Now some buildings are all in steel structure.Today, we need to talk about preheating before welding of steel structure engineering. Preheating before welding is very important to ensure welding quality.The welding of important components, alloy steel and thick parts requires preheating before welding.The main functions of preheating before welding are as follows:

　　1.Preheating in steel structure engineering can slow down the cooling rate after welding, which is conducive to the escape of diffusive hydrogen in weld metal and avoid hydrogen induced cracks.At the same time, the hardness of weld and heat affected zone is reduced, and the crack resistance of welded joint is improved.

　　2.Preheating can reduce welding stress.Uniform local preheating or overall preheating can reduce the temperature difference (also known as temperature gradient) between the weldments in the welding area.In this way, on the one hand, the welding stress is reduced, on the other hand, the welding strain rate is reduced, which is conducive to avoid welding cracks. 

　　3Preheating can reduce the restraint degree of welded structure in steel structure engineering, especially for the angle joint. With the increase of preheating temperature, the crack rate decreases. 

The selection of preheating temperature and interpass temperature is not only related to the chemical composition of steel and electrode, but also related to the rigidity of welding structure, welding method, ambient temperature, etc., which should be determined after comprehensive consideration of these factors. 

In addition, the uniformity of preheating temperature in the direction of steel plate thickness and in the weld area has an important influence on reducing the welding stress of steel structure workshop.The width of local preheating shall be determined according to the binding degree of the parts to be welded. Generally, it shall be three times of the wall thickness around the weld area and shall not be less than 150-200mm.If preheating is not uniform, not only welding stress will not be reduced, but also welding stress will be increased