The microstructure and properties of 17-4PH martensitic precipitation hardening stainless steel modified by plasma nitrocarburizing

17-4PH martensitic precipitation hardening stainless steel was plasma nitrocarburized at 430 °C and 460 °C for 8 h. The nitrocarburized layers were characterized by optical microscope, scanning electron microscope, X-ray diffractometer, microhardness tests, pin-on-disc tribometer and the anodic polarization method in a 3.5% NaCl solution. The results show that the microstructure of plasma nitrocarburized layer is characterized by a compound layer with no evident diffusion zone. The phases in the 430 °C treated layer are mainly of γ′-Fe₄N, nitrogen and carbon expanded martensite (α′_N), and some incipient CrN phases. When the temperature increases up to 460 °C, there is no evidence of α′_N phase. The processes of bulk precipitation hardening and surface treatment by plasma nitrocarburizing can be successfully combined in a single-step process on this steel. The hardness of modified layer can reach up to 1186HV, which is 3 times higher than that of untreated steel. The wear and corrosion resistance of the specimens can be apparently improved by plasma nitrocarburizing. The 460 °C/8 h treated specimen has the best wear and corrosion resistance in the present test conditions.