Stepwise S-N curves were obtained under rotating bending fatigue tests at high temperatures for an low alloy steel, as well as fatigue tests at room temperature for surface hardened specimens by shot peening or carburizing treatment. In the high cycle region, internal fracture with fish-eye pattern was dominant. As for mechanisms of internal fatigue fracture for 1Cr-0.5Mo steel, under high cycle region at high temperatures, following results can be obtained. (1) Oxidized surface layer itself

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... no effect on preventing fatigue crack from initiating at the specimen surface. (2) Specimen surface hardens during rotating bending fatigue test at 673K due to dynamic strain aging. Because of this phenomenon, it is difficult for Stage I cracks to initiate at specimen surface. (3) Effects of surface improvement by dynamic strain aging on fatigue strength can be recognized.