某汽车仅使用2 d（天）、行驶56 km，其右后拖曳臂轴即发生断裂。采用断口宏观和微观检验、硬度测试、金相检验、化学成分分析等方法，对该右后拖曳臂轴断裂的原因进行了分析。结果表明：该轴断裂性质为沿晶脆性断裂，断裂的主要原因是其锻造组织存在缺陷及未经调质处理，使轴的强度大大降低；次要原因是轴表面因磨削加工过热形成了表面磨削淬火层，增加了轴表面的脆性及拉应力，使微裂纹在轴表面产生。

When an automobile used for two days and traveled 56 km, it was found that the shaft of the right rear trailing arm fractured. The fracture reasons of the shaft of the right rear trailing arm were analyzed by macro and micro fracture examination, hardness testing, metallographic examination, chemical composition analysis and so on. The results show that:the main reasons for the fracture of the shaft were that the defects existed in the forging structure and the quenching and tempering heat treatment didn't carry out, which greatly reduced the strength of the shaft; the secondary reason was that the surface grinding hardening layer formed on the surface of the shaft due to the overheating grinding, which increased the brittleness and tensile stress of the shaft surface, and made the micro cracks appear on the shaft surface.