Surface and Grain Boundary Segregation of Antimony and Tin - Effects on Steel Properties Segregacija antimona in kositra na površini in po mejah zrn - vpliv na lastnosti jekel H. J. Grabke1, Max-Planck-lnstitut, Dusseldorf, Germany Dedicated to Prof. Dr. F. Vodopivec on the occasion of his 65th birthday. Prof. dr. Francu Vodopivcu za njegov 65. rojstni dan. Prejem rokopisa - received: 1996-10-01; sprejem za objavo - accepted for publication: 1996-11-04 The tramp elements Sb and Sn have a strong tendency to surface segregation on iron. By LEED and AES surface structures and concentrations of Sb and Sn segregated on single crystal vvere determined. The surface segregation is strongly dependent on orientation. therefore recrystallization of steel sheet is affected since the surface energies of different grains are reduced to different extent - this effect may be used to obtain advantageous textures of electrical steel sheet and deep dravving steels. Surface segregation of Sb and Sn retards surface reaction kinetics as vvas shovvn for the gas carburization of čase hardening steels. Surface segregation of tin in creep cavities of turbine steels vvas shovvn to accelerate the creep fracture. The grain boundary segregation of both elements in iron is minor, and furthermore Sb and Sn are displaced from grain boundaries by carbon so that most steels are not endangered by grain boundary embrittlement due to Sb and Sn, but some low alloy turbine steels are susceptibie to temper and long-term embrittlement. Key vvords: surface and grain boundary segregation Fe-Sb alloys, Fe-Sn alloys, Fe-Sb-C alloys, Fe-Sn-C alloys, intergranular fracture embrittlement Elementa v sledeh Sb in Sn močno segregirata na površini železa. Površinska struktura in koncentracija Sb in Sn v segregirani plasti sta bili določeni z metodami LEED in AES. Površinska segregacija je odvisna od kristalografske orientacije, rekristalizacija jeklenih pločevin je aktivirana, ker imajo posamezna kristalna zrna različno znižano površinsko energijo - pojav se lahko uporabi za pridobivanje prednostnih tekstur elektro pločevin in pločevin za globoki vlek. Površinska segregacija Sb in Sn zavira kinetiko površinske reakcije kar je prikazano pri procesu naogljičevanja jekel. Površinska segregacija kositra v vdolbinah pri lezenju jekel za turbine povzroča pospešenje lezenja do preloma. Segregacija obeh elementov po mejah kristalnih zrn je v železu minimalna zato ker Sb in Sn na mejah zrn izpodrine ogljik. Tako večina jekel ni ogroženih zaradi krhkosti kristalnih mej. ki bi jih povzročala Sb in Sn, le nekatera nizka ogljična turbinska jekla so občutljiva na popuščno krhkost. Ključne besede: površinska segregacija, segregacija po mejah zrn, Fe-Sb zlitine. Fe-Sn zlitine, Fe-Sb-C, Fe-Sn-C, interkristalna krhkost 1 Introduction 1.1 The role of tramp elements in steels The effects of the so-called tramp elements in steels, Ni, Cu, P, S, Pb, As, Sb, Sn etc. are generally deleterious, the greatest problems they cause are 'hot shortness' and 'temper embrittlement' of steels. The hot shortness, a lack of hot workability can have different reasons, one possible reason is the copper enrichment due to surface scaling1-2. Beneath the scale the more noble elements Cu, As, Sb, Sn are enriched and form a liquid phase which causes surface cracking by grain boundary penetration. Sb and Sn greatly reduce the solubility of Cu in austenite and hence lead to precipitation of a molten phase and its grain boundary penetration, under conditions of much less enrichment and down to lower temperatures. The enrichment of tramp elements below the oxide scale upon reheating or hot rolling of steels and could be detected by electron microprobe (EPMA). This enrichment also can have strong effects on the scale adherence and mor- 1 Prof.Dr.Sc. Hans Jiirgcn GRABKE Max-Planck.Inslilut fiir Eisenfurschung GmhH Postfach 140444. 40074 Dussedldorf. Germany phology as has been studied extensively by F. Vodopivec et al. in vvork started at the IRSID311: by the presence of the more noble elements Cu, Ni, Sb, Ag, S the scale adherence is enhanced vvhereas the elements Si, Al, P and B which are oxidized and form silicate, aluminate, phos-phate or borate layers cause formation of voids and cavities at the scale/metal interface. The other way of enrichment which leads to deleterious effects of tramp elements is equilibrium segregation, so the 'temper embrittlement' is caused by segregation of P, Sn or Sb in the temperature range 400 - 700°C to the steel grain boundaries, e.g. during sIow cooling after tempering, but also during application of steels in this temperature range. It vvas suspected since long that temper embrittlement is caused by grain boundary segregation, but this suspect could be confirmed only after the arrival and spreading of interfacial analysis by Auger-electron spectroscopy (AES) in the eighties. But the tramp elements do not have only deleterious effects, e.g. it is knovvn that Cu can enhance the resistance against atmospheric corrosion. Even positive effects of Sb and Sn vvere detected and studied at the IMT Ljubljana and the MPI fiir Eisenforschung Diisseldorf12"20, these tramp elements can improve the texture and magnetic proper- ties of nonoriented silicon steel sheets, caused by surface segregation and its effect on surface energies as dis-cussed in the following chapter. 1.2 Fundamentals of surface and grain boundary segregation In this revievv the equilibrium segregation of Sb and Sn will be described and only the effects will be dis-cussed which are caused by equilibrium surface and grain boundary segregation. Most elements which are dissolved in iron tend to enrich at elevated temperatures at surfaces, grain boundaries and interfaces21"25, and dis- tribution equilibria are established at sufficiently high temperature. A (dissolved) A (segregated) (D There are different driving forces for such equilib-rium segregation: 1. free bonds at the surface or interface can be satu-rated by interaction vvith the atoms A 2. the iron surface may be covered with a layer of atoms A which has a lower surface energy than the initial iron surface 3. the release of atoms A from the bulk solution leads to release of elastic energy, especially in the čase of in- Gibbs In a Langmuir - Mc lean fl - 1*Ka O. 4_L RT* R ln a [n a Figure 1: Schematic diagrams on the Gibbs isotherm (a-c) and the Langmuir-McLean isotherm (d-f) a) surface energy y vs activity a of the adsorbed or segregated element A, b) y vs In a and c) the latter plot for two orientations vvith different surface energies and different adsorption or segregation behaviour - upon increasing activity a and coverage 0 the surface firstly instable becomes stable, a reason for tertiary recrystallization or facetting, d) degree of coverage 0 vs activity of the absorbed or segregated element A, e) plot for the evaluation of studies at constant activity or concentration of the element A, f) isosteres for determination of the thermodynamic data at constant coverage Slika 1: Shematski diagrami Gibbsove izoterme (a-c) in Langmuir-McLeanove izoterme (d-f) a) površinska energija y v odvisnosti od aktivnosti a adsorbiranega ali segregiranega elementa A, b) y v odvisnosti od ln a in c) zadnji grafikon za dve orientaciji z različnimi površinskimi energijami in različno adsorbcijo oziroma segregacijo - po zvišanju aktivnosti a in pokritja ©je sprva nestabilna površina postala stabilna, vzrok za terciarno rekristalizacijo ali facetiranje, d) stopnja pokritja 0 v odvisnosti od adsorbiranega ali segregiranega elementa A, e) grafični prikaz za ovrednotenje študij pri konstantni aktivnosti ali koncentraciji elementa A, f) izostere za določitev termodinamičnih podatkov pri konstantnem številu atomov A terstitial atoms or substitutional atoms larger than the iron atoms. The latter effect is certainly true for Sb and Sn since both elements have large atoms, causing a strain in the iron lattice and increase of lattice parameter26. In fact, ali equilibrium segregation processes should lead to a de-crease in surface energy (or interfacial energy) according to Gibbs' law dy d ln a, = -RTT4 (2) where y is the surface energy, aA the thermodynamic ac-tivity of the segregating species A and Ta the surface concentration (mol/cm2), R gas constant, T temperature (K) (Figure la-c). The effect of adsorption or segregation on surface energy can be measured by the so-called zero creep method27 but onIy at very high temperatures. One example of the result for a measurement on Fe-Sn foils vvith different Sn concentrations at 1420°C28'29 is given in Figure 2a. Combining such a study with measuring 'grain boundary grooving', i.e. the dihedral angle of the thermally etched grain boundary grooves at the surface gave the ratio of the grain boundary to surface energies and thus the dependence of grain boundary en-ergy was derived as a function of the bulk tin content (Figure 2b). From these 'Gibbs isotherms' also the iso-therms for surface resp. grain boundary segregation could be derived27"29. Hovvever, these techniques were time consuming, difficult and tedious and since the arri-val and spreading of AES they are no more used. In manv cases, segregation can be described by a simple equation, the Langmuir-McLean isotherm (Figure ld-f), describing segregation to a limited number of sites which leads to a maximum coverage TAsat when ali sites are occupied, and with a free energy AGa which is independent of coverage. Then the degree of coverage o = FA/rA is given by 0A/(1-0A) = Xa exp (-AGa/RT) (3) (4) Since AGa = AHa - T ASa (5) this leads to the form of the Langmuir-McLean equation ln- ©A 1-0. AHa AS, RT R • + ln x. (6) vvhich is used to derive the enthalpy and entropy of segregation from measurements of ©a at a constant bulk concentration xa of the segregating species in dependence on temperature. Such measurements have been conducted, e.g. for the surface segregation of C, Si, N, P and S on iron and also for the grain boundary segregation of P, Sb and Sn (see Figure 6 and 8). The surface analyses were conducted by AES, observing the concentrations in situ on single or polycrystalline surfaces in dependence on temperature30"40. E.D. Hondros, M.P Seah 1970 0.2 04 tin (wt.%) mt1 icrl tin (wt.%) Figure 2: a) Surface energy and b) grain boundary energy of iron-tin alloys at 1420°C plotted as a function of the bulk tin content28,29 in (b) also the grain boundary segregation isotherm is given, which can be derived from the measurements Slika 2: a) površinska energija in b) energija kristalnih mej zlitine železo-kositer pri 1420°C kot funkcija vsebnosti kositra v osnovni zlitini28'29 (b) podana je tudi izoterma segregacije po mejah zrn, ki jo lahko izračunamo iz meritev The grain boundary analyses are also performed by AES, but after annealing the specimens for sufficient time at elevated temperature, then introducing them into the UHV system and fracturing in-situ by impact or ten-sile test35,36,39'40. The analysis of intergranular fracture facets yields the grain boundary concentration, assuming that the content of impurity A has been distributed equally to both sides upon fracture. The sites and structures attained in surface segregation can be elucidated using LEED (=low energy elec-tron diffraction). In most cases the elements A are en-riched on Fe(100) up to half a monolayer, corresponding to a c(2x2) structure, only for oxygen a complete mono-layer and p(lxl) structure is attained. At grain bounda-ries rather high coverages are possible, for P in ferrite coverages nearly up to one monolayer have been observed. The observation of the LEED structures on single crystal surfaces gives a good possibility for calibrating the AES measurements, also at grain boundaries, since the coverage for the saturated LEED structures in known. Further information on segregated species can be ob-tained using photoelectron spectroscopy (XPS), the pho-tolines obtained can indicate the ionization state of ions and the charge transfer between substrate and segregated atom4'"45. Generally, there is a transfer of negative charge (electrons) to the segregated atoms, which means that these (C, N, S, O, P etc.) are present as negatively charged atoms (anions) on the metal surface. This most probab!y is also the čase in the grain boundary segregation, and it is supposed that such charge transfer vveakens the cohesion of grain boundaries46'47 - leading to temper embrittlement of steels. In the čase that two elements are segregating simulta-neously to a surface or a grain boundary, there is gener-ally a competition for the sites available and the relative amount of both species in the surface depends on their free energy of segregation and concentrations in the bulk. Cases of competitive segregation have been studied on the iron surface for carbon and silicon38, and at grain boundaries: carbon-phosphorus36, carbon-sulfur48, nitro-gen-phosphorus37... The simple formalism for competitive segregation without further energetic interaction of the segregating species is given by 0A / (1 -0A-0B) = xA ■ exp (-AGa / RT) (7) 0B / (1-©A-0B) = xB ■ exp (-AGB / RT) (8) which could be applied in the cases mentioned above. In the literature on temper embrittlement there is a lot of fuss about 'cosegregation', the mutually enhanced segregation of two species where attractive energetic interaction is to be assumed. In some cases the enhanced segregation can be explained in a different way - in other cases which are important here (Ni-Sn, Ni-Sb) formation of two- or three-dimensional phases at the grain boundaries may be suspected (see below, chapters 2.2 and 3.4). 1.3 Systems Fe-Sn and Fe-Sb The solid solutions of Sn in a-Fe vvere determined by lattice parameter measurements49,50. Accordingly, the solubility ranges from a maximum at 9,2 at% (17,7 wt%) at 900°C to 3,2 at% (6,56 wt%) at 600°C. The solubility limit in y-Fe has been determined24'26 the y-loop extends to 0,92 at% (1,93 wt%). Own investigations on Fe-0,054 wt% Sn and Fe-0,080 wt% Sn [unpublished], however, showed precipitation of Sn-rich particles on the grain boundaries after long-term annealing at 550°C; accord-ingly, there are uncertainties on the solubility at temperatures <600°C. The solubility of Sb in a-Fe has been determined by several authors, the results are in substantial agree-ment49. The solubility at 900°C is 4,19 at% Sb (8,71 wt%) decreasing at 600°C to 2,58 at% (5,46 wt%). The Y-loop extends till 1,1 at% Sb (2,36 wt%). Experimental and theoretical studies have been con-ducted on the effects of other alloying elements on the antimony solubility, they vvere found to be the largest for M = Ti, Mn and Ni and small for M = Cr, Co. The pres-ence of Ni e.g. reduces the solubility strongly, the phase precipitating is a hexagonal NiAs type: Fe96Sb2Ni2. A cubic CaFei type Fe97Sb2Ti is formed with Ti, which reduces the solubility at 900°C to 1,91 at%52. Strong interaction of Ni and Sb is also observed in surface segregation53. 2 Interfacial segregation of Sn and Sb on and in iron and steels 2.1 Surface segregation of Sn and Sb on iron The surface segregation of tin on Fe-Sn single crys-tals has been studied in the temperature range 450°C to 650°C, mainly on crystals vvith relatively high Sn concentrations so that always saturation coverages vvere observed, no dependence of coverage on temperature, so that the segregation enthalpy was not obtained54 55. Each of the low index orientations exhibits a characteristic behaviour of the segregating Sn, the coverages attained are governed by segregation kinetics (Figure 3a). After heating the specimen for a short time a c(2x2) structure is observed, corresponding to half a monolayer coverage. But the segregation continues which leads to an order-disorder transition and a coverage somevvhat higher than a monolayer (corresponding to 1,4 • 1015 atoms Sn/cm2). The transition is accompanied by a shift of the photoli-nes observed by XPS to values closely corresponding to the values characteristic for pure elemental tin, Figure 3b. Most probably the transition can be explained by formation of a two-dimensional nearly close packed layer of tin on Fe(100) vvith a high surface mobility. This segregation behaviour is different from the segregation in most systems Fe-A (A = C, N, S, P, Sb...) vvhich always leads to a saturation at a surface coverage of 0,5. The driving force for the segregation of tin to higher coverages is probably the strong decrease of surface energy by the presence of a layer of tin. This layer at high coverage has properties similar to a layer of pure molten tin on iron, as indicated by the results of the XPS measurements. The segregation behaviour on Fe-Sn(lll) is similar, there is an inflection point in the kinetics vvhen the p(lxl) structure vvith one monolayer coverage is reached, vvhich corresponds to 7 ■ 1014 atoms Sn/cm2 on Fe(lll). After this surface structure is reached, further Sn segregation occurs, an order-disorder transition is observed and a Sn monolayer is attained. The segregation behaviour is different on Fe-Sn(llO), here no intermedi-ate adsorption structures vvere observed, but only structures vvith high Sn content, firstly a hexagonal structure corresponding to one monolayer of grey tin. Upon fur- Figure 3: a) Kinetics of the tin surface segregation on Fe-4 wt% Sn(100) during heating to 650°C54, at the inflection point indicated the structuraJ phase transition from the ordered monolayer c(2x2)Sn to the disordered multilayer occurs; b) photolines observed during increasing surface concentration demonstrating the shift caused by the transition Slika 3: a) Kinetika površinske segregacije na zlitini Fe-4 ut.% Sn(100) med žarjenjem do 650°C54, prevoj označuje strukturni fazni prehod iz urejene monoplasti c(2x2)Sn v neurejeno večplastnost; b) fotolinije med naraščanjem površinske koncentracije prikazujejo kemijski premik, nastal zaradi prehoda ther segregation a structure is formed which corresponds to a layer of the intermetallic compound FeSn of one unit celi thickness, Figure 4a. The segregation behaviour of Sb on Fe-4 wt% Sb55'56 is similar on the orientations (100) and (111) to the behaviour of tin, on both orientations on ordered adsorp-tion structure is formed, c(2x2) on (100), see Figure 5, and p(lxl) on (111) but upon continued segregation no elevated Sb surface concentration were observed, in con-trast to Sn. On Fe(l 10) the presence of Sb caused facet-ing, the LEED patterns indicated formation of (111) and (111) planeš, Figure 4b. Accordingly, the segregation enthalpy of Sb to Fe(lll) must be very exothermic (negative), due to a strong decrease of the surface energy of Fe(l 11) which compensates the increase of total surface area by the faceting. Figure 4: Phenomena on the Fe(l 10) face caused by segregation of Sn or Sb; a) Supposed structure of the surface compound 'FeSn' formed by epitaxial stabilization on Fe-Sn(l 10) as the final saturation structure ; b) Faceting on Fe-Sb(llO) under formation of (111) faces due to Sb segregation56 Slika 4: Pojav na Fe(110) ploskvi, ki gaje povzročila segregacija Sn ali Sb; a) predpostavljena struktura zlitine na površini 'FeSn', ki je nastala z epitaksialno stabilizacijo na Fe-Sn(llO) kot končna nasičena struktura54; b) facetiranje na površini monokristala Fe-Sb(llO), zaradi segregacije Sb se tvorita (111) in (111) ploskvi Three possibilities are demonstrated in the systems Fe-Sn and Fe-Sb for the behaviour upon segregation, (i) formation of adsorption structures such as c(2x2) or p(lxl), (ii) formation of surface phases such as two-di-mensional grey tin and two-dimensional FeSn, or (iii) formation of facets to attain surface energies. 2.2 Grain boundary segregation of Sn and Sb A fundamental study on grain boundary segregation in Fe-Sn alloys has been conducted after annealing in the temperature range 500-750°C for up to 5000 h39. The results of the grain boundary analyses show a wide scatter Ek (eVl Figure 5: Surface segregation of Sb on Fe-Sb (100)56; a) Auger spectrum after segregation at 640°C, corresponding to surface segregation; b) model for the Fe-Sb (100) c(2x2) structure derived from LEED study of the saturated surface Slika S: Površinska segregacija Sb na monokristalu Fe-Sb orientacije (100)56; a) AES spekter posnet po segregaciji Sb pri 640°C; b) model za Fe-Sb (100) c(2x2) strukturo dobljen z metodo LEED na nasičeni površini (Figure 6a) which may be caused by the strong dependence of tin segregation on grain boundary orientation. Ali data have been obtained for Sn concentrations within the a-solid solution range, no precipitates of intermetal-lic compounds should have formed. The tin concentrations are always below a monolayer, in contrast to the surface segregation behaviour. In spite of the large scat-ter the data were evaluated according to the Langmuir-McLean equation (Figure 6b), yielding the values for segregation enthalpy and entropy AH = -22,5 kJ/mol AS = 26 J/mol K for 550°C results in good agreement with previous results of E. D. Hondros and M. P. Seah28 29. The enthalpy value is relatively low (P: AH = -34,3 kJ/mol35'36), this indicates the rather low tendency for grain boundary segregation of Sn! Furthermore, due to o 0.20 %Sn ^ 0.08 '/.Sn o 0.054% Sn x 0022% Sn •----■--- 0-g- * . i temperature (°CI Figure 6: a) Grain boundary concentrations of tin in Fe-Sn alloys after annealing at elevated temperatures, measured by AES on intergranular fracture faces39; b) evaluation of the measurements in (a) applying the Langmuir-McLean equation (6) Slika 6: a) Koncentracija kositra v segregirani plasti na mejah zrn po žarjenju pri povišanih temperaturah, merjeno z metodo AES na interkristalnih prelomnih ploskvah39; b) ovrednotenje meritev (a) z uporabo Langmuir-McLean enačbe (6) its low segregation enthalpy Sn is kept from the grain boundaries effectively by the presence of carbon such as in plain carbon steels (Figure 7). As described in the in-troduction, an equilibrium of site competition between Sn and C occurs according to C(dissolved) + Sn(segregated) = C(segregated) + Sn(dissolved) In the presence of some ppm dissolved carbon, the tin is effectively removed from the grain boundaries. Hovvever, in low alloy steels the concentration of dissolved C is reduced due to the formation of less soluble carbides vvith Cr and Mn. Tin segregation is possible if not Sn is displaced from the grain boundaries by segre-gated phosphorus. For rotor steels, CrMoV steels it is even dangerous to have too low phosphorus contents, since in application at high temperature if Sn segregation prevails, this easily leads to formation of creep cavities, due to the strong tendency for surface segregation of tin. The surface segregation of Sn decreases the surface en-ergy of pores and cavities, stabilizes such defects and ac-celerates their growth (see chapter 3.2). In must be kept in mind that the data given above are average values and have an integral character, since the grain boundary segregation of tin in iron is strongly de- 0.10 0.05 cn - Q \ \ \ \ \ A ir-\ * / v / / -— C S / / __ \ ' ' / \ 1 > \ ' ! \// / /'V / !// o / 550 °C o * -0 Sn Fe-Fe- 0.08 V.Sn 0.054 V.Sn / Fe- 0.022 V.Sn ' » • 20 40 60 [Cl (ppm) 550 600 650 700 750 annealing temperature (°C) Figure 7: Grain boundary segregation of Sn and C in Fe-Sn-C alloys in dependence on the bulk carbon concentration after equilibration at 550°C, demonstrating the displacing effect of carbon on segregated Sn39 Slika 7: Segregacija Sn in C po mejah zrn v Fe-Sn-C zlitinah v odvisnossti od koncentracije ogljika v osnovnem materialu pri ravnotežju pri 550°C pendent on the misorientation, increasing with the tilt angle of misorientation betvveen the grains57. Sn causes grain boundary hardening, excess hardness extending to many mtcrons on either side of the grain boundary, also increasing with the misorientation. This is a well docu-mented effect but not well understood57-58. For temper embrittled Ni-Cr steels there are strong indications that Sn is present at the grain boundaries cou-pled with Ni in an bidimensional phase corresponding to an intermetallic compound such as Ni3Sm, this has been concluded from Mossbauer spectroscopy and TEM work59"61. The grain boundary segregation of Sb was investi-gated for Fe-Sb and Fe-Sb-C alloys after equilibration at temperatures betvveen 550°C for sufficient time62,63. The analysis of intergranular fracture faces by AES calibrated on the base of the surface segregation studies shovvs relativen low interfacial concentrations, see Figure 8, and a wide scatter of results. The plot of the data according to the Langmuir-McLean equation leads to the values for segregation enthalpy and entropy: AH = -19 kJ/mol AS = 28 J/mol K Thus, the segregation enthalpy is even lower than for Sn, which emphasizes the Iow tendency for grain bound-ary segregation of Sb. However, even small grain bound-ary concentrations of Sb cause marked grain boundary embrittlement and prevailing intergranular fracture. Also the segregant Sb is effectively displaced from grain .o LO X XI LO CD X) LO CD b) ■ 0.012% Sb • 0.049% Sb * 0.094% Sb 0,95 1,00 1,05 1,10 103 / T I K 1,15 -11 1,20 1,25 Figure 8: a) Grain boundary concentrations of Sb in Fe-Sb alloys, plotted vs equiIibration temperature62; b) plot of the data in (a) according to the Langmuir-McLean aquation (6) Slika 8: a) Segregacija Sb po mejah zrn v Fe-Sb zlitini v odvisnosti od ravnotežne temperature62; b) prikaz podatkov v (a), ki ustrezajo Langmuir-McLean enačbi (6) boundaries by carbon, small concentrations of dissolved carbon < 60 wtppm can shift the displacement equilib-rium to low Sb segregation and also lead to a marked reduction of intergranular fracture, see Figure 964. Carbon not only removes Sb from the grain boundaries, but also enhances the grain boundary cohesion and enforces transgranular fracture. The effect of carbon also was demonstrated by notch-impact tests on Fe-Sb-C alloys, see Figure 10. As in the čase of Sn, for unalloyed carbon steels the danger of embrittlement by Sb is minor, there will be always enough dissolved and segregated carbon to avoid Sb grain boundary segregation. Only for alloyed steels, in which the carbon is tied up by carbide forming elements, Cr, Mn, etc., embrittlement is possible during heat treatment or use of steels in an elevated temperature range. Several authors have claimed an effect of nickel, en-hancing the grain boundary segregation of Sb, however, this effect could not be reproduced in recent studies on ai k_ 3 u O 10 carbon 20 30 concentration 40 50 ! wt.ppm 80 rs 70 F o 60 —i 50 40 m aj 30 C a> 20 i . o n 10 LL E 0 Fe - 0.05 % 6 ppm C -100 -50 50 100 temperature (°C) Figure 10: Results of notch impact tests on an Fe-Sb al!oy with different carbon concentrations . The ductile-brittle transition temperature is shifted to lower temperatures by carbon, due to the removal of Sb from the grain boundaries and increase of grain boundary cohesion by segregated carbon Slika 10: Rezultati udarnih preizkusov na zlitini Fe-Sb z različnimi vsebnostmi ogljika6-1. Temperatura prehoda duktilno-krhko je premaknjena k nižjim temperaturam zaradi ogljika, le-ta izpodrine Sb z mej zrn in poviša kohezijo a a. 3 C a L. cn k_ a« C Figure 9: a) Grain boundary segregation of Sb and C in Fe-Sb-C alloys after equilibration at different temperatures, plotted in dependence on the bulk concentration, demonstrating the displacing effect of carbon on segregated Sb62,63; b) intergranular part of fracture in dependence on bulk carbon concentration Slika 9: a) Ravnotežna koncentracija Sb v segregirani plasti na mejah zrn v zlitini Fe-Sb-C pri različnih temperaturah, prikazana v odvisnosti od koncentracije C v osnovnem materialu, prikazuje pojav ko ogljik izrine Sb v segregirani plasti62,63; b) interkristalna ploskev preloma v odvisnosti od koncentracije ogljika v osnovnem materialu Fe-Ni-Sb alloys64. In earlier studies65-66 of Fe-Sb and Fe-Ni-Sb alloys at 560°C an increase of Sb segregation was observed with hte Ni-content and Ni also segregates to the grain boundaries, its segregation being only slightly affected by the presence of Sb. For low alloy Ni-Cr steels the authors65'66 conclude that the Sb-segregation is a complex function of the total alloy composition. When Mn is present in these steels it causes precipitation of an antimonide and greatly reduces Sb-segregation. A de-tailed investigation of a 3,5 Ni-ICr-steel after embrittle-ment at 480°C demonstrates a dependence on the microstructure67. Intergranular embrittlement in a quenched and tempered martensitic microstructure was associated with the segregation of phosphorus, which is possible since the carbon activity is reduced by precipitation of chromium rich carbides at the grain boundaries. the embrittlement in the bainitic microstructure was associated with the segregation of antimony, since the carbon activ-ity is relatively high due to the formation of cementite type carbides. Prolonged embrittlement of the bainite produced a low energy fracture. Increased nickel and an-timony concentrations at the grain boundaries vvere associated with the formation of a fine grain boundary pre-cipitate. The increased carbon activity continued to prevent appreciable P segregation but could not inhibit the 'cosegregation' of Ni and Sb67. 2.3 Segregation of Sb and Sn at internat interfaces Sb can be trapped by TiC precipitates in Fe. A dense dispersion of TiC, produced by ion implantation and annealing at 600-700°C, ties up Sb effectively68. Continued annealing leads to slow release of Sb into the matrix in a diffusion and trapping process. The Sb is present at the interface TiC/ferrite, and not in the TiC, the binding en-thalpy is -35,6 kJ/mol68"70. This interfacial segregation may provide a means for keeping Sb from grain boundaries in ferritic steels to suppress embrittlement. Similar trapping has been observed at TaC and Cu precipitates in Fe at 600°C71. Trapping or segregation of Sn at MnS particles has been observed in Fe-3% Si doped vvith tin. The Sn was clearly enriched compared to the grain boundaries, this segregation retards the growth rate of the MnS particles so that in Sn doped alloy they are much smaller than in Sn-free Fe-3% Si72. The size of the precipitates affects the primary and secondary recrystallization, thus influ-encing the magnetic properties of Si steels, see chapter 3.1. In the eutectoid transformation of austenite to čast iron, minor additions of Sb (0,08 wt%) or Sn (0,12 wt%) 0 10 20 30 40 50 60 carbon concentration (wt.ppm) » 700°C .--•-- 650°C —600°C were found to inhibit the y —> a + graphite and the Fe.iC —> a + graphite reaction paths, but did not significantly affect the metastable y —> a + FejC reaction73. Scanning Auger microprobe analysis indicated that Sn and Sb ad-sorb at the graphite/metal interface. The segregated layer acts as a barrier for the access of carbon to the graphite nodules. With the graphite disabled as a sink for carbon, the metal transforms as a nongraphite steel. 3 Effects of interfacial segregation of Sn and Sb on steel properties 3.1 Effects of surface segregation on the texture of electrical sheet A (100) [001] texture of Fe-Si can be achieved vvith the aid of adsorption or segregation of different species: O, S, Sb, Sn etc. The (100) [001] texture cannot compete lossvvise vvith the (110) [001] texture if unidirectional magnetization is important. In applications vvhere the magnetization must occur in ali directions in the plane of the sheet such as in motors or generators the (100) [001] texture is favourable since the plane of the sheet does not contain the hard (111) direction of magnetization, but even in transformers lovver losses can be obtained by using some (100) [001] texture. After the primary recrystal-lization, the grovvth of grains is governed by the surface energy, preferential grovvth of grains vvith a low surface energy occurs in the secondary recrystallization. In ab-sence of oxygen or other adsorbing or segregating species yno is the lovvest surface energy and (110) [001] grains grovv. When sufficient oxygen or sulfur is present yno and (100) grains become stable in the surface74"76, see also Figure 1. Presence of oxygen and sulfur is not vvell possible in the production process of non-oriented electrical sheet. The annealing for secondary recrystallization is done in dry hydrogen at about 900°C. Presence of sulfur vvould cause precipitation of MnS particles in the steels vvhich may hinder the reorientation of the magnetic domains. Thus, other elements such as Sn and Sb vvere success-fully used as alloying additions to improve the texture and magnetic properties of non-oriented steel sheet77-78. The alloying additions may not be too high to obtain the vvanted (100) [001] texture, for too high activities and surface coverages the surface energies of nearly ali ori-entations are decreased so strongly that no preferential grovvth of (100) is attained. Sb has proved to have an-other advantageous effect, it suppresses widely the inter-nal oxidation of the alloying elements Si, Al and Mn vvhich is possible during the decarburization treatment and causes increasing permeability deterioration vvith in-creasing subscale depth79. Also in the production of high induction and high permeability grain oriented Fe-Si, the presence of Sb and Sn can have positive effects, yielding a more precise (110) [001] secondary recrystallization texture than in conventional Fe-Si. In earlier vvork it vvas assumed that Sb and Sn are effective on the primary re-crystalIization, retarding primary grain grovvth in coop-eration vvith BN and S, less S being necessary than vvith-out Sb and Sn. But in recent studies it vvas found that grain boundary segregation of Sb and Sn is negligibly lovv in the silicon steel sheet after the usual thermal treatment. Obviously, the effect of Sb and Sn is caused by the surface segregation during recrystallization annealing. The surface segregation decreases the surface energy of grains vvith (100) orientation in the plane of the steel sheet and the grains vvith lovv surface energy grovv on ac-count of grains vvith other space orientation in the sheet plane. The role of the surface segregation has been con-firmed by extended studies on silicon steel doped vvith Sb and Sn12"20. Only a controlled surface segregation promotes the vvanted selective grain grovvth. For too high Sb and Sn concentrations the surface energy of ali orien-tations are strongly decreased and no preferential grovvth of (100) is obtained. For steels vvith a high Sb content rather the unvvanted grovvth of (111) is to be expected, since the surface concentration on that plane is highest56. Furthermore, it has been stated that Sb and Sn retard the decarburization79, vvhich is also a very important process in the production of electrical steel sheet - so this again vvould be a negative effect of Sb and Sn surface segregation. 3.2 Effect of surface segregation in creep of steels Due to their strong tendency to surface segregation Sn and Sb can very negatively affect the creep behaviour of heat resistant CrMo- and CrMoV- steels used for turbine rotors and blades80. The failure of such steels occurs by formation of creep cavities at the grain boundaries and in the steel matrix and the coalescence of the cavities to cracks. The nucleation of the cavities mostly starts at inclusions, such as sulfides (MnS) and oxides81. But the nucleation is favoured and accelerated by the presence of Sn or Sb vvhich vvill immediately segregate to the free metal surface of a pore forming at an inclusion or at a grain bondary. The segregation decreases the surface en-ergy, the pores are stabilized and can grovv to cavities un-der further surface segregation. This effect of Sn has been observed for a CrMoV- steel39-40 measuring creep curves for melts doped vvith different Sn-concentrations. The higher the Sn content of the steel the earlier they failed by rupture in the creep test, Figure 11. 3.3 Effects on the carburization of čase hardening steels Surface segregation on Sn and Sb can effectively retard the carburization of čase hardening steels82. The carburization is generally conducted at about 930°C in CO-H2-H2O atmospheres. In the beginning its rate is controlled mainly by the surface reaction sequence CO(g) = C (dissolved) + O (adsorbed) O (adsorbed) + H2(g) = H,0(g) Figure 11: Creep curves for 1% CrMoNiV- steel at 300 MPa and 500°C39, effect of different Sn-contents - vvith increasing Sn-content the rupture tirne is markedly decreased Slika 11: Krivulje lezenja za jeklo 1% CrMoNiV pri 300 MPa in 550°C39, vpliv različnih vsebnosti Sn - z naraščajočo vsebnostjo Snje prelomni čas opazno znižan and later on a coupled surface reaction and diffusion control determines the rate of carburization. The surface reaction rate can be described by r - P([C]e„ - [C],) where [C]eq and [C]s are the equilibrium and the actual surface concentration of carbon and (3 is the carbon transfer coefficient, which contains dependencies on partial pressures and temperature83. Extended thermo-gravimetric studies of carburization on steels doped vvith Sn, Sb, Cu, P or Pb demonstrated a strong effect of Sb on the coefficient (3 (see Figure 12a), vvhereas the effect of the other elements is much less. This retarda-tion of carbon transfer is caused by the blocking of surface sites for reaction, the adsorption and dissociation of CO, by segregated Sb. The surface segregation of Sb and Sn on the čase hardening steels vvas demonstrated by AES studies, after exposure in the carburization atmosphere at 900°C, see Figure 12b. Segregation in the UHV chamber leads to displacement of Sb and Sn by sulfur, hovvever, in the carburization atmosphere the sul-fur vvould be removed by the reaction S (absorbed) + H2(g) = H2S(g). The presence of too high levels of Sb in čase hardening steels vvould lead to too lovv carbon con-tents after the usual carburization period and insufficient hardening of the vvorkpieces, see Figure 12c. Thus, a specification for Sb-content < 25 wt ppm vvas recom-mended for čase hardening steels, vvhereas concentration of the other tramp elements may be in the usual range84. 3.4 Temper embrittlement Reversible temper embrittlement occurs upon slowly cooling of steels through the temperature range 550 to 350°C after annealing (tempering) at higher temperatures or during application of steels in this range. Temper em- mass% Sb orSnICu mass%/10) eV distance from the surface (mm) Figure 12: Effects of Sb, Sn and Cu on the gas carburization of a case-hardening steel at 930°C82,84, a) carbon transfer coefficient p in dependence on bulk concentrations of Sb, Sn or Cu, b) Auger spectrum of the Sb-doped steel after heating in hydrogen to 930°C, c) carbon concentration profiles after gas carburization of samples in an industrial furnace Slika 12: Vpliv Sb, Sn in Cu na plinsko naogljičenje jekla za cementacijo pri 930°C82-84, a) (3 prenosni koeficient ogljika v odvisnosti od koncentracije Sb, Sn ali Cu v osnovnem materialu, b) AES spekter jekla legiranega z Sb po žarjenju v vodiku pri 930°C, c) koncentracijski profil ogljika po plinskem naogljičevanju vzorcev v industrijski peči brittlement is caused by grain boundary segregation of P, Sn, Sb and As61"72 but severe embrittlement is observed only if the alloying elements Ni, Cr and Mn are present, such as in low alloy turbine steels. In earlier years this fact was explained by 'cosegregation', e.g. of Cr and P, however especially for this čase it could be clearly shown that Cr alone has no enhancing effect on P-segre-gation35,36. In fact, Cr and Mn decrease the carbon solu-bility in steels, and the effect of carbon on P-segregation, i.e. removal of P from the grain boundaries by displace-ment by C, is reduced in the presence of Cr and Mn, thereby allovving more P-segregation. 'Cosegregation' was also suspected for Ni and Sb, and Ni and Sn, but most probably the strong effect of these combinations on embrittlement are due to interfacial formation of inter-metallic compounds of these elements. Steels without Ni do not show embrittlement by Sn or Sb85"96. Temper embrittlement is a particular problem for low alloy steels, e.g. Ni-Cr-Mo-V rotor steels and Cr-Mo pressure vessels. Temper embrittlement does not occur in plain carbon steels vvith less than 0,5% Mn. At high Mn concentrations, hovvever, P-segregation is possible in plain carbon steels and also a 'cosegregation' of Mn and Sb is supposed to occur. Hovvever, the effect of Mn can easily be explained by the reduction of carbon activity caused by formation of Mn-rich carbides97. Thereby, carbon segregation is reduced vvhich allovvs grain bound-ary segregation of P, Sn and Sb. 3.5 Hydrogen induced cracking The threshold stress intensity for cracking of a Ni-Cr-Mo steel is strongly reduced by grain boundary segregation of Sb, Sn and p98"100. In the presence of hydrogen this threshold stress intensity is lovvered further, but it is the impurity effect vvhich is dominant, the hydrogen merely accentuates the tendency for brittleness already present. If must be emphasized again that for embrittlement of steels by Sb the presence of Ni and Cr is necessary. Sb causes intergranular fracture in the constant strain rate test, it is five times more effective in inducing intergranular fracture at cathodic potentionals than S, the results are consistent vvith H-permeation studies in Fe as affected by Sb and S101-102. 3.6 Possible effects of grain boundary segregation in in- terstitial free steels One may expect that grain boundary segregation of Sn and Sb is possible in interstitial free steels (i.f. steels). Such steels have very lovv concentrations of C and N in order to attain good deep dravving properties, and thus the tramp elements are not kept away from the grain boundaries by segregated carbon (see Figure 10). The effects of Sn and Sb in deep dravving steels vvere not studied as yet, but a behaviour similar to p103'104 may be expected. Brittle behaviour vvas found for steels vvith very lovv C content (< 300 ppm), caused by P at grain boundaries. Some i.f. steels are alloyed vvith Ti to tie up the interstitial elements, but Ti is also effective in scav-enging the phosphorus forming a very stable phosphide. Also TiC as a precipitate is able to trap phosphorus and to keep it from the grain boundaries to a certain extent, such trapping effect has also been reported for Sb at the TiC/ferrite interface68"70. Anyway, similar as P can be deleterious for the properties of certain deep dravving steels vvith lovv C and no TiC or excess Ti, also Sn and Sb may adversely affect the ductility of such steels. Es-pecially if steels vvith high Sn and/or Sb contents are slowly cooled after batch annealing or coiling, they may segregate to grain boundaries and cause embrittlement. On the other hand, also positive effects may occur on the texture, as in the čase of electrical steel sheet. Effects of Sb and Sn on the texture of deep-dravving steels are cur-rently investigated105. 4 Conclusions Generally, tramp elements such as Sn and Sb can have effects on steel properties only if they enrich at in-terfaces, the enrichment by equilibrium segregation leads to coverages in the range of a monolayer depending on bulk concentration and decreases vvith temperature. Util-izing Auger-electron spectroscopy the thermodynamics of segregation to surfaces and grain boundaries can be elucidated. The solubilities of Sn and Sb in the ferritic matrix are relatively high, the solubility is strongly decreased in the presence of some elements such as Ni vvhich form inter-metallic compounds vvith Sn and Sb. The tendency for surface segregation of Sn and Sb is very high, as yet no thermodynamic data have been determined since always saturation vvas observed. The segregation coverages and structures are very different for different crystallographic orientations, therefore the decrease of surface energy vvill be strongly dependent on orientation and marked effects of Sn and Sb on the sta-bility of different crystallographic planeš are to be ex-pected. Sn and Sb segregate to grain boundaries in ferrite, the extent of segregation strongly depends of the misfit of the grains. The tendency for grain boundary segregation of Sn and Sb is relatively lovv, as indicated by the results on equilibrium segregation in binary alloys in the temperature range 500 to 750°C. This can also be seen from the segregation enthalpies: -22,5 kJ/mol Sn and -19 kJ/mol Sb. Sn and Sb also segregate to interfaces, for Sb at inter-faces ferrite/TiC and for Sn at the interface ferrite/MnS. In the annealing of steel sheet the surface segregation of Sn and Sb affects the stability of certain orientations. For intermediate concentrations the (100) orientation ap-pears to be stabilized, for higher contents the (111) orientation becomes stable. These effects are of importance in the production of electrical Fe-Si steel sheet and may also be useful in the production of deep-drawing steels. The strong tendency for surface segregation of Sn (and Sb) plays a role in the creep of heat resistant steels, since formation and growth of creep cavities is enhanced by surface segregation decreasing the surface energy of the cavities. This was demonstrated for Sn doped CrMoV-steels. Surface segregation of Sn and Sb affects the carburi-zation of čase hardening steels. Especially Sb strongly retards the carbon transfer and may cause insufficient carburization. Grain boundary segregation of Sn or Sb causes em-brittlement. Temper embrittlement of low alloy steels only occurs in the presence of Ni and Cr. Obviously, re-duction of carbon activity by Cr and formation of inter-metallics NixSny resp. 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