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

Abstract

Hydrogen diffusion during rolling contact fatigue (RCF) is considered a potential root cause or accelerator of white etching cracks (WECs) in wind turbine gearbox bearing steels. Hydrogen entry into the bearing steel during operation is thought to occur either through the contact surface itself or through cracks that breach the contact surface, in both cases by the decomposition of lubricant through catalytic reactions and/or tribochemical reactions of water. Thermal desorption analysis (TDA) using two experimental set-ups has been used to measure the hydrogen concentration in non-hydrogen-charged bearings over increasing RCF test durations for the first time. TDA on both instruments revealed that hydrogen diffused into the rolling elements, increasing concentrations being measured for longer test durations, with numerous WECs having formed. On the other hand, across all test durations, negligible concentrations of hydrogen were measured in the raceways, and correspondingly no WECs formed. Evidence for a relationship between hydrogen concentration and either the formation or the acceleration of WECs is shown in the rollers, as WECs increased in number and severity with increasing test duration. It is assumed that hydrogen diffusion occurred at wear-induced nascent surfaces or areas of heterogeneous/patchy tribofilm, since most WECs did not breach the contact surface, and those that did only had very small crack volumes for entry of lubricant to have occurred.

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

“…It is hypothesised that the occurrence of these features is a result of either, or a combination of; (1) an increase in traction between contacts at later stages of RCF operation, resulting in a rise of the maximum subsurface shear stress zone towards the surface. This would be significantly increased in the case of insufficient lubrication and higher surface roughness; (2) microindentations at the contact surface causing local regions of mixed/boundary lubrication regimes increasing the traction coefficient; (3) areas of localised increase in contact pressure, and (4) increase in the concentration of mobile diffusible hydrogen over longer RCF operation [44], where locally at the nearsurface, higher localised penetration and concentrations may exist and aid in the acceleration of these features. Figure 15 illustrates the hypothesised mechanisms for the formation of these features.…”

“…Two factors to explain this could be: (1) steel cleanliness has shown that the raceway is 'cleaner' than the roller (see Sect. 2.3.1); (2) a lack of hydrogen being available to accelerate WEC formations [44]; and (3) the raceway is ~ 23% softer (590 HV) than the rollers (765 HV); therefore, the raceway is less prone to cracking due to an increased toughness. The importance of high toughness steel has been highlighted in the reduction in intergranular subsurface cracking and the subsequent movement of crack faces in generating WEA [41].…”

“…12, 13), a ramped increase seen at the later stages of RCF operation (12-18 h). This could be due to WECs coalescing to form larger crack networks resulting in a 'weakening' of the surrounding steel accelerating WEC growth, this being heightened in the event of a sufficient threshold concentration of diffusible hydrogen being reached [44]. Variance in severity is also observed between rollers.…”

“…(4) Due to the rapid growth of the crack and crack volume, the time and action available for WEA development is alleviated and thus a reduction or non-existent presence of WEA is seen. In the event of hydrogen diffusion, hydrogen acts to decrease the Mode I/II stress limits for crack growth and propagation [66,67], it may be reasoned that this step increase in WEC formations is due to a threshold concentration of hydrogen being reached for a decrease in Mode II crack growth [44].…”

“…There is an accompanying piece of work [44] to this investigation that explores the role of hydrogen diffusion in the RCF tests conducted in this study.…”