Heat Treatment for Pressure Vessels - Post-Weld Heat Treatment

Heat treatment, as a traditional and effective method for improving and restoring metal properties, has long been a relatively weak link in the design and manufacturing processes of pressure vessels. This discussion will focus on the heat treatment involved in pressure vessels—post-weld heat treatment (also known as stress-relief heat treatment).

Objectives

1. Relieve residual welding stresses.

2. Stabilize the shape and dimensions of the structure, reducing distortion.

3. Improve the properties of the base material and the welded zone, including:

a. Enhancing the plasticity of the weld metal.

b. Reducing the hardness of the heat-affected zone.

c. Improving fracture toughness.

d. Enhancing fatigue strength.

e. Restoring or improving the yield strength that has been reduced during cold forming.

4. Increase resistance to stress corrosion.

5. Further release harmful gases, especially hydrogen, from the weld metal to prevent delayed cracking.

Post-weld heat treatment utilizes the reduction in the yield limit of metal materials at elevated temperatures, allowing plastic flow to occur in areas with high stress, thereby achieving the goal of eliminating residual welding stresses. Simultaneously, it improves the plasticity and toughness of the welded joint and the heat-affected zone, enhancing resistance to stress corrosion. This stress-relief method is widely adopted in pressure vessels made of carbon steels and low-alloy steels with a body-centered cubic crystal structure.