There are three purposes of post-weld heat treatment

There are three purposes of post-weld heat treatment: eliminating hydrogen, eliminating welding stress, and improving 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.

Stress-relieving heat treatment is to reduce the yield strength of the welded workpiece at a high temperature to achieve the purpose of relaxing the welding stress. There are two commonly used methods: one is the overall high temperature tempering, that is, the whole weldment is put into a heating furnace, slowly heated to a certain temperature, then kept for a period of time, and finally cooled in the air or in the furnace. With this method, 80%-90% of welding stress can be eliminated. Another method is local high-temperature tempering, that is, only heating the weld and its nearby area, and then slowly cooling it to reduce the peak of the welding stress, so that the stress distribution is relatively gentle, and the purpose of partially eliminating the welding stress.

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, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.

The hydrogen elimination treatment is to keep the temperature in the heating temperature range of 300 to 400 degrees for a period of time. The purpose is to accelerate the escape of hydrogen in the welded joint, and the effect of hydrogen elimination treatment is better than that of low temperature heat. Post-welding and post-weld heat treatment, prompt post-weld heat and hydrogen elimination treatment are one of the effective measures to prevent welding cold cracks. For thick-walled pressure vessels and other important product components with a thickness of more than 100mm, in the welding process, to prevent Hydrogen-induced cracks caused by the accumulation of hydrogen in multi-layer welding of thick plates should be subjected to 2 to 3 intermediate hydrogen elimination treatments.

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