Rolling and Sliding Contact Fatigue Process under High Pressure in Carburized SCM420 Steel

It is important to improve fatigue life for pitting of gear in order to attempt down-sizing of gear unit and low fuel consumption. Rolling and sliding contact fatigue under high pressure in carburized SCM420 steel is studied. The ultimate aim is to understand the effect of material properties on the fatigue life. In this research, there was considerable interest in change of the material in the contact surface of specimens depending on the shift in tangential force acting on a moving roller in the direction of a tangent to the path of the roller during the fatigue test. Three lubricants were used in these fatigue tests for the purpose of changing the degree of tangential force. A roller tested under a lubricant that the tangential force is shifted higher compared with that of the other lubricant produced a pitting failure earlier. The roller also promoted to decrease the hardness and to narrow the half width of the (211) peak of alpha iron at the contact surface. The transition of these characteristics related to the creation of fine grain layer. That is regarded as worked structure formed during fatigue process. The depth of fine grain layer expands according to repeated loading. Fatigue crack propagation depth was almost equal to the depth of fine grain layer. Not only well-known softening caused by increase in temperature but also microstructural change caused by repeated stress loading have an effect on the life through the involvement in fatigue crack initiation and propagation behavior.