某分动箱的20CrMnTi钢齿轮在工作过程中发生断裂。采用宏观分析、微观分析、化学成分分析、硬度测试、硬化层深度测量、非金属夹杂物分析、金相检验等方法对齿轮的断裂原因进行了分析。结果表明：齿面上残留的加工刀痕导致应力集中，在周期载荷的作用下，疲劳裂纹源首先在残留的加工刀痕较深处形成，随后裂纹逐渐扩展，最终齿轮发生疲劳断裂。

The fracture occurred in the working process of 20CrMnTi steel gear in a transfer case. The fracture causes of the gear were analyzed by means of macro analysis, micro analysis, chemical composition analysis, hardness test, depth measurement of hardened layer, analysis of non-metallic inclusions and metallographic examination. The results show that the residual tool marks on the tooth surface led to stress concentration. Under the action of periodic load, the fatigue crack source firstly formed in the deep part of the residual tool mark, then the crack gradually expanded, and finally the gear fatigue fracture occurred.