Post-weld heat treatment (PWHT) is typically required to accomplish what?

• A. Increase weld metal hardness
• B. Shorten welding time
• C. Reduce residual stresses and improve toughness; relieve hydrogen-induced cracking and improve ductility
• D. Change filler metal composition

Answer: (C)

Post-weld heat treatment is used to relieve the stresses locked into the weld and heat-affected zone, which helps the metal respond more toughness and ductility. Welding creates high internal stresses and can trap hydrogen in some steels; those stresses can lead to brittle fracture or hydrogen-induced cracking if not managed. By heating the welded piece to a controlled temperature, holding it to allow stress relaxation and hydrogen to diffuse out, and then cooling slowly, the metal becomes less prone to cracking and more able to absorb energy without breaking. This combination—lower residual stresses, improved toughness, and reduced risk of hydrogen-related cracking—explains why post-weld heat treatment is performed. It does not aim to increase hardness, shorten welding time, or change the filler metal composition.