When welding structural steel, especially when the thickness is greater than 25mm, the preheating temperature of the weldment should be determined according to the chemical composition, thickness, ambient temperature, etc., and through the weldability test and production test. Preheating can increase the welding speed, thus reducing the residence time of the molten pool metal at high temperature or reducing the burnout of alloying elements. At the same time, it can increase the stirring and fusion ability of the molten pool, accelerate the diffusion and escape of hydrogen, and prevent the generation of pores. Preheating can also appropriately reduce the welding current, which is convenient for beginners to operate. The following weldments or environments require preheating of weldments:
① Weldments with higher welding strength
② Welding of steels with a tendency to harden
③ Welding materials with particularly good thermal conductivity
④ Weldments with larger thickness
⑤ When the ambient temperature of the welding area is lower than 0℃
⑥ Weldments specified in the technical requirements of the design drawings
The preheating temperature of aluminum is between 150 and 250 °C, generally not more than 350 °C, and the preheating temperature of copper should be higher, up to 500 °C. The preheating zone should have a certain width to ensure uniformity, generally within the range of 150~250mm on both sides of the weld, using back heating (if possible), the preheating temperature should not be too high, otherwise the molten pool will be too large , Pockmarks are formed on the surface of the weld, and the welder is exposed to heat radiation, which affects the welding environment and deteriorates the working conditions. In general, the starting temperature of alloy steel should not be lower than 15 ℃, otherwise, cracks are easy to form due to the uneven temperature of the starting point.
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