Post Weld Heat Treat
Post weld heat treat (PWHT) is the application of uniform heat after welding to a structure or component at temperatures low enough to avoid phase changes. It is often performed to relax residual stresses in the material, reduce the brittleness of certain microstructures and remove hydrogen contamination from a weld.
What are the 5 basic heat treatment?
PWHT is an essential part of the metallurgical process, and can significantly reduce the risk of weld failures and improve the strength and quality of a weld. It is especially important in reducing the susceptibility to brittle trans-granular cracking (TGRC) in welded pressure vessels and components, as well as for the risk of stress corrosion cracking.
TGRC is caused by a combination of load stresses and high levels of residual stresses in the material. Post weld heat treat is effective at lowering these levels of stress, but it cannot eliminate them completely. If not properly executed, PWHT can also reduce the tensile strength, creep strength and notch toughness of a weld, as well as increase its susceptibility to corrosion.
The most common reason for performing PWHT is to relieve a weld’s residual stresses. This is particularly true for carbon steels, which tend to form hard and brittle weld microstructures in the HAZ during the welding process due to rapid cooling as they heat up. Post weld heat treat can restore the ductility of these welds by heating the steel and allowing it to cool slowly.
Other reasons for PWHT may include normalizing (to refine a coarse weld microstructure and reduce stresses), tempering (to reduce the temperature range at which martensite is formed during welding) or regaining original properties for precipitation hardening alloys after welding.