Important components of alloy steel and thick parts must be preheated before welding.
The purpose of spiral steel pipe preheating before welding is as follows:
(1) Preheating can slow down the cooling rate after welding so that hydrogen diffuses in the weld metal and escapes to avoid cracking caused by hydrogen. It can also reduce the degree of hardening of the weld and heat-affected zone to improve the crack resistance of the welded joint.
(2) Preheating can reduce welding stress. Uniform local or global preheating reduces the weld zone temperature (also known as temperature gradient) between welders. 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 bonding degree of the welded structure, especially the bonding degree of the 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 steel and electrode, stiffness of the welded structure, welding method, ambient temperature, etc. but also these factors should be considered and determined. In addition, the uniformity of the preheating temperature has an important influence on the uniformity of the welding area in the thickness direction of the steel and the reduction of welding stress. The width of local preheating should be the possibility of the welder's restraint, generally three times the wall thickness around the weld area, and not less than 150-200mm. If preheating and unevenness, not only will not reduce the welding stress but will increase the welding stress.