Abstract: Steel rack in automotive steering systems requires high bending fatigue life, and which is significant affected by the straightening process after surface induction heat treatment. Main factors affecting the bending fatigue life and fatigue fracture failure mechanisms were analyzed by comparing the effects of the straightening direction and deformation amount on the tooth root microstructure, hardness distribution, root surface residual stress and corresponding bending fatigue life of the S45SC steel rack. The surface residual stress was detected by an X-ray diffractometer, and the fatigue life was tested by the four-point bending fatigue method. The results show that when the straightening pressing position is the tooth back, the residual compressive stress on the tooth root surface increases, and vice versa. The fatigue life demonstrates the same law. The rack failure fracture shows typical fatigue fracture characteristics, and the cracks originate from the tooth root surface. The key to improving the bending fatigue life of the rack lies in the control of the direction and amount of distortion caused by heat treatment. When the rack end is controlled to bend in the direction to the tooth surface, and the pressing position is the tooth back, the bending fatigue life requirements of the rack can be met with a high probability.

Key words: rack, S45SC steel, straightening, residual stress, bending fatigue life