Fatigue properties of gas nitrided austenitic stainless steel pre-treated with fine particle peening

Abstract

The nitrided layer of austenitic stainless steel is typically extremely thin due to the existence of a passive film at the surface. In order to facilitate the diffusion of nitrogen into the material, a fine particle peening (FPP) treatment was performed prior to gas nitriding. The effects of the FPP treatment on gas nitriding behavior and consequent fatigue properties of AISI 316 steel were evaluated. Surface microstructures of the FPP treated specimens prior to gas nitriding were characterized using a micro-Vickers hardness tester, scanning electron microscope (SEM), optical microscope, electron probe micro analyzer (EPMA) and X-ray diffraction (XRD). The FPP treatment generated a local stratification pattern which exhibits high dislocation densities. During the subsequent nitriding process, nitrogen diffused into this unique structure, resulting in an increase in the surface hardness. Fatigue tests on a number of specimens with differing surface treatments were performed at room temperature using a rotational bending fatigue testing machine. The FPP treated specimen prior to gas nitriding exhibited the highest fatigue strength. After the fatigue tests, fracture surfaces were observed using an SEM in order to investigate the high-cycle fatigue properties.