Modern gears are made from cleaner steels, so contact fatigue in the form of pitting initiated at subsurface oxide inclusions rarely occurs now. Instead, pitting initiated from surface micropitting has become more prevalent. As a result, there is a greater need to study the mechanism by which micropitting is created and ultimately leads to the formation of larger and more detrimental pits. Earlier studies indicated that near surface microstructural changes can be linked to the onset of micropitting. The near-surface microstructural changes observed were believed to be caused by a martensite decay process as a result of the contact stress exerted upon the meshing gears under heavy load. To understand better the nature of the martensite decay process, we investigated the influence of surface finishing condition and the length of applied contact stress on subsurface microstructural changes in helical gears and spur gears, which were subjected to contact fatigue tests. We found that qualitatively the extent of microstructural changes are related to the length of the applied contact stress. These results suggest that the modified martensite regions observed could be the nucleation site of micropitting in gears.
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