White Etching Crack Failure Mode in Roller Bearings: From Observation via Analysis to Understanding and an Industrial Solution

Some roller bearing applications are prone to the white etching crack (WEC) failure mode. The applications seem to have in common that they work under dynamic operating conditions. The specific feature of this failure mode is that the subsurface microstructure of a failed bearing contains modified material structures near cracks which are white after a nital etching test. In case of a WEC failure, the real lifetime of the bearing is much lower than the theoretical lifetime calculation. The hypotheses of fatigue overload, hydrogen, and accumulated plastic microstrain are evaluated and a root cause hypothesis is developed based on observations. The white etching material structures are interpreted as adiabatic shear bands generated by an impact load mechanism. We developed the root cause hypothesis that the dynamic operation of a roller bearing is generating a bearing internal pressure peak causing loads at high strain rate which result in material damage and initiate the WEC failure mode. Impact tests reveal a high sensitivity of through hardened martensitic and bainitic bearing steels for the adiabatic shear band failure mode. The origin of the bearing internal pressure peak is further explained based on available ElastoHydrodynamic Lubrication (EHL) experimental and simulation results. The generation of butterflies and WEC networks is interpreted as recrystallisation driven by high stress after many load cycles or a moderate stress combined with a high strain rate loading. The industrial experience is analysed from the perspective of the root cause hypothesis. The Weibull curve of a WEC bearing failure case is explained based on the material parameter full width at half-maximum (FWHM) at the raceway surface. The potential solutions of an optimised microstructure, black oxidized treatment, and hot assembly are identified based on positive industrial experience.