White Etching Crack Root Cause Investigations

Abstract

White etching crack (WEC) failure is distinct to classical fatigue and driven by the composition of lubricants under special loading conditions; for example, slippage and electricity. The white etching area (WEA) within WEC contains carbon supersaturated ferrite (bcc-iron) and carbides, with a size of a few nanometers. This article presents investigations supporting the hypothesis that WEC processes start within a failure-free period by successive accumulation of a structural distortion. This can be measured by acoustic emission. Failure statistics show a steep ascent in the Weibull diagram (ß values beyond 1) leading to the assumption that WEC processes start unsuspicious, as one would see as a failure-free period, but imply a hidden subsurface accumulation of material defects. It is suggested and supported by the evidence presented within this article that WEC is neither related to the presence of nonmetallic inclusions nor related to other impurities in the steel. Instead, the failure is a sequence and accumulation of plastic deformations in the microstructure. Within the SAE 52100 material as discussed in this article, this accumulation is located in the microstructure around cementite, seen in a turn of hard magnetization toward soft magnetization proven by Barkhausen noise measurements. This decay is caused by the plastic deformation of such domains. Distortions in the vicinity of a cementite first would lead to carbon supersaturation by diffusion processes and later to a plastic deformation of the carbides. In the end, the complete distorted region will release the accumulated energy by downsizing the microstructure toward WEC.

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

“…These areas have also been found to coincide with zones of highest concentrations of hydrogen [50,59,60]. Supporting evidence for the slip energy criterion has been shown on FAG-FE8 tests where WECs appeared firstly at areas of high frictional energy, this also being demonstrated in tests using angular contact ball bearings [29,58]. A number of inclusion-WEC interactions were also recorded during fine serial sectioning corresponding to the 2-3-mm zone of high slip energy dissipation and asperity accumulation (see Fig.…”

“…11, 12, 13 and Video 2). Slip has been shown to influence the formation of WECs in both FAG-FE8 and three ring roller micropitting rig (MPR) tests using the same 'special' oil known used in this study [29,55,56], where evidence for the influence of negative slip being more dominant in WSF over positive slip is provided [56]. More recently the influence of slip on WEC formations has also been shown in a two-disc test rig set-up, where again negative slip showed dominance in WEC production in contrast to positive [57].…”

“…Differences in the severity between outer and inner roller halves at 18 h are also shown, the outer half having a greater severity than the inner. Condition monitoring on FAG-FE8 tests [29] has suggested a rapid release of subsurface WECs occurring at ~ 20% outstanding RCF test time before WSF failure. This is proposed to be due to the steel experiencing a failure-free period (no WECs detected), in which energy is absorbed (explained by Barkhausen noise (BN) measurements [54]), a limit being reached with a sudden release of WECs.…”

“…The proposed initiation and propagation mechanisms for WSF/WECs are: (1) surface initiation through two opposing mechanisms, (1) shear stress-induced fatigue microcracks [17] and (2) localised high circumferential tensile stress spontaneously induced cleavage-like axial cracks that initiate independently [5,17,18], at defects such as inclusions [17][18][19][20][21] or due to corrosion, machining defects or electrical erosion pits [21]; (2) subsurface initiation by non-metallic inclusions (NMIs) [5][6][7][22][23][24][25][26], perhaps in some cases due to tensile stresses [27]; (3) adiabatic shear banding independent or including defects through impact events, cracks forming after microstructural changes occur [2,28]; (4) self-charging of lubricants triggering localised transient current flow causing local electromagnetic induction that crosses the contact surface leading to electrothermal mechanisms triggering subsequent WEA microstructural change [29,30]; (5) a multistage initiation of WECs as a result of migration of carbon under shear stress and high localised energy [31].…”

Thermal Desorption Analysis of Hydrogen in Non-hydrogen-Charged Rolling Contact Fatigue-Tested 100Cr6 Steel

“…A number of additives found in lubricants have been shown to promote WSF occurrence, these include; extreme pressure (EP) and anti-wear (AW) additives consisting of sulphur and phosphorus compounds [66,67], where sulphur aids in hydrogen diffusion by preventing atomic hydrogen recombination [68] and formulations of AW zinc dithiophosphates (ZDDP/ZnDTP/ZnDDP) with detergent/rust preventative calcium sulphonate additives [6,61,[69][70][71][72][73][74].…”