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

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

• A. To change the chemical composition of the weld metal.
• B. To increase surface roughness for paint adhesion.
• C. To reduce weight of the joint.
• D. To relieve residual stresses and improve toughness of the heat-affected zone.

Answer: (D)

Welding creates large residual stresses from the rapid heating and cooling, especially in the heat-affected zone. Post-weld heat treatment is done to relieve those stresses by heating the joint to a suitable temperature and holding it to allow the metal to relax, then cooling slowly to avoid introducing new stresses. This tempering also softens hard microstructures formed during welding, improving toughness and reducing the risk of brittle cracking in the joined area. It can also help reduce distortion and, in some materials, mitigate hydrogen-related cracking. It does not change the chemical composition of the weld metal, nor is it intended to alter surface roughness or reduce weight. Hence, the primary purpose is to relieve residual stresses and improve toughness of the heat-affected zone.

Welding creates large residual stresses from the rapid heating and cooling, especially in the heat-affected zone. Post-weld heat treatment is done to relieve those stresses by heating the joint to a suitable temperature and holding it to allow the metal to relax, then cooling slowly to avoid introducing new stresses. This tempering also softens hard microstructures formed during welding, improving toughness and reducing the risk of brittle cracking in the joined area. It can also help reduce distortion and, in some materials, mitigate hydrogen-related cracking. It does not change the chemical composition of the weld metal, nor is it intended to alter surface roughness or reduce weight. Hence, the primary purpose is to relieve residual stresses and improve toughness of the heat-affected zone.