A Study On The Very High Cycle Fatigue And Fracture Behavior Of Bearing Steel By Ultrasonic Nanocrystal Surface Modification

This study was to investigate the influence of ultrasonic nanocrystal surface modification (UNSM) effects on very high cycle fatigue(VHCF) strength and fracture mechanism of a bearing steel. In case of mirror finished specimens with and without UNSM treatment after machined surface, the duplex S-N curves were obtained, but in case of unpolished machined surface condition, the duplex S-N curves were not formed. The UNSM treated specimen showed a tendency to increase about 28% in the fatigue limit of 10(6) cycles due to the refined structure to nanocrystal, the increased surface hardness, the decreased surface roughness and the increased compression residual stress. In the range of HCF, the fracture was caused by the surface crack generated by the surface defects originating from the surface. In the VHCF range, the dominant fracture mechanism was the fish-eye crack in which the internal originating fracture type was formed. The central part of the fish-eye where cracks initiated was inclusion consist of TiN. The stress intensity factor range (Delta K) of inclusions and fish-eye cracks approached 5 MPa root m and 10 MPa root m respectively.