某核电厂500 kV母线上的承压螺母发生断裂失效,采用化学成分分析、金相检验、硬度测试、断口形貌观察以及能谱分析等方法,对螺母的断裂原因进行分析。结果表明:开裂螺母材质为奥氏体不锈钢,由于碳含量较高,在晶界处析出了大量碳化物,造成晶界贫铬,提高了应力腐蚀敏感性。此外,螺母在生产加工过程中还存在加工硬化,进一步增加了螺母的应力腐蚀敏感性,同时,螺母所用原材料中存在较多B类和D类非金属夹杂物,促进了裂纹的萌生和发展。在海洋大气和预紧力共同作用下,最终导致螺母发生沿晶应力腐蚀开裂而失效。

The fracture failure of the pressure bearing nut on the 500 kV busbar of a nuclear power plant occurred. The fracture cause of the nut was analyzed by chemical composition analysis, metallographic examination, hardness test, fracture morphology observation and energy spectrum analysis. The results showed that the cracked nut was made of austenitic stainless steel. Due to the high carbon content, a large amount of carbide precipitated along the grain boundary, resulting in chromium depletion at the grain boundary, which increased the stress corrosion sensitivity. In addition, there was work hardening in the production process of nuts, which further increased the stress corrosion sensitivity of nuts. At the same time, there were many non-metallic inclusions of type B and D in the raw materials used for nuts, which promoted the initiation and development of cracks. Under the combined action of ocean atmosphere and preload, the nuts eventually failed due to intergranular stress corrosion cracking.