Abstract: The transmission output shaft broke during the comprehensive durability test of a certain vehicle. Via chemical composition detection, microstructure observation, fracture morphology and energy spectrum analysis, the fracture causes of transmission output shaft made of 20CrMnTi are analyzed. The results show that during the casting and solidification process of raw materials, chromium (Cr) and manganese (Mn) elements converge in the dendrites at a slow diffusion rate, and the solute elements in the liquid phase region cause element segregation at the grain boundaries, thereby precipiting Cr and Mn compounds and weakening the grain boundaries. Under the comprehensive action of internal stress and phase transition stress generated by subsequent heat treatment process, micro-cracks occur on the weakened grain boundaries. When subjected to alternating load, the microcracks gradually spread inward along the original austenite grain boundary, and finally lead to the fracture of the parts. Users can learn from the direction of failure analysis introduced in this paper to control the occurrence of such failure problems at the user end.

Key words: 20CrMnTi; brittle fracture; grain boundary segregation of Cr and Mn elements; phase change stress; fracture microcrack

摘要： 某车型综合耐久试验过程中变速器输出轴发生断裂。通过化学成分检测、显微组织观 察、断口形貌及能谱分析等方法，对由 20CrMnTi 制成的变速器输出轴断裂原因进行分析。 结果表明，原材料熔铸凝固过程中铬（Cr）、锰（Mn）元素扩散速率较慢在枝晶间汇集，晶 间液相区溶质元素在晶界造成元素偏析，从而析出 Cr、Mn 化合物使晶界弱化，在后续热处 理工艺产生内应力和相变应力的综合作用下，弱化的晶界上产生微裂纹。当承受交变载荷时， 微裂纹沿原奥氏体晶界逐步向内扩展，最终导致零部件断裂。工程使用端可借鉴文章介绍的 失效分析方向，控制使用端此类失效问题的发生。

关键词: 20CrMnTi；脆性断裂；Cr、Mn 元素晶界偏析；相变应力；断口微裂纹