Redistribution of carbon caused by butterfly defects in bearing steels

Butterfly defects initiate from inclusions in the subsurface of steel bearing components subject to rolling contact. The white etching matter (WEM) microstructure is a characteristic of butterflies and is related to the dissolution of carbides and thus generally believed to be enriched with carbon, in supersaturated solid solution, relative to the parent microstructure. Here, several butterflies are investigated using wavelength dispersive spectroscopy (WDS), soft x-ray emission spectroscopy (SXES) and electron microscopy (EM). Contrary to established thinking, in all cases investigated the butterfly-neighbouring WEM was found to be depleted in carbon, relative to parent material, by around 27% (measured in counts). Furthermore, the carbon level was shown to be lower than the matrix itself, suggesting that solute carbon is also expelled from the WEM during its formation, possibly due to the low level of solubility of carbon in ferrite. This was observed in both AISI 52100 and 18NiCrMo14-6 bearing steels. In spite of this depletion, nano-indentation found that WEM in both alloys was ~17% harder than the parent material. This may explain the strings of micro-voids observed near the WEM-parent interface, which appear to play a role in the growth of the butterfly cracks. It is suggested that the increased hardness of the WEM is mainly due to microstructural changes, rather than changes in solute carbon concentration.