Abstract: Reduction bevel gears were found to have cracks on the inner surface of the axle journal during magnetic powder inspection after processing. The property and cause of the cracks were analyzed by macro and micro observation, metallographic examination, hardness test, residual stress test and hydrogen content analysis. The results show that the cracks of the gears are hydrogen-induced cracks. The hydrogen mainly came from the chrome plating process of the surface of the axle journal. The external reason is that the machining method of the inertia friction welding position of the axle journal was changed from polishing to turning, resulting in the existence of residual tensile stress. Under the co-effect of hydrogen and residual tensile stress, cracks formed at this position. It is verified that controlling the time of chrome plating, timely hydrogen removal and stress relief, and strict implementation of polishing procedures at the welding position can effectively prevent such failure from happening again.