| Rolling contact fatigue of high-carbon chromium bearing steel, STB 2 (AISI 52100 or JIS SUJ 2), with matrix of lower bainite has been studied in comparison with that of the steel with conventional matrix of tempered martensite. Rolling contact fatigue test was conducted in the lubricant at a shaft speed of 500 rpm under maximum Hertzian shear stress of 557Kg/㎟, using thrust-type rolling contact fatigue tester. It was observed that spherical carbides dispersed in the matrix of lower bainite formed by partial austenitizing at 840℃ for 30 min and isothermal holding at 250℃ for 2 hrs. Local softening occurred between 300 and 600㎛ below the raceway in the matrix of tempered martensite due to rolling contact fatigue. On the other hand, microhardness in the matrix of lower bainite was markedly increased within the region 400㎛ in depth below the raceway. Propagation of subsurface crack in the lower bainite was retarded by deflection, creating plastic zone along the front of crack. The fatigue life of the specimen with the lower bainite was improved in comparison with that of the specimen with the tempered martensite. |
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