The invention discloses a device for preheating before welding and post-welding heat treatment of tank. The device includes a first roller, two electric heating belts, calipers, a thermostat, two positioners and hinges. Both of the electric heating belts are arranged around the tank and distributed on both sides of the welded seam. A number of calipers are provided on both sides of each electric heating belt, one end of each caliper can operatively clamp the side of the electric heating belt, another end of each caliper is rotatably connected to a first roller. The hinge is connected to the electric heating belt, and one end of the hinge away from the electric heating belt is in a fixed state. The heating temperature of the electric heating belt is controlled, thereby realizing preheating of the tank before welding, heat preservation during welding and post-welding heat treatment without transferring. The electric heating belt will not rotate with the tank to ensure heat evenly, and the equipment connected with the electric heating belt will not be knotted, which improves the practicability.
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