钟形壳花键断裂分析

汽车左后轮总成产品中的钟形壳花键在行驶过程中发生断裂，采用宏观观察、扫描电镜观察分析、断口分析、金相分析及理化测试分析等试验方法对钟形壳花键断口进行了化学成分、非金属夹杂、硬度、硬化层深度、金相组织、断口形貌特征进行了分析。结果表明:花键为脆性断裂，且有数个裂纹源同时扩展。电镜下可观察到有明显的脆性特征，花键部位的化学成分、非金属夹杂物、硬度均符合技术要求，但马氏体较粗（2级），不符合技术要求的M3-6级。花键齿顶部位的晶粒粗大，易造成该部位的韧性不足，脆性过大，抗冲击过载能力不足，易产生脆性起始裂纹。因此晶粒粗大是造成花键断裂的主要原因。针对深层次原因，提出了应对花键淬火温度加以监测控制的改进措施。

Fracture Analysis on Spline of Bell-like Shell

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.