The preheating is total or partial heated during welding work. Usually using medium carbon steel, high carbon steel weldment quench cracking tendency or temperature lower ground conditions, a large difference when welding, as well as the thickness of the weldment need to warm up.
purpose of preheating in welding:
(1)Preheating can reduces the cooling rate of the welded joint, which is conducive to the diffusible hydrogen in the weld metal escape to avoid hydrogen induced cracking.
(2)Preheat can extend the cooling time of the temperature range of 800 -500 `C at affecting zone. Welded joints is from just solidified high temperature to room temperature in the cooling process, the microstructure changes austenite from 800 ℃ to shift into the ferritic and pearlite or troostite, cooling slowly,avoid martensite hardened tissue, thus, improve the welded joints crack resistance, thereby avoiding the weld cracking.
(3)Preheating can reduce welding stress. The preheating (partial the preheated or overall preheating) can reduce the temperature difference between the overall temperature of the weld zone and the weldment value (also referred to the temperature gradient), the smaller the value of the temperature difference between the temperature of the weld zone and the weldment structure unevenness smaller, As a result, on the one hand, reducing the welding stress, on the other hand reducing the welding strain rate and help to avoid weld cracking.
(4)Preheated can reduce the lower bound of welded structures, especially reduce the angle joint restraint intensity, with the improvement of the preheating temperature, the crack rate.