Why is post-weld heat treatment (PWHT) used on nuclear components?

• A. To relieve residual stresses from welding and improve toughness.
• B. To increase creep resistance.
• C. To change alloy composition.
• D. To inhibit corrosion completely.

Answer: (B)

Post-weld heat treatment is used to relieve the residual stresses created by welding and to improve the toughness of the welded joint. Welding subjects the material to rapid heating and cooling, which locks in tensile stresses in the weld metal and heat-affected zone. If left, these stresses can promote brittle fracture, fatigue cracking, or cracking under service loads. PWHT allows atoms to diffuse and rearrange, reducing those residual stresses and tempering any hard, brittle microstructures formed during welding, which boosts toughness and resistance to cracking. It can also help hydrogen diffusion out of the steel, mitigating hydrogen-related cracking in some cases. PWHT doesn’t change the alloy’s composition and isn’t primarily aimed at increasing creep resistance or completely preventing corrosion.