Analysis on Early Fracture of Steering Knuckle

Early fracture frequently occurred to a type of steering knuckles in cars. In order to find out the failure cause, macro and micro observation, microstructure examination, chemical composition analysis and hardness testing were carried out. The results show that the failure mode of the steering knuckle was fatigue fracture, and the micro crack initiated from the transition zone by induction quenching. The thicker induction hardened layer of the steering knuckle and the lower heating rate during induction quenching resulted in larger internal stress in the transition zone, which is the main cause for the poor fatigue resistance of the steering knuckle. Then, according to the geometry structure of the steering knuckle, the induction quenching process got adjusted and the technique of extended hardened layer was adopted to eliminate the potential danger at the transition section between the knuckle cylinder and the end of the flange, which effectively prevented such failure.