Failure analysis of the balance ball pin in the car steering system

A balance ball pin used in the car steering system was found broken into two pieces. An evaluation of the failed part was investigated to determine the cause of failure and assess its integrity. Mechanical properties tests of the material are conducted first. Macroscopic examinations, scanning electronic microscopy(SEM) examination, metallographic examination,hardness measurement and chemical composition analysis were all conducted. The results indicated that fatigue fracture is the main fracture mode, and the uneven quench treatment led to the inhomogeneous thickness of quench-hardened case. As a result, in the fluctuations load, microcracks along the circumferential direction emerged on the subsurface of the quench-hardened case. Moreover, due to poor assembly, the stabilizer bar didn't turn smoothly in the working and fitting surface was worn badly, where produced the large resistance, which is the main torque to the ball pin.