Improvements in microstructure homogenization and mechanical properties of diffusion-alloyed steel compact by the addition of Cr-containing powders

Abstract

The use of diffusion-alloyed powders for fabricating powder metal parts, despite alleviating the segregation problem of the alloying elements while retaining good compressibility, still cannot attain homogeneous microstructure in as-sintered products. The presence of soft Ni-rich areas and pores causes poor mechanical properties compared to those of wrought steel counterparts. This study investigated the effects of adding 0.5 wt pct Cr, which was introduced in the 316L stainless steel powder form, on the microstructure and mechanical properties of diffusion-alloyed Fe-4Ni-1.5Cu-0.5Mo-0.5C (Metal Powder Industries Federation (MPIF) FD-0405) steels. The results show that weak Ni-rich areas were present in the Cr-free specimen when sintered at 1120 °C and 1250 °C. These areas were lean in carbon because of the strong repelling effect between Ni and C. With the addition of 316L powders, the Cr was uniformly distributed and helped eliminate the soft Ni-rich areas, particularly in specimens sintered at 1250 °C. The distribution of carbon also improved. With a more uniform distribution of Ni and C, and more homogeneous microstructure, which consisted mainly of bainite and martensite, the mechanical properties of the Fe-4Ni-1.5Cu-0.5Mo-0.5C diffusion alloy steels were improved significantly.