某叉车桥壳端轴在使用约1 000 h后发生断裂，通过对断裂件的断口、化学成分、力学性能、显微组织等进行分析，找出了断裂原因。结果表明：该叉车桥壳端轴断裂属于疲劳断裂；断裂处过渡圆角半径过小且加工刀痕明显，导致此处产生应力集中；另外热处理工艺控制不当；一方面导致材料显微组织严重不均匀产生较大的内应力，另一方面未进行调质处理使材料力学性能和疲劳强度大幅度下降；上述因素是造成桥壳端轴疲劳断裂的主要原因。

The end axle of a forklift axle housing fractured after 1 000 h of use, and the fracture, chemical compositions, mechanical properties, microstructure of the fractured part were analyzed to identify the fracture reasons. The results show that the fracture of the end axle was fatigue fracture. Small fillet radius and evident machining marks at the fracture position resulted in serious stress concentration here. In addition, Due to the improper control of the heat treatment process, the microstructure of the material was seriously uneven and large internal stress was produced. On the other hand, the mechanical properties and the fatigue strength of the material decreased greatly because of no quenching and tempering treatment. The above mentioned factors were the main causes for the fatigue fracture of the end shaft of the forklift axle housing.