Preheating by induction will take place when a voltage or electromotive force is induced in a workpiece by exposing it to an alternating current magnetic field. The induced voltage will cause current to flow in the workpiece, heating it by its resistance to the flow of current. Usually, a water-cooled copper coil is placed close to the workpiece or area to be heated or wrapped around the part. If 60 Hz ac is to be used, the coil can

be connected to a step-down transformer of adequate kVA rating. If higher frequency is to be used, then a high-frequency generator of adequate kVA rating will be needed.

One major advantage of preheating by induction is that it can be applied to very large items or systems such as pressure piping, boilers and valves for power plants. These items are made from high-carbon, high-alloy steels designed for high-temperature service.

They require preheat for welding and could not be welded satisfactorily without preheat. Induction heating rapidly develops the heat within the workpiece as well as at the surface.

 

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