What causes white etching cracks to develop?
In looking at bearing failure analysis, the root cause of white etching cracks (WEC) in bearings can be traced to bearing rolling contact fatigue as well as accelerated rolling contact fatigue.
In prematurely spalled bearings, two physical parameters will accelerate rolling contact fatigue:
Stresses higher than anticipated
Strength lower than anticipated
The material strength of a bearing can be negatively influenced by environmental factors that are suspected to generate hydrogen. These could include water contamination, corrosion, electrical stray currents and more. In these cases, moderate loading conditions may lead to a premature failure (fig.1).
While still being discussed in the material science community, the findings of bearing failure analysis detailed below strongly support that WEC occur at the end of the failure chain, and are a natural consequence of crack networks in failed bearings.
For small, high-loaded, and long-running bearings (very high cycle fatigue), it is known that bearings can run through several stages of fatigue until failure.
The first stage is the shake-down stage, leading to microplastic deformation, work hardening and eventually, buildup of residual stress. During the shakedown, the surface of the bearing may also undergo some microplastic deformation where asperities are flattened.
After the shake-down, the major part of the bearing life begins, and is characterized by gradual changes in the microstructure. During this stage, the carbide distribution is changing due to microplastic deformation. Also, retained austenite may decay, and all microstructure changes are accompanied by a build-up of residual stress.
In an advanced state of bearing rolling contact fatigue, dark etching regions (DER) can be found as well as white etching, high-angle bands (HAB) and low-angle bands (LAB) (fig.2). Although HAB and LAB are white etching too, they have a different appearance compared to irregular WEC formations found in early bearing failures. This has been leading to the conclusion that irregular WEC formation is not part of RCF (rolling contact fatigue). However, the micro-structure of these white etching areas is not very different in crystal structure than the white etching areas that are observed in premature failures.
For medium-to-larger bearings, the effects listed above will not necessarily occur in the same manner as in small, high-loaded bearings. Like other mechanical components, these bearings typically fail due to breakage of the weakest link (i.e. pre-existing deviations in the material structure such as inclusions and porosities). As explained in ISO/TR 1281-2_2008, the fatigue limit will decrease with bearing size above a bearing mean diameter of 100 mm. In addition, when comparing the effects of contact pressure on smaller vs. larger bearings, the affected stress volume increases in the larger bearings, as do the negative influences of weak links. One example is inclusions, which are a natural part of all bearing steels.
WEC were reported in rolling-contact-fatigued bearings in the 1960s (see also work of SKF in 1980s, fig.3a). Post-investigations of medium-to-large failed bearings (either from highly accelerated life tests or from endurance testing) have confirmed that the occurrence of extended, irregular WEC networks is a natural by-product in rolling-contact-fatigued bearings (fig.3b).
The difference between premature spalling (often interpreted as WEC failures in industry) and bearing rolling-contact fatigue can be found in the length of time it takes for the different events to occur before spalling begins. In addition, compared to endurance testing or normal rolling-contact fatigue, premature failures are often associated with crack initiations at several locations/areas as revealed by bearing failure analysis (fig.4).
The reasons for crack initiation in bearing steel can be different, and they can accelerate in cases of higher stresses, or strength reduction due to environmental effects, such as the ingress of hydrogen into the steel (fig.1). Once crack nucleons are generated (sometimes associated with the occurrence of dark etching regions (DER) a rubbing process across the crack faces leads to the transfer of material from one side of the crack to the other. This results in a meandering crack, which accumulates a white etching microstructure on the receiving side of the crack.
The white etching area (WEA) development also depends on the orientation of the crack in the sub-surface, which can be related to acting internal forces and deformation modes. This is why WEA are more often found in horizontal-oriented cracks (parallel to the raceway), whereas the vertical-oriented parts of the crack frequently show less WEA indication. Additionally, the generation of WEA depends on the interspace between crack faces and the number of stress cycles, as well as the internal stress state in the material. (fig.5)
Testing provides insights on premature bearing failure and WEC
Research on premature bearing failure and WECs has been conducted with several external partners, including SKF University Technology Centers. Investigations were conducted related to field bearings, endurance testing and WEC testing. Although not fully clarified and part of ongoing research, it was shown that WEC can be reproduced, found and associated with the following test conditions:
- in bearing endurance tests of medium-to-larger bearings (rolling contact fatigue)
in bearing tests with rings being exposed to higher-than-normal structural tensile stresses
- in bearing tests being exposed to brief, heavy loads
in bearing tests being exposed to contamination with water (fig.6)
in bearing tests running under mixed friction and high kinematic slip conditions using specific lubricants (fig.7)
- in bearing tests with hydrogen charged components
in bearings that suffered from electrical current damage (electrical erosion)
- Each premature bearing failure is unique: The "one & only root cause" does not exist and each failure case needs to be reviewed in the light of the corresponding operating conditions.
Counter measures can be identified to significantly increase bearing performance.