Important components of alloy steel and 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, avoiding cracking caused by hydrogen. It can also reduce the hardening degree of the weld and heat-affected zone to improve the crack resistance of the welded joint.
(2) Preheating can reduce welding stress. Uniform local preheating or overall preheating will reduce the temperature of the welding zone between welders (also known as the 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 bonding degree of the welded structure, especially the bonding degree of the corner joint, 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 the steel and welding rod, the stiffness of the welded structure, the welding method, the ambient temperature, etc., but also these factors should be considered, and determined. In addition, the uniformity of 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 local preheating should be the possible constraint of the welder, generally three times the wall thickness around the weld area, and should not be less than 150-200mm. If the preheating is uneven, it will not only fail to reduce the welding stress but will increase the welding stress.
Post time: Sep-02-2024