Activated gas nitriding of 17-4 PH stainless steel

Results of the investigation of nitrided precipitation-hardened steel 17-4 PH surface have been presented. The layers have been created as a result of the gas nitriding process in a partly dissociated ammonia. Hydrogen chloride admixture to ammonia was used as a steel surface activator. The influence of the steel heat treatment before nitriding on the diffusive process has been considered. 17-4 PH stainless steel in various states of heat treatment, i.e. in supersaturated or precipitation-hardened state was nitrided. The nitrided layers were studied by scanning electron microscopy, X-ray microanalysis (EDX and WDX), and X-ray diffraction. Microhardness tests of the layers using Vickers method has been done too. Hardness of the generated layers reached 1700 HV. The optimal temperature range of the thermo-chemical treatment was determined. The influence of precipitation processes taking place during the heat treatment before nitriding on kinetics of diffusive nitriding process, surface geometry and the created layer properties has been proved. The tests results showed, that nitrided layers thickness is bigger on a base of the 17-4 PH stainless steel in precipitation-hardened, than in supersaturated state. Increase of steel ageing temperature before nitriding caused an increase of the nitriding process intensity. Differences of surface morphology, thickness, phase composition and nitrogen concentration of the diffusion layers created at the same nitriding conditions, but various conditions of 17-4 PH steel heat treatment before nitriding have been observed.