UDK 621.762:620.17 ISSN 1580-2949 Strokovni ~lanek MTAEC9, 37(6)377(2003) DETERMINATION OF THE EFFECTIVE LOAD-BEARING CROSS-SECTION OF SINTERED STEEL PRODUCTS DOLO^ANJE DELE@A DUKTILNEGA PRELOMA SINTRANIH JEKLENIH IZDELKOV Matja` Godec, Borivoj Šu{tar{i~ Institute of Metals andTechnology, Lepi pot 11, 1000 Ljubljana, Slovenia matjaz.godec. imt.si Prejem rokopisa – received: 2003-10-20; sprejem za objavo – accepted for publication: 2003-11-18 The mechanical properties of sintered steel products depend mostly on their porosity and the bonds between the steel powder particles. Increasing the cohesion interconnection significantly improves the mechanical properties. Using scanning electron microscopy (SEM) with its high depth resolution it is possible to determine the fraction of ductile fracture which is honeycomb-like in appearance andis effective load-bearing cross-section. Key words: ductile fracture, sintered steel, warm pressed Mehanske lastnosti sintranih jeklenih izdelkov so odvisne predvsem od njihove poroznosti in vezi med jeklenimi pra{nimi delci. S pove~anjem kohezivne povezave mednjimi mo~no izbolj{amo mehanske lastnosti. Uporaba vrsti~nega elektronskega mikroskopa (SEM) zaradi velike globinske lo~ljivosti omogo~a dolo~itev dele`a duktilnega preloma, ki je glede morfologije podoben ~ebeljemu satovju in je nosilni del v nekem prerezu. Klju~ne besede: duktilni prelom, sintrano jeklo, toplo stiskanje cross-section which indirectly controls the mechanical 1 INTRODUCTION properties. Most of the steel products used in cars are manufacturedby the classical powder metallurgy route 2 EXPERIMENTAL PROCEDURE of pressing andsintering. The products have a very good surface quality with excellent dimensional tolerances. Two series of sintered steel products were indu-The mechanical properties of these sinteredsteel strially preparedusing coldpressing andwarm pressing products depend primarily on porosity. Higher porosity on a Dorst 500 kN hydraulic press (Figure 1). The means lower density and correspondingly worse products were safety element of a servo system and were mechanical properties 1,2. By influencing the volume producedfrom Höganäs’ Distaloy AB andDensmix 6 for fraction andthe morphology of the pores better coldandwarm pressing, respectively. One can notice mechanical properties are obtained 2. A reduction in the that two different materials were compared due to total porosity results not only in improvedstrength but specific demandfor coldandwarm pressing. Both also in better ductility 3. The total porosity can be reduced, but not eliminated, by double pressing or using the newly developed technology of warm pressing 4. Insteadof using metallographic techniques, fracto-graphic techniques were proposedfor describing the porous microstructures 5. The sinteredproducts with low porosity show isolatedpores while the high-porosity products show interconnectedpores andisolatedsintered contacts. Goodmechanical properties correspond directly to the cohesion of sintered powder particles. This paper deals with the method of load-bearing cross section determination according to the internal standards for sintered steel products used in the car industry. The fraction of ductile and brittle fracture at random places was measured by planimetry of SEM fractographs. Selectedexamples show how the method of sinteredsteel manufacture influences the load-bearing MATERIALI IN TEHNOLOGIJE 37 (2003) 6 M. GODEC, B. ŠU[TAR[I^: DETERMINATION OF THE EFFECTIVE LOAD-BEARING CROSS-SECTION ... Table 1: The pressing conditions andthe compact performances, obtainedby pressing on a Dorst 500 kN press. The characteristics of the sintered products using classical sintering powder metallurgy and warm pressing are compared. Tabela 1: Pogoji stiskanja in lastnosti stiskanca, dobljeni na stiskalnici Dorst 500 kN. Podana je primerjava lastnosti sintranih izdelkov, dobljenih po klasi~ni metodi sintranja prahov in s toplim stiskanjem. Pressing conditions Temperature (°C) (tool/ powder) Pressing device Density . (g/cm3) 6 Loadbearing cross-section (%) Fracture strength (kN) 8 Hardness HB green sintered ductile brittle green sintered coldpressing Room temperature Dorst 500 kN 7.05 7.11–7.13 24-29 0 0.40-0.45 12.2-12.5 106/117 warm pressing 110/110 Dorst 500 kN 7.33 7.32–7.34 31-45 1-4 0.50-0.54 14.9-15.3 125/132 materials consists of iron particles, but for warm pressing a lubricant is added. The pressing conditions are shown in Table 1. Some physical properties were comparedandanalysed. The fraction of ductile fracture Figure 2: (a) Typical fracture surface of a sinteredsample showing areas of porosity as well as ductile and brittle regions. (b) The area of the ductile and brittle fracture surface is determined by a planimetric technique using manual mode in analySIS-PRO software. The ductile andbrittle areas are 31 % and14 %, respectively. Slika 2: (a) Tipi~na prelomna povr{ina sintranega vzorca, na kateri so poleg duktilne in krhke prelomne povr{ine prikazane tudi pore. (b) Plo{~ino duktilnega in krhkega dela preloma smo dolo~ili z ro~nim planimetriranjem ob uporabi ra~unalni{kega programa analySIS-PRO. Dele` duktilnega preloma je 31-odstoten in dele` krhkega preloma 14-odstoten. area – the effective load-bearing cross-section – on both series of samples was determined using a SEM Jeol JSM-35 andimage analysing software analySIS-PRO 3.1 from Soft Imaging System, Germany. 2.1. Method for the determination of the load-bearing cross-section For detecting the sintered material performances different static as well as dynamic mechanical para­meters can be measured. The procedure for determining the porosity of sintered steel products is standardised 7,8. Additionally to all this parameters the car industry also check the load-bearing cross-section which is a very goodindicator of the mechanical properties. The proce­dure to determine the load-bearing cross-section which M. GODEC, B. ŠU[TAR[I^: DETERMINATION OF THE EFFECTIVE LOAD-BEARING CROSS-SECTION ... has been usedin our research andis very similar to those used in car industry is described as follows: • Sinteredsteel products are fracturedalways by pressuring or bending at the same ordered critical place, performedin the same way. • The fracturedarea is observedunder the SEM at magnification 400-times. • The number of examinedplaces shouldbe at least 25, which have to be randomly chosen and uniformly distributedon the fracturedsurface, 15 of them must be selectedfrom the border area. • The observed area is photographed. The ductile area can be measureddirectly in the SEM or with photographs. • The area of the ductile fracture surface – the load-bearing cross-section – is determined by a planimetric technique. This can be done by the application of image-analysing software in automatic mode or manual mode. • 30 % of the fracture surface area must be of a ductile nature. 3 RESULTS AND DISCUSSION In Figure 2 a the fracturedsurface area of a sintered steel product is shown. There are three different types of area: 1. Ductile, which is honeycomb-like in appearance and acts as an actual effective load-bearing cross-section; 2. Brittle, which has cleavage lines andis flat; 3. Pores. For exact measurement, the ductile areas are measured in the manual mode, that means every ductile fracture area is markedby handandthe markedareas are measured(Figure 2 b). The automatic mode can be used, and this is much faster, but the correct thresholding of gray scale image must be applied. Sometimes this is not possible, because the ductile area are not always within a specific defined gray value range. Sinteredsteel products manufacturedby warm pressing have a higher density and a lower porosity. By decreasing the porosity level the static as well as the dynamic mechanical performance (hardness, strength, ductility and fatigue life) is improved. In Table 1 a comparison of some mechanical properties is MATERIALI IN TEHNOLOGIJE 37 (2003) 6 M. GODEC, B. ŠU[TAR[I^: DETERMINATION OF THE EFFECTIVE LOAD-BEARING CROSS-SECTION ... summarisedfor sinteredsteel products manufacturedby classical sintering powder metallurgy and warm pressing. It is clear that the ductile fracture area increases with increasing density or decreasing porosity of the sinteredsteel product. In Figure 3 the porosity of cold andwarm-pressedsamples is shown. The porosity of these cold-andwarm-pressedsamples are 9.0 % and3.6 %, respectively. Figure 4 shows the microstructure of samples after etching with 2 % nital. The ferrite-perlite grains are smaller andthe microstructure is more homogeneous comparedto the cold-pressedsamples. Both porosity andmicrostructure are improvedby warm pressing. The mechanical properties can be directly comparedto the fraction of ductile fracture area. As a consequence, the car industry have introduced the measuring of load-bearing cross-sections as an additional test methodfor controlling the quality of sintered products. Figure 5 shows the fracture surface of cold-andwarm-pressedsamples. The difference in ductile fracture area is obvious. 4 CONCLUSIONS A method for the determining the load-bearing cross-section is described. It is an appropriate additional methodfor controlling mechanical properties, particularly useful because sinteredsteel products are very small andhave complicatedshapes which make them unsuitable for standard mechanical test procedures. The results have shown that warm-pressedsintered products have a much higher area of ductile fracture surface andsmaller porosity resulting in a better microstructure andmechanical properties. 5 REFERENCES 1 Klar, E. et al., Metal Handbook, 9th Edition, Powder Metallurgy, Volume 7, Ohio USA, American Society for Metals, 1984 2 Danninger, H.: Acta Metallurgica Slovaca, 2 (1996)4,310 3 Haynes, R.: Mechanical Properties of SinteredMetals, Freund Publishers, London, 1981 4 Engström, U.: Proc. PM’94 Paris, SF2M EPMA, Les Editon de Physique, Les Ulis Cedex a, France, I (1994), 57 5 Šlesár, E. Rudnayová, D. E.: Powder Metall. Int. 24 (1992), 232 6 http://www.hoganas.com/ 7 ISO 2738-1999, DIN 30911-90 Teil 3, MPIF (Metal Powder Industries Federation), Standard Test Methods for Metal Powders and Powder Metallurgy Products, Metal Powder Industries Federation, Edition 1985/1986, New Jersey 8 ISO 2739-1973, Determination of Radial Crushing Strength