Abstract
The formation of white etching cracks (WECs) in steel rolling element bearings can lead to the premature rolling contact fatigue (RCF) failure mode called white structure flaking. Driving mechanisms are still debated but are proposed to be combinations of mechanical, tribochemical and electrical effects. A number of studies have been conducted to record and map WECs in RCF-tested samples and bearings failed from the field. For the first time, this study uses serial sectioning metallography techniques on non-hydrogen charged test samples over a range of test durations to capture the evolution of WEC formation from their initiation to final flaking. Clear evidence for subsurface initiation at non-metallic inclusions was observed at the early stages of WEC formation, and with increasing test duration the propagation of these cracks from the subsurface region to the contact surface eventually causing flaking. In addition, an increase in the amount of associated microstructural changes adjacent to the cracks is observed, this being indicative of the crack being a prerequisite of the microstructural alteration.
Highlights
Rolling element bearings used in wind turbine gearboxes suffer from a premature failure mode called white structure flaking (WSF)
It must be taken into account that the FAG-FE8 test rig has differing dynamics to those experienced by wind turbine gearbox bearings (WTGBs) during service, e.g. the relatively high slip experienced in the cylindrical roller thrust bearings (CRTBs)
Inclusion types found to interact with the large white etching cracks (WECs) networks found at the later stages of rolling contact fatigue (RCF) duration (18 h) are consistent with those found at the early infant stages
Summary
Rolling element bearings used in wind turbine gearboxes suffer from a premature failure mode called white structure flaking (WSF). This typically occurs in 1–20% of the bearing’s L10 life, where the wind turbine lifetime is reduced from the predicted 20 years to < 2 years [1, 2]. WSF is due to the formation of white etching cracks (WECs) typically ~ 1 mm below the contact surface. WECs are networks of microcracks with an associated microstructural alteration called white etching area (WEA) which borders or is intermixed with the WEC. Amorphous-like phases have been shown to be present in WEA, forming first before WEA is generated [8, 14, 15]. WEA has been proposed to exist in two ways: deformed WEA consisting predominantly of nanocrystallites and transformed WEA consisting of co-existence between nanocrystallites and amorphous phase [16]
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