What Does Post Weld Heat Treatment (PWHT) Mean?
A post weld heat treatment is a process that involves elevating the temperature of a material or materials following a welding process. A post weld heat treatment is performed to alleviate residual stresses, increase the strength, increase or decrease the hardness, and reduce the risk of cracking. An array of heating processes can be used to carry out post weld heat treatment.
Corrosionpedia Explains Post Weld Heat Treatment (PWHT)
Post weld heat treatment is critical for precipitation hardened alloys that have been welded because they rely on precipitates to block dislocations in the crystalline structure of the metal to increase their strength and hardness. These precipitates are formed to the correct size through a heat treatment process known as artificial aging. When a precipitation hardened metal is exposed to the heat from welding, the precipitates in the heat affected zone of that metal are likely to have been changed and rendered less effective. Therefore, it is important to post weld heat treat these materials so that the precipitates are of the correct size and to reduce the risk of weaker material near the weld area.
Post weld heat treatment is used to alleviate residual stresses following a welding process. Welding, especially in thick sections of materials that are highly restrained, causes residual stresses to form because of the localized heating and rapid cooling during weld puddle solidification. The residual stresses left by welding can create an opportunity for hydrogen-induced cracking to occur. To prevent this, these residual stresses should be removed.
Not only can residual stress cause hydrogen cracking, but so too can a susceptible microstructure, particularly brittle microstructures such as martensite or bainite. When welding is performed on a steel with high amounts of carbon, the rapid heating and cooling of the welding process can cause these hard, brittle microstructures to form. These microstructures are more susceptible to hydrogen cracking than that of ferrite or pearlite. To reduce the risk of hydrogen cracking, a post weld heat treatment is used to heat the steel and allow it to cool slowly to reduce the amount of brittle microstructure in the steel, thus reducing the risk of hydrogen cracking. This also may increase the ductility of the steel.