Dual-strengthening of steel surface and bulk via synergistic effect of plasma nitriding: a case study of M50 steel

Abstract Duplex-hardened M50 steel is used widely for jet engine shaft bearings. To reduce time and cost in duplex hardening, simultaneous plasma nitriding and tempering is performed upon as-solutioned M50 steel in this work. This heat-treating step can achieve high surface hardness and through-hardened core at the same time. Compared with the conventional “tempering + nitriding” method, the one-step heat treating method in this work can significantly increase the nitrided layer thickness by more than 61% and form a high hardness (>1000 HV0.1) layer spanning half of the total nitrided layer, which leads to reduced friction coefficient and improved wear resistance. A comprehensive study of the nitrided layer was carried out by slicing the surface along the diffusing direction. The phase distribution characters along the diffusing direction were investigated by XRD and SEM. A nitrided layer with gradient nano-structure was observed by the TEM, which experimentally explained the observed supersaturation of nitrogen in the sub-surface layer as a result of spinodal decomposition.