The present work studies the sintering and hardening response of the prealloyed steel powder Anchorsteel® 737 SH with added copper and carbon. Referred to as SH737 throughout, the finding of particular interest is the energy-saving process of sinter hardening.

In recent years, powder metallurgical (P/M) components are increasingly being utilized for automotive and structural applications.[1] As compared to conventional casting techniques, P/M processing offers advantages such as lower processing temperature, near-net shaping, high final density, greater material utilization (>95%) and a more refined microstructure that provides superior material properties.[2] In addition, P/M products have greater microstructural homogeneity. The significant advances in powder-production technology, new alloy design with novel properties, compaction and developments in sintering-furnace technologies boost the growth of powder metallurgy.

Table 1. Sintered densities (both % theoretical and in g/cm) and densification parameter of SH737-2Cu-0.9C at different sintering temperatures – 1120,1180 and 1250°C

Alloying Elements in P/M Materials

The main thrust toward higher performance in P/M alloys has been achieved by introducing alloying additions such as Mo, Ni, Mn and Cu. Most alloying additions enhance the strength through solid solution hardening during sintering. In addition, these alloying elements also enhance the hardenability by shifting the continuous cooling transformation curve to the right. Subsequent heat treatment results in enhancing the mechanical properties of ferrous alloys.[6-10] In addition, alloying can improve oxidation or corrosion resistance. Some applications rely on alloying to secure special magnetic properties or high-temperature strength.

The alloying methods that are used for production of ferrous P/M parts can be divided into three groups:
  • Admixture of elements to a plain iron powder
  • Diffusion-bonded or partially prealloyed powders
  • Completely prealloyed powders
Elementally admixed materials heavily suffered from segregation problem, whereas prealloyed iron powder is effective but significantly decreases the compressibility of the powders. In conventional P/M processing, “diffusion-alloyed” powder has typically been used. This process involves heat treatment of iron powder and alloying elements in reducing atmosphere, which allows partial diffusion and metallurgical-bond formation prior to pressing and sintering. Due to partial diffusion, these powders have higher compressibility and fewer tendencies for small alloying elements to agglomerate for better homogeneity. The influence of chemical and microstructural homogeneity on the mechanical properties of sintered material has been studied by a number of authors.[3-5]

A common alloying metal in powder metallurgy is copper, which is not sensitive to oxidation and causes sufficient increase in strength. A great number of sintered components are made for automotive applications from mixing of copper and carbon with prealloyed iron. This material is sintered with transient liquid phase when copper content is less than 8%.[6] Hence, formation of secondary pores at the site of original copper particles is an inevitable consequence of transient liquid-phase sintering.

This study focuses the investigation of one such alloy SH737 (designated), which has nominal composition of Fe-1.25 Mo-1.4 Ni-0.42 Mn (wt%). The composition has been tailored with a view to alter the CCT-curve characteristics in such a manner that during post-sintering cooling itself the sintered compacts undergo transformation in the bainitic/martensitic region. Such grades of powder are also referred to as sinter hardening. As the name suggests, sinter hardening achieves sinter and hardening in the single step.

Table 2. Densification behavior, dimensional change and mechanical property response for furnace-cooled and sinter-hardened SH737-2Cu-0.9C

Experimental Procedure

For the present investigation, partially prealloyed powder mixture of SH737 admixed with 2% Cu and 0.9% C (graphite) – a proprietary process developed by Hoeganaes Corp. – was used as starting material.

Powders were pressed at 600 MPa in a 50-ton uniaxial hydraulic press, and densities of compacted specimens were between 6.99 and 7.02 g/cm3. To minimize friction, the compaction was carried out using zinc stearate as a die-wall lubricant. The powder contains 0.75% acrawax, which was added to facilitate its compaction during sintering. The sintering response on densification and microstructures were evaluated on cylindrical pellets (16 mm dia. x 6 mm high). The green compacts were dewaxed in a tubular SiC furnace under N2-20H2 atmosphere. All the green samples were delubed at 850°C (1560°F) for 30 minutes.

To prevent cracking of green compacts by thermal shock, they were heated at 3°C/min. Then the compacts were sintered at three different temperatures – 1120, 1180 and 1250°C – for 30 minutes in a tube furnace with a SiC heating element. Delubing was not carried out for samples processed in the sinter-hardening furnace. Samples were sintered at 1120°C (2050°F) for 30 minutes before cooling rapidly (2.0°C/s) in the sinter-hardening furnace. For all sintering processes, a mixture of nitrogen and hydrogen in the ratio 85N2-15H2 was used as the protective atmosphere.

The sintered density was obtained by dimensional measurements. The densification parameter was calculated to quantify the densification that occurred during sintering. It is expressed as:

Densification parameter = (sintered density - green density)/(theoretical density - green density)

The samples sintered conventionally at different temperatures were heat treated at 900°C (1650°F) for one hour and subsequently cooled by four different methods – furnace cooling (annealing), air cooling (normalizing) and oil and brine quenching. The samples were polished to mirror finish, ultrasonically cleaned and etched in 3% Nital. Bulk hardness of the samples was measured by a Vickers hardness tester at 10 kgf load. The observed hardness values are the averages of five readings taken at random spots throughout the sample.

The oil- and brine-quenched samples sintered at different temperatures were tempered at four different temperatures – 200, 400, 600 and 700°C for one hour. The microstructural analyses of the samples were carried out using an optical microscope.

Fig. 1. Hardness of SH737-2Cu-0.9C samples sintered at (a) 1120°C, (b) 1180°C and (c) 1250°C and cooled by different methods

Results and Discussion

Steels can be assessed in terms of the carbon equivalent (CE), which scales the concentration of each element by its ability to retard the austenite/martensite transformation. It is also used to express the hardenability of alloy steels in terms of equivalent plain-carbon steel. It is calculated as follows.

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

For SH737 with a composition of:

94.03%Fe + 1.25%Mo + 1.4%Ni + 0.42%Mn + 2%Cu + 0.9%C (% by weight)

CE = 0.9 + (0.42/6) + (0+1.25+0)/5 + (1.4+2)/15 = 1.38.

The calculated CE value implies extreme complications. Weld cracking is very likely, and, therefore, preheat in the range 100-400°C and low-hydrogen electrodes are required.

Table 1 shows the effect of sintering temperature on the densification response of SH737 alloys. It is clear that the sintered density improves ~0.5% with an increase in sintering temperature from 1120 to 1250°C (2050-2280°F). To take into account the effect of composition, all the sintered densities were normalized with respect to the theoretical densities. The sintered sample contains nearly 11-12% porosity. It can be inferred that the temperature has a marginal affect on the densification of these alloys. In order to account for the effect of green density on the sintered density, the densification response was qualified in terms of densification parameters. Variation of densification parameter with temperature is shown in Table 1. The densification parameter is negative for the sintering temperatures 1120°C and 1180°C (2156°F), thus confirming compact swelling, and is positive for 1250°C. It is clear from the figure that the densification parameter marginally increases with the sintering temperature.

Fig. 2. Effect of tempering temperatures on hardness of (a) oil quenched and (b) brine quenched SH737-2Cu-0.9C samples sintered at different temperatures

Densification behavior, dimensional change and mechanical-property response for furnace-cooled samples and sinter-hardened samples are shown in Table 2. Sintered density of furnace-cooled samples is higher as compared to sinter-hardened samples. This is because sinter-hardened samples, cooled at 2.2°C/s, are transformed into bainite and martensite. Furnace-cooled samples transform into pearlite, which is dense as compared to martensite. Variation in dimensional change in sinter-hardened samples is lower than in furnace-cooled samples. Because sinter hardening transforms austenite into hard phases like bainite and martensite, hardness for sinter-hardened samples was found to be significantly higher than hardness of furnace-cooled samples. Tensile properties for rapidly cooled samples were found to be higher than for furnace-cooled samples.

From Fig. 1, it is quite evident that furnace-cooled samples have the lowest hardness regardless of sintering temperature. The trend of hardness is as follows:

Furnace cooled<Air cooled<Oil quenched<Brine quenched

This can be attributed to the formation of martensite and bainite for higher rates of cooling (as in oil and brine quenching).


Fig. 3. Optical micrograph of furnace-cooled material

Figure 1 also shows the effect of sintering temperature on the hardness for the SH737 alloy samples cooled by various methods. It is clear from the graph that bulk hardness improves with increasing sintering temperature from 1120 to 1250°C. This can be attributed to the sintered density as there was a marginal densification achieved with increasing sintering temperature. Higher densification implies lower porosity, hence better heat transfer leading to high hardness. A higher sintering temperature further enhances the microstructural homogeneity due to interdiffusion of alloying elements. This also contributes to the enhancement of the bulk hardness. Bulk hardness also increases with the temperature by forming bainite at a higher sintering temperature due to comparatively faster cooling.

From Fig. 2, we infer that with increasing tempering temperature the hardness first increases, reaches a maximum around 600°C (1110°F) and then decreases. This is due to secondary hardening taking place in the quenched samples. Secondary hardening is taking place in the samples due to the presence of sufficient amounts of carbide formers like Mn and Mo. Above 500°C (930°F), these elements have high diffusivity to nucleate and grow to form a fine dispersion of alloy carbides, causing secondary hardening. Secondary hardening is a process similar to age hardening, in which coarse cementite particles are replaced by new and much finer alloy carbide dispersion of Mo2C and Mn2C. The critical dispersion causes a peak in the hardness. But as the carbide dispersion slowly coarsens with further increase in tempering temperature, hardness decreases rapidly.

Fig. 4. Optical microstructures of SH737-2Cu-0.9C cooled at different cooling rates after sintering at 1120°C for 30 min. – (a) 1.0°C/s, (b) 1.5°C/s and (c) 2.0°C/s

An optical micrograph representing material sintered at 1120°C (2050°F) for 30 minutes and furnace cooled is shown in Fig. 3. Martensite and bainite are clearly visible in sinter-hardened samples (cooled at 2.0°C/s), whereas pearlite is predominantly seen in furnace-cooled samples. The effect of different cooling rates is seen in Fig. 4. Bainite dominates over martensite when cooling rate is reduced to 1.5°C/s. Hard phases are not found on further reduction in cooling rate to 1°C/s, and only pearlite is found in these samples. Presence of martensite and bainite impart significantly higher hardness to samples cooled at 2.2°C/s. Figure 5 shows optical micrographs for tempered samples after brine and oil quenching.

Advancements in sintering furnaces for rapid cooling provide great flexibility to tailor microstructure as required for specific applications.

Fig. 5. Optical micrographs for brine-quenched and oil-quenched samples after tempering

Acknowledgment

The authors gratefully acknowledge the technical feedback from Dr. K.S. Narasimhan and his colleagues from Hoeganaes Corporation, USA.

For more information: Contact the corresponding author, A. Udadhyaya, at the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, INDIA; tel: 91-512-2597672; fax: 2597505; e-Mail: anishu@iitk.ac.in

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: Sintered SH737-Cu-C Steel, heat treatment, transient liquid phase sintering, quenching, tempering, secondary hardness

ABSTRACT

The present study examines the change in hardness of SH737-2Cu-0.9C sintered at different temperatures, heat treated by various methods and then tempered at different temperatures. The samples were transient liquid-phase sintered at 1120, 1180 and 1250°C. The sintered samples were then characterized for density and densification parameter. The samples were austenitized at 900°C and cooled by four different methods – furnace cooling (annealing), air cooling (normalizing) and oil and brine quenching. The samples were then hardness tested using Vickers hardness at 10 kgf load. The hardness trend observed was minimum for air-cooled and maximum for brine-quenched samples. For the case of the sample sintered at 1250°C, relatively higher hardness was observed. Sinter hardening was done in a specially designed furnace with provision to cool sintered samples at cooling rate 2.0°C/s after holding samples at 1120°C for 30 minutes.

Densification response and hardness behavior of sinter-hardened samples was compared with heat-treated samples, and it was found that hardness obtained after sinter hardening is comparable with that obtained by oil quenching or brine quenching. The oil- and brine-quenched samples were then tempered at 200, 400, 600 and 700°C. The hardness pattern observed typically showed secondary hardness taking place (due to presence of Mn and Mo), reaching the maximum around 600°C.