40CrNiMo钢主轴在使用过程中发生断裂。采用宏观观察、化学成分分析、硬度测试及显微组织分析等方法，分析了该40CrNiMo钢主轴断裂的原因。结果表明：主轴的倒角曲率半径偏小且材料中存在氮化物，使应力集中程度严重，导致裂纹源产生；粗大的晶粒降低了主轴的解理断裂强度，使疲劳扩展速率加大，最终导致主轴发生早期疲劳断裂。最后根据主轴失效原因提出了改进措施。

A main shaft of 40CrNiMo steel fractured during use. The fracture reasons of the main shaft of 40CrNiMo steel were analyzed by means of macroscopic observation, chemical composition analysis, hardness testing, and microstructure analysis. The results show that:the chamfer curvature radius of the main shaft was relatively small, and the nitrides existed in the material, which made the stress concentration more severe and caused the generation of the crack source; coarse grains decreased the cleavage fracture strength of the main shaft and increased the fatigue spreading rate, and finally the early fatigue fracture occurred to the main shaft. In the end, the improvement measures were put forward according to failure reasons of the main shaft.