Post weld heat treatment
- The material strength of a part needs to retain after welding. To do so, Post Weld Heat Treatment (PWHT) is often performed. PWHT is a method of hardness control, and can also reduce residual stresses.
- Generally, after welding, materials go through the Post Weld Heat Treatment. PWHT has different objectives other than preheat or inter-pass heating. Engineers need to ensure that temperatures do not drop below specified degree. Hence, you may have to apply PWHT method with the work piece at temperatures up to 600° F (316° C).
- Engineers need to perform Post Weld Heat Treatment (PWHT) in the right way and also, should not neglect the process. Otherwise, residual stresses combine with load stresses, and exceed materials’ design limitations.
Limitations may lead to:
- Weld failures
- High cracking potential
- Susceptible to brittle fracture
Benefits of PWHT can be primary and consequential.
- Relaxation of residual stresses
- Hydrogen removal
- Avoidance of hydrogen induced cracking
- Dimensional stability
- Improved ductility toughness
- Corrosion resistance
Post Weld Heat Treatment (PWHT) encompasses different types of potential treatments. Most common types discussed below are:
One common occurrence during welding is Hydrogen Induced Cracking. This occurs when high levels of ambient hydrogen permeate into a material. Heating the material after welding diffuses hydrogen from the welded area. This method of preventing Hydrogen Induced Cracking is the post heating process. This process should begin immediately after the weld is complete. Even after cooling, the material should maintain a hot temperature for a couple of hours. This depends on the type and thickness of the material.
After completion, the welding process might leave a large number of residual stresses. This increases the potential for stress corrosion and Hydrogen Induced Cracking. PWHT releases these stresses and cracking. In PWHT, temperature of the material is set to change whenever necessary.
Decision on applying PWHT process to a material depends on several factors. These factors include a check for previous heat treatments and alloying system. PWHT may cause damages to certain materials, but essential for most of them.
Temperature gradients essential to local PWHT produces sub-critical, inter-critical and super-critical temperature regions. Prior heat treatment to the material results in harmful effects. These affect certain properties such as, tensile/yield strength and impact toughness. This includes local irregularities as well. Also, reduced material strength at supercritical temperatures creates a greater likelihood for distortion. Post Weld Heat Treatment is carried out in Bogie Hearth Furnace and Chamber Furnace