Study of M50NiL steel under carburizing and nitriding duplex treatment

Abstract

To better understand the behavior of M50NiL steel under carburizing and nitriding duplex treatment, different carburizing temperatures (ranging from 890 to 980 °C) were used for the carburizing pretreatment. The surface microstructures were characterized by X-ray diffraction (XRD), laser scanning confocal microscopy, microhardness testing, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the proportions of γ′ and large submicron precipitates increase with increasing carburizing temperature, which corresponds to an increase in large primary carbides. The increase in the number/size of large primary carbides promotes the diffusion of nitrogen. The resulting changes in the nanometric precipitates after nitriding lead to different levels of solid solution by nitrogen (reaching a maximum at a 920 °C carburizing pretreatment). The growth mechanism of the submicron precipitates is proposed. The effect of various precipitates and the diffusion behavior of C and solid solution of N on the hardening behavior are discussed.