Abstract
The fatigue strength of steel is generally proportional to the hardness up to a certain point. However, the relationship between hardness and fatigue strength is more complicated for the case of high strength material, mainly because the defects in the material sensitively reduce the strength. The threshold notch depth a_<n th>, which is the limit not to reduce the fatigue strength, is obtained by the fatigue test. The surface layer is observed by using scanning electron microscope and electron probe micro analyzer to clarify the characteristics of latent defects. A strength evaluation method for the carburized gear is proposed using those results.
