Nitriding behavior and mechanical properties of carburizing and nitriding duplex treated M50NiL steel

Abstract

In this study, carburizing and nitriding duplex treatments were carried out for 10 h at various nitriding temperatures in the range of 460–540 °C on M50NiL steel. The results show that an increase in nitriding temperature greatly increases the thickness of the nitrided layer, but a further increase in the nitriding temperature to 540 °C does not obtain a deeper nitrided layer. Moreover, an increase in nitriding temperature promotes the dissolution of grainy carbides and leads to a precipitation of intergranular precipitates. The pre-existing carbides are easier to be transformed into M(N,C) at a low nitriding temperature. The compound layer is a mixed phase of γ′ (Fe4N) and ε (Fe23N). With increasing the nitriding temperature, the magnitude of the residual compressive stresses decreases, the surface roughness increases, and the proportion of γ′ phase increases. The results of the wear tests carried out at different loads demonstrate that the wear mechanism controlling the wear rate transfers from oxidative wear to abrasive wear when the load increases. The sample nitrided at 500 °C shows the best wear resistance to abrasive wear under a high load due to its thicker compound layer, high ε proportion and high surface residual compressive stress.