某核电厂海水循环泵齿轮箱连接螺栓在服役过程中发生断裂。采用宏观观察、化学成分分析、硬度测试、金相检验、扫描电镜及能谱分析等方法对该螺栓断裂原因进行了分析。结果表明：该螺栓发生了疲劳断裂，螺栓表面脱碳层中铁素体降低了螺栓的表面硬度和耐磨性；螺栓在拆装过程中与螺栓孔边缘摩擦产生了折叠或微裂纹，这些折叠或微裂纹成为螺栓早期裂纹形成与扩展的疲劳源。

The connecting bolt of seawater circulating pump gearbox in a nuclear power plant fractured during service. The fracture reasons of the bolt were analyzed by macroscopic observation, chemical composition analysis, hardness test, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the bolt had fatigue fracture, and the ferrite in the decarburized layer on the bolt surface reduced the surface hardness and wear resistance of the bolt. During the disassembly and assembly of the bolt, the folding or microcracks were generated by friction with the edge of the bolt hole, and these folding or micro cracks became the fatigue source of early crack formation and propagation of the bolt.