White etching cracks

White etching cracks (WEC), or white structure flaking or brittle flaking, is a type of rolling contact fatigue (RCF) damage that can occur in bearing steels under certain conditions, such as hydrogen embrittlement, high stress, inadequate lubrication, and high temperature.

WEC has been observed in a variety of applications, including wind turbine gearboxes, automotive engines, and other heavy machinery.

The exact mechanism of WEC formation is still a subject of research, but it is believed to be related to a combination of microstructural changes, such as phase transformations and grain boundary degradation, and cyclic loading.

[8][6] Ultimately, the formation of WECs appears to be influenced by a complex interplay between material, mechanical, and chemical factors,[3] including hydrogen embrittlement, high stresses from sliding contact, inclusions,[9] electrical currents,[10] and temperature.

[8] Hydrogen can come from a variety of sources, including the hydrocarbon lubricant or water contamination, and it is often used in laboratory tests to reproduce WECs.

[13] Hydrogen generation is influenced by lubricity, wear width, and the catalytic reaction of a fresh metal surface.

When the traction force opposed the direction of over-rolling (referred to as negative sliding), it consistently led to the development of WECs.

Furthermore, the number of WECs appeared to correlate with variations in contact severity, including changes in surface roughness, rolling speed, and lubricant temperature.

[17] Under certain conditions, when the current densities are low (less than 1 mA/mm2), electrical discharges can significantly shorten the lifespan of bearings by causing WECs.

[18] The analysis revealed that different reaction layers form in the examined areas, depending on the electrical polarity.

[10] In the case of AC, the rapid change in polarity involves the creation of a plasma channel through the lubricant film in the bearing, leading to a momentary, intense discharge of energy.

The localised heating and rapid cooling associated with these discharges can cause changes in the microstructure of the steel, leading to the formation of WEAs and WECs.

The constant flow of current can also cause local heating, leading to thermal gradients within the bearing material.

Representatives from academia and industry gathered to discuss the mechanisms behind WEC formation in wind turbines, focusing on the fundamental material processes causing this phenomenon.

WEC initiation at MnS inclusions in steel [ 1 ]
WEC initiation at MnS inclusions .
EDAX spectrum showing the chemical composition of MnS inclusion. [ 1 ]
Electron microscopy analysis of an axial cross-section (rolling direction in/out of the page) approximately 200 μm below the raceway surface: a) BSE image of the WEC selected for further investigation with EBSD. b) Higher magnification BSE image of the ROI with the WEM and untransformed material highlighted; c) BC EBSD map of the ROI. d) IPF Z orientation map of the ROI. The crack, which, like the WEM, is also a non-indexing feature, was segmented from the BSE images and overlaid in white. [ 7 ]