One of the typical failures due to the surface fatigue of steel is pittings on surface which are caused by the repetitive rolling contact with sliding component being in bearings, gears or contacts between rail and wheel. In this work, detailed investigations on the pitting failure have been tried experimentally by fatigue tests using two cylindrical test pieces contacted each other under a rolling condition with sliding. Materials adopted for the tests were annealed S45C and SCM21 steels (by JIS) which were typical gear materials, and two kinds of surface condition, as turned and as ground, were prepared in each steel. As the result, it was clarified that the pitting failure originated from surface fatigue cracks which were able to propagate only on the surface of the testing cylinder with lower rotational speed, independent of the surface roughness and hardness. It was also clarified that the pitting failure was enhanced by the surface roughness of testing cylinder with higher rotational speed. In this paper, in addition to these results, the initial stage of pitting cracks will be discussed in some detail based on optical micrographic observations of fatigued surfaces.
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