Influence of initial heat treatment of 17-4 PH stainless steel on gas nitriding kinetics

Results of the investigation of nitrided layers on 17-4 PH type precipitation hardening stainless steel are presented in this paper. The layers have been produced in the process of gas nitriding in a partly dissociated ammonia at temperatures between 410 and 570 °C. Hydrogen chloride admixture to active atmosphere was used as a surface activator. Structure of the nitrided layers were examined using scanning and transmission electron microscopy, X-ray microanalysis (EDX and WDX), and X-ray diffraction. The influence of the initial steel heat treatment on the nitriding kinetics has been considered. 17-4 PH stainless steel was nitrided at various heat treatment conditions, i.e. after solution treatment or ageing at different temperatures. The influence of precipitation processes taking place during the heat treatment before nitriding on the diffusive process kinetics was proven. It was found that, that increasing of steel ageing temperature up to 600 °C before nitriding effects on an increasing of the nitriding kinetics.