Unusual intermediate layer precipitation in low-temperature salt bath nitrocarburized 316L austenitic stainless steel

This study focuses on investigating the precipitation behavior of the expanded austenite during low-temperature salt bath nitrocarburizing by adjusting the treatment temperature and time. The results indicate that the nitrocarburizing layer consists of a N-rich expanded austenite layer near the surface, and a C-rich expanded austenite layer closer to the substrate. Notably, all precipitates occur within the N-rich expanded austenite. When the temperature exceeds 430 °C, Cr₃C₂ and M_2-3N emerge in the near-surface region. Furthermore, at 470 °C, with the prolongation of time, the M_2-3N near the surface gradually decreases and even disappears, as the increase in carbon content within the N-rich expanded austenite enhances its stability. Specifically, at 430 °C, due to the high chemical potential of carbon and the nitrogen-induced lattice expansion, M₅C₂ forms near the interface between the N-rich layer and the C-rich layer. However, when the temperature rises to 470 °C, a significant amount of thermodynamically more stable M₇C₃ and M₂₃C₆ precipitates with the assistance of chromium diffusion. These findings may provide new insights for the process design and performance optimization of nitrocarburizing.