PWHT

PWHT (Post Weld Heat Treatment) is necessary to relieve internal stresses caused by the quenching of weld metal from molten condition to frozen condition or due to metallurgical changes that occur during solidification of metal at high rate of quenching of metal or both. In a pipe butt weld, the V-joint is around the pipe circumference and this does not allow the weld to freely contract in any preferential direction, thus leaving residual stresses in the weld joint. PWHT allows the stresses to be relieved by the process of re-heating and cooling slowly to room temp. Transformation of grain structure from Austenite to Ferrite at high speed during quenching of high carbon steels or Alloy Steels causes formation of Martensitic sturcture (needle like structure caused by trapped carbon atoms) which is brittle and prone to cracking. PWHT transforms the martensite to ferritic structure by allowing the carbon atoms to freely migrate and relieves the stresses.