Which statement about post-weld heat treatment and its purpose is true?

• A. PWHT creates a protective oxide layer
• B. PWHT is used to relieve residual stresses and improve ductility in certain steels and critical welds.
• C. PWHT cools the weld immediately to room temperature
• D. PWHT adds filler material after welding

Answer: (B)

Post-weld heat treatment is a controlled heating and cooling process used to relieve the stresses built up during welding and to temper the microstructure in the heat-affected zone, which improves ductility and toughness in certain steels and critical welds. By heating to a specified temperature, holding, and then cooling slowly in a controlled way, internal residual stresses are redistributed rather than locked in, reducing the risk of brittle fracture, distortion, and cracking and helping the weld meet toughness and code requirements. It’s not about creating a protective oxide layer, which is a surface effect not the goal of PWHT. It also doesn’t involve cooling the weld to room temperature in an uncontrolled way, since the cooling must be controlled to achieve the desired material properties. And it doesn’t add filler material after welding; PWHT is separate from the welding process itself. The main idea is to improve the weld’s performance by reducing residual stresses and increasing ductility in the appropriate steels and welds.