Abstract
Aiming at an enhanced fatigue strength of quenched and tempered SCM435, surface treatment called cutting and rubbing process was developed. In the processed specimens, the compressive residual stress on the surface decreased at the beginning of the rotating bending fatigue tests but remained at a high value of -1800 MPa or more even after 107 cycles at the fatigue limit stress. Consequently, fatigue limit stress of the processed specimens was enhanced to approximately 1.5 times of a smooth specimen finished with #1000 emery paper, and most of the fractures were fisheye fractures. Then, fatigue tests were carried out on the processed specimens from which residual stress was removed by heat treatment. As a result, it was found that the effect of compressive residual stress is larger than that of the processing layer for enhancing fatigue strength. In order to investigate the effect of residual stress on the internal fatigue crack of the processed specimen, the fatigue test was carried out to 5×107 cycles at the fatigue limit stress. The internal crack was not obtained by electrolytic polishing and replica methods. However, the actual fatigue limit is much higher than the calculation result obtained from the fatigue limit prediction equation, which implies that the compressive residual stress near the surface layer promotes non-propagating internal fatigue cracks.
