In-Situ Observations on the Fracture Mechanism of Diffusion-Alloyed Ni-Containing Powder Metal Steels and a Proposed Method for Tensile Strength Improvement

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.