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 °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 °C for 5–30 h and 430 °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 γ′-Fe4N with lamellar and granular shapes, whose intensity increased with increasing nitriding temperature or time. In the case of a lower temperature (390 °C) and shorter time (1−3h) of plasma nitriding, the austenite in the work-hardened layer transformed to the S phase (expanded austenite), and α'N (expanded martensite) was formed. As the nitriding temperature or time further increased, the expansion of the α'N and S phase increased, eventually both the S phase and α′N disappeared. The phase evolution occurred: S phase transfers to CrN in the work-hardened layer, while α′N phase transfers to γ′-Fe4N or CrN, and γ′-Fe4N phase transfers to α + CrN in the substrate. Among these phases, α′N was distributed next to γ′-Fe4N, the α phase was surrounded by γ′-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.