White structure flaking (WSF) in wind turbine gearbox bearings: effects of 'butterflies' and white etching cracks (WECs)

The actual service life of wind turbine gearboxes is often well below the desired 20 years. One of the prevalent failure modes in gearbox bearing raceways is white structure flaking (WSF) by the formation of butterflies and white etching cracks with associated microstructural change called white etching areas. Despite these failures having been observed for two decades in various industries, the detailed reasons and mechanisms for their formation are not fully understood. In this review, white etching area formation mechanisms are discussed, specifically grain refinement, and effects of carbon/carbide in a range of bearing steels of widely differing carbon content. The review also highlights the severe transient, cyclic loading and tribochemical operating conditions of gearbox bearings and explains how these may act as drivers to produce WSF. Much previous research has focused on the detrimental effects of hydrogen, but other work suggests that hydrogen is not the only cause for WSF. Possible methods for preventing WSF are discussed, with attention paid to special steels such as high chromium steels, low carbon stainless nitrogen alloy steels and carbonitrided steels. Beneficial compressive residual stresses, surface coatings and enhanced lubrication and additive packages are shown to offer degrees of prevention, although the mechanisms leading to improvements are not fully understood.