The role of carbon in the white etching crack phenomenon in bearing steels

Since white etching crack (WEC) phenomena primarily occur in high carbon steels, we elucidate the role of carbon in this failure mechanism in bearings. The nano-crystalline ferritic regions that make up the white etching area (WEA) are formed by crack surface rubbing leading to complete decomposition of the initial microstructure by severe plastic deformation. In order to analyze local carbon compositions on mu m-nm length scales, we employ electron probe microanalysis, transmission electron microscopy and atom probe tomography. We focus on a 100Cr6 wind turbine gearbox bearing which failed in service due to extensive formation of WEC networks below the raceway surface and subsequent spalling. Our results show a significantly lower carbon content in the WEA as compared to the nominal alloy composition. At the same time, we find carbon deposits with a carbon content of > 85 at%, which are heterogeneously distributed across WEAs. We explain this observation by assuming segregation of excess carbon from the WEA to the open crack surfaces during crack surface rubbing. Further, the presence of a lubricating carbon film at the WEC surfaces might explain the accelerated failure by WECs as compared to classical rolling contact fatigue. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The white etching crack (WEC) phenomena in high carbon steels affect bearings, and research shows that it is associated with the carbon element. Analyzing the carbon content and distribution in 100Cr6 bearings can provide insights into the formation mechanism of white etching cracks.