The mechanical properties of Ni-containing powder metal (PM) steels are usually inferior to those of their wrought counterparts. The main problem is attributed to the non-uniform Ni distribution, in addition to the problems caused by porosity. The effects of this non-uniform alloying on the mechanical properties were investigated in this study using mini tensile bars that were made of diffusion-alloyed FD-0405 (Fe-4Ni-1.5Cu-0.5Mo-0.5C) powders. In-situ observations under a scanning electron microscope (SEM) were carried out during the tensile testing. The results indicated that the soft Ni-rich/C-lean area around the pore, which was identified as ferrite using electron backscattered diffraction (EBSD) analysis, was responsible for the failures. By adding Cr in the form of prealloyed powders, the distribution of Ni and C became more uniform, and the Ni-rich/C-lean ferrite was replaced by bainite and martensite. After sintering at 1250°C for 1 hour, the tensile strength of the Fe-4Ni-1.5Cr-0.2Mo-0.5C and Fe-4Ni-3Cr-0.5Mo-0.5C reached 1178 and 1323 MPa, respectively, without employing any accelerated cooling system. These properties are higher than all those reported to date in the literature for sinter-hardened PM alloys that were rapidly cooled after sintering. This significant improvement was attributed to the homogeneous alloying, particularly due to the reduction of the repelling effect between Ni and C, as was explained through the thermodynamics analysis using the Thermo-Calc program.

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.

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