Post-weld heat treatment (PWHT) is a crucial step in the welding process as it helps to reduce residual stresses that can build up in the welded joint and reduce brittle fracture.
Residual stress can cause distortion and cracking in the material, potentially leading to failure of the welded structure. PWHT can help to reduce the stress by heating the material to a specific elevated temperature for a certain period of time, allowing the metal to relax and release the stress.