Microstructures and mechanical properties of surface-hardened layer produced on SKD 61 steel by plasma radical nitriding

Plasma radical nitriding was performed to harden the surface of SKD 61 steel for 1–10 h at temperature range of 450–550 °C. The microstructures and material properties of the radical nitrided layer were characterized in order to investigate the effects of various radical nitriding processing parameters. No compound layer was formed during this process except the experiment carried out at 500 °C for 10 h. A diffusion depth increased with increasing treatment temperature and time (up to about 150 μm). The surface hardness of radical nitrided layer was two times higher than that of the untreated surface. The main phases produced in the diffusion zone were identified to be ɛ-Fe2-3(N,C) and γ′-Fe4(N,C). The residual stress of the diffusion layer also increased with increasing treatment temperature and time due to the increase of precipitates. In addition, plasma radical nitriding produces better surface roughness, compared with conventional ion nitriding.