某核电厂溢流阀螺栓发生断裂。通过分析和观察断裂螺栓的化学成分、硬度、显微组织及宏微观形貌，探讨了螺栓断裂的原因，并提出了相应的建议。结果表明：失效螺栓的化学成分及组织正常，螺栓等级为12.9级，硬度接近标准上限；螺栓的断口具有沿晶特征形貌，断裂性质是氢致应力腐蚀开裂，为环境促进开裂机制作用的结果。

The bolts of overflow valve in a nuclear power plant fractured. Through analysis and observation of chemical composition, hardness, microstructure and macro/micro morphology of the fractured bolts, the reasons for bolt fracture were discussed. And some corresponding suggestions were proposed. The results show that the chemical composition and microstructure of the bolts were normal, the grade of the bolts was 12.9, and the hardness of the bolts was close to the upper limit of standard. The fracture of the bolt had an intergranular characteristic morphology, and the fracture property belonged to hydrogen-induced stress cracking, which was the result of environment promoting cracking mechanism.