroupa T. 125 Kulekci M. K. 215 Kuskulic, T. 121 Kvackaj, T. 121 L La{ V. 125, 171 Lamut M. 39 Leonti} B. 105 Leskov{ek V. 251 Lukatela J. 105 M Ma~ek J. 51 Makovec D. 179, 285 Mamuzi} I. 59 Mandrino D. 39, 251 Mandziej S. T. 13 Margolin B. Z. 237 Marin{ek M. 85 Martínek L. 33, 135, 175, 183 McGuiness P. 117 Molnarova M. 121 Muhi~ M. 203, 211 Muhi~ T. 211 N Nme~ek S. 135 Nikolic I. 45 Novosel B. 51 P Panjan J. 277 Pe~ek M. 277 Pedraza F. 157 Petkovi~ Habe N. 221 Pipi} D. 111 Pleterski M. 203, 211 Podlogar M. 69 Podmilj{ak B. 117 Poga~nik @. 27 Pokorny, I. 121 Pribo{i~ I. 221 R Radmilovic V. 45 Radovi} N. 191 Re{kovi} S. 257 Re~nik A. 69 Repetto L. 143 Rozman N. 65 S Sagbas A. 215 LETNO KAZALO – INDEX MATERIALI IN TEHNOLOGIJE 42 (2008) 6 303 Semenov A. 3 Sholklapper T. Z. 45 Smiljani} I. 105 Smontara A. 105 Sochor L. 33, 135, 183 Stani} D. 105 Staroselsky A. 3 Stránský K. 175 Suchánek P. 131 [ [krlep L. 179 [kulj I. 117 [olar M. 13 T Torkar M. 39 Tu{ek J. 203, 211 Turecký V. 135 V Valbusa U. 143 Vasi} V. 243 Veskovi~ Bukudur S. 221 Vodopivec F. 257 Vratnica M. 191 Výrostková A. 13 W Weiss M. 121 Z Zem~ík R. 171 Zupani~ F. 65, 111 @ @elezný R. 33, 183 @nidar{i~ A. 79 LETNO KAZALO – INDEX 304 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 A Abed M. 13 Akbulut H. 55, 56 Akgün S. 129 Alp A. 55, 56 Altuncu E. 49 An`el I. 26, 34 Aslan S. 55, 56 Ayday A. 51, 57 B Babul T. 89 Bala`ic M. 159 Bari{i} B. 93 Ben-Hamida J. 14 Betiuk M. 72, 74 Betzwar-Kotas A. 104, 106 Bizjak M. 108 Bleck W. 99 Br~i} M. 93 Brni} J. 93 Bruncko M. 26 Buchmeister B. 159 Burdyñski K. 72, 74 Burzi} D. 110 Burzi} M. 110 Býndal C. 60 C Cackovic D. 167 Cajner F. 41 Cajner H. 41 Canale L. C.F. 23, 153 Cassteletti L.C. 71 Chang S. S. 52 Chen C. 52 Collins T. 32 Czinege I. 171 ] ]urkovi} L. 65 ^ana|ija M. 93 ^ ^ekada M. 65, 69 ^esnik D. 108 D Danninger H. 104, 106 De Flora M. G. 95 Devrim Caliskanoglu Z. 119 Djilali A. 18 Dolin{ek S. 161 Durman M. 51 E Ebner R. 114 Ecker W. 114 Erdogan G. 54 F Fedrizzi A. 121 Felde I. 154,171 Filetin T. 131, 151, 167 G Ga~o D. 110 Gierl C. 104, 106 Godec M. 131, 161 Gogte C. L. 45 Gor{}ak \.65, 69, 131, 167 ]urkovi} L. 69 Grant J. T. 143 Grilec K. 131 Grum J. 123 Grzelecki W. 89 Gubeljak N. 116 Gül H. 55, 56 H Hanson M. 138 Hansson P. 97 Harksen S. 99 Haxhiraj A. 169 Hogmark S. 138 I Iljki} D. 154 Ivanov R. 16 Iyer K. M. 43, 45 J Jacobson S. 138 Jenko M. 116, 143, 161 K Kapudija D. 131 Kasemi V. 169 Kiefer J. 86 Kiliç F. 55, 56 Kladari} I. 21, 112 Kneissl A. C. 26 Kova~i~ M. 149 Kowalski S. 72 Kozak D. 21 Krug T. 76 Krumes D. 21, 112 Kumruolu L. C. 57 Kurnaz C. 51 L Lanc D. 93 Landek D. 41, 151 Leindl M. 114 Lenger H. 119 Leskov{ek V. 41, 133, 143, 161 Li{~i} B. 11, 151 Lingenhöle K. 39 Lisjak D. 151 Lombardi Neto A. 71 Lübben T. 151 M Maamar H. 18 Mari} G. 135 Marsoner S. 114 Maru{i} V. 135 Matijevi} B. 81 Mayrhofer P. H. 84 Michalski J. 72, 74 Milfelner M. 159 Mohamed K. 18 Molinari A. 36 Muñoz Riofano R. M. 71, 153 Mýmarolu A. 57 N Nabil D. 18 Nakonieczny A. 72, 74, 87 O Obuchowicz Z. 89 Otmani Rafik R. 18 Özel A. 51, 57 P Pa P.S. 58, 145 Pal~i~ I. 159 LETNO KAZALO – INDEX MATERIALI IN TEHNOLOGIJE 42 (2008) 6 305 Panjan P. 65, 69 Papa F. 76 Pellizzari M. 36, 95, 121 Pepelnjak T. 93 Perko J. 29, 119 Perucca M. 78 Piekarski B. 101 Podgornik B. 133, 136 Pokorska I. 87 Predan J. 116 Pu~ko B. 19 R Reithofer G. 29, 32 Réti T. 171 Rink M. 14 Rosso M. 80 Rozman J. 108 S Sacher G. 67 ahin S. 129 Sánchez Sarmiento G. 153 Schneider R. 29, 32 Semoli~ B. 159 Sen~i~ B. 116 Smokvina Hanza S. 154 Smoljan B. 154, 172 Sohar C. R. 104, 106 Spies H.-J. 67 Stein R. 24 Stratton P. 39 Stupni{ek M. 81 Surberg C. H. 39 [ [u{tar{i~ B. 161 T Tacikowski J. 72 Tietema R. 76 Torres E. 78 Totten G. E. 23, 71, 153, 154 Toufik F. 18 Turk A. 60 Turkalj G. 93 U Ugues D. 78 Üstel F. 49, 54, 61, 129 V Vatavuk J. 23 Vi`intin J. 133, 136 Vitez I. 112 Vojtìch D. 63 Vojvodi~ Tuma J. 116 Vukeli} G. 93 W Wach P. 72, 74 Weiss B. 104, 106 Wingens T. 174 Wu H. C. 52 Z Zabett A. 13 Zadra M. 121 Zenker R. 67 Zieger B. 24 Zupan~i~ M. 123 LETNO KAZALO – INDEX 306 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 MATERIALI IN TEHNOLOGIJE / MATERIALS AND TECHNOLOGY VSEBINSKO KAZALO / SUBJECT INDEX LETNIK / VOLUME 41, 2008 Kovinski materiali – Metallic materials A failure criterion for single-crystal superalloys during thermocyclic loading Merilo za prelom monokristala superzlitine pri termocikli~ni obremenitvi L. Getsov, A. Semenov, A. Staroselsky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Accelerated creep testing of new creep resisting weld metals Preizkusi pospe{enega lezenja zvarov novega jekla, odpornega proti lezenju S. T. Mandziej, A. Výrostková, M. [olar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Developing and testing a new type-8K mould for tool-steel ingot casting Razvoj in preizkus nove kokile vrste 8K za ulivanje ingotov iz orodnega jekla M. Balcar, L. Sochor, R. @elezný, P. Fila, L. Martínek, L. Kraus, D. Ke{ner, J. Ba`an . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 An AES investigation of brushed AISI 304 stainless steel after corrosion testing AES-preiskave krta~enega nerjavnega jekla AISI 304 po korozijskem preskusu M. Torkar, D. Mandrino, M. Lamut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Using a FIB to prepare Al(OH)3 samples for the TEM Uporaba FIB za pripravo vzorcev Al(OH)3 za TEM I. Nikolic, V. Radmilovic, T. Z. Sholklapper, D. Blecic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Lubrication flow during the rolling of seamless tubes Tok maziva pri valjanju brez{ivnih cevi D. ]ur~ija, I. Mamuzi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 The influence of cooling rate on the microstructure of an Al-Mn-Be alloy Vpliv ohlajevalne hitrosti na mikrostrukturo zlitine Al-Mn-Be N. Rozman, T. Bon~ina, I. An`el, F. Zupani~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 A metallographic examination of a fractured connecting rod Metalografska preiskava preloma ojnice R. Celin, B. Arzen{ek, D. Kmeti~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 On the different nature of time-dependent and time-independent irreversible deformation O razli~ni naravi ~asovno odvisne in ~asovno neodvisne ireverzibilne deformacije Getsov L.B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 The action of a laser on an aluminium target Obsevanje aluminijaste tar~e z laserjem V. Hen~-Bartoli}, T. Bon~ina, S. Jakovljevi}, D. Pipi}, F. Zupani~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Changes in the microstructure of Fe-doped Gd5Si2Ge2 Spremebe v mikrostrukturi zlitine Gd5Si2Ge2, dopirane z Fe I. [kulj, P. McGuiness, B. Podmilj{ak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Development of microstructure during the hot plastic deformation of high clean steels for power plants Razvoj mikrostrukture med vro~o plasti~no deformacijo visoko ~istega jekla za energetske naprave Kuskulic, T., Kvackaj, T., Fujda, M., Pokorny, I., Bacsó J., Molnarova, M., Kocisko, R., Weiss, M., Bevilaqua, T. . . . . . . . . . . . . . . . . 121 The influence of carbon content on the corrosion of MGO-C refractory material caused by acid and alkaline ladle slag Vpliv vsebnosti ogljika na korozijo ognjevzdr`nega materiala MGO-C v kisli in bazi~ni pe~ni `lindri Z. Adolf, P. Suchánek, I. Husar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 An evaluation of the properties of rotor forgings made from 26NiCrMoV115 steel Ocena lastnosti izkovkov za rotorje iz jekla 26NiCrMoV115 M. Balcar, V. Turecký, L. Sochor, P. Fila, L. Martínek, J. Ba`an, S. Nme~ek, D. Ke{ner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Applications of focused ion beam in material science Uporaba fokusiranega ionskega curka v znanosti materialov L. Repetto, G. Firpo, U. Valbusa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 LETNO KAZALO – INDEX MATERIALI IN TEHNOLOGIJE 42 (2008) 6 307 Low energy-high flux nitridation of metal alloys: mechanisms, microstructures and high temperature oxidation behaviour Nitriranje kovinskih zlitin s fluksom z majhno energijo in veliko gostoto: mehanizmi, mikrostrukture in visokotemperaturno oksidacijsko vedenje F. Pedraza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Application of the theory of physical similarity for the filtration of metallic melts Uporaba teorije fizikalne podobnosti za opis filtriranja kovinske taline K. Stránský, J. Ba`an, J. Dobrovská, M. Balcar, P. Fila, L. Martínek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 The development of a chill mould for tool steels using numerical modelling Razvoj kokile za orodna jekla z uporabo numeri~nega modeliranja M. Balcar, R. @elezný, L. Sochor, P. Fila, L. Martínek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 The effect of compositional variations on the fracture toughness of 7000 Al-alloys Vpliv sprememb v sestavi na `ilavost loma aluminijeve zlitine vrste 7000 M. Vratnica, Z. Cvijovi}, N. Radovi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Wear mechanism of duplex-coated P/M Vanadis 6 ledeburitic steel Mehanizem obrabe ledeburitnega jekla P/M Vanadis 6 z dupleksno prevleko P. Jur~i, M. Hudáková . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 Analiza toplotnih razpok na orodjih za tla~no litje aluminija Analysis of thermal cracks on die casting dies D. Klob~ar, J. Tu{ek, M. Pleterski, L. Kosec, M. Muhi~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Lasersko reparaturno varjenje termorazpok na orodjih za tla~no litje aluminija Laser repair welding of thermal cracks on aluminium die casting dies M. Pleterski, J. Tu{ek, L. Kosec, D. Klob~ar, M. Muhi~, T. Muhi~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Use of artificial neural networks in ball burnishing process for the prediction of surface roughness of AA 7075 aluminum alloy Uporaba umetnih nevronskih mre` za napoved hrapavosti povr{ine pri krogelnem glajenju aluminijeve zlitine AA 7075 U. Esme, A. Sagbas, F. Kahraman, M. K. Kulekci . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 On the determination of safety factors for machines using finite element computations O dolo~itvi faktorjev varnosti za naprave pri izra~unu z metodo kon~nih elementov L. B. Getsov, B. Z. Margolin, D. G. Fedorchenko . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Characterization of multilayer PACVD TiN/Ti(B-N)/TiB2 coatings for hot-worked tool steels using electron spectroscopy techniques Karakterizacija ve~plastne PACVD TiN/Ti(B-N)/TiB2 prevleke za orodna jekla za delo v vro~em s tehnikami elektronske spektroskopije M. Jenko, D. Mandrino, M. Godec, J. T. Grant, V. Leskov{ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 An investigation of the stretch reducing of welded tubes Raziskava iztezne redukcije varjenih cevi S. Re{kovi}, F. Vodopivec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Experimental analysis of crack initiation and growth in welded joint of steel for elevated temperature Eksperimentalna analiza nastanka in rasti razpoke v zvaru jekla za povi{ano temperaturo M. Burzi}, @. Adamovi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 The role of chloride salts on high temperature corrosion of 321 stainless steel Vloga kloridnih soli pri visokotemperaturni koroziji nerjavnega jekla 321 N. Amin, M. M. Amin, S. B. Jamaludin, K. Hussin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Centreline formation of the Nb(C,N) eutectic in 0.15 % C; 0.0071 % N; 0.022 % Nb; 0.033 % Al and 0.003 % S structural steel Sredinsko izcejanje in nastanek evtektika Nb(C,N) v konstrukcijskem jeklu z 0,15 % C; 0,0071 % N; 0,022 % Nb; 0,033 % Al in 0,003 % S J. Berneti~, B. Brada{kja, G. Kosec, B. Kosec, E. Bricelj . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Anorganski materiali – Inorganic materials The interaction of SOFC anode materials with carbon monoxide Reakcije med anodnimi materiali SOFC in ogljikovim monoksidom B. Novosel, M. Avsec, J. Ma~ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Tailoring the microstructure of ZnO-based ceramics Kontrola razvoja mikrostrukture v ZnO keramiki S. Bernik, M. Podlogar, N. Daneu, A. Re~nik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 A mechanism for the adsorption of carboxylic acids onto the surface of magnetic nanoparticles Mehanizem adsorbcije karboksilnih kislin na povr{ino magnetnih nanodelcev A. Drmota, A. Ko{ak, A. @nidar{i~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 LETNO KAZALO – INDEX 308 MATERIALI IN TEHNOLOGIJE 42 (2008) 6 Analysis of the temperature profiles during the combustion synthesis of doped lanthanum gallate Analiza temperaturnih profilov med zgorevalno sintezo dopiranega lantanovega galata M. Marin{ek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Low-temperature transport properties of the ε-phases Al-Pd-(Mn, Fe, Co, Rh) Nizkotemperaturne transportne lastnosti ε-FAZ Al-Pd-(Mn, Fe, Co, Rh, …) D. Stani}, I. Smiljani}, N. Bari{i}, J. Dolin{ek, A. Bilu{i}, J. Lukatela, B. Leonti}, A. Smontara . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Merjenje obrabne obstojnosti strukturne keramike Al2O3 Wear-resistance measurement of structural Al2O3 ceramics M. Ambro`i~, S. Veskovi~ Bukudur, T. Kosma~, K. Krnel, D. Eterovi~, N. Petkovi~ Habe, I. Pribo{i~ . . . . . . . . . . . . . . . . . . . . . . . . . 221 Polimeri v beli tehniki Polymer materials in white goods industry V. Vasi} . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Raztapljanje CO2 v embalirani vodi ali brezalkoholni pija~i in s tem povezane mo`ne po{kodbe Problems associated with the dissolution of CO2 in the case of bottled water and non-alcoholic beverages D. Drev, M. Pe~ek, J. Panjan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 Priprava Co-feritnih nanodelcev z ozko porazdelitvijo velikosti z metodo termi~nega razpada oleatov Preparation of Co-ferrite nanoparticles with a narrow size distribution by the thermal decomposition of oleates S. Gyergyek, D. Makovec, M. Drofenik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Polimeri – Polymers The off-axis behavior of a unidirectional fiber-reinforced plastic composite Zunajosno obna{anje enosmernih z vlakni oja~enih plasti~nih kompozitov T. Kroupa, V. La{ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Numerical and experimental analyses of the delamination of cross-ply laminates Numeri~na in eksperimentalna analiza delaminacije v kri`nih plo{~atih laminatih R. Zem~ík, V. La{ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Priprava nanokompozita za biomedicinske aplikacije Preparation of nano-composites for biomedical applications S. ^ampelj, D. Makovec, L. [krlep, M. Drofenik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Gradbeni materiali – Materials in civil engineering Zveza med analiznimi rezultati – karbonatna bomba in termi~na analiza Connection between analysis results – carbonate bomb and thermal analysis @. Poga~nik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Informatika – Informatics Materials and Technology: historical overview Materiali in tehnologije: zgodovinski pregled N. Jamar, J. Jamar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 LETNO KAZALO – INDEX MATERIALI IN TEHNOLOGIJE 42 (2008) 6 309