某40Cr钢汽车转向弯臂出现断裂故障，通过宏观分析、微观分析、化学成分分析、硬度测试、金相检验、受力分析、强度校核等方法对转向弯臂的断裂原因进行了分析。结果表明：转向弯臂断裂形式为双向弯曲疲劳断裂。断裂的根本原因是在弯臂R角表面存在机加工刀痕，产生了应力集中，且感应淬火表面热处理强化作用不足，使截面变化的过渡区R角处未能有效淬火而存在残余拉应力，导致裂纹在此处萌生，在转向循环应力作用下裂纹扩展直至发生疲劳断裂。

The 40Cr steel steering automobile bending arm fractured. The causes of fracture on steering bending arm were analyzed by means of macro analysis, micro analysis, chemical composition analysis, hardness test, metallographic inspection, stress analysis and strength check. The results show that the fracture form of steering bending arm is bidirectional bending fatigue fracture. The root cause of the fracture was that there were machining tool marks on the R corner surface of bending arm, resulting in stress concentration. And the strengthening effect of induction quenching surface heat treatment was insufficient, so that the R corner of the transition area with cross-section change can not be effectively quenched and there was residual tensile stress, resulting in crack initiation here, and the crack expanded until fatigue fractured under the action of steering cyclic stress.