I. Preheating before first welding
Pre-welding and post-welding heat treatment are very important to ensure the welding quality. Welding of important components, welding of alloy steel and welding of thick components 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 weld metal, and avoid hydrogen-induced cracking. At the same time, the hardening degree of weld and heat affected zone is reduced, and the crack resistance of welded joint is improved.
2) Preheating can reduce welding stress. Uniform local or global preheating can reduce the temperature difference (also known as temperature gradient) between the welded workpieces in the welding area. In this way, on the one hand, the welding stress is reduced, on the other hand, the welding strain rate is reduced, which is conducive to avoiding welding cracks.
3) Preheating can reduce the restraint of welded structures, especially for corner joints. With the increase of preheating temperature, the incidence of cracks decreases.
The selection of preheating temperature and interlayer temperature is not only related to the chemical composition of steel and welding rod, but also to the rigidity of welding structure, welding method and environmental temperature, which should be determined after comprehensive consideration. In addition, the uniformity of preheating temperature in the direction of steel plate thickness and in the weld zone has an important impact on reducing welding stress. The width of local preheating should be determined according to the binding degree of the welded workpiece. Generally, it should be three times the wall thickness around the weld zone and not less than 150-200 mm. If the preheating is not uniform, not only does not reduce the welding stress, but also increases the welding stress.
II. Post-weld Heat Treatment
There are three purposes of post-weld heat treatment: eliminating hydrogen, eliminating welding stress, improving weld structure and comprehensive performance.
Post-weld dehydrogenation treatment refers to the low-temperature heat treatment after the completion of welding, when the weld has not cooled to below 100 C. The general specification is to heat to 200 - 350 C for 2 - 6 hours. The main function of post-weld dehydrogenation treatment is to accelerate the escape of hydrogen in weld and heat affected zone, which has a remarkable effect on preventing welding cracks of low alloy steel during welding.
In the welding process, due to the inhomogeneity of heating and cooling, and the restraint or additional restraint of the component itself, the welding stress will always occur in the component after the completion of the welding work. The existence of welding stress in the component will reduce the actual bearing capacity of the welded joint area, produce plastic deformation, and even lead to the damage of the component when it is serious.
Stress relief heat treatment is to reduce the yield strength of welded workpiece at high temperature so as to relax the welding stress. There are two commonly used methods: one is the whole high temperature tempering, that is, putting the whole welding parts into the heating furnace, slowly heating to a certain temperature, then holding for a period of time, and finally cooling in the air or furnace. This method can eliminate 80%-90% welding stress. Another method is local high temperature tempering, that is, only heating the welding seam and its adjacent area, then slowly cooling, reducing the peak value of welding stress, so that the stress distribution is relatively flat, so as to partially eliminate the welding stress.
After welding, some alloy steels will have hardened joints, which will deteriorate the mechanical properties of the materials. In addition, under the action of welding stress and hydrogen, the hardened structure may lead to joint failure. After heat treatment, the metallographic structure of the welded joint is improved, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.