The Role of Preheating Before Welding and Heat Treatment After Welding

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The welding of important components, the welding of alloy steel and the welding of thick parts all require preheating before welding. The main functions of preheating before welding are as follows:

1. Preheating can slow down the cooling rate after welding, facilitate the escape of diffusive hydrogen in the weld metal, and avoid hydrogen-induced cracks. At the same time, it also reduces the hardening degree of the weld and the heat-affected zone, and improves the crack resistance of the welded joint.

2. Preheating can reduce welding stress. Even partial preheating or overall preheating can reduce the temperature difference (also called temperature gradient) between the welded workpieces in the welding area. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, which is beneficial to avoid welding cracks.

3. Preheating can reduce the degree of restraint of the welded structure, especially for reducing the degree of restraint of corner joints. With the increase of preheating temperature, the incidence of cracks decreases.

The choice of preheating temperature and interlayer temperature is not only related to the chemical composition of the steel and welding rod, but also related to the rigidity of the welded structure, welding method, and ambient temperature, which should be determined after comprehensive consideration of these factors.

In addition, the uniformity of the preheating temperature in the thickness direction of the steel plate and the uniformity in the weld area have an important effect on reducing welding stress. The width of the local preheating should be determined according to the restraint of the workpiece to be welded. Generally, it should be three times the wall thickness around the weld area, and it should not be less than 150-200 mm.

If the preheating is uneven, not only will it not reduce the welding stress, but will increase the welding stress.

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 value of the welding stress, so that the stress distribution is relatively gentle, and the purpose of partially eliminating the welding stress is achieved.

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.

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