The Purpose of Preheating The Steel Pipe before Welding The Spiral Steel Pipe

Alloy steel and critical components of thick parts must be preheated before welding. The purpose of preheating before welding is as follows:

(1) Preheating can slow down the cooling rate after welding, allowing hydrogen to diffuse and escape in the weld metal, thus avoiding 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 preheating or global preheating reduces the temperature of the welding zone (also called the temperature gradient) between welding machines. 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 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 the steel and welding rod, the stiffness of the welding structure, the welding method, the ambient temperature, etc., 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 reducing the welding stress. The width of local preheating should be as close as possible to the welder's restraint, which should generally be three times the wall thickness around the weld area and should not be less than 150-200mm. If the preheating is uneven, not only will the welding stress not be reduced, but it will be increased.