Fracture reason of high temperature stud bolt of 20Cr1Mo1VNbTiB steel for steam turbine

The 20Cr1Mo1VNbTiB steel high temperature stud bolt used for steam turbine in a fossil-fuel power station fractured. The cause of bolt fracture was analyzed using methods such as macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis, mechanical property testing. The results show that there was a coarse grain zone at the root of the first thread of the stud. The microstructure changed from tempered bainite to martensite phase bainite. The plasticity and toughness of the material decreased, while the brittleness increased. Under the action of high temperature stress and alternating load, intergranular cracking and continuous expansion occurred in the coarse grain zone and finally led to brittle fracture of the stud. It was recommended to strictly control the smelting process and heat treatment process, standardized the installation and disassembly procedures of the studs, used non-destructive testing and hardness testing and other methods to carry out factory inspection of the studs to ensure their performance.