Abstract
For PM steel precision parts, high mechanical loading is very common in service. In particular for automotive applications, fatigue up to very high loading cycle numbers may be encountered. Advanced alloying concepts for sintered steels are required, involving also alloy elements with high oxygen affinity. In the present study, hybrid alloyed sintered steels based on prealloyed steel powder Fe-1.8%Cr were prepared by the press-and-sinter route. Mn and Ni, respectively, were admixed as well as a masteralloy containing Mn and Si, the alloy element content being varied, and the combination hardness vs. impact energy was assessed. From the most promising variants, fatigue test specimens were prepared, sinter hardened and characterized, fatigue testing being done by ultrasonic resonance up to Nmax = 10E10 cycles. Ni alloying proved to be positive both for the impact energy and the gigacycle fatigue strength while Mn resulted in high hardenability but low impact and fatigue strength, in part because of intergranular embrittlement. The grade with Mn-Si masteralloy was slightly less ductile than the Ni alloyed variant, but in particular at N > 10E8 the fatigue endurance strength was similar to that of the Ni alloyed type, however with significantly lower scatter.
1. Introduction
In the last decades, the range of applications for powder metallurgy (PM) precision parts has increased, especially in the automotive industry (Whittaker, Citation2015). In many cases, parts used in engines as well as transmission systems are subjected to cyclic stresses with loading cycle numbers frequently >10E8. Therefore, the interest for the high cycle fatigue behaviour of PM steels consistently increased, also because the literature concerning high cycle fatigue is still limited (Hadrboletz & Weiss, Citation1997). Numerous endurance strength data are available for Nmax = 2* 10E6 or at best 10E7, which are of limited significance for loading up to higher N (Danninger & Weiss, Citation2001; Sonsino, Citation2005, Citation2007; Zafari & Beiss, Citation2008).
However, this focus on automotive applications, specially in internal combustion engines, also bears the risk that because of the current shift to alternative drivetrain systems, traditional markets will be lost (Kotthoff & Leupold, Citation2017), and therefore new ones have to be found. This requires improved properties to overcome the weakness regarding mechanical properties caused by the inherent porosity in pressed and sintered parts. On the other hand, the strengths of the press-and-sinter route, geometrical complexity and high precision as well excellent material and energy utilization, have to be maintained. The demand is for higher hardness, yield and fatigue strength in combination with high toughness.
All these requirements have to be met in a cost-effective way. Sinter hardening, i.e., gas quenching of the parts directly after sintering, within the sintering furnace, which is a one-step process, combines good manufacturing economy with the ability to achieve unique combinations of mechanical properties and is environmentally friendly compared to e.g., oil hardening (Bocchini, et al., Citation2002; Brian James, Citation1998; Engström, Citation2000; Karamchedu, Hatami, Nyborg, & Andersson, Citation2014; Ratzi & Orth, Citation2000; Stoyanova & Molinari, Citation2004). On the other hand, the cooling rates attained in gas quenching are significantly lower than in oil hardening, and therefore the materials used have to offer improved hardenability, Thus, tailoring the material composition based on the requirements and the processing conditions is of special importance. Chromium is a very attractive alloying element in PM steels (Berg & Maroli, Citation2002; Lindqvist, Citation2001). It is a ferrite stabilizer in steels, which increases strength, hardness and hardenability and forms hard carbides. It can also increase the wear resistance. However, the advantages of using chromium-containing PM steels are to some extent offset by the necessity to reduce the Cr based oxides present on the powders, which requires higher sintering temperatures than the classical alloy elements for PM steels, Ni, Cu and Mo (Danninger, et al., Citation2002; Hojati, Danninger, & Gierl-Mayer, Citation2021; Kremel, Danninger, & Yu, Citation2002; Kulecki, Lichańska, & Sułowski, Citation2015; Sułowski, Kulecki, & Radziszewska, Citation2014).
Nickel is another excellent alloying element to work with when a combination of strength and ductility is targeted. The addition of Ni is known to improve the mechanical properties of PM steel components by increasing strength, toughness, impact resistance, abrasion resistance and fatigue performance (Behera, Tripathi, & Chaubey, Citation2018; Nabeel, Frykholm, & Hedström, Citation2014; Wu, Tsao, Shu, & Lin, Citation2012); in particular the diffusion alloyed variants are widely used (Lindskog, Citation2013). Some previous studies have, however, reported that presence of the soft austenitic phase, i.e., Ni rich areas, can be detrimental to mechanical properties such as tensile strength (Carabajar, Verdu, & Fougeres, Citation1997; Wu, Hwang, & Huang, Citation2007). Formation of these areas is due to slow diffusion of Ni in iron as compared to other alloying elements such as C, Cu and Mo (Bergmark & Alzati, Citation2005). During the last two decades, the price of Ni has been volatile and showed an increasing trend for the time being, especially in the powder form, also as a consequence of the political situation, Russia being a major supplier of this metal. In addition, Ni allergy is another issue that limits the usage of this alloying element. Furthermore, the demand for Ni, as for Cu, and thus also the price, must be expected to further increase as a consequence of electrification. Therefore, it is of great interest to look at alloying systems without or with lower nickel content.
Manganese is another alloying element that is commonly used in the steel industry and could be an option for the production of highly stressed PM components with high static and dynamic characteristics (Šalak & Selecká, Citation2012). It offers increased strength – Mn is strongly hardening ferrite – and hardenability at much lower cost than many other alloy elements such as Cu, Ni and Mo (ASM International. Handbook Committee, Citation1990). However, there are some features of Mn that make the industrial production of Mn alloyed PM steels through the press-and-sinter route difficult. One is the high oxygen affinity of Mn which increases the risk of oxygen pickup from the atmosphere during sintering and renders the removal of the natural oxides present on the powder surfaces more difficult. High purity sintering atmospheres with an extremely low oxygen potential are required for the reduction of oxides covering the surfaces of these powders (Danninger, Pöttschacher, Bradac, Salak, & Seyrkammer, Citation2005; Šalak & Selecká, Citation2012). The second problem is the high vapor pressure of Mn which results in Mn loss during sintering, primarily at the part surfaces where the positive effects of Mn are needed most (Jalili Ziyaeian, Citation2008). A. Šalak showed that Mn evaporation during sintering is able to reduce the risk of oxygen pickup from the atmosphere by the “self-getter effect” (Šalak & Selecká, Citation2012), though at the expense of Mn loss. (Fortunately, this effect is less probable in industrial production than in laboratory experiments (Danninger et al., Citation2005)). Mn can therefore on the one hand lower oxygen pickup from the atmosphere, by forming a protective vapour shell, but on the other hand retain the oxygen that is already present in the compact, mainly on the base powder surfaces, necessitating higher reduction temperatures.
Both problems might be avoided or at least alleviated by using the masteralloy (MA) route for introducing the alloy elements. Masteralloys are powders that combine several alloy elements and are admixed to a base powder to introduce these elements simultaneously. This could be an attractive way in particular for introduction of oxygen sensitive elements such as Si, Mn and Cr, as recognized already in the 1970s (Albano-Müller, Thümmler, & Zapf, Citation1973; Banerjee, Gemenetzis, & Thümmler, Citation1981; Schlieper & Thümmler, Citation1979; Zapf & Dalal, Citation1977), since the chemical activity of these elements, and thus their reactivity to oxygen, is significantly lower than in case of admixing elemental powders. Furthermore, the composition can be tailored such to result in transient liquid phase during sintering, thus accelerating homogenization. It also suggests some more benefits such as better compressibility, more flexibility in the selection of the final composition of the steel and the potential reduction of the overall cost compared to the prealloyed systems. Today, the earlier problems of high die wear and low output of sufficiently fine fractions have been overcome by advanced powder production techniques (De Oro Calderón et al., Citation2016). In Danninger (Citation2021) and Danninger et al. (Citation2021) it was shown that introducing Mn not as elemental powder but through suitable masteralloys results in lower final oxygen content and also significantly less Mn evaporation (Šalak & Selecká, Citation2012).
In the present study, hybrid alloyed sintered steels – combining prealloying and mixing (Geroldinger, Oro Calderon, Gierl-Mayer, & Danninger, Citation2021) – were prepared based on Cr prealloyed steel powder, and elemental Mn and Ni, respectively, were admixed as well as a masteralloy containing Mn and Si, which combination has been shown to yield interesting properties (Klein, Oberacker, & Thümmler, Citation1985). In the first series of experiments, steel grades with varying contents of alloy elements, but identical nominal carbon, were prepared and investigated in the as sintered state, in order to assess the potential for optimum mechanical properties and also for sinter hardening. In a second series, the monotonic and cyclic properties were investigated in the sinter hardened state. Since for many applications, fatigue loading up to very high cycle numbers is common, ultrasonic resonance fatigue testing was applied to Nmax = 10E10 cycles.
2. Experimental procedure
The base powder chosen for this study was Astaloy CrA, which is a prealloyed steel powder containing 1.8% Cr (supplied by Höganäs AB, Sweden). The elemental Ni powder was carbonyl grade (Ni 287) and the elemental Mn was electrolytic grade supplied by Poudmet. The masteralloy used in this research was the experimental grade H46 produced by Ultra High-Pressure Water Atomization (supplied by Atomising Systems Ltd., UK, chemical composition: see ). Carbon was introduced as natural graphite (grade UF4, Kropfmühl) to a nominal level of 0.6%; for the masteralloy-containing variant some C was also contributed by the MA, see . All the metallic additives were sieved to the size < 20 µm and then were added to the mixtures. The powders were mixed in a Turbula mixer for 60 min; mixing was done all in one step, together with graphite. The composition and designation of the mixtures used for test series 1 and 2 are presented in and .
Table 1. Chemical composition of the masteralloy used (wt %).
Table 2. Composition of the mixtures used in study 1.
Table 3. Designation and composition of the mixtures used in study 2.
The steel powder mixes were compacted uniaxially at 700 MPa to bars with the dimensions 55 × 10 × 10 mm for series 1 (Charpy bars ISO 5754) and to bars 90 × 12 × 12 mm for series 2, both in tools with floating die. Die wall lubrication was afforded using Multical sizing fluid as lubricant. Sintering of the samples was performed in an SiC rod heated electrical laboratory furnace equipped with a gas-tight Kanthal APM superalloy tube muffle. The sintering runs were done at 1250 °C for 1 hr (isothermal sintering, push in-push out method) in N2-10% H2 for 60 min, cooling being subsequently done in the water-jacketed exit zone of the furnace, the linearized cooling rate being approx. 0.7 K/s. For heat treatment of the larger specimens (simulating the sinter hardening process) the samples were reheated at 1000 °C in N2 for 30 min and then gas quenched with pressurized nitrogen at a cooling rate (linearized) of 3 K/s. Heat treatment of the samples was completed by tempering at 150 °C for 1 hr. For the fatigue test specimens, heat treatment was performed after machining and polishing the fatigue test bars, also to remove any compressive residual stresses, which are particularly crucial with heat treated steels, more than with as sintered ones, see (Sohar, Betzwar-Kotas, Gierl, Weiss, & Danninger, Citation2008; Williams, Deng, & Chawla, Citation2007).
The as-sintered ISO Charpy bars (55 × 10 × 10 mm) and the heat treated sintered bars (90 × 12 × 12 mm) were characterized by standard techniques. Green density was calculated from mass and dimensions, and the sintered density was determined by water displacement after impregnation. Charpy impact tests (unnotched) and measuring the dynamic Young’s Modulus of the samples were performed on the rectangular bars using a resonance test method in accordance with ASTM E1876 were done. (please note that for series 2, the cross section was larger than given in ISO 5754 for standard impact test bars). Cross sections of the samples were examined by metallography, and hardness measurements were done in the sections. The oxygen content was measured through hot fusion analysis using a LECO TC 400 analyzer and the combined carbon content through combustion analysis in a LECO CS-230 device.
In order to investigate the fatigue behaviour of the PM steels, dumbbell-shaped specimens were machined from the sintered bars (). Before shaping (by turning) the sintered bars were soft annealed at 650 °C for 1 hr. After turning and surface finishing, which included grinding and polishing (finally with 2400 mesh grinding paper), the fatigue specimens were heat treated as described above. Ultrasonic fatigue testing was performed in push-pull mode (R= −1) at 20 kHz in a resonance testing system. The equipment of the ultrasonic fatigue testing is schematically shown in ; details can be found e.g., in Danninger, Xu, Khatibi, Weiss, and Lindqvist (Citation2012) and Danninger and Weiss (Citation2001).
Figure 1. Final geometry (machined and polished) of the fatigue samples for Astaloy CrA + 0.6%C + 3%Ni sinter hardened at 1250 °C.
Figure 2. Equipment for ultrasonic fatigue testing.
Figure 3. Hardness, impact energy and oxygen content of PM steels with 0.6% admixed carbon, compacted at 700 MPa, sintered 60 min at 1250 °C in N2-10% H2.
3. Results and discussion
3.1. Sintered steels with varying alloy element content (as-sintered)
In this part of the work, the effect of varying contents of alloy elements Ni and Mn as well as of the Mn-Si masteralloy H46 were investigated. For Ni and Mn the concentration range 1–3% was chosen, for MA, higher additive contents of 2–4% were selected, to compensate for the lower alloy element content in the MA (4.0%MA yield the same Mn content as 1.6% elemental Mn). The as-sintered properties are shown in ; for hardness, impact energy and the oxygen content, also graphic depictions are shown to better illustrate the results (). For comparison, also plain Cr steel, without further additives, was investigated.
Table 4. As-sintered properties of PM steels AstaloyCrA-x-0.6%C (x = Ni, Mn, MA in varying contents). Compacted at 700 MPa, sintered 1 hr at 1250 °C in N2-10%H2.
The green density values do not differ too much. Ni addition slightly increases the green density while both Mn and MA decrease it, but the differences are marginal. For the sintered density, in contrast, quite pronounced differences are observed: Ni increases the density, as also evident from the marked shrinkage, i.e., the well-known effect of Ni to enhance shrinkage (see e.g Hausner & Mal, Citation1982; Engström, Citation1986) is thus evident also here. Mn, in contrast, promotes expansion during sintering and thus lowers the sintered density, also this effect, “Mn swelling”, being well known (Danninger et al., Citation2005; Danninger & Gierl, Citation2001). It is a consequence of the unique homogenization of Mn during sintering through gas phase transport, which is a “one-way” process, the vapour pressure of Mn being several orders of magnitude higher than than of Fe. Thus, Mn is transported to Fe and alloyed there, but the reverse is not possible. The masteralloy does not show this effect since here, homogenization is primarily attained through transient liquid phase, the vapour pressure of Mn above the masteralloy being significantly lower than above elemental Mn. In all cases, however, the dimensional changes are small, <1% linear, and thus can be expected to be uncritical regarding dimensional precision of the sintered parts.
The dynamic Young’s modulus is correlated well to the sintered density, as described e.g., in (Azadbeh, Danninger, & Gierl, Citation2006). For the Ni alloyed steels, it is slightly lower than expected from the density, which can be attributed to the presence of Ni-rich austenitic areas, as a consequence of the slow homogenization of Ni in a ferrous matrix (Garcia, Sainz, & Castro, Citation2010). The hardness values show very pronounced differences as a consequence of alloying type and content: while the plain Cr steel, the reference material, is relatively soft as sintered, with about 170 HV30, Ni addition results in increased hardness. At 1 and 2%Ni the effect is not yet too pronounced, only at 3%Ni a real sinter hardening effect being observed, with about 360 HV30. Mn, in contrast is much more effective; here, at 2% and 3% hardness levels of around 400 HV30 are attained, with agrees with the much higher Grossmann factor of Mn compared to Ni (e.g., Krauss, Citation2005; Salak, Citation1995). In case of the masteralloy, sinter hardening is attained at 4% but not at 3%, which agrees with the results obtained with the Mn steels: about 1%Mn is still too low for sinter hardening, but 2% or, in case of 4%MA, 1.6% are sufficient.
These results agree well with the as-sintered microstructures revealed by metallography (): here the plain Cr steel (CrA-0.6C) shows a pearlitic microstructure. In case of 3%Ni, the fraction of martensite phase is increased significantly compared to 2%Ni and a heterogeneous bainitic-martensitic microstructure is observed with numerous pearlitic as well as austenitic islands, indicating the slow Ni diffusion into the cores of the larger base powder particles. The steel containing 2 and 3%Mn is almost fully martensitic, with only a few pearlitic areas, while in case of 1%Mn the microstructure is mostly pearlitic. The 4%MA steel shows very fine martensite with only some pearlite. Comparing with 3%MA, this indicates that this composition is just at the threshold for sinter hardening under the cooling conditions in the sintering furnace; it could however be expected to become fully martensitic if sinter hardening – i.e., gas quenching – would be applied.
Figure 4. Metallographic sections of PM steels with 0.6% admixed carbon, compacted at 700 MPa, sintered 60 min at 1250 °C in N2-10% H2; as-sintered, Nital etched.
The impact energy data show mostly an inverse image to the hardness. The fairly ductile plain Cr steel (CrA-0.6C) stands out here, with almost 40 J.cm−2, as do the steels with 1 and 2%Ni. However, also the grade with 3%Ni still shows an impact energy >30 J.cm−2, despite its high hardness. This illustrates the well-known positive effect of Ni on the toughness, as described e.g., by F. V. Lenel (Citation1980). The well hardenable Mn steels, in contrast, show a poor performance here, only the 1%Mn steel exceeding 20 J.cm−2, while the sinter hardenable grades with 2 and 3%Mn attain just 12 and 5 J.cm−2, respectively. The masteralloy grade is markedly better here, even the highly hardenable grade with 4%MA still exceeding 20 J.cm−2. This once more shows the high relevance of the alloying route in particular for the introduction of Mn into sintered steels.
The reason for these differences can be deduced from fractographic images (see ): the impact fracture surfaces of the plain Cr steel (CrA-0.6C) exhibit ductile fracture with relatively coarse dimples, which agrees with the high impact energy. The 3%Ni steel shows predominantly ductile fracture, too, with a few cleavage facets, and the fracture surface of the masteralloy grade looks similar, but with more and larger cleavage facets. In the 3%Mn steel, in contrast, intergranular fracture dominates, which explains the low impact energy values. At 2%Mn the proportion of this failure mode is lower, and at 1%Mn ductile and cleavage fracture are observed, which shows the huge effect of the Mn content. The formation of intergranular fracture in sintered Mn steels has been described in the literature for Fe-Mn-C (Danninger et al., Citation2005; Hryha, Citation2007; Hryha, Nyborg, Dudrova, & Bengtsson, Citation2010), in particular when using elemental Mn. E. Hryha (Citation2007) explains it by the interaction of Mn vapour with the oxygen covering the base powder particles: Mn vapour reacts with the oxygen-bearing surface of the base powder particles, i.e., an oxygen transfer from iron (or iron-chromium) oxides to Mn- or at least Mn-containing oxides occurs. It is not surprising that this effect is still more pronounced with steels from Cr prealloyed base powders than for such from plain Fe since the oxygen content of the base powders is both higher and more stable, Cr oxides being more difficult to reduce than Fe oxides. Therefore, they cannot be reduced by H2 at low temperatures but remain stable up to temperatures at which Mn vapour is formed that can reduce the Cr oxides manganothermically, forming layers of Mn base oxides at the grain boundaries. This effect can be regarded as another variant of the “internal getter” effect, to some degree a combination of both variants of this phenomenon described in Gierl-Mayer, Calderon, and Danninger (Citation2016).
Figure 5. Impact fracture surfaces of PM steels with 0.6% admixed carbon, compacted at 700 MPa, sintered 60 min at 1250 °C in N2-10% H2.
3.2. Sinter hardened steels – Fatigue tests
Based on the results described above, 3 compositions were selected for further investigations, the additive contents being 2%Mn, 3%Ni and 4%masteralloy, respectively: In this series, all specimens were gas quenched and tempered, equivalent to the conditions for industrial sinter hardening.
The as-heat treated properties of the steels are given in . The results indicate that also here, alloying the steel with Ni resulted in the highest values of dimensional change (−0.67%) which means higher shrinkage and sintered density. This resulted in the highest value for the Young’s modulus as well. In contrast to Ni, the Mn alloyed steel once more showed swelling, with positive values of 0.05% for the dimensional change, which as described above is the consequence of a “one-way” homogenization of Mn through the gas phase and agrees well with previous studies which showed the expanding effect of this alloying element on PM steels (Danninger et al., Citation2005; Danninger & Gierl, Citation2001). Formation of the larger pores in the Mn steel compared to two other grades, as presented in , confirms these results as well. This steel has the lowest Young’s modulus compared to the other grades, as reported in , which once more confirms the close relationship between Young’s modulus and density (Azadbeh et al., Citation2006). The results also show that introducing Mn through the masteralloy route did not lead to such swelling, which consequently resulted in higher sintered density and Young’s modulus of the MA steel compared to the Mn steel. Also the impact energy shows a clear correlation with the density; here, however, also the positive effect of Ni on the toughness must be considered, as described e.g., by F. V. Lenel (Citation1980).
Figure 6. As-polished OM micrographs of hybrid alloyed steels; the larger pores in Mn steel compared to the two other grades are clearly visible.
Table 5. Sintered properties of hybrid alloyed steels; compacted at 700 MPa, sintered 60 min at 1250 °C in N2-10%H2, heat treated by gas quenching and tempering
The fairly high apparent hardness values – above 450 HV30, see – for all the steels indicate the potential of these hybrid alloyed steels for sinter hardening, which agrees with their martensitic microstructure as presented in . Another information which could be obtained from the microstructures is the homogeneity of the phases. The metallographic study of these steels showed that in contrast to the Mn and MA steel – with mainly martensitic, locally bainitic microstructure – in the microstructure of the Ni alloyed steel in addition to these phases some soft Ni-rich austenitic regions can be found. This can be attributed to powder agglomeration and also to the slow diffusion rate of Ni into the iron lattice (see ). When comparing the impact energy data with those obtained after sintering (see ) it stands out clearly that not only the hardness but also the impact energy is improved by sinter hardening for all materials. Compared to the as sintered state, in which the hardness ranges between 360 and 400 HV, after sinter hardening it is >450 HV in all cases, and the impact energy is increased by about 6 J.cm−2 for all investigated materials. This improvement of both strength and impact energy clearly underlines the positive effect of the sinter hardening treatment.
Figure 7. Nital etched OM micrographs of different steels show martensitic matrix after sinter hardening.
Figure 8. Soft Ni-rich austenitic area in the microstructure of CrA-3Ni-0.6C.
Fracture surfaces of the steels after Charpy impact testing are presented in with the corresponding data for impact energy and oxygen content. As can be seen, in the Mn alloyed steel, despite the lower oxygen content compared to the MA steel, weaker sintering contacts are visible, and as also observed in the as sintered state, the fracture is mostly intergranular, which led to the lowest value for the impact energy (19 J.cm−2), which is however still markedly higher than as sintered. In the fracture surface of the two other steels (Ni and MA), transgranular cleavage facets – which means stronger sintering bonds – are dominant. In these two grades, in addition to the cleavage facets, some ductile dimples are also discernible. The fraction of dimple areas in the Ni alloyed steel is much higher than in the MA steel, which led to the highest impact energy of 39 J.cm−2. However, also the MA alloyed variant is an attractive solution with sound interparticle bonding that results in an impressive combination of hardness and toughness.
Figure 9. Impact fracture surfaces of different steels, sinter hardened (broken at RT).
The S/N graphs of the steels as obtained by ultrasonic resonance testing are plotted in . First of all, it is evident that the graphs drop consistently up to the gigacycle range, i.e., there is no “fatigue limit” up to 10E10 cycles, which agrees e.g., with the statement of C. M. Sonsino (Citation2005). Comparing the fatigue behaviour of the steels containing different alloying elements it stands out clearly that addition of Ni results in the highest fatigue strength data below 10E8 cycles, while at N > 10E8 the fatigue endurance is similar to the MA alloyed steel. The good fatigue behaviour of the Ni alloyed steel at N < 10E8 could be explained by its higher density and its stronger sintering necks. However, it seems that at higher loading cycles (N > 10E8) the Ni alloyed grade, which showed to be quite promising in the high amplitude (lower N) range, does not retain this advantage into the gigacycle range and also presents a very large scatter of the values. It has already been shown that high strength/low ductility metallic materials are sensitive to singular defects in case of high cycle fatigue (Danninger, Spoljaric, & Weiss, Citation1997; Danninger & Weiss, Citation2003), therefore in this case as well, the larger austenitic areas (with low hardness) as shown in might well cause crack initiation. The figure also shows that the steel alloyed with 2% Mn has the lowest fatigue strength, however it showed the same trend as the MA alloyed steel and a parallel curve. In this case, the lower density of the Mn alloyed steel and in particular the lower grain boundary cohesion can be supposed to be the reasons for this low fatigue strength.
Figure 10. S/N curves of different steels, compacted at 700 MPa, sintered 60 min in N2-10% H2 at 1250 °C, gas quenched and tempered.
A point that should not be neglected here is the role of the secondary pores formed in the Mn alloyed PM steels due to Mn evaporation; also these might lower the fatigue strength. This assumption is supported by previous studies with Mo steels containing secondary pores (e.g., Danninger, Citation1996). As mentioned before, the MA steel in this series showed the same trend and slope as the Mn alloyed steel, however the S/N curves shows that the fatigue strength of this steel is approximately 60 MPa higher than of the steels alloyed with Mn, which could be explained by higher density as well as stronger sintering contacts.
Fractographic studies () showed that there is hardly any difference between the steel grades at and near the crack initiation sites. In all cases, larger pores or pore clusters can be identified as starting features, and the surfaces around the initiation sites are very smooth, almost like metallographic sections, which is a consequence of the extremely low initial crack growth rate da/dN in case of gigacycle fatigue testing. The final fracture areas, in contrast, show marked differences, similar to the impact fracture surfaces, since the generation, by one loading event, is similar, i.e., mostly intergranular fracture for the Mn steel, ductile rupture for the Ni alloyed grade and mixed fracture for the masteralloy type. There is even a slightly higher tendency to intergranular fracture compared to impact fracture, not only in the Mn steel, but some localized areas are found also in the masteralloy variant, which however do not affect the lifetime of the specimens, in particular in the high N range where the MA variant performs surprisingly well.
Figure 11. Fatigue fracture surfaces of hybrid alloyed sintered steels, compacted at 700 MPa, sintered 60 min in N2-10% H2 at 1250 °C, gas quenched and tempered.
4. Conclusions
Hybrid alloyed steels based on Cr prealloyed base powder and admixed with different alloy powders were produced by pressing, sintering and in part heat treatment and were characterized regarding the dimensional and mechanical properties as well as the hardening behaviour both during cooling after sintering and by gas quenching, simulating industrial sinter hardening.
Dimensional change and sintered density showed that the addition of manganese to the alloy causes swelling during sintering and thus decreases the sintered density, while in the other two steels there were negative dimensional changes that led to an increase of the density after sintering. This increase of the density was more pronounced in the Ni steel than in the MA steel. Pore morphology showed larger pores in the Mn steel compared to the other two steels, which agreed with the dimensional behaviour of the steel. In all cases however the dimensional changes were sufficiently small to avoid precision problems.
In the as sintered state, hardening during cooling was found to be promoted most by addition of Mn; Ni and the Mn-Si masteralloy required higher contents to attain martensitic microstructures, at least 3 and 4 wt%, respectively, compared to 2 wt% for Mn. On the other hand, the impact toughness was best for the Ni steel, and also the MA alloys steel proved to be sufficient, while Mn addition lowered the impact energy the more, the higher the Mn content was – and thus the hardness –; the reason was increasing tendency to intergranular fracture, attributed to Mn oxide films at the grain boundaries.
For the intentionally sinter hardened, i.e., gas quenched and tempered, state, metallography and hardness testing showed martensitic microstructure and hardness above 450 HV30 for all the steels, which means good sinter hardenability of these materials. Also in this state, impact testing and fractography showed stronger sintering necks and higher impact energy in the Ni steel than in the other two steels; in that respect, the MA steel was superior to the Mn alloyed grade. However, in terms of microstructure the nickel steel showed less homogeneity, with some austenitic areas of nickel enrichment, attributable to the lower diffusion coefficient of nickel in iron and homogenization entirely by solid state diffusion. In general, the sinter hardened steels showed higher hardness and in particular higher impact energy values compared to the respective as sintered materials, which underlines the positive effect of defined sinter hardening treatment.
Ni alloying proved to be positive for the gigacycle fatigue strength while Mn was less effective, in part because of intergranular embrittlement. The Mn-Si masteralloy, which showed better impact behaviour compared to Mn steels, indicates better cyclic properties as well, and in particular at cycle numbers above 10E8 the fatigue endurance strength was similar to that of the Ni alloyed type. In all cases the S/N graphs dropped consistently up to 10E10 cycles, i.e., a true “fatigue limit” was not observed with any of the materials.
Therefore, for fatigue loaded PM precision components the Ni alloyed hybrid steel looks attractive, however with the problems associated with Ni, as described above. Mn alloying, at least through elemental powder, is less recommendable, while the masteralloy type seems to be an interesting alternative that competes well with the Ni alloyed steel, but without the disadvantages linked to Ni.
Acknowledgement
The authors wish to thank Atomising Systems Ltd., Sheffield, UK, for producing and supplying the masteralloy powder used.
Disclosure statement
No potential conflict of interest was reported by the authors.
Additional information
Notes on contributors
Milad Hojati
Milad Hojati is currently a postdoctoral researcher at Technische Universität Wien, Vienna, Austria. He did his PhD with the title of “Manufacturing and characterization of sintered steels with improved mechanical and functional properties” under supervision of Professor Herbert Danninger. He also holds a bachelor in Metallurgy Engineering (2010) and a master in Materials Engineering - Corrosion and Protection of Materials (2013), both from Ferdowsi University of Mashhad, Iran. He also worked as R&D manager in Mashhad Powder Metallurgy Company (2010-2018).
Christian Gierl-Mayer
Herbert Danninger is retired professor for Chemical Technology of Inorganic Materials at Technische Universität Wien, Vienna, Austria. From 2011 to 2019 he was Dean of the Faculty of Technical Chemistry. He has been active in powder metallurgy research and education for more than 40 years and is author/co-author of 550+ publications as well as several books and book chapters. From 2009 to 2020 he was also chairman of the “Gemeinschaftsausschuss Pulvermetallurgie”, the PM association of the German-speaking countries He holds honorary doctoral degrees of Technical University Cluj-Napoca (Romania), Universidad Carlos III de Madrid (Spain) and Universitatea din Craiova (Romania) and is Fellow of APMI and EPMA. In 2020 he was awarded the “Ivor Jenkins Medal” of IOM3 (London, UK) and in 2022 the “Richard Zsigmondy Medal” of TU Wien.
Herbert Danninger
Christian Gierl-Mayer studied Technical Chemistry at TU Wien, got his Master in 1996 and his PhD in 2000 from TU Wien. After 3 years in private research institute (ofi-Austrian Research Institute of Chemistry and Technology) he re-joined the powder metallurgy group of Prof. Herbert Danninger as senior researcher. He got his habilitation in 2019 for “Thermoanalytical Investigation of Interactions between Powder Metallurgy Steels and the Atmosphere during Sintering”, and became Associate Professor in 2019 and full Professor for “Chemical Technology of Metals” 2022. He is currently leading the research group Powder Metallurgy at TU Wien and the research unit Chemical Technologies, Institute of Chemical Technologies and Analytics. His publication record is about 260 publications in journals and conference proceedings, 4 book chapters and 7 patents.
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