We know that the welding of general important parts, th […]
We know that the welding of general important parts, the welding of alloy steel and the welding of thick parts require preheating before welding.
The main functions of preheating before welding are:
(1) Preheating can reduce the cooling rate after welding, which is beneficial to eliminate hydrogen diffusion in the weld metal and avoid hydrogen induced cracking. At the same time, the degree of hardening of the weld and the heat affected zone is reduced, and the crack resistance of the welded joint is improved.
(2) Preheating can reduce welding stress. Even local preheating or overall preheating reduces the temperature difference (also known as temperature gradient) between welders in the weld zone. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is lowered, which is advantageous for avoiding the occurrence of welding cracks.
(3) Preheating can reduce the degree of bonding of the welded structure, and the joint degree of the joint is particularly conspicuous. As the preheating temperature increases, the incidence of cracks decreases. The preheating temperature and the choice of interlayer temperature are not only related to the chemical composition of the steel and the electrode, but also related to the welded structure, the welding method and the rigidity of the ambient temperature. Therefore, we should consider these factors comprehensively. In addition, the preheating temperature is in the thickness direction of the steel plate. Uniformity and uniformity of the weld zone have an important effect on reducing weld stress. The width of the local preheating shall be determined according to the degree of limitation of the welder, which is generally three times the wall thickness around the weld zone and shall not be less than 150-200 mm. If the preheating is uneven, not only will the welding stress not be lowered, but also the welding stress will increase.