某电厂在停炉检修时发现一主汽门阀盖螺栓断裂失效，采用宏观分析、化学成分分析、硬度试验、金相分析、断口以及能谱分析等方法对螺栓断裂原因进行了分析。结果表明：螺栓断裂的主要原因为螺栓显微组织粗大且不均匀，在长期高温和应力作用下，局部位置的晶界发生蠕变产生蠕变孔洞；然后在腐蚀介质的作用下，裂纹从晶界蠕变孔洞处萌生并沿晶界扩展，最终造成一次性沿晶脆性断裂。

The main steam valve cover bolt in a power plant was found fractured when the boiler was shut down for maintenance. Means such as macro analysis, chemical composition analysis, hardness test, metallographic analysis, fracture and energy spectrum analysis were used to analyze the causes for the bolt fracture. The results show that the main reason for the bolt fracture was that the microstructure of the bolt was coarse and uneven. Under the action of long-term high temperature and stress, creep happened and creep cavities appeared at local grain boundary. Then under the action of the corrosive medium, cracks generated from the grain boundary creep cavities and expanded along the grain boundary, which eventually resulted in the one-time intergranular brittle fracture.