Effect of the carburized layer on the plasma nitriding behavior of duplex treated M50NiL steel

Abstract

In this study, carburizing and nitriding duplex treatment was carried out at various carburizing pressures in the range of 1000–2500 Pa on M50NiL steel. The modified layers were characterized by laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD) and microhardness testing. The results show that increasing the carburizing pressure to 2000 Pa leads to a decrease in the thickness of compound layer and nitrided layer, and resulted in a great increase in the number of spherical precipitates M(C,N) in the nitrided layer. The compound layer consists of ε phase and γ′ phase as well as a trace of Fe3O4. The change of carbon content in the surface layer after carburizing and the increase in carbon content in the surface layer caused by the transformation from carbides into nitrides during nitriding greatly decreased the content of γ′ phase in the compound layer when the carburizing pressure increased from 1000 Pa to 1500 Pa. The effects of the solid solution by alloying elements, the precipitates and the diffusion of carbon atoms on the diffusion rate of nitrogen atoms are discussed.