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The mechanism for fatigue failure in extremely high cycle fatigue in the regime of N > 10^7 is studied on a bearing steel, JIS SUJ2. Special focus was given to the fracture morphology in the vicinity of fracture origin (subsurface non-metallic inclusion) of a heat treated bearing steel (Specimen QT). The particular morphology looks dark during optical microscopic observation. Specimens with short fatigue life of the order of N_f=10^5 did not have such a dark area, ODA (optically dark area). To investigate the influence of the hydrogen trapped by nonmetallic inclusions on fatigue properties, specimens heat treated in a vacuum followed by quenching and tempering (Specimen VQ) were prepared. Specimens VQ contained 0.07ppm hydrogen as compared to 0.80ppm hydrogen for conventional Specimens QT. Specimens VQ had a slightly smaller ODA than Specimens QT. Hydrogen was detected by a Secondary Ion Mass Spectrometer around the inclusion at fracture origin of Specimens QT and Specimens VQ. Thus, it can be concluded that the formation of ODA is closely related to hydrogen trapped by nonmetallic inclusions. Estimations of fatigue limit by the √areaparameter model based on the original size of inclusions for fatigue limit defined for 10^7 cycles are ,10% unconservative. Considering the size of ODA into fatigue limit estimation, the √area parameter model can predict the mechanical fatigue threshold for small cracks without influence of hydrogen. The mechanism of duplex S-N curve is also discussed.
