Preheating before welding can slow down the cooling rate of weld metal and heat affected zone of welding, reduce the hardening tendency of steel, and reduce the diffusive hydrogen content in weld metal, which is one of the important measures to prevent welding cold cracks. However, local preheating during field assembly welding has its particularity. If the temperature is too low, it can not play the role of preheating, and if the temperature is too high, it will make the weld additional thermal stress, that will worsen the crack resistance of the joint. Only when an optimal preheating temperature range is determined, can the preheating function be brought into full play and the purpose of preventing cold crack in welding be achieved.
The heat treatment process of steel tube
Preheating means to heating the steel within the range of 75-100mm on both sides of the bevel end to a required preheating temperature. It can be heated by flame, power frequency induction heating or infrared heating. The preheating temperature of low carbon steel and common low alloy steel is 100-200 ℃. When determining the preheating temperature, the following points are mainly considered:
- The welding ability of workpiece,mainly depends on carbon content and alloy element content;
- Thickness of weldment, type of welding joint and degree of structural restraint;
- Hydrogen content of welding materials;
- Ambient temperature.
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