During rolling contact, hydrogen entering the steel could occur by two mechanisms: (1) through surface cracks where water contamination or lubricant enters surface cracks allowing local release of hydrogen ions by tribochemical reactions at crack tip nascent surfaces; and (2) through wear induced nascent surfaces where hydrogen is generated by decomposition of lubricants through catalytic reactions and tribochemical reactions of water 7 8.Researchers 6 9 10 11 support the understanding that WEC mechanism to be considered as two steps; initiation and propagation respectively. They were in the consensus that WEC propagation is caused due to hydrogen embrittlement. The WEC initiation depicted in Figure 2 are caused by hydrogen permeation in steel surface through tribochemical parameters like lubrication or slippage on the surface, and through near surface microcracks mostly caused by temporary local high tensile stresses, inclusions or contamination dents. Once the hydrogen enters the steel, part of it stays trapped at crack tips and/or defects in the lattice while the rest remain diffusible. Due to this local hydrogen embrittlement at the crack tips, WEC networks can propagate at moderate contact stresses beyond the Hertzian zone.
As the crack propagate, horizontal crack flacks in the maximum shear stress zone also appear in the white etching microstructure due to crack rubbing stresses and hydrogen enhanced localized plasticity (HELP), which further leads to martensite transformation into nano-grained ferrite. The intense localization of plastic deformation occurs because the cracked region has a lower compliance, so the surface of the cracks repeated beat or slide against each other. The intense deformation breaks up the pro-eutectoid cementite and forces it into the surrounding area, thus increasing the carbon content and consequently increasing the surrounding hardness by approximately 170HV. The initial crack region would be greatly embrittled by this dissolved carbon and the crack would propagate as the white etching region grows but not beyond the confines of the white area 12.
It’s also believed a corrosion fatigue crack propagation process (CFC) is triggered by the microstructural changes in the steel as the hydrogen is introduced in the crack faces. This transformation creates white etching areas and the typical irregular WEC networks as depicted in
