Why do we need post-weld heat treatment during welding

There are three purposes of post-weld heat treatment: eliminating hydrogen, eliminating welding stress, optimizing weld structure and overall performance.

The post-welding hydrogen elimination treatment refers to the low-temperature heat treatment performed after the welding is completed, when the weld has not been cooled to below 100°C. The general specification is to heat to 200~350℃ and keep it for 2-6 hours. The main function of post-welding hydrogen elimination treatment is to accelerate the escape of hydrogen in the weld and heat-affected zone, which is extremely effective in preventing weld cracks during low-alloy steel welding.

In the welding process, due to the unevenness of heating and cooling, as well as the constraints or additional constraints of the component itself, welding stress will always be generated in the component after the welding work is completed. The existence of welding stress in the component will reduce the actual load-bearing capacity of the welded joint area and produce plastic deformation. In severe cases, it will also lead to the destruction of the component.

After some alloy steel materials are welded, their welded joints will have a hardened structure, which will deteriorate the mechanical properties of the material. In addition, this hardened structure may cause damage to the joint under welding stress and hydrogen. After heat treatment, the metallographic structure of the joint is improved, which improves the plasticity and toughness of the welded joint, thereby optimizing the comprehensive mechanical properties of the welded joint.

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