Reasons for Fracture of Overflow Valve Bolts in Nuclear Power Plant

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.