Indian Institute of Technology, Department of Materials and Metallurgical Engineering
Kanpur, India 208016
The present study examines the change in hardness of sintered hardened steel (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℃ respectively. The sintered samples were characterized then for density and densification parameter. The samples were austenitized at 900℃ and cooled by four different methods viz. furnace cooling (annealing), air cooling (normalizing), oil quenching, and brine quenching. The samples were then tested for their hardness using Vickers's hardness at 10 kgf load. The trend of hardness observed was found minimum for air cooled and maximum for brine quenched. In case of sample sintered at 1250℃, relatively higher hardness was observed. The oil and brine quenched samples were then tempered at 200℃, 400℃, 600℃ and 700℃. The hardness pattern observed typically showed secondary hardness taking place (due to presence of Mn and Mo) and reaching the maximum around 600℃.
Keywords: Sintered SH737-Cu-C steel, heat treatment, transient liquid phase sintering, quenching, tempering, secondary hardness
In recent years, powder metallurgical (P/M) stainless steel components are increasingly being utilized for automotive and structural applications. 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. In addition P/M products have greater microstructural homogeneity. The significant advances in powder production technology, new alloy design with novel properties, compaction and sintering furnace technologies boost up the growth of powder metallurgy.
|Table 1. Qualitative ranking of alloying elements in prealloyed materials||Table 2. Composition of the powder|
The main thrust towards higher performance in powder metallurgy alloys has been achieved by introducing alloying additions such as Mo, Ni, Mn, and Cu. Most of the 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 toughness of ferrous alloys. In addition, alloying can improve oxidation or corrosion resistance. Some applications rely on alloying to secure special magnetic properties or high temperature strength. Table 1 qualitatively shows that the alloys that are efficient in improving hardening tend to reduce the compressibility and high affinity for oxygen.
The alloying methods used for the production of ferrous P/M parts are divided into three groups: admixture of elements to a plain iron powder, diffusion bonded or partially prealloyed powders and completely prealloyed powders. Elementally admixed materials suffer heavily from segregation problems. Prealloyed iron powder though effective against segregation, significantly decreases the compressibility of the powders. In conventional powder metallurgy processing, "diffusion alloyed" powders have typically been used. This process involves heat treatment of iron powder and alloying elements in a reducing atmosphere, allowing 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, hence 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. Several sintered components are made for automotive applications by mixing copper and carbon with prealloyed iron. This material is sintered with a transient liquid phase, when copper content is less than 8%.[6-9] Formation of secondary pores at the site of original Cu particles is an inevitable consequence of transient liquid phase sintering.
This study focuses on the investigation of one such alloy, sintered hardened grade steel SH737 (designated), which has a 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, the sintered compact undergoes a transformation in the bainitic/martensitic region. This grade of powder has also been referred to as a sinter-hardening grade. As the name suggests, sinter-hardening achieves sinter and hardening in a single step.
2. Experimental Procedure
For the present investigation, a partially prealloyed powder mixture (Fe, 1.4 wt% Ni, 1.25 wt% Mo, 0.42 wt% Mn, 2 wt% Cu, and 0.9 wt% graphite), produced using a proprietary process developed by Hoeganaes Corp, was used as a starting material. [10,11] The as-received powder was characterized for its flow behavior as well as apparent and tap density using set MPIF standards. The results are shown in Tables 2 and 3 and in Figure 1.
The received powder was compacted at 600 MPa in a 50 ton uniaxial hydraulic press (APEX Construction Ltd, UK). To minimize friction, the compaction was carried out using zinc stearate as a die wall lubricant. The density of compacted specimens was between 6.99 and 7.02 g/cm3. The powder contained 0.75 wt. % acrawax, which was added to the powder to facilitate its compaction during sintering. The sintering response on densification and microstructures was evaluated on cylindrical pellets (16 mm diameter and 6 mm height). The green compacts were dewaxed in a tubular silicon carbide (SiC) furnace under a 80% N2-20% H2 atmosphere. The lubricant was removed from the green samples using a heat treatment at 850℃ for 30 min. To prevent cracking of green compacts by thermal shock, the compacts were slowly heated at 5℃/min. They were then sintered at three different temperatures, i.e. 1120℃, 1180℃ and 1250℃ respectively for 30 min in a tube furnace with a SiC heating element. The thermal profiles for the sintering are shown in Figure 2. All the sintering was carried out under a 80% N2-20% H2 atmosphere.
The sintered density was obtained by dimensional measurements. The densification parameter was calculated to determine the amount of densification occurred during sintering. It is expressed as:
The sintered samples were heat treated at 900℃ for one hour and subsequently cooled by four different methods: furnace cooling (annealing), air cooling (normalizing), oil quenching and brine quenching. The samples were polished to mirror finish and ultrasonically cleaned in acetone, followed by etching in 3% Nital. Bulk hardness of the samples was measured by LECO V - 100 - C1 Vickers hardness tester at 10 kgf load using a pyramidal shaped diamond indenter. The load was applied for 10 seconds. The recorded 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℃) for 2 hours. The microstructural analysis of the samples was carried out through optical microscopy, and then bulk hardness was measured using Vickers hardness tester at 10 kgf load.
3. Results and Discussion
Figure 3 shows the effect of sintering temperature on the densification response of SH737-2Cu-0.9C alloys. It is quite evident from the figure that the sintered density improves (~0.5%) with increased in sintering temperature from 1120℃ to 1250℃. To take into account the effect of composition, all the sintered densities were normalized with respect to the respective theoretical densities. The sintered sample contains nearly 11-12% porosity. It can be inferred that the temperature has marginal effect on the densification of these alloys.
|Figure 3. Effect of sintering temperature on densification: a) density b) percent theoretical density|
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 Figure 4. The densification parameter is negative for the sintering temperatures 1120℃ and 1180℃, thus confirming compact swelling. The parameter is positive for 1250℃, thus exhibiting marginal increase in densification parameter with sintering temperature.
|Figure 4. Effect of sintering temperature on the densification response||Table 4. Bulk hardness values of SH737-2Cu-0.9C sintered at three different temperatures (1120℃, 1180℃ and 1250℃) and then cooled by different methods|
Table 4 shows that furnace cooling leads to 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 in the case of higher rate of cooling (as in oil and brine quenching).
|Figure 5. Bulk hardness of SH737-2Cu-0.9C samples sintered at different temperatures and cooled by different methods||Figure 6. Effect of tempering temperatures on hardness of oil quenched SH737-2Cu-0.9C samples sintered at different temperatures|
Figure 5 shows the effect of sintering temperature on the hardness of SH737-2Cu-0.9C alloy samples cooled by various methods. It is evident from the graph that bulk hardness improves with increasing sintering temperature from 1120℃ to 1250℃, which can be attributed to the sintered density, because there was a marginal densification achieved with increasing sintering temeprature. Higher densification implies lower porosity, hence better heat transfer leading to high hardness. A higher sintering temperature further enhances the microstructural homogeneity due to inter-diffusion of alloying elements. This contributes to the enhancement of the bulk hardness, which also increases with the formation of bainite at higher sintering temperature due to comparatively faster cooling.
Figure 6 and 7 show the hardness pattern of tempered oil and brine quenched samples. The hardness increases up to 600℃, attains a maximum, and then decreases, which is due to secondary hardening taking place in the quenched samples. Secondary hardening occurs due to presence of carbide formers like Mn and Mo above 500℃. These elements have high diffusivity to nucleate and form a fine dispersion of alloy carbides producing secondary hardening. Secondary hardening is a process similar to age hardening, where coarse cementite particles are replaced by a new and much finer dispersion of Mo2C and Mn2C stable carbides. The critical dispersion causes a peak in the hardness as the carbide particles slowly coarsen, causing their strengthening ability to decrease, leading to an overall decrease in hardness.
The hardness values in Table 5 and 6 show that the as-sintered samples have the same hardness as that achieved by brine quenching followed by tempering at 600℃.
Sintered hardened grades have an advantage over simple carbon steels, as they do not require secondary processing, which is evident from the above data. The hardness of sintered SH737 samples is the same as that obtained from heat treatment and tempering, allowing the secondary hardness to take place. Sinter hardened steels have an enormous processing time advantage over simple carbon steels.
The authors gratefully acknowledge Dr. Anish Upadhyaya, Associate Professor, Department of Metallurgical & Materials Engineering., IIT Kanpur, for his expert supervision and Dr. Narasimhan, Hoeganaes Corporation for providing us with the working powders.
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