THE ROLE OF SPHEROIDAL CEMENTITE PARTICLE ON SAE 52100 BEARING STEEL FOR GIGACYCLE FATIGUE

摘要

A method of metal improvement, ultrasonic nanocrystalline surface modification (UNSM) treatment, has been applied to the SAE 52100 bearing steel which is made up of ferrite and spheroidized cementite, in order to increase performances in terms of lifetime and safety. Although ultrasonic fatigue testing (UFT) is becoming more time consuming and costly with current design products, the long-time fatigue life test was performed on a piezoelectric system. It is the aim of our research to develop better understanding of fatigue properties of the untreated and UNSM-treated specimens in the gigacycle domain. Strain-induced microstructure evolution, including the grain refinement of ferrite, was examined. The ultrasonic fatigue test results show that the gigacycle fatigue life of the UNSM-treated specimens was prolonged by spheroidal cementite particles, including increased surface microhardness in the range of 200 HV to 300 HV up to depth of 100 μm from the surface. Moreover, surface structure was observed using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) as well as fracture surface was analyzed using scanning electron microscopy (SEM). The observations show that the cementite particles density were compared and developed in ferrite grains, which evolved gradually into sub-boundaries and high-angle misoriented grain boundaries at increasing strain, leading to the grain refinement of ferrite. The improvement in gigacycle fatigue life of the UNSM-treated specimen was longer than the untreated one.