Post Weld heat treatment (PWHT), or stress relief as it is sometimes known, is a method for reducing and redistributing the residual stresses in the material that have been introduced by welding.
The extent of relaxation of the residual stresses depends on the material type and composition, the temperature of PWHT and the soaking time at that temperature. A commonly used guideline for PWHT is that the joint should be soaked at peak temperature for 1 hour for each 25mm (1 inch) of thickness, although for certain cases a minimum soak time will be specified.In addition to reduction and redistribution of residual stresses, PWHT at higher temperatures permits some tempering, precipitation or ageing effects to occur. These metallurgical changes can reduce the hardness of the as-welded structure, improving ductility and reducing the risks of brittle fracture. In some steels, however, ageing/precipitation processes can cause deterioration in the mechanical properties of the steel, in which case, specialist advice should be taken on the appropriate times and temperatures to use.
The necessity for PWHT depends on the material and the service requirements. Other factors that influence the need for PWHT are the welding parameters and the likely mechanism of failure. In some standards, PWHT is mandatory for certain grades or thicknesses, but where there is an option, cost and potential adverse effects need to be balanced against possible benefits. The energy costs are generally significant due to the high temperatures and long times involved, but costs associated with time delays may be more important. Detrimental effects include distortion, temper embrittlement, over-softening and reheat cracking, which means that control of heating and cooling rates, holding temperature tolerances and the times at temperature are extremely important, and must be carefully controlled in order to realize the full benefit of the process.