Why is post-weld heat treatment (PWHT) sometimes performed?

Post-weld heat treatment (PWHT) is performed primarily to relieve residual stresses that can develop during the welding process and to enhance the mechanical properties of the welded joint. When materials are welded, they experience rapid heating and cooling cycles, leading to thermal expansion and contraction. These temperature fluctuations can introduce internal stresses within the weld and the surrounding base material.

By applying heat through PWHT, these residual stresses can be alleviated, reducing the risk of defects such as cracking and distortion. Additionally, PWHT can improve mechanical properties, such as toughness, ductility, and hardness, ultimately increasing the performance and lifespan of the welded structure. This process is particularly important in high-stress applications or for materials that are sensitive to the effects of residual stresses, such as alloy steels, pressure vessels, and pipelines.

Other options do not accurately reflect the primary purpose of PWHT. Enhancing the aesthetic appearance of a weld or increasing the speed of the welding process are not goals of heat treatment. Cooling the weld faster contradicts the purpose of applying heat in PWHT, as the treatment aims to stabilize the weld through controlled heating rather than quick cooling.