Sintering and Heat-Treatment Response of SH737-2Cu-0.9C Steels

Tag : structural steels
The main thrust toward higher performance in P/M alloys has beenachieved by introducing alloying additions such as Mo, Ni, Mn andCu. Most alloying additions enhance the strength through solidsolution hardening during sintering. In addition, these alloyingelements also enhance the hardenability by shifting the continuouscooling transformation curve to the right. Subsequent heattreatment results in enhancing the mechanical properties of ferrousalloys.[6-10] In addition, alloying can improve oxidation orcorrosion resistance. Some applications rely on alloying to securespecial magnetic properties or high-temperature strength. The alloying methods that are used for production of ferrous P/Mparts 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 segregationproblem, whereas prealloyed iron powder is effective butsignificantly decreases the compressibility of the powders. Inconventional P/M processing, “diffusion-alloyed” powderhas typically been used. This process involves heat treatment ofiron powder and alloying elements in reducing atmosphere, whichallows partial diffusion and metallurgical-bond formation prior topressing and sintering. Due to partial diffusion, these powdershave higher compressibility and fewer tendencies for small alloyingelements to agglomerate for better homogeneity. The influence ofchemical and microstructural homogeneity on the mechanicalproperties of sintered material has been studied by a number ofauthors.[3-5] A common alloying metal in powder metallurgy is copper, which isnot sensitive to oxidation and causes sufficient increase instrength. A great number of sintered components are made forautomotive applications from mixing of copper and carbon withprealloyed iron. This material is sintered with transient liquidphase when copper content is less than 8%.[6] Hence, formation ofsecondary pores at the site of original copper particles is aninevitable 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-1.4Ni-0.42 Mn (wt%). The composition has been tailored with a view toalter the CCT-curve characteristics in such a manner that duringpost-sintering cooling itself the sintered compacts undergotransformation in the bainitic/martensitic region. Such grades ofpowder are also referred to as sinter hardening. As the namesuggests, sinter hardening achieves sinter and hardening in thesingle step.