Preheat steel structure before welding.
Preheating before welding is very important to ensure welding quality. Welding of important components, welding of alloy steel and 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, which is conducive to the escape of diffusible hydrogen in the weld metal and avoids hydrogen cracks. At the same time, the degree of hardening of the weld and heat affected zone is reduced, and the crack resistance of the welded joint is improved.
Preheating reduces welding stress. Uniform localized preheating or global preheating reduces temperature differences (also known as temperature gradients) between welded workpieces in the welding area. In this way, the welding stress is reduced on the one hand, and the welding strain rate is reduced on the other hand, which is beneficial to avoid welding cracks.
3) Preheating can reduce the constraints of welded structures, especially the constraints of corner joints. As the preheating temperature increases, the incidence of cracks decreases.
The choice of preheat temperature and interpass temperature is not only related to the chemical composition of the steel and electrode, but also to the stiffness of the welded structure, the welding method and the ambient temperature. In addition, the uniformity of the preheating temperature in the thickness direction of the steel plate and the weld zone has an important influence on reducing the welding stress. The width of local preheating should be determined according to the degree of restraint of the welding workpiece, generally it should be three times the wall thickness around the welding area, not less than 150-200mm. If the preheating is not uniform, not only will it not reduce the welding stress, but it will increase the welding stress.
