Abstract
Sub-surface crack networks in areas of altered microstructure are a common cause for bearing failures. Due to its appearance under light microscopy, the damage pattern is referred to as White Etching Cracks (WEC). The root causes leading to the formation of WEC are still under debate. Nevertheless, it has already been shown that atomic hydrogen can have an accelerating effect on the formation and propagation of WEC. In addition to hydrogen pre-charging, hydrogen can be released and absorbed during rolling/sliding due to the decomposing of the lubricant and water. The current work focuses on the analysis of the hydrogen content of cylindrical roller thrust bearings after testing in a FE8 type test rig using two different lubricants. Within the framework of this work, two different hydrogen analysis methods were used and assessed regarding their applicability. The results show that the so-called Hydrogen Collecting Analysis (HCA) is more suitable to investigate the correlation between lubricant chemistry and hydrogen content in the test bearings than the Local Hydrogen Analysis (LHA). The measurements with the HCA show a continuously increasing freely movable and diffusible hydrogen content under tribological conditions, which leads to the formation of WEC. Comparative tests with an oil without hydrogen showed that the tendency of the system to fail as a result of WEC can be reduced by using a lubricant without hydride compounds.
Highlights
The estimation of the service life of rolling bearings is usually carried out following DIN ISO 281 [1] and assumes a failure caused by subsurface initiated rolling contact fatigue (RCF)
The sub-surface damage pattern is characterized by regions of altered microstructure areas that appear white under the light microscope due to their etching resistance, the name White Etching Areas (WEA)
Like the Local Hydrogen Analysis (LHA), the Hydrogen Collecting Analysis (HCA) at 210 °C measures the freely moving hydrogen content; the measured values cannot be compared with one another
Summary
The estimation of the service life of rolling bearings is usually carried out following DIN ISO 281 [1] and assumes a failure caused by subsurface initiated rolling contact fatigue (RCF). Mechanisms that lead to WEC have been controversially described by various hypotheses It is still unclear whether crack formation occurs first [6, 8] or the white etching areas precede the crack formation [9, 10]. [13, 14, 16,17,18], the main source of hydrogen is water in the lubricant. Kohara et al [19] report that certain basic components and additives in oil are responsible for the diffusion of hydrogen into steel. It should, be noted that the formation of WEC can occur using hydrogen-free lubricants [20]. The driving effect of hydrogen on the formation and promotion of WEC is still unclear
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