The importance of preheating

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

(1) Preheating can slow down the annealing speed after welding, which is conducive to the escape of hydrogen in the weld metal and avoids hydrogen-induced cracks. At the same time, it reduces the welding and heat-affected zone, the degree of quenching, and improves the crack resistance of the welded joint.

(2) Preheating can reduce welding stress. Even partial preheating or overall preheating, the welding area can reduce the temperature difference between welders (also called temperature gradient). 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 degree of restraint of the welded structure, especially when reducing corner joint restraint. As the preheating temperature increases, the incidence of cracks decreases.

The choice of preheating and interpass temperature is not only related to the chemical composition of steel and welding, but also related to the rigidity of the welded structure, welding method, and ambient temperature. These factors should be considered comprehensively. In addition, the uniformity and uniformity of the preheating temperature of the steel plate in the thickness direction in the welding area has an important impact on reducing welding stress. The local preheating width should be based on the constraints of the welder. Generally, it should be three times the thickness of each layer around the weld zone and not less than 150-200 mm. If the preheating is not uniform, it will not only reduce the welding stress, but also increase the welding stress.