Rotating bending fatigue tests were carried out for notched specimens of a SUS316NG austenitic stainless steel at 300°C. Nevertheless any non-propagating crack was not recognized at the root of the notch for specimens endured at 108 stress cycles, the fatigue strength at 108 cycles did not decrease continuously with increasing the stress concentration factor. An area of the root of notch hardened during fatigue test at 300°C by dynamic strain aging. The hardening behavior became remarkably with increasing the stress concentration factor. Effects of the stress concentration factor and hardening behavior on the fatigue strength cancel each other, and then dependency of the fatigue strength on the stress concentration factor becomes insensitive.
