Q. Explain about Postweld Heat Treatment?

Pressure vessels shall be postweld heat treated per applicable sections, in accordance with applicable material specifications or when specified by the User, due to service such as ammonia, caustics, amines, or wet hydrogen sulfide. Alternative Postweld Heat Treatment Requirements For Carbon And Low Alloy Steels, shall not be employed.

PWHT provides reduction of residual stresses, due to forming and welding, and results in softening of heat affected zones. Some steels can be damaged within certain temperature zones below PWHT temperature. A Materials Engineer shall be consulted regarding the need for postweld heat treatment, and dependent on service conditions. The resulting recommendation shall be included on the Data Sheet.