汽车变速箱二轴在工作中发生多起断裂事故。采用金相检验、扫描电镜和化学成分分析等手段，对断裂变速箱二轴进行了检验分析。结果表明，轴的断口具有明显的疲劳特征，疲劳源位于二轴花键槽根部。由于渗碳淬火工艺不当，在二轴花键槽根部的组织中产生了网状碳化物和粗大的针状马氏体，在外力作用下形成沿晶显微裂纹并扩展，是导致二轴疲劳断裂的主要原因。

Fracture accidents occurred on the second shaft of automobile gearbox during operation. The study of premature fracture failure of these shafts was made by means of SEM inspection, microstructure examination and chemical composition analysis. The result shows that the fracture has appeared fatigue feature, with initiation sites at the root grooves of the spline of the shafts. The network carbide and acicular martensites that existed in the microstructure of the root grooves of the spline due to improper carburating and quenching process of the shafts, led to the initiation and propagation of the intergranular microcracks and were the main reason that caused the early fracture of the shafts.