Bending fatigue behavior of metastable and stable austenitic stainless steels with different surface morphologies

The surface morphology has a significant influence on the fatigue behavior of components. For austenitic stainless steels (ASSs), this issue is even more pronounced due to their metastability. Based on the complex deformation mechanisms of metastable ASSs, which include dislocation slip, deformation twinning, and deformation-induced martensitic phase transformation, the metastable stainless steel AISI 347 was investigated in this study together with the stable AISI 904L as a reference material. Four-point bending fatigue tests with load ratio R = 0.1 and testing frequency f = 10 Hz at ambient temperature were carried out on specimens with five technically relevant surface morphologies: mechanical polished, milled, microshot peened, laser shock-peened, and ultrasonic modified. Systematic material characterizations were carried out to elucidate the crucial role of surface roughness and deformation-induced α′-martensite in the fatigue behavior of both metastable and stable materials. While surface roughness is a well-known key factor in conventional fatigue cases, deformation-induced martensite layers implemented by various surface modification methods were proven to improve the fatigue life in metastable austenitic steels, opening new perspectives to extend the lifetime of ASS components.