The purpose of preheating the spiral steel pipe before welding

The purpose of preheating the spiral steel pipe before welding

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

(2) Preheating can reduce welding stress. Uniform partial preheating or overall preheating will reduce the temperature of the weld zone between welders (also called temperature gradient). 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 degree of bonding of the welded structure, especially the degree of bonding of the corner joints, reduce the occurrence of cracks, and increase the preheating temperature.

The selection of welding preheating temperature and interlayer 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 influence on the uniformity of the welding area and the reduction of welding stress. The width of the local preheating should be the possibility of welder restraint. Generally, it should be 3 times the wall thickness around the weld zone, and should not be less than 150-200mm. If the preheating is not uniform, not only will it not reduce the welding stress, but will increase the welding stress.