Abstract: In order to investigate the effect of ground layer on fatigue behavior of high carbon chromium bearing steel JIS SUJ2 in ultra-long life regime, rotating-bending fatigue tests are performed in laboratory air at room temperature using hour- glass-shaped emerized and electro-polished specimens. The emerized specimen and the electro-polished one are obtained by removing a partial ground layer with surface roughness and the whole ground layer from ground ones, respectively. The results show that S-N curves are composed of surface fracture mode curve in the region of short life and internal fracture mode curve in the region of long life. The S-N curves for surface fracture mode are affected by surface roughness and compressive residual stress. The S-N curves for internal fracture mode are not affected by surface conditions, which is an inherent characteristic of the material. For the surface fracture mode, the emerized specimen shows the largest fatigue limit, about 1.11 times larger than for the electro-polished one and 1.20 times larger than for the ground one. The fatigue limits of surface fracture mode affected by the surface compressive residual stress can be expressed by modified Goodman diagram. Fur- thermore, conditions for internal crack initiation and pro- pagation are discussed. The fatigue limit in ultra-long life regime is estimated.

Key words: Bearing steel, Fish-eye, Ground layer, Internal crack, Nonmetallic inclusion, S-N curve, Surface crack, Ultra-long life fatigue