Post-weld heat treatment is performed to what purpose?

• A. To adjust the chemical composition of the weld.
• B. To re-weld defective areas.
• C. To remove oxide layers.
• D. To relieve residual stresses and improve properties.

Answer: (D)

Post-weld heat treatment is used to relieve the residual stresses that build up in and around the weld during cooling and to improve the weld’s mechanical properties. Heating the welded area to a controlled temperature below melting allows time for the metal's internal stresses to relax and for the microstructure to become more stable, which reduces the risk of cracking, lowers hardness, and enhances toughness and ductility. This makes the joint more reliable under service conditions and can help prevent hydrogen-assisted cracking in susceptible steels. It doesn’t change chemical composition, repair defects by re-welding, or remove oxide layers, which are handled by other processes.