What is the purpose of post-weld heat treatment?

The purpose of post-weld heat treatment (PWHT) is to improve the mechanical properties and reduce residual stresses in welded joints. Welding involves localized heating and cooling, which can create high residual stresses in the weld metal and adjacent base metal. These stresses can reduce the weld's strength, ductility, and resistance to fatigue and corrosion. PWHT involves heating the entire welded component to a specific temperature, holding it at that temperature for a specified time, and then cooling it down at a controlled rate. This process relieves residual stresses by allowing the metal to creep or deform slightly, redistributing the stresses more evenly. PWHT can also improve the weld's microstructure, making it more uniform and less susceptible to cracking. In some cases, PWHT is required to temper or harden the weld metal, improving its strength and wear resistance. There are several different PWHT methods, including stress relieving, annealing, normalizing, and tempering. The specific method used depends on the type of metal, the size and shape of the component, and the desired properties. For example, PWHT is often used on high-strength steel weldments to prevent hydrogen-induced cracking, a type of cracking that can occur after welding due to the presence of hydrogen in the weld metal. PWHT is also used on pressure vessels and piping systems to ensure that the welds meet the required strength and toughness.