What is the primary purpose of post-weld heat treatment (PWHT)?

• A. Improve corrosion resistance
• B. Relieve residual stresses and improve toughness
• C. Increase welding speed
• D. Eliminate the need for fillers

Answer: (B)

Relieving residual stresses and improving toughness is the main purpose of post-weld heat treatment. Welding creates high temperature gradients, so as the metal cools, it traps internal stresses that can lead to distortion and brittle fracture, especially in thicker sections or high-strength steels. PWHT heats the weld region to a controlled temperature and holds it long enough for the metal to relax those stresses and temper the heat-affected zone, reducing hardness and increasing ductility. This combination lowers the risk of cracking and improves overall toughness, which is why PWHT is done. It isn’t primarily about corrosion resistance, it doesn’t speed up the welding process, and it doesn’t replace fillers.