What is post-weld heat treatment (PWHT) and when is it used?

• A. Cooling the weld immediately after welding
• B. Painting the weld with primer
• C. Controlled heating after welding to relieve stresses and improve toughness
• D. Inspecting with magnetic particle testing

Answer: (C)

Post-weld heat treatment is a controlled heating of a welded joint after welding to relieve residual stresses and improve toughness and ductility. By heating to a specified temperature, holding for a set time, and then cooling in a controlled way, the internal stresses formed during welding are reduced and the microstructure in the weld and heat-affected zone becomes more stable. This helps prevent cracking, reduces hardness near the weld, and improves performance in service, especially for thicker sections or high-strength steels and other critical welds. It’s used when codes, standards, or service conditions require reduced residual stress or increased toughness. It’s not about cooling, painting, or inspection.