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.
