White-Etching Matter in Bearing Steel. Part II: Distinguishing Cause and Effect in Bearing Steel Failure
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ITE-STRUCTURE flaking is a bearing failure mechanism associated with microstructural damage known commonly as ‘‘white-etching matter’’ (WEM), that leads eventually to flakes of material detaching from the raceway surface. Such damage has been identified as one of the possible causes of the premature failure of large bearings, such as those in wind-turbine gearboxes.[1] WEM is a generic term describing microstructural damage that exhibits lighter contrast relative to the surrounding material, when an nital-etched sample is observed using optical microscopy. This diminished contrast arises because the wavelength of light (390 to 770 nm) is large compared with the size of crystallites in the affected regions (5 to 500 nm).[2] White-etching regions include the so-called butterfly wings, some cracks, white-etching spheres, bands, all caused by rolling contact fatigue (RCF) in the subsurface of bearing raceways or white-etching layers caused again by RCF on railway tracks or by hard turning, as reviewed in Reference 3. In the case of bearings, RCF refers to the cyclic loading of the raceways by the repeated passage of rolling elements, which induce Hertzian contact stresses responsible for phenomena such as the decomposition of retained austenite,
W. SOLANO-ALVAREZ, Ph.D. Student, and H.K.D.H. BHADESHIA, TATA Steel Professor of Metallurgy and Director of the SKF University Technology Centre, are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, U.K. Contact e-mail: [email protected] Manuscript submitted December 11, 2013. Article published online July 11, 2014 4916—VOLUME 45A, OCTOBER 2014
smoothing of surfaces, steady-state plastic strain, texture development, residual stress development, work hardening, and surface or subsurface crack initiation and propagation. WEM can be classified into two categories, hard and soft.[3] Soft white-etching matter such as that associated with bands, is depleted in carbon,[4] can be transgranular,[5] and is composed of fine, equiaxed grains that have recrystallized.[6] Such regions can also contain amorphous patches and microvoids, while the material adjacent to the band has a high dislocation density.[7] It is emphasized that these soft white-etching bands are not associated with cracks or surface discontinuities, unlike the harder versions. The latter occurs as butterflies, cracks, spheres, or layers and are some 30 to 50 pct harder than the unaffected matrix.[8,9] They are generally supersaturated in carbon as a result of carbide dissolution,[10] have a structure of equiaxed but very fine grains,[7,11,12] and are free of large carbides.[11,12] In the case of butterflies, the hard WEM wings originates at voids, non-metallic inclusions, carbides, or microcracks already at 5 to 10 pct of the L10 life, which represents the number of loading cycles when 10 pct of the bearings in a population fail before their estimated life time.[1,13] Several mechanisms have been proposed to explain the formation of WEM. The hard WEM is associated with severe, localized plas
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