Fracture analysis of 600 MW steam turbine valve bolts based on ultrasonic technology for coarse grains detection

摘要

Failure analysis of valve high temperature 20Cr1Mo1VTiB bolts used in a 600 MW steam turbine has been carried out. The valve bolts fractured with evidence of coarse grains. Chemical analysis, micro-structural characterization, fractography, mechanical properties measurement and ultrasonic detection were used for the analysis. The microstructure is frame structured bainite, coarse grains with serious directional were found. Which lead to the inferior impact toughness. It was concluded that all these factors produced brittle failure. In the meantime, the ultrasonic technology was be found which could efficiently to detect coarse grains of 20Cr1Mo1VTiB bolt. It is recommended to first guarantee the microstructure and mechanical properties of material. Secondly, use ultrasonic technology could detect coarse grains phenomenon of bolt. Finally, it is necessary to use the ultrasonic technology and metallographic examination technology to detect the coarse grains of all in-service 20Cr1Mo1VNbTiB or 20Cr1Mo1VTiB bolts, in order to avoid the same type of bolts fracture accident.