Austempering Effects on the Rolling Contact Fatigue Characteristics of Bearing Steels

Conventional studies of bearing steels and heat treatments have dealt with quenching and tempering (Q/T) in 52100 steel. Despite all the research, however, the latest trend in automotive parts is having a long life span, high quality, and reliability. Car components must be developed to meet the above car manufacturer's requirements. The increase in service life of wheel bearings, by improving the rolling contact fatigue (RCF) life of bearing steels, was investigated. Bearing steels for wheel units are commonly treated by through or case hardening to strengthen the steel. This study suggests a new trial to increase the strength of bearing steels by austempering in phases after general heat treatments such as Q/T or carburizing. This special heat treatment was also designed to produce fine austenite grains to increase the fatigue strength. The experiment is carried on with conventional 5120 and 52100 bearing steels, with the former applied to carburizing, while the latter is applied to WT. Austempering was then additionally conducted in each steel to investigate the effect of fine austenite generation in each bearing steel. Aside from the aforementioned trial, this study also focuses on the microstructural behavior taking place after austempering. In this study, a newly designed heat treatment concept is introduced and the probabilistic property of fatigue life was investigated using the RCF life test and the L-10 approach. The effect of microstructural behavior on these properties is also analyzed using scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD). In addition, the dilatometer is used to determine the bainite transformation temperature point in both cases.