The technical means for heating the whole or welded area of the large diameter seamless steel tube before welding is called preheating. On the welding strength level higher, hardening tendency of large diameter seamless steel tube, heat conduction function particularly outstanding data and thick welding parts, and when the welding area around the ambient temperature is too low, heavy caliber seamless steel pipe before welding usually need to butt welding preheating were carried out.

The primary intention of the large diameter seamless steel pipe welding preheating is to decrease the cooling rate of the welded joint, and the preheating temperature. As can be seen from the table, the preheating can fall in the cooling rate, but it does not affect the high temperature stay at the moment, which is very ideal. So when welding with the large diameter seamless steel pipe, the tendency to reduce the cooling rate of the primary technical approach to reduce the hardening tendency is to preheat, rather than increase the line energy.