Preheating means a process that heat weldments in whole or in weld areas before welding. Particularly good material for welding high strength level, hardening tendency of steel, thermal conductivity, thickness larger weldments, and when the ambient temperature is too low,welding zone often requires butt to warm up before welding.
The purpose of preheating is to reduce the speed of the cooling of the welded joints and preheating temperature. It can be seen from the table, warm-up can reduce the cooling rate, but did not influence the time to stay in the high temperature, which is very desirable. So when welding steel with a hardening tendency, to reduce the cooling rate decreases the hardening tendency of the main process measures, is to warm up, rather than increase the energy input.