Composition, microstructure, and phase evolution of 17-4PH stainless steel with a work-hardened layer in the low-temperature plasma nitriding process

In this study, we investigated the composition, microstructure, and phase evolution of (390-430 degrees C for 1-30 h) 17-4PH stainless steel with a work-hardened layer in the low-temperature plasma nitriding process. Scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were used for the analysis. The results suggested that crack-like structures appeared with the nitriding conditions of 410 degrees C for 5-30 h and 430 degrees C for 3-30 h. The activation energy was relatively low (32 kJ/mol) at the initial stage of nitriding due to the presence of a work-hardened layer, which increased with elevated nitriding temperature and prolonged time. The nitrided layer was mainly composed of gamma '-Fe4N with lamellar and granular shapes, whose intensity increased with increasing nitriding temperature or time. In the case of a lower temperature (390 degrees C) and shorter time (1-3h) of plasma nitriding, the austenite in the work-hardened layer transformed to the S phase (expanded austenite), and alpha'N (expanded martensite) was formed. As the nitriding temperature or time further increased, the expansion of the alpha'N and S phase increased, eventually both the S phase and alpha ' N disappeared. The phase evolution occurred: S phase transfers to CrN in the work-hardened layer, while alpha ' N phase transfers to gamma '-Fe4N or CrN, and gamma '-Fe4N phase transfers to alpha + CrN in the substrate. Among these phases, alpha ' N was distributed next to gamma '-Fe4N, the alpha phase was surrounded by gamma '-Fe4N, and CrN was precipitated at the normal grain boundaries but not at the twin boundaries. This work can provide a theoretical basis for the investigation of the wear and corrosion properties of the nitrided material in their application domains.

Automated summary

This study investigated the composition, microstructure, and phase evolution of 17-4PH stainless steel with a work-hardened layer in the low-temperature plasma nitriding process. The results showed that crack-like structures appeared with certain nitriding conditions, and the intensity of γ'-Fe4N phase increased with increasing nitriding temperature or time.