On-Site Post Weld Heat Treatment for Grade 91 Stellite Main Steam Control Valve Seat Crack Problem Repairing

Main steam control valves are one of the critical components in power plants as they are the last components in the steam piping system before entering the steam turbine. Any damaged parts from these valves can penetrate and severely damage steam turbines. Recently, as the power plants are forced to be operated under cyclical loading, the risk of control valve seat crack problem is accelerated due to differential expansion of the Stellite surface and the underneath surface which is usually made of Grade 91 steel due to elevated temperature operation. To protect the security of the power supply, power plants does not have extended duration, thus, on-site service is needed.

This paper demonstrates the on-site Stellite seat post weld heat treatment (PWHT) technique. With the aid of heating pad arrangement and induction heater, the final PWHT temperature specified in the welding specification procedure (WPS) of 740°C can be achieved. This enables the on-site service of valve seat repair and can be applied to other power plants.