Uncovering the microstructure of White Etching Area around White Etching Cracks in bearing steels
Predicted life time of wind turbine gearboxes is 20 years. However, often they do not achieve the desired lifespan. The damage in the bearings in the gearboxes is often associated to a phenomenon known as White Etching Cracks (WECs). WECs are subsurface cracks often formed around (debonded) non-metallic inclusions situated in the area of high Hertzian stresses that subsequently develop and cause spalling of the raceway surface. Microstructural features around WECs are altered and consist of nanocrystalline ferrite also known as White Etching Area (WEA). Despite extended research on this topic no consensus has been achieved on the exact mechanism of damage. Many mechanisms have been proposed in the literature, but still there is a lack of agreement in chronology of appearance of WEC and WEA. In this work, a failed hydrogen charged deep-groove ball bearing tested in laboratory conditions was analyzed with the help of Transmission Electron Microscopy. Three lamella were chosen, one in parent material, without any damage and two in the zone of WEC and WEA. It was found that WEA is mainly situated on one side of the WEC, with a very small amount on the other side of the WEC. Moreover, within WEA martensite laths were observed which indicates that the formation process of WEA is not completed. Different grain size within WEA was observed and coupled with EDS analysis. It was found that smaller grain and bigger grain size corresponds to chromium enriched and chromium depleted zone, respectively. Average grain size of chromium enriched zone is 17.4nm and chromium depleted zone is 18.9nm. The findings of WEA on one side of the WEC is in line with previous research, while to the best of the author’s knowledge, different grain sizes within WEA are reported for the first time.