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Ultrasonic shot peening was applied to type 304 austenitic stainless steel and four-point rotating bending fatigue tests were performed at room temperature (R.T.) and 573K up to 10 8 cycles. The fatigue strengths at 573K were much lower than those at R.T. irrelevant to shot peening process. The shot-peened specimens exhibited higher fatigue strengths than untreated ones at both R.T. and 573K, indicating that shot peening had beneficial effect even at the elevated temperature of 573K. Clear fatigue limits were recognized in both shot-peened and untreated specimens at R.T. On the contrary, at 573K, fatigue failure occurred at high cycle fatigue (HCF) region (N > 2×10 6 cycles) in the shot-peened specimens, while not in the untreated ones. Fatigue crack initiated at the specimen surface due to cyclic slip deformation in the untreated specimens at R.T. and 573K and in the shot-peened ones at R.T. However the shot-peened specimens showed sub-surface crack initiation with a typical fish-eye pattern in the HCF failures at 573K. Inclusions were not recognized at the center of the fish-eye. The transition of crack initiation mechanism at 573K of shot-peened specimens could be attributed to the combined effects of the surface hardening by shot peening and the softening of material at the elevated temperature.

It is generally recognized that one of the reasons for improvement on fatigue durability by shot peening is surface compressive residual stress. And a shot peening condition decides a residual stress profile on surface layer. But, at this moment the method to confirm this profile is only measurement by X-ray difractowith the experimental value every shot peening condition of this experiment.