Formation mechanisms of white etching cracks and white etching area under rolling contact fatigue

Abstract

The formation of white etching cracks in the 1 mm zone beneath the contact surface in steel rolling element bearings causes a premature wear failure mode called white structure flaking. The formation drivers of white etching cracks are contested, as are the initiation and propagation mechanisms of the cracks. Hydrogen diffusion into bearing steel sourced from the hydrocarbon lubricant or water contamination and transient operating conditions have been suggested as formation drivers. Extensive work has been conducted at Southampton to further understanding of white structure flaking and this paper summarises these evidences and the conclusions made. Serial sectioning has been used to map subsurface wear volumes of wind turbine gearbox bearings from service and large-scale test rigs, test specimens/bearings from laboratory under hydrogen charged conditions and non-hydrogen charged conditions. The process involves polishing of cross sections of test specimens/bearings at ∼3–5 µm material removal intervals typically over hundreds of slices, and this was used to map white etching cracks in their entirety for the first time. Serial sectioning has allowed a comprehensive investigation of the initiation and propagation mechanisms of white etching cracks and thresholds for their formation with respects to concentration of diffusible hydrogen, contact pressure and number of rolling cycles. From these studies it has been found that white etching cracks can form by subsurface crack initiation at inclusions under hydrogen charged and non-hydrogen charged conditions; hence it has been confirmed that this is one mechanism of WEC formation. Small/short sized sulfide inclusions, globular manganese sulfide + oxide inclusions and small globular oxide inclusions between ∼1 µm and 20 µm in diameter/length predominated as crack initiators. In addition, detailed focused ion beam/transmission electron microscopic studies have been conducted to enhance the understanding of butterfly crack and white etching area formation mechanisms.