Preheating is required before welding important components, alloy steels parts and thick parts. The main role of preheating before welding is as follows:

The choice of preheating temperature and interlayer temperature is not only related to the chemical composition of steel and welding rod, but also to the rigidity of welding structure, welding method, ambient temperature and so on, and these factors should be considered together. In addition, the preheating temperature uniformity in the direction of the thickness of the steel plate and the uniformity in the weld area, to reduce the welding stress has an important impact. Partial preheat width, should be based on the degree of restraint of the workpiece is welded, generally around the weld zone wall thickness of three times, and not less than 150-200 mm. If the preheating is not uniform, not only does not reduce the welding stress, but will increase the welding stress situation.