Abstract: The spline of the bell-like shell of the left rear wheel assembly fractured while a car was running.To find out the failure cause,macro and micro observation,chemical composition analysis,microstructure examination,hardness testing,non-metallic inclusions inspection and hardened-layer depth measurement were carried out.The results show that the fracture of the spline is brittle fracture with more than one source.Brittle fracture characteristics can be observed by SEM,and the chemical composition,non-metallic inclusions and hardness of the spline are all in accordance with the technical requirements.However,there are coarser martensites of grade M2,which does not meet the technical requirements of grade M3-6.The grains in the spline teeth are too coarse,which can easily lead to inadequate toughness and higher brittleness,and lower resistance to overload,so brittle cracks easily initiate.The coarse grains are the main cause for the spline fracture.Accordingly,improvement measures were put forward to control the quenching temperature.