Important components of alloy steel and thick parts must be preheated before welding. The purposes of preheating spiral steel pipes before welding are as follows:

(1) Preheating the spiral steel pipe slows down the cooling rate after welding, allowing hydrogen to diffuse and escape from the weld metal, thus preventing hydrogen-induced cracking. It also reduces the hardening degree of the weld and heat-affected zone, improving the crack resistance of the welded joint.
(2) Preheating the spiral steel pipe reduces welding stress. Uniform local or overall preheating lowers the temperature of the welding zone between welders (also known as the temperature gradient). This reduces both the welding stress and the welding strain rate of the spiral steel pipe, thereby preventing welding cracks.
(3) Preheating the spiral steel pipe reduces the bonding strength of the welded structure, especially at corner joints, reducing the occurrence of cracks and increasing the preheating temperature.

The selection of preheating temperature and interpass temperature for spiral welded steel pipes is not only related to the chemical composition of the spiral welded steel pipe and welding rod, the rigidity of the welded structure, the welding method, and the ambient temperature, but these factors should also be considered and determined.

Furthermore, the uniformity of preheating temperature in the thickness direction of the spiral welded steel pipe within the weld area has a significant impact on reducing welding stress. The width of the local preheating area should be within the welder’s constraint limits, generally three times the wall thickness around the weld zone, and not less than 150-200 mm. Uneven preheating will not only fail to reduce welding stress but may even increase it.