Why is the spiral steel pipe preheated before welding

　　The purpose of the spiral steel pipe preheating before welding is as follows:

　　(1) Preheating can slow down the cooling rate after welding, make hydrogen diffuse in the weld metal and escape, and avoid cracking caused by hydrogen. It can also reduce the degree of hardening of welds and heat-affected zones to improve the crack resistance of welded joints.

　　(2) Preheating can reduce welding stress. Uniform local or general preheating reduces the weld zone temperature (also known as temperature gradient) between welders. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, 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 interpass temperature is not only related to the chemical composition of steel and electrode, the stiffness of welding structure, welding method, ambient temperature, etc., but these factors should also be considered and determined. In addition, the uniformity of 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 welder restraint, generally three times the wall thickness around the weld zone, and not less than 150-200mm. If the preheating is uneven, not only will the welding stress not be reduced, but it will increase the welding stress.