某40Cr钢齿轴在使用过程中发生早期断裂失效，通过宏观检验、化学成分分析及金相检验的方法，对齿轴断裂的原因进行了分析。结果表明: 齿轴在加工过程中，由于切削刀具吃刀量过大或刀具过钝，使齿轴表面产生铁屑翻皮卷曲，导致表层晶粒脱落及切削挤压微裂纹。同时原材料中非金属夹杂物过多，材料的强度大幅度降低，脆性显著增大，淬火应力集中形成开裂。齿轴服役承载时，淬火形成的裂纹继续扩展，最终导致齿轴断裂失效。

The 40Cr steel gear shaft was fractured incipiently during use. The fracture cause of the gear shaft was analyzed by macrographic inspection, chemical composition analysis and metallographic inspection. The results show that during the machining process, the flanging and curling scrap was generated on the surface of gear shaft due to too large cutting depth or blunt cutting tool. The surface grain shed down and cutting extrusion produced the microcrack. Meanwhile, too much nonmetallic inclusions existed in the raw material. As the result of it, the strength degraded and the brittlement increased significantly, and cracks were formed due to quenching stress concentration. The cracks continued to expand under the service load, and eventually led to the fracture failure of the gear shaft.