Improving fatigue life and surface properties of AISI 316 stainless steel through plasma nitriding

The current study investigates the rotating-bending fatigue behavior of plasma-nitrided austenitic stainless steel (AISI 316) with two effective case depths of 40 and 80 µm. The nitrided layer thickness and associated microstructural evolution were characterized using optical and scanning electron microscopy. The microstructure revealed a distinct dark-etching surface layer, confirming the formation of nitrogen-expanded austenite (S-phase) without evidence of detrimental carbide precipitation. Tensile testing revealed an enhancement in strength after nitriding, accompanied by a marginal reduction in ductility. The surface hardness increased significantly from 185 HV for the untreated AISI 316 to 542 HV and 625 HV for specimens nitrided to depths of 40 and 80 µm, respectively. Rotary bending fatigue tests conducted under varying stress amplitudes demonstrated a substantial improvement in fatigue life, particularly at higher stress levels. Fractographic analysis indicated delayed crack initiation and refined fracture morphologies in the nitrided specimens. The results suggest that plasma nitriding enhances the surface integrity and fatigue performance of AISI 316 stainless steel, establishing it as a viable surface engineering approach for applications demanding superior wear and fatigue resistance.