The residual stresses arise in the weld joints as a consequence of strains caused by solidification, phase change and contraction during welding.  The residual stresses cause detrimental effects to welded structure.

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Post weld heat treatment is required to relieve the residual stresses.  PWHT is done in accordance with the requirements for heat treatment referred to the relevant standards such as ASME B31.8, ASME B31.3, ASME B31.1, API 650, ASME Section VIII Division 1, AWS D10.10, etc..