PWHT is used primarily to reduce what?

• A. Increase ductility
• B. Reduce residual stresses
• C. Improve corrosion resistance
• D. Eliminate porosity

Answer: (B)

Post weld heat treatment is used to relieve residual stresses that remain after welding. Welding creates temperature gradients, so the weld metal and surrounding heat-affected zones cool and contract differently, locking in tensile stresses. These stresses can make the joint susceptible to cracking under service loads, especially in pressure vessels and piping. By heating the welded assembly to a controlled temperature and holding it, the metal can relax these stresses and rearrange its microstructure to a more stable, less brittle state. This stress relief is the primary benefit and helps improve toughness and reduce distortion.

Porosity is a defect in the weld metal that heat treatment doesn’t fix; it’s related to how the weld was made, not the subsequent heat treatment. Corrosion resistance is mainly about alloy composition and microstructure at the surface, which PWHT may influence indirectly but not as its primary purpose. Increasing ductility can occur as a secondary effect of stress relief and tempering, but the main reason PWHT is used is to reduce residual stresses.