某油田HY级抽油杆在服役过程中于扳手方和外螺纹台阶过渡处发生断裂。采用断口形貌观察、化学成分分析、显微组织观察、硬度测试及有限元模拟等方法，对其断裂原因进行分析。结果表明：失效抽油杆的断裂方式为脆性断裂，裂纹源位于扳手方和外螺纹台阶过渡位置的棱角处；断裂原因在于碳、钼元素含量低于标准要求，组织中出现含量较高的铁素体，导致抽油杆淬硬性和强度不足，以及扳手方与外螺纹台阶的过渡圆角半径不符合标准要求且过渡位置表面存在凹坑缺陷，导致较大应力集中。

A grade HY sucker rod in an oilfield fractured at the trasition between wrench side and external thread step during service. The fracture cause was studied by fracture morphology observation, chemical composition analysis, microstructure observation, hardness test and finite element simulation. The results show that the fracture mode of failed sucker rod was brittle fracture, and the crack source was located at the edge corners of the transition position between the wrench side and the external thread step. The cause of fracture was the content of carbon and molybdenum elements lower than that of the standard requirement and higher content of ferrite in microstructure led to insufficient hardenability and strength of sucker rods, and the transition fillet radius between the wrench side and the external thread step did not meet the standard requirement and there were pit defects on the surface of the transition position, resulting in greater stress concentration.