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

• A. Relieve residual stresses
• B. Increase hardness
• C. Decrease ductility
• D. Reduce need for shielding gas

Answer: (A)

Post-weld heat treatment is used to relieve the residual stresses that are locked into the weld and surrounding metal during welding. When metal cools after welding, different sections contract at different rates, creating internal stresses. If these stresses remain, they can lead to distortion, cracking, or brittle failure in service. By heating the welded area to a controlled temperature and then cooling it slowly, the material can relax these stresses and achieve a more stable, tougher microstructure.

While PWHT can also influence hardness and toughness (e.g., tempering that reduces hardness in some steels), its primary purpose is stress relief. Increasing hardness would typically be undesirable after welding, and decreasing ductility is not the goal of PWHT. Shielding gas is a process parameter of the welding operation, not the heat treatment that follows. Therefore, the best answer is relief of residual stresses.