Friction Property under Lubrication for Case Hardening Steel Subjected to Combined Thermomechanical Treatment with Excess Vacuum Carburizing and Subsequent Severe Plastic Deformation and Induction Hardening

Friction property of the case hardening steel subjected to excess vacuum carburizing and subsequent severe plastic deformation and induction hardening was evaluated by the traction test. The purpose of this study is to clarify the effect of fine microstructure on the friction property, focusing on the interaction between the fine microstructure and the lubricating oil additives. The vacuum carburizing treatment is performed at the hyper-eutectoid composition of 1.0 mass% C. Subsequently, the carburized surface was formed the white layer by the surface-nanostructured wearing (SNW) process, and the specimen having the initial microstructure was subjected to induction hardening. The microstructure of the condition with SNW was finer compared to that with SNW-less. According to the traction test, traction coefficient (μ) in the specimen having the fine microstructure on the rolling contact surface decreased. Therefore, it was found that the decrease of μ could be achieved by the application of high-density lattice defects (grain boundaries in this study). After the test, the rolling contact surface of the specimen with fine microstructure became smooth, and the surface showed high reactivity with the lubricating oil additives and formed the compound film of Fe-O-P system having a fine, spherical morphology. The surface roughness was improved by the presence of the wear particles on the surface. Therefore, it was thought that the μ was decreased because the transition to a mild friction condition was caused due to the dispersion of the contact pressure.