EFFECT OF SINTERING CONDITIONS ON PARTICLE CONTACTS AND MECHANICAL PROPERTIES OF PM STEELS PREPARED FROM 3%Cr PREALLOYED POWDER

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1 Powder Metallurgy Progress, Vol.2 (2002), No EFFECT OF SINTERING CONDITIONS ON PARTICLE CONTACTS AND MECHANICAL PROPERTIES OF PM STEELS PREPARED FROM 3%Cr PREALLOYED POWDER S. Kremel, H. Danninger, Y. Yu Abstract Chromium alloyed sintered steels offer considerable potential with regard to mechanical properties both in the as sintered and heat treated state. The main problem, esp. when using prealloyed steel powders, is oxygen at the sintering contacts that can both originate from the starting powders or be introduced during sintering, e.g. by using impure atmospheres. Here it is shown that oxide layers in the interparticle contacts inhibit formation of sufficiently strong sintering necks which adversely affects the mechanical properties, in particular the impact energy. Removal of the oxide layers is obtained primarily by carbothermic reduction through admixed graphite, the temperatures necessary for Cr alloy steels being markedly higher than in the case of plain carbon steels. High temperature sintering is particularly beneficial here, resulting in more thorough deoxidation and in a much better property profile. The effect of the carbothermic reduction is well visible from the fracture behaviour of the specimens, esp. in the case of impact loading, impact energy and the appearance of the fracture surfaces being effective criteria here. It is also found that the carbon loss (decarburisation) is homogeneous through the whole volume, and thus can be compensated by adding an extra amount (20-40% more) of graphite in the mix. Keywords: sintered steel, chromium, reduction, sintering contacts, mechanical properties INTRODUCTION Conventional low alloy structural steels produced by ingot metallurgy contain mostly Cr, Mn, and Si, with some addition of carbide formers such as V and Mo. In powder metallurgical parts manufacturing, in contrast, high strength alloy steels used e.g. for sintered engine and transmission components traditionally are alloyed with Cu, Ni, and in part Mo [1]. The reason for using these elements which are considerably more expensive than e.g. Cr, Mn, Si and V employed in the wrought steels, is the high oxygen sensitivity of the latter group of metals that requires relatively pure sintering atmospheres. Otherwise, removal of the oxide layers covering the powder particle surfaces is difficult, and even further oxidation can be expected which is particularly detrimental due to the large specific surface of pressed powder compacts. Such phenomena are well known from sintering of stainless steels for which very low oxygen contents in the atmosphere are required; typically in hydrogen a dew point of <-40 C [2]. On the other hand, chromium is a cheap and effective alloy element that gives good heat treating response; it is the primary alloy Sabine Kremel, Herbert Danninger, Vienna University of Technology, Institute of Chemical Technologies and Analytics, Vienna, Austria Yang Yu, Höganäs AB, Höganäs, Sweden

2 Powder Metallurgy Progress, Vol.2 (2002), No element with wrought low alloy steels such as DIN 42CrMo4 / AISI 4140 or bearing steels such as DIN 100Cr6 / AISI [3]. Cr alloy steels are therefore very attractive also for PM components. The use of Cr alloy steels in the PM industry has been helped by progress of PM parts manufacturing in general. With increasingly tight requirements towards carbon control during sintering, the criteria for sintering atmosphere purity have also become more strict which primarily means less oxygen and water content. This resulted in improvements both in furnace technology and atmosphere preparation. Sintered steels alloyed with elements of higher oxygen affinity have therefore been regarded as more feasible. This holds e.g. for Mn alloy steels pioneered e.g. by Šalak [4, 5], and for Si-Mn alloy grades promoted by Thümmler et al. [6] but in particular for Cr alloys steels. These steels can be manufactured from elemental powder mixes, in which case however microstructural homogenisation has to be attained during sintering, and this required fairly intense sintering [7, 8]. The other variant is using prealloyed steel powders containing the alloy elements in solid solution. In this case, the main problem is manufacturing the steel powder with a sufficiently low content mainly of oxygen and carbon. Cr-Mn-Mo (AISI 4100 grade) steel powders with low oxygen content have been successfully produced by oil atomisation [9, 10] and by water atomisation with a subsequent vacuum anneal [11]. The powder grades result in excellent mechanical properties; however there are technical and economical drawbacks that have prevented large scale use of these powders, and the oil atomized grades even quickly disappeared from the market. Recently, however, increased interest in Cr alloy sintered steels has emerged, and Höganäs AB reacted by introducing a prealloyed steel powder containing 3% Cr and 0.5% Mo that is produced by water atomisation and reducing anneal under suitable conditions to result in appropriately low oxygen content while enabling the powder, designated Astaloy CrM, to be available at a comparatively low price. Within this work, the relationships between sintering and heat treatment of the materials prepared from this powder and the resulting properties are described, as compared to standard unalloyed PM steel. EXPERIMENTAL DETAILS Cr prealloy starting powder Astaloy CrM was mixed with varying amounts of carbon as natural graphite and ethylene bisstearoylamide (HWC) as pressing lubricant. The powders were blended in a tumbling mixer for 60 min, and then compacted in pressing tools with floating die to form tensile test bars (ISO 2740) and impact test bars (ISO 5754), respectively. In part, warm compacted specimens were prepared at Höganäs AB using a Densmix grade based on Astaloy CrM. Sintering was done in part in an industrial belttype furnace at Höganäs AB, in part in a laboratory pushtype furnace (Degussa Baby ), in both cases the atmosphere being N 2-10% H 2. In the belt furnace, 0.1% CH 4 was added to the atmosphere to adjust the carbon level; in the pusher furnace, getter boats with Ferro-Al getter were used to prevent decarburization. Green compacts produced with die wall lubrication were sintered in a pushrod dilatometer in rotary pump vacuum in order to study the degassing and reduction behaviour, the pressure in the vacuum system was recorded. Characterization of the sintered specimens included measuring the density through water displacement, tensile testing, Charpy impact testing of the unnotched specimens (ISO 5754), Vickers macrohardness testing, and standard metallographic and fractographic investigations.

3 Powder Metallurgy Progress, Vol.2 (2002), No STUDY OF THE DEGASSING AND REDUCTION BEHAVIOUR As stated above, one critical point with sintering of Cr alloy steels is the removal of the surface oxides covering each individual powder particle. Such oxides are present also on plain iron or e.g. Mo alloy steel powders; however, it has been shown that oxygen removal occurs relatively fast in these cases. Unalloyed or Mo alloyed steels exhibit 3 degassing maxima during sintering under inert atmosphere, in the temperature ranges of C, 700 C, and C [12, 13, 14]. The latter two maxima are caused by formation of CO and some CO 2, according to Boudouard s equilibrium, thus indicating carbothermic reduction of oxides present in the metal powders. In any case, the removal of the oxygen takes place well below the standard sintering temperatures, and thus are complete in a rather early stage of sintering. For Cr prealloyed steels (produced by water atomisation with subsequent vacuum anneal), it has been shown that these CO/CO 2 maxima emerge at significantly higher temperatures, which is understandable from the thermodynamical viewpoint when considering the higher stability of Cr oxide compared to iron oxides, and also the higher reducing power of C at higher temperatures. The same phenomenon can be expected also for the prealloyed Cr-Mo steel. Degassing experiments carried out with AstaloyCrM-0.5%C (Fig.1a) showed that with this material there is some slight degassing in the low temperature range, but virtually none in the range of about 700 C where gas formation is most pronounced in the case of unalloyed steel (Fig.1b) and also for e.g. Mo alloyed grades [13]. However there is a strong degassing maximum at about 1000 C and another one in the range of 1240 C. It has been assumed that the first of these degassing maxima indicates the removal of the surface oxygen, and the second one removal of oxygen from within the powder particles [14, 15]. In this case it can be concluded that only after the surface oxygen has been removed i.e. after a temperature threshold of 1000 C has been exceeded metallic sintering contacts can be formed and significant mechanical strength is observed. For unalloyed steels, in contrast, contact formation can be expected to occur already above 700 C. Fig.1a. Astaloy CrM-0.5%C Fig.1b. Fe-0.5%C Fig.1: Dilatometric and degassing graphs of steel compacts. p c = 600 MPa, heating rate 10 K.min -1, vacuum INFLUENCE OF THE TEMPERATURE ON CONTACT FORMATION It therefore could be concluded that in the case of Cr prealloy steel powder a marked increase of the mechanical properties would be expected if the sintering temperature exceeded about 1000 C, and there would also be a noticeable effect on the

4 Powder Metallurgy Progress, Vol.2 (2002), No structure of the fracture surfaces. In order to check if this assumption would come true, impact test specimens Astaloy CrM-0.5% C were sintered at temperatures varying between 600 and 1200 C; in order to obtain a reasonable comparison to less oxygen sensitive sintered steels, also unalloyed steel ASC % C was investigated. Sintering was done in a laboratory furnace with an ODS superalloy (PM2000) muffle, and high purity N 2 (99.999%) was employed as the atmosphere in order to avoid oxidation/decarburization due to atmosphere effects during sintering. The properties of the sintered specimens are given in Fig.2 as a function of the sintering temperature. a. Sintered density b. Hardness c. Charpy impact energy (unnotched) Fig.2. Properties of AstaloyCrM-0.5%C and ASC %C, compacted at 700 MPa and 60 min sintered at varying temperatures in N 2 (99.999%). It stands out clearly that the density of the Astaloy CrM specimens is lower than that of the unalloyed ones, which is attributable to the lower compactibility of the alloyed steel as a consequence of solid solution hardening. With both materials the density tends to increase at higher sintering temperatures. This trend is still more pronounced for the dimensional change: here the Cr-Mo steel exhibits a significant expansion in the temperature range C in which range the sintered specimens also showed a dark surface and in the metallographic sections also dark layers in the pressing contacts (Fig.3), indicating some oxidation yet at higher temperatures showing markedly more shrinkage than the unalloyed variant. This leads to the conclusion that for obtaining some densification during sintering, higher temperatures are more helpful with Astaloy CrM than with the unalloyed steel. The mechanical properties clearly reveal the different response of the two materials to the sintering temperature, as expected from the degassing behaviour. Regarding the hardness, a very special feature of Astaloy CrM is the rather high sintering temperature necessary to obtain the hardness levels expected for this Cr alloyed steel. The metallographic sections here revealed that in Astaloy CrM, reasonable dissolution of carbon in the matrix material occurs only at T>1000 C, as compared to >850 C for the unalloyed

5 Powder Metallurgy Progress, Vol.2 (2002), No steel (see also [16, 17]). This indicates that the oxide layers are not only barriers for the formation of sintering contacts but also for the dissolution of the graphite in the metallic matrix, which is not really surprising since in both cases diffusion processes play a major role; apparently diffusion of carbon through an oxide layer is almost as difficult as is that of the metallic elements. Furthermore, it can be concluded that carbothermic reduction of the oxide layers both in unalloyed and Cr alloyed steels takes place from the elemental C, i.e. the graphite. Ts = 800 C Ts = 950 C Ts = 1000 C Ts = 1200 C Fig.3. Metallographic sections of AstaloyCrM-0.5%C compacted at 700 MPa, sintered 60 min in N 2 (99.999%) For the Charpy impact energy the differences between the two steels are particularly pronounced which is not surprising since the impact energy is the single mechanical property most sensitive to interparticle bonding. Here, the much higher temperature necessary for obtaining measurable impact energy values in the case of the Cr alloy steel is clearly visible. Once more 1000 C can be regarded as a sort of transition temperature above which sintering of Astaloy CrM has to be carried out, in any case, to result in measurable strength. Fractographic investigations (Fig.4) confirmed that formation of noticeable sintering contacts takes place above about 800 C in plain Fe-C, while in the case of Astaloy CrM at least 1000 C is necessary. This underlines that the essential precondition for successful sintering of Cr prealloyed steels is the removal of the oxide layers on the powder particles [18].

6 Powder Metallurgy Progress, Vol.2 (2002), No Astaloy CrM-0.5%C, Ts = 700 C Astaloy CrM-0.5%C, Ts = 850 C Astaloy CrM-0.5%C, Ts = 1000 C Astaloy CrM-0.5%C, Ts = 1200 C Fe-0.5%C, Ts = 700 C Fe-0.5%C, Ts = 850 C Fe-0.5%C, Ts = 1000 C Fe-0.5%C, Ts = 1200 C Fig.4. Fracture surfaces of Astaloy CrM-0.5%C as compared to Fe-0.5%C, sintered for 60 min in N 2 at different temperatures.

7 Powder Metallurgy Progress, Vol.2 (2002), No SINTERING TEMPERATURE AND MECHANICAL PROPERTIES The results shown so far have indicated that a minimum temperature of about 1000 C is necessary to obtain sintering neck formation and to result in reasonable carbon dissolution. However, this only gives a minimum value for the temperature and does not yield any information for how e.g. a further increase of the temperature affects the mechanical properties. Here it must be kept in mind that as shown in Fig.1a, further degassing occurs up to a temperature of about 1250 C, and in practice it can be expected that the reduction processes will take place more slowly than in the case of the dilatometric runs performed under rather ideal conditions (small specimens, pumping off the gases generated, ). Furthermore, it has been shown both for plain iron and high strength steels that higher sintering temperature result in markedly better (esp. dynamical) properties, at least at higher relative density levels [19-21]. In order to check the response of the Cr alloy steel to the sintering temperature, sintering tests were carried out with tensile and impact test bars compacted uniformly at 700 MPa but parallel by cold and warm compaction [22], resulting in significant differences of the green density. Sintering was done both for 30 min at 1120 C to simulate belt furnace conditions - and for 60 min at 1250 C, the atmosphere being 90% N 2-10% H 2. In order to cover different applications of the steel, the carbon level was widely varied. Although the cooling rates in the furnaces were slightly different - the pusher furnace affording faster cooling -, the microstructures were rather similar, and any differences in the properties were thus attributed to the role of sintering contacts/pore morphology rather than to that of the matrix microstructure, which is also supported by the findings given in [23]. The properties obtained are shown in Fig.5. Here it can be seen clearly that, while the sintered density of course is generally higher for the warm compacted materials (Fig.5a), the increase in density during sintering as indicated also by the dimensional change, is drastically enhanced by the higher sintering temperature. This indicates that for the Cr alloy steel, the response to the sintering conditions is particularly pronounced; therefore, improvement of the mechanical properties by sintering at higher temperatures should be very effective here, as well. One point that should not be neglected is, however, the effective (= combined) carbon content in the sintered materials. As shown above, higher sintering temperatures result in more complete reduction, i.e. lower oxygen content. However, since the only reducing agent available in inert atmosphere is carbon, at least in N 2 or a vacuum, more complete reduction also means more carbon loss during sintering. In N 2 -H 2 atmosphere there might also be some contribution of hydrogen to the reduction, but as shown e.g. by Campos et al. [24], also in reducing atmosphere the main agent for oxygen removal is carbon, the efficiency of the reduction and the properties of the sintered materials - being directly related to the carbon loss. Carbon analysis of the sintered materials once more confirmed this relationship, showing that the carbon loss after sintering at 1120 C is negligible, at 0.2% C nominal, and in the range of about 0.05% at 0.35% and 0.5% C nominal (which is of course partly attributable to the addition of methane to the atmosphere). Sintering at 1250 C, in contrast, resulted in a significantly higher carbon loss, which is also an indicator for more complete reduction.

8 Powder Metallurgy Progress, Vol.2 (2002), No Green density Sintered density Dimensional change during sintering Combined carbon, as-sintered Apparent hardness Tensile strength Yield strength Elongation Charpy impact energy (unnotched) Fig.5. Properties of Astaloy CrM-x%C, cold/warm compacted at 700 MPa, sintered 30 min at 1120 C and 60 min at 1250 C, respectively, in N 2-10%H 2

9 Powder Metallurgy Progress, Vol.2 (2002), No Theoretically, the higher carbon loss could also be attributable to surface decarburization, which is definitely a problem during high temperature sintering. However, metallographic investigations as well as hardness measurement, both on the surfaces and in cross sections of the specimens, confirmed that there is virtually no carbon gradient from the surface to the core, i.e. all the carbon loss observed is attributable to the reaction of carbon with the natural oxygen content of the starting powders, which latter is evenly distributed and therefore also results in homogeneous carbon loss. Nevertheless, this higher carbon loss at higher temperatures results in different compositions after sintering at 1120 and 1250 C, respectively, and basically, at higher temperatures lower hardness and strength but higher ductility could be expected. Surprisingly however, it showed that the hardness of the high temperature sintered materials is at least at the same level as that of the low temperature sintered ones; at 0.35% and 0.5% C nominal it is even slightly higher. In these cases the effect of the sintering temperature even exceeds that of the green density: cold compaction combined with sintering at 1250 C results in higher hardness than warm compaction and sintering at 1120 C. In principle this high hardness might also be due to faster cooling from 1250 C in the pusher furnace, but in this case some adverse effect on the ductility and impact energy should be discernible, which is apparently not the case. For the tensile strength and yield strength, the higher carbon loss at 1250 C results in some drop of R m and R p0.2 in the case of the lowest carbon content; for higher nominal carbon levels however, there is virtually no difference between specimens sintered at 1120 and 1250 C, respectively. On the other hand, the lower carbon content positively affects the elongation to fracture which is drastically higher with the materials sintered at 1250 C; the difference is however not completely attributable to the different C levels but also to the better interparticle bonding. The impact energy finally shows that for 30 min sintering at 1120 C there is virtually no difference between cold and warm compacted specimens, the values being in the range of about 20 J. After 60 min sintering at 1250 C however, even after cold pressing, a markedly higher impact energy is measured which is further increased in the case of the warm compacted specimens, levels of >50 J being attained here which is exceptional for sintered steels, especially when considering the hardness of >250 HV30. The effect of the sintering conditions stands out still more clearly if the hardness and impact energy are plotted as a function of the combined carbon content (Fig.6); in this case it is also particularly visible that warm compaction does not result in significant improvement when sintering at 1120 C but very much when sintering is done at 1250 C. For optimum assessment of the properties, plotting strength vs. ductility is useful (see [25]), for sintered steels, taking hardness vs. impact energy proved to be more significant. If this is done for the steels investigated here (Fig.7), the very pronounced effect of the sintering intensity (i.e. temperature and time) is most evident, and surprisingly it has been found that, at least in the as-sintered state, higher carbon levels result in better hardness but surprisingly also in higher impact energy, which may be due to the higher relative sintering temperature, i.e. normalized to the respective solidus temperature which, of course, drops with increasing carbon content. These results clearly indicate that also for Astaloy CrM, superior mechanical properties can only be obtained when combining high density with sufficiently intense sintering. High green density by itself is not particularly useful unless the specimens are appropriately sintered. This phenomenon has been shown also for other PM steels, e.g. for the fatigue behaviour of Mo alloy steel [21], but it seems to be particularly pronounced with Astaloy CrM, probably due to the additional effect of the more complete oxygen removal.

10 Powder Metallurgy Progress, Vol.2 (2002), No The inevitable side effect of higher carbon loss can be compensated for in advance by adding an extra amount of graphite. Fig.6. Properties of Astaloy CrM-x%C as a function of the combined carbon content a) Hardness, b) Impact energy Fig.7. Impact energy vs. hardness of Astaloy CrM-x%C, differently manufactured CONCLUSIONS Cr alloy sintered steel Astaloy CrM containing % C nominal can be sintered to excellent mechanical properties if sufficiently high sintering temperatures are chosen. The absolute minimum temperature is that above which carbothermic reduction of the surface oxides takes place; below this temperature virtually no sintering contacts are formed. Under rather ideal conditions the critical temperature is in the range of 1000 C. If sintering is done in a proper atmosphere, reasonable mechanical properties are obtained by sintering at 1120 C, however the full capability of this new Cr alloy steel is only exploited if sintering is done above 1200 C. At higher temperatures more complete reduction occurs, which results in a somewhat higher carbon loss (that has to be taken into account when defining the admixed carbon content); nevertheless, the much stronger interparticle bonding compensates for this effect with regard to hardness and strength while elongation and impact energy are very positively affected. Only through high temperature sintering the full benefit of high green density, as obtained e.g. by warm compaction, can be exploited as well. Acknowledgement This work was performed within the international project Höganäs Chair. The financial and logistic support given by Höganäs AB, Sweden, is gratefully acknowledged.

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