What is the primary purpose of PWHT (Post Weld Heat Treatment)?

What is the primary purpose of PWHT (Post Weld Heat Treatment)?

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

Answer: (B)

Post weld heat treatment is performed to relieve the residual stresses built up in the welded joint and surrounding heat-affected zone during welding. By heating to a prescribed temperature and then cooling slowly, the metal’s internal stresses relax, which reduces the risk of distortion, cracking (including hydrogen-induced cracking in susceptible steels), and brittle behavior. This stress relief is the main reason PWHT is used. Other effects can occur, such as tempering the microstructure to adjust hardness and toughness, which can improve ductility in some cases. However, improving ductility, while beneficial, is not the primary purpose. PWHT does not remove porosity, which is a defect formed during welding, nor is its main aim to directly improve corrosion resistance.

Post weld heat treatment is performed to relieve the residual stresses built up in the welded joint and surrounding heat-affected zone during welding. By heating to a prescribed temperature and then cooling slowly, the metal’s internal stresses relax, which reduces the risk of distortion, cracking (including hydrogen-induced cracking in susceptible steels), and brittle behavior. This stress relief is the main reason PWHT is used.

Other effects can occur, such as tempering the microstructure to adjust hardness and toughness, which can improve ductility in some cases. However, improving ductility, while beneficial, is not the primary purpose. PWHT does not remove porosity, which is a defect formed during welding, nor is its main aim to directly improve corrosion resistance.