After prolonged cyclic stressing in rolling contact, AISI 52100 bearing steel parts develop extensive regions of microstructural alteration, designated as white etching areas. These are oriented in predictable directions relative to the rolling track. Lenticular carbides are always associated with these areas. Evidence is presented indicating that the boundaries of lenticular carbides constitute planes of weakness which may be preferred planes of fatigue cracking. In the transmission electron microscope the martensitic structure appears gradually transformed into a cell like structure by the action of cyclic stress. The size of crystallites is greatly reduced in this process. The density of microstructural change is found increased with cycling and is distributed in depth along a curve resembling that of the calculated maximum unidirectional shear stress with little or no visible change in the region of maximum orthogonal (alternating) shear stress.

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Nucleation points for contact fatigue failures in rolling bearings are identified. In cases where bearings are made from exceptionally clean steel, fatigue failures are frequently nucleated at minute surface imperfections. These have been classified as grinding furrows, pits, and dents. Regions of plastically deformed martensite are found in the vicinity of defects that are active or potential failure nuclei. Plastic deformation is also seen in the bulk material. The plastically deformed martensite is examined by light and electron microscopy and its role in fatigue failures is described.

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