Important component of the welding, alloy steel welding and thick parts of the welding, are required to preheat before welding. The main function of preheating before welding is as follows:

(1) preheating can slow down the cooling rate after welding, which is beneficial to the diffusion of hydrogen in the weld metal and avoid the hydrogen induced cracking. It also reduces the degree of quenching of weld and heat affected zone, and improves the anti cracking of welded joints.

(2) preheating can reduce the welding stress. Evenly preheat or integral preheating can reduce the welding area is between the welding workpieces temperature difference (also known as the temperature gradient). Thus, on the one hand, it can reduce the welding stress, on the other hand, it reduces the strain rate, and it is beneficial to avoid welding cracks.

(3) preheating can reduce the binding degree of the welded structure, which is particularly evident, with the increase of the preheating temperature, the crack occurrence rate decreases.

Preheating temperature and interlayer temperature not only with the chemical composition of the steel and welding, and welding structure of rigidity, welding method, environment temperature and so on, should be comprehensive consideration of these factors is determined. In addition, the preheating temperature in the steel plate thickness direction uniformity and the uniformity in the weld region, the reduction of the welding stress has an important impact. Preheat the width should be according to welding the restraint of the situation, the general should be for weld zone around the times of the wall thickness, and shall not be less than 150 mm. If the preheating is not uniform, not only does not reduce the welding stress, but will appear to increase the welding stress.