Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an inductor that creates an electromagnetic field within the coil to heat up and possibly melt steel, copper, brass, graphite, gold, silver, aluminum, or carbide. An important feature of the induction heating process is that the heat is generated inside the object itself, instead of by an external heat source via heat conduction. Thus objects can be heated very rapidly. In addition, there need not be any external contact, which can be important where contamination is an issue. Induction heating is used in many industrial processes, such as heat treatment in metallurgy, Czochralski crystal growth and zone refining used in the semiconductor industry, and to melt refractory metals that require very high temperatures. It is also used in induction cooktops. An induction heater consists of an electromagnet and an electronic oscillator that passes a high-frequency alternating current (AC) through the electromagnet. The rapidly alternating magnetic field penetrates the object, generating electric currents inside the conductor called eddy currents. The eddy currents flow through the resistance of the material, and heat it by Joule heating. In ferromagnetic and ferrimagnetic materials, such as iron, heat is also generated by magnetic hysteresis losses. The frequency of the electric current used for induction heating depends on the object size, material type, coupling (between the work coil and the object to be heated), and the penetration depth.
Applications
Induction heating allows the targeted heating of an applicable item for applications including surface hardening, melting, brazing and soldering, and heating to fit. Due to their ferromagnetic nature, iron and its alloys respond best to induction heating. Eddy currents can, however, be generated in any conductor, and magnetic hysteresis can occur in any magnetic material. Induction heating has been used to heat liquid conductors (such as molten metals) and also gaseous conductors (such as a gas plasma—see Induction plasma technology). Induction heating is often used to heat graphite crucibles (containing other materials) and is used extensively in the semiconductor industry for the heating of silicon and other semiconductors. Utility frequency (50/60 Hz) induction heating is used for many lower-cost industrial applications as inverters are not required.
Furnace An induction furnace uses induction to heat metal to its melting point. Once molten, the high-frequency magnetic field can also be used to stir the hot metal, which is useful in ensuring that alloying additions are fully mixed into the melt. Most induction furnaces consist of a tube of water-cooled copper rings surrounding a container of refractory material. Induction furnaces are used in most modern foundries as a cleaner method of melting metals than a reverberatory furnace or a cupola. Sizes range from a kilogram of capacity to a hundred tonnes. Induction furnaces often emit a high-pitched whine or hum when they are running, depending on their operating frequency. Metals melted include iron and steel, copper, aluminium, and precious metals. Because it is a clean and non-contact process, it can be used in a vacuum or inert atmosphere. Vacuum furnaces use induction heating to produce specialty steels and other alloys that would oxidize if heated in the presence of air.
Welding
A similar, smaller-scale process is used for induction welding. Plastics may also be welded by induction, if they are either doped with ferromagnetic ceramics (where magnetic hysteresis of the particles provides the heat required) or by metallic particles. Seams of tubes can be welded this way. Currents induced in a tube run along the open seam and heat the edges resulting in a temperature high enough for welding. At this point, the seam edges are forced together and the seam is welded. The RF current can also be conveyed to the tube by brushes, but the result is still the same—the current flows along the open seam, heating it.
Manufacturing In the Rapid Induction Printing metal additive printing process, a conductive wire feedstock and shielding gas is fed through a coiled nozzle, subjecting the feedstock to induction heating and ejection from the nozzle as a liquid, in order to refuse under shielding to form three-dimensional metal structures. The core benefit of the use of induction heating in this process is significantly greater energy and material efficiency as well as a higher degree of safety when compared with other additive manufacturing methods, such as selective laser sintering, which deliver heat to the material using a powerful laser or electron beam.
Cooking
In induction cooking, an induction coil inside the cooktop heats the iron base of cookware by magnetic induction. Using induction cookers produces safety, efficiency (the induction cooktop is not heated itself), and speed. Non-ferrous pans such as copper-bottomed pans and aluminium pans are generally unsuitable. By thermal conduction, the heat induced in the base is transferred to the food inside.
Brazing
Induction brazing is often used in higher production runs. It produces uniform results and is very repeatable. There are many types of industrial equipment where induction brazing is used. For instance, Induction is used for brazing carbide to a shaft.
Sealing
Induction heating is used in cap sealing of containers in the food and pharmaceutical industries. A layer of aluminum foil is placed over the bottle or jar opening and heated by induction to fuse it to the container. This provides a tamper-resistant seal since altering the contents requires breaking the foil.
Heating to fit
Induction heating is often used to heat an item causing it to expand before fitting or assembly. Bearings are routinely heated in this way using utility frequency (50/60 Hz) and a laminated steel transformer-type core passing through the centre of the bearing.
Heat treatment
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