Post Weld Heat Treatment

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Post weld heat treatment, commonly referred to as stress relief or low tension annealing is a necessary process in the welding of metals. This is because during the welding process high temperatures and rapid cooling creates stresses in the metallurgical structures of the material being welded. The PWHT removes these unwanted stresses to ensure that the material can perform as intended in the final product under dynamic loading conditions.

What is heat treatment and why it is done?

The PWHT process involves heating the material to a certain temperature and allowing it to soak at that temperature for a period of time before slowly cooling it back down. The purpose is to reduce the levels of hydrogen induced cracking (HIC) that can occur when the weld and the heat affected zone (HAZ) are allowed to cool down to room temperature rapidly.

HIC occurs when a steel has an impurity that can cause hydrogen to diffuse into the weld and the HAZ area and weaken those areas of the material making them susceptible to cracking. This is often caused by a hard, brittle microstructure of the steel such as martensite or bainite, which are more susceptible to hydrogen cracking than a ductile microstructure such as ferrite or pearlite. The slow cooling that occurs during PWHT helps to reduce this risk by converting these brittle microstructures into a softer, more ductile microstructure.

The process also allows for some tempering, precipitation or artificial ageing effects to take place during the PWHT cycle that is used which can further improve the mechanical properties of a welded joint, such as toughness and ductility. This is particularly important for pre-hardened alloys that are welded, which rely on precipitates to block dislocations in their crystalline structure and increase their strength and hardness.

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