Characterization of simultaneous aged and plasma nitrided 17-4 PH stainless steel

Microstructural characteristics (using TEM, SIMS, SEM, optical profilometry and XRD) and residual stress (using nanoindentation and XRD) of simultaneously aged and plasma nitrided martensitic 17-4 PH stainless steel were investigated. It was observed that morphology and microstructure produced during plasma nitriding of 17-4 PH stainless steel were very dependent on the treatment temperature and time. An unetchable nitrided layer formed on specimen surface nitrided at 400 °C which contained expanded martensite, whereas nitrided layers at 450 and 500 °C contained dark phases of nitrides in addition to expanded martensite. Nitride precipitates and expanded martensite laths were observed in TEM images and their corresponding diffraction patterns. Lack of incoherent copper rich precipitates in TEM confirmed that aging and plasma nitriding were performed simultaneously. SIMS profiles revealed that the total diffusion depth of nitrogen is 3-4 times thicker than the nitrided case. The lowest and highest surface residual stresses were contributed to PN400C5h and PN400C10h, respectively. Although hardness values increased by increasing temperature/time of plasma nitriding process, i.e. increasing nitrided layer thickness, residual stress was a function of residual stress relaxation at high temperature in addition to thickness of nitrided layer. Residual stress depth profile showed that maximum compressive residual stress at the surface, dropped to zero just below the nitrided case, then increased to a maximum value of tensile residual stress; then decreased gradually to zero. Hardness depth profile measurements revealed that hardness was very dependent on residual stress induced by nitrogen atoms diffused interstitially during plasma nitriding. Core hardness of specimens made it evident that aging took place during plasma nitriding.