Purpose of preheating before welding :
(1) Preheating can reduce the cooling rate of the welded joints, weld metal diffusible hydrogen escaped to avoid hydrogen induced cracking.
(2) Preheating may be extended heat-affected zone of 800 -500 `C temperature range of cooling time, the welded joints from just solidified high temperature to room temperature in the cooling process, the microstructure will change, from 800 ¡ã C austenite began change, when the cooling is slow, it is transformed into ferrite and pearlite or troostite martensite hardened tissue so that you avoid, improve the crack resistance of welded joints, in order to avoid welding cracks.
(3) Preheating can reduce welding stress. Warm-up (local warm-up or overall warm-up) can reduce the temperature difference between the welding zone, weldments overall temperature values (also known as temperature gradient), the smaller the value of this temperature difference, the temperature non-uniformity in the welding zone and weldment structure the smaller the As a result, on the one hand, to reduce welding stresses, on the other hand to reduce the welding strain rate, to avoid welding cracks.
(4) Preheating can reduce the binding degree of welded structures, especially to reduce the degree of angle joint binding, with the preheat temperature increased, the crack rate.