Formation mechanism of weak ferrite areas in Ni-containing powder metal steels and methods of strengthening them

The mechanical properties of Ni-containing powder metal steels are typically inferior to those of their wrought counterparts due to the presence of pores and soft Ni-rich/C-lean ferrite. To improve the mechanical properties, the distribution of alloying elements and the formation mechanism of the soft Ni-rich/C-lean areas were investigated. The results of diffusion couple experiments showed that a soft ferrite layer formed between martensite and pearlite phases. When Cr and Mo were added, the distribution of Ni and C improved and the ferrite disappeared. As a result, the tensile strength increased because the fracture path changed by passing through pearlite or along pearlite/martensite boundaries, instead of passing through ferrite. Based on the microstructure and diffusion couple results, schematics were presented to explain the formation mechanism of the weak Ni-rich/C-lean ferrite and its elimination by adding Cr and Mo.