Serial sectioning investigation of butterfly and white etching crack (WEC) formation in wind turbine gearbox bearings

Premature wind turbine gearbox bearing failures in the form of white structure flaking (WSF) can occur in as little as 6–24 months of operation. WSF is not fully understood but is thought to be due to hydrogen release and diffusion into the bearing steel and/or transient operating conditions not fully understood. The initiation mechanisms of white etching cracks (WECs) are contested, where amongst others mechanisms, subsurface initiation at non-metallic inclusions (perhaps associated with extension of butterfly cracks) and surface crack initiation are cited. For the first time this study applies serial sectioning to map WEC networks in wind turbine gearbox bearings to elucidate WEC initiation mechanisms. A comparison is made between WEC data for inner rings of an industrial transient test gearbox bearing and a planet bearing that spalled in service. It is proposed that one mechanism of WEC formation in wind turbine gearbox bearings is due to subsurface WEC initiation from inclusions, either in a butterfly manner or non-butterfly manner; where these small WECs link together to form larger WEC networks, these eventually propagating to the surface resulting in WSF. Small size/length inclusions were found to be likely WEC initiators, therefore the data suggests that steel cleanliness standards analysing inclusion density (as opposed to maximum inclusion lengths) are more relevant in understanding butterfly/WEC initiation in wind turbine gearbox bearings. However standards used should be able to differentiate pure sulfides from sulfides+oxide encapsulations and record inclusions that are only a couple of mircometer’s in length/diameter.