Inductoheat - An Inductotherm Group Company

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Induction heating the internal surfaces of a workpiece can be used in applications such as hardening, tempering, annealing, shrink fitting, stress relieving, coating, drying, and brazing. Application specifics requires the use of a variety of different inductor styles to heat internal surfaces including solenoid-type, cylindrical single- and multi-turn coils, hairpin inductors, C-core inductors, and others.

Single- and multi-turn solenoids are the most popular inductors for heating internal surfaces. Such inductors are called ID coils or internal coils. Internal coils are typically made of copper tubing that is spiral wrapped the same way a solenoid is wrapped. In some cases, the head of the internal inductor is machined from a solid copper bar.

Installation of a magnetic flux concentrator inside the internal inductor is frequently necessary to increase coil efficiency and reduce coil current, particularly for heating internal surfaces of small to moderate diameters. The use of magnetic flux concentrators on internal coils provides a noticeable reduction in required coil current and power, reduces coil water-cooling requirements, and often simplifies load matching of the induction coil and inverter.

The figure shows the results of computer modeling of the dynamics of induction scan hardening of inside diameter of carbon steel pipe using two-turn solenoid inductor with a U-shape magnetic flux concentrator. The pipe ID and OD are 11.8 and 7.8 in. (300 and 200 mm), respectively. Spray quenching follows induction heating. Because of the pipe symmetry, only the top-half of the system was modeled using Inductoheat’s proprietary FEA software. Temperature variations at four selected areas of the pipe are shown at different locations.

Computer modeling shortens the inductor development time and helps reveal many important process subtleties, which would be costly and time-consuming to determine experimentally.

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Further reading:

V. Rudnev, et al., Handbook of Induction Heating, Marcel Dekker, NY, 2003.