WEB The role of carbon in the phenomenon of white etching cracks
We investigated the microstructure and 3-D chemical evolution of a white etching crack (WEC) affected roller bearing made of high carbon bearing steel 100Cr6. The initial hardened microstructure consisted of bainite and µm sized spherical cementite precipitates. However, the WEC formation leads to nanoscale reconstitution of the phases, mainly by the formation of nano-sized grains of ferrite directly adjacent to subsurface cracks through severe plastic deformation. The small grain size (<10 nm is found) of white etching areas (WEAs) is stabilized by carbon grain boundary segregation. In order to determine carbon content and distribution we used electron probe micro analysis, transmission electron microscopy and atom probe tomography in a complementary manner. We found that WEAs are generally carbon depleted if compared to the average concentration inside the alloy. As net carbon loss cannot occur if cracks are not connected to the surface we propose that carbon could in principle present at crack surfaces. Finally, we discuss potential effects this might have on crack propagation in WECs.