Relationships Between Rotating Bending Fatigue Strength and Static Strength of Small-Diameter JIS-SUS304 Ultrafine Grain Austenitic Stainless Steel

The grain size of austenitic stainless steel JIS-SUS304 stainless steel was refined by rolling process and heat treatment to achieve higher strength. Rotating bending fatigue tests and tensile strength tests were conducted at room temperature. As a result，it was confirmed that both static strength and fatigue properties were improved by grain size refinement．In particular，for heat-treated GS1.6 or larger，the fatigue limit was significantly higher than the 0.2% proof stress，suggesting that plastic deformation occurred in the surface layer．The fatigue limits of all these specimens were significantly higher than those estimated from their hardness and tensile strength. This trend was especially pronounced in the heat-treated materials, which did not follow the conventional empirical rules．X-ray diffraction and hardness testing of these heat-treated specimens revealed that cyclic stress induces work hardening and stress-induced martensitic transformation in the surface layer．On the other hand, such progression was not observed in the GS0.9 specimen， although residual plastic strain from rolling may remain．Therefore，when estimating the fatigue limit of fine-grained stainless steels based on static strength，it is necessary to take the influence of microstructure into account.