Very-High-Cycle Fatigue Behaviors for Bearing Steel Microstructural Transformation

It has been reported that the multiphase structure with martensite and a mixed structure obtained by the quenching and tempering of bearing steel shows high strength and ductility. However, there appears to be no study on the effects of very-high-cycle fatigue (VHCF) and ultrasonic nanocrystal surface modification (UNSM) of a new bearing steel required for the durability design of next-generation bearing steel. This study analyzed the characteristics of microstructure transformation associated with heat treatment cycles and studied and evaluated the fatigue strength characteristics by the UNSM. Fisheye cracks occur from small inclusions inside the specimen only in martensitic microstructure materials in the long-life range. And the characteristic of a double S-N curve in which the fatigue limit decreased in two stages appeared. However, fisheye cracks and double S-N curves did not appear in pearlite and mixed-structure materials.