Abstract: The reasons of the fatigue failure of flexspline are analyzed by observation analysis， observation and hardness determination of fracture morphology of microstructure. The results show that the micro-structure of the flexspline of the failed harmonic reducer are tempered troostite， and there is more severe grade 3 banded structure on the flexspline . Although the 4.42 μm micropores caused by MnS and Al₂O₃ inclusions exist， they are not the main cause of breakage due to its small size and number . Meanwhile the overall grain size of the sample， especially the grain size at fracture crack， is coarse， and the grain at the fracture crack is thicker and more uneven. the grain size at the fracture crack is 1.4 times that away from the fracture grain. The coarse grain assembly leads to smaller grain boundary area， which not only leads to more uneven plastic deformation， but also more prone to stress concentration， which causes primary cracks， and is difficult to hinder crack expansion， resulting in fracture failure of the sample.Therefore， the primary austenite grain is the main cause for fatigue fracture of flexspline. The grain refinement can be achieved by circulating heat treatment or adding normalizing pretreatment.

Key words: Harmonic Reducer, Flexspline, Fracture Failure

摘要： 通过显微组织的观察分析、断口形貌的观察和硬度测定，分析造成柔轮疲劳断裂失效的原因。结果表明，失效柔轮的显微组织为回火屈氏体，柔轮上存在较严重的3级带状组织。虽然存在MnS和Al₂O₃夹杂物引起的4.42 μm微孔，但由于其尺寸和数量较小，不是导致其断裂的主要原因。样品的晶粒整体上较粗大，断口裂纹处晶粒更粗大且更不均匀，断口裂纹处的晶粒尺寸是远离断口晶粒的1.4倍。晶粒粗大使晶界面积减小，导致塑性变形不均匀，易产生应力集中，从而产生初生裂纹，同时也不利于阻碍裂纹的扩展，导致样品发生断裂失效。因此，原奥氏体晶粒粗大是导致柔轮疲劳断裂主要原因。可通过循环热处理或增加正火预处理，达到细化晶粒的作用。

关键词: 谐波减速器, 柔轮, 断裂失效