Failure Analysis on Frature of a Main Steam Valve Cover Bolt

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.