Mechanical Properties of White Etching Areas in Carburized Bearing Steel Using Spherical Nanoindentation
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components operate under demanding conditions related to large applied loads, complexloading patterns and thin lubrication films. Wind turbines gearboxes, which experience unpredictable weather-dependent loading cycles and severe environmental conditions, exemplify these difficult operating conditions and the current challenge in determining operating guidelines to reduce maintenance downtime and prevent premature bearing failure. The failure of bearings, particularly those found in wind turbine gearboxes, has been attributed to rolling contact fatigue (RCF): the gradual degradation of material under repeated rolling contact loading. Failure of bearing components, accounting for 76 pct of all failures
JONATHAN F.W. LEUNG is with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332. Contact e-mail: [email protected] VIKRAM BEDEKAR and ROHIT VOOTHALURU are with the The Timken Company, North Canton, OH 44720. RICHARD W. NEU is with the School of Materials Science and Engineering, Georgia Institute of Technology and also with the George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332. Manuscript submitted January 1, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
in wind turbine gearboxes,[1] is challenging to predict due to the complex microstructural transformations and tribological drivers for RCF. On inspection of failed bearings, extensive crack networks surrounded by microstructural-transformed regions resulting from RCF have been found throughout the subsurface region. These regions, which appear white under optical microscopy after dilute Nital etching are known as white etching areas (WEAs). For carburized bearing steels, it has been found that WEAs form readily in the case-hardened region under repeated rolling and sliding contacts.[2–4] The WEA is characterized as a nanocrystalline carbide-free ferrite that is supersaturated with carbon atoms[5,6] and possesses a reported grain size of 5 to 300 nm,[6–12] compared to the larger matrix grain size of 5 to 10 lm in these bearing steels. Current hardness studies of WEAs have indicated that the relative hardness of the WEA is 15 to 50 pct higher than the adjacent matrix.[4,6,8,13] Despite the pervasiveness of the WEAs in failedbearing components and its suspected link to subsurface crack formation,[3,4,14] comprehensive mechanical testing which can be used to determine the constitutive response of WEAs is limited due to the small-length scale of WEAs, which ranges from 10 to 100 lm,[15–17] and the service history dependency of WEA formation restricts the variety of applicable mechanical tests and thus hinders the acquisition of meaningful mechanical properties beyond the current hardness values. Furthermore, although the occurrence of WEAs is widespread, there is yet to be consensus on the root causes of WEA formation. Thus, the formation of WEAs has been attributed to multiple factors including the hydrogen embrittlement due to the breakdown of lubricants,[18–20] loca
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