The Study of the Evaluation of the Fracture Initiation and Propagation on the Rolling Contact Fatigue in Bearing Steels

Abstract

The bearings used in various industrial machines need to have good rolling contact fatigue resistance. It is known that the fatigue resistance in clean lubricating oil is decreased when there is a nonmetallic inclusion in the bearing steel. For this reason, various studies to reduce the size and quantity of inclusions have been carried out. On the other hand, to obtain a new approach to suppress the influence of the nonmetallic inclusions, it is important to understand the influence of each nonmetallic inclusion type on the rolling contact fatigue resistance. Therefore, we have worked on evaluations of the fracture initiation and propagation on the rolling contact fatigue in bearing steels. In the previous symposium, we reported that we had created an oxide type inclusion controlled steel (OTICS) using the melting furnace in our laboratory. This steel has an excellent rolling contact fatigue life. In this paper, we will report fracture initiation and propagation on the rolling contact fatigue in OTICS and conventional steel evaluated using an ultrasonic test and an acoustic emission test. We have investigated the defects under the rolling contact surface by the ultrasonic test, with loadings applied at certain times in the thrust-type rolling contact fatigue test. During the test, we also measured the acoustic emissions generated when a fracture occurred. From the ultrasonic test, we succeeded in detecting nonmetallic inclusions with the fractures. Furthermore, the time before defects were detected in the OTICS was longer compared to conventional steel. The same tendency was observed in the acoustic emission test. Longer load times were required before acoustic emissions were detected in OTICS. According to these results, it can be considered that OTICS has a greater rolling contact fatigue life than conventional steel due to the inhibition of the fracture initiation from the nonmetallic inclusion.