A pressure vessel fabricated from P365GH steel (a pressure vessel grade carbon steel) with a wall thickness of 30 mm has just been welded. The engineering drawing specifies post-weld heat treatment (PWHT). What is the primary purpose of performing PWHT on this pressure vessel?

Question: A pressure vessel fabricated from P365GH steel (a pressure vessel grade carbon steel) with a wall thickness of 30 mm has just been welded. The engineering drawing specifies post-weld heat treatment (PWHT). What is the primary purpose of performing PWHT on this pressure vessel?

Answer options:

• To increase the hardness of the weldment for improved wear resistance.
• To enhance colour matching between the weld and base metal. ✅ To reduce residual stresses from welding and improve toughness.
• To prevent solidification cracking during the cooling phase.

Correct answer: To reduce residual stresses from welding and improve toughness.

Explanation: The primary purpose of PWHT is to reduce residual stresses that build up during welding, which can lead to distortion or cracking in service. It also refines grain structure and improves the toughness of the weld and heat-affected zone. Increasing hardness is generally not a goal for PWHT in pressure vessels, and PWHT occurs after solidification, so it cannot prevent solidification cracking.