Effect of Austempering Time on Contact Fatigue Performance of SAE8620 Bearing Steel for Railway Locomotive

The surface carburized SAE8620 bearing steel was austenitized at 855℃, then subjected to austempering in a salt bath at 225℃, and then tempered at 225℃. The effects of austempering time (7, 21 h) on the microstructure, phase composition, hardness and contact fatigue properties of the test steel were investigated. The results show that the surface microstructure of the test steel was composed of bainitic ferrite, retained austenite, martensite and carbides when austempered for 7 h. When austempered for 21 h, the martensite in the surface structure disappeared, the average width of bainite laths increased, the content of acicular bainitic ferrite increased and that of the retained austenite decreased. The test steel austempered for 7 h had higher surface hardness and a longer contact fatigue life than the steel austempered for 21 h, which was mainly related to the better resistance to plastic deformation due to the more retained austenite, smaller average width of bainite laths in the surface and higher surface hardness.