PWHT is a controlled process in which a welded metal area is reheated to a temperature below its lower critical transformation temperature, and then held at that temperature for a specified amount of time. Heat treatment performed after welding has a specific meaning within the oil, gas, petrochemical, and nuclear industries. Industrial codes, such as the ASME Pressure Vessel and Piping Codes, often require mandatory PWHT on certain metals to ensure a safe design with optimal mechanical and metallurgical properties.
Resistance heating is widely used for PWHT. Ceramic tiles placed on the base metal are heated from energy released by the conductive wire resistance to electrical current. The heated ceramic tiles transfer heat from the attached conductive wire to the base metal through radiant and conductive heat where the pads are in contact with the metal. PWHT temperature is controlled by regulating the electricity running through the conductive wire.
