Why do we need to preheat before welding during welding

Preheating before welding and heat treatment after welding are very important to ensure welding quality. The welding of important components, the welding of alloy steel and the welding of thick parts all require preheating before welding. The main functions of preheating before welding are as follows:

(1) Preheating can slow down the cooling rate after welding, facilitate the escape of diffusive hydrogen in the weld metal, and avoid hydrogen-induced cracks. At the same time, it also reduces the hardening degree of the weld and the heat-affected zone, and improves the crack resistance of the welded joint.

(2) Preheating can reduce welding stress. Even partial preheating or overall preheating can reduce the temperature difference (also called temperature gradient) between the welded workpieces in the welding area. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, which is beneficial to avoid welding cracks.

(3) Preheating can reduce the degree of restraint of the welded structure, especially for reducing the degree of restraint of corner joints. With the increase of preheating temperature, the incidence of cracks decreases.

The choice of preheating temperature and interlayer temperature is not only related to the chemical composition of the steel and welding rod, but also related to the rigidity of the welded structure, welding method, ambient temperature, etc., which should be determined after comprehensive consideration of these factors. In addition, the uniformity of the preheating temperature in the thickness direction of the steel plate and the uniformity in the weld area have an important effect on reducing welding stress. The width of the local preheating should be determined according to the restraint of the workpiece to be welded. Generally, it should be three times the wall thickness around the weld area, and it should not be less than 150-200 mm. If the preheating is uneven, not only will it not reduce the welding stress, but will increase the welding stress.

https://www.ceramicpadheater.com/