New microstructure of butterfly white etching area in rolling contact fatigue of bearing steel

Abstract

Butterfly white etching area (WEA) formed at subsurface inclusions under rolling contact fatigue (RCF) of bearing steel was investigated by using the Scanning Electronic Microscopy and Transmission Electron Microscopy. Two kinds of butterfly WEAs with different orientations were observed at non-metallic inclusions: largely sheared and narrow band morphologies. Both involve body-centered-cubic (bcc) to face-centered-cubic (fcc) transformation, which is opposed to the previously reported butterfly WEAs consisting of only ferrite nanocrystallines. The largely sheared WEA is composed of nanocrystallines with the grain size of 15 nm averagely. In contrast, apart from the nanocrystallines, the narrow band WEA also includes well-developed equiaxed dynamic recrystallization grains with the grain size ten times larger than the nanocrystallines. This indicates that the WEA involves not only grain refinement, which is commonly reported, but grain growth as well due to dynamic recrystallization. It was suggested that the bcc to fcc phase transformation in nanocrystallines is mechanically controlled, while the phase transformation in equiaxed dynamic recrystallization grains is governed by thermally-assisted plastic deformation.