White-Etching Matter in Bearing Steel. Part II: Distinguishing Cause and Effect in Bearing Steel Failure

Abstract

The premature failure of large bearings of the type used in wind turbines, possibly through a mechanism called "white-structure flaking", has triggered many studies of microstructural damage associated with "white-etching areas" created during rolling contact fatigue, although whether they are symptoms or causes of failure is less clear. Therefore, some special experiments have been conducted to prove that white-etching areas are the consequence, and not the cause, of damage. By artificially introducing a fine dispersion of microcracks in the steel through heat treatment and then subjecting the sample to rolling contact fatigue, manifestations of hard white-etching matter have been created to a much greater extent than samples similarly tested without initial cracks. A wide variety of characterisation tools has been used to corroborate that the white areas thus created have the same properties as reported observations on real bearings. Evidence suggests that the formation mechanism of the white-etching regions involves the rubbing and beating of the free surfaces of cracks, debonded inclusions, and voids under repeated rolling contact. It follows that the focus in avoiding early failure should be in enhancing the toughness of the bearing steel in order to avoid the initial microscopic feature event.

The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 Steel

“…Similarities have been shown between microstructural alterations in WECs, and those alterations found in dark etch regions [40]. An experimental approach by artificially inducing microcracks into the steel prior to RCF has also shown that hard WEAs formed in close proximity to the microcracks, providing an experimental validation that cracks can be a precursor to WEA formations [12]. In an investigation to study the effect of brittleness on the generation of WEA, modified AISI 8620 steel was intentionally heat treated to produce intergranular embrittlement [41].…”

“…The appearance of WEA is revealed when etched in nital solution (2% nitric acid in ethanol). WEA is a nanocrystalline ferrite structure of grain sizes ~ 5-300 nm, ~ 10-50% harder than the surrounding matrix and comprised of wholly or partially dissolved spherical carbides found to be part of the WEA formation process [3][4][5][6][7][8][9][10][11][12][13]. Amorphous-like phases have also been shown to be present in WEA, forming first before WEA is generated [8,14,15].…”

Degradation of nanostructured bainitic steel under rolling contact fatigue

“…It is worth mentioning that no white-etching matter was seen in any of the nanobainitic samples studied, even after 948 h, suggesting that the ductility of this alloy and its lack of carbides restrict the operation of white-etching matter formation mechanisms such as deformation localization, mechanical solution of fine carbides, and recrystallization of ferrite [27,36].…”

Elucidating white-etching matter through high-strain rate tensile testing

“…In this interpretation, the damage by cracking must occur first, as has been demonstrated in rolling-contact experiments on bearing steels. 16,[25][26][27] In other words, those white bands, which are more likely band-like white-etching cracks, are not the cause but the symptom of the damage mechanism.…”