(1) Preheating can slow down the cooling rate after welding, make hydrogen diffuse and escape from the weld metal, and avoid cracking caused by hydrogen. It can also reduce the degree of hardening of the weld and the heat-affected zones to improve the crack resistance of the welded joint.
(2) Preheating can reduce welding stress. Uniform partial preheating or overall preheating will reduce the temperature of the welding zone 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, thereby avoiding welding cracks.
(3) Preheating can reduce the degree of bonding with the welded structure, especially the degree of bonding with corner joints, reduce the occurrence of cracks, and increase the preheating temperature.
The selection of welding preheating temperature and inter pass temperature is not only related to the chemical composition of the steel and welding rod, the rigidity of the welding structure, the welding method, the ambient temperature, etc. but also these factors should be considered and determined.
In addition, the uniformity of the preheating temperature in the thickness direction of the steel has an important effect on the uniformity of the welding area and the reduction of welding stress. The width of the local preheating should be the degree of welder restraint. Generally, it should be three times the wall thickness around the weld zone, and it should not be less than 150-200mm. If preheating and uneven, not only will not reduce the welding stress but will increase the welding stress.