Induction Heating Offers Many Processing Opportunities
Industrial applications of induction heating continue to emerge due to advancements in the design of solid-state induction power sources, the reduction in equipment size, the advantages of integrating computer technology with the induction process and the capability of more accurate and consistent results over other techniques. Some less well-known processes that use induction heating include semisolid forming, shrink fitting, crystal growing, degassing and gettering. From adhesive curing to zone refining, induction heating extends into almost every manufacturing field.
Induction heating of a metallic component is achieved by passing high-frequency electric current through a coil surrounding a workpiece, which in turn induces a high-frequency electromagnetic field in the piece. The magnetic field induces eddy currents in the workpiece, and the electrical resistance of the piece to the flow of current causes the piece to heat up. The depth at which the current flows is dependant on the frequency of the magnetic field; that is, shallow depth at higher frequencies and deeper at lower frequencies. This frequency/case depth relationship is known as the penetration, or reference, depth. The depth of heating is a function of the electrical frequency applied, the heating time and the power density applied to the component being heated.
Induction heat treating
Probably the largest use (in terms of numbers of units) of the induction heating process is in heat treating. Conventional heat treating requires that parts be processed in large quantities, limiting the potential to fit into in-line cell manufacturing. Induction processing provides that advantage with one-piece flow, making it easy to integrate into an in-line process.
All induction heating systems heat treat parts either with the part being heated in place or scanned horizontally or vertically. Induction heat treated parts include shafts, camshafts, crankshafts, push-rods, front wheel drive components, ring gears, cogs, gears of most types, connecting rods and pinions. Induction heat treating processes include hardening, tempering, and annealing.
The basic factors associated with the selection of suitable equipment are:
- Power source output frequency
- Power density
- Scanning rate
- Inductor length
- Type of material being heat treated
- Inductor configuration
- Quench arrangement
Set-up factors of an induction system often are finally determined by actual testing of the components to be heat treated. Typical heat-treating parameters are shown in Table 1, which may be used as a guide to initial system capabilities.
Although the initial cost of induction heating equipment often is higher than that of a conventional furnace, the payback period can be short because the equipment is only on when needed, is more environmentally friendly and fits into today's manufacturing processes more readily.
Heating for forming and forging
Fast, even heating of a billet prior to the forming operation yields the best forging. The inherently fast heating time of induction heating at the correct frequency provides that advantage. Heating for forging requires applying a low power density to the bar or billet, ensuring uniform through heating, resulting in a forging having good flow characteristics. The frequency selection is different from that chosen for heat treating.
The amount of power required is dependant on the material being heated, the required temperature, the induction coil design and the type of material handling being used. A typical operating efficiency when heating steel from ambient to a temperature of 2200 F (1200 C) in a helically wound coil is 6 to 6.5 lb/kWh (~3 kg). The efficiency when heating brass from ambient to 1400 F (760 C) in a similar helical coil arrangement is about 7 to 7.5 lb/kWh (~3.5 kg).
Both production rates are based on heating the materials continuously; that is, parts heated one after the other. Efficiency is reduced in systems where this is not practical or when different coil arrangements are used. For example, a channel, or "C" type, coil arrangement may only be 65% as efficient resulting in a rate of approximately 4 lb/kWh (~2 kg).
Compared with gas-fired furnaces typically used in forges, induction heating provides advantages of lower operating costs, vastly improves die life, uses less metal due to limited scale and instant on/off operation, avoiding the need for the furnace to be on at all times. As the price of gas fuel continues to increase, the payback period for electric induction is becoming much shorter.
The forging/forming market represents one of the larger uses of induction power with power levels of up to 10,000 kW.
This technology uses induction heating to uniformly heat various metals including aluminum (the most popular), copper, nickel, stainless steel and ductile iron to cast the metal while it is in a semisolid state. The greatest concern in this process is to ensure that heat uniformity is maintained throughout the heating process. End effects caused by the induction-heating field are compensated by careful design of the induction coil. Process advancements include the use of mathematical modeling to determine the electromagnetic fields.
Induction heating for shrink fitting is faster and can provide localized heat compared with conventional heating methods. Process temperatures typically range from 400 to 600 F (200 to 315 C). Typical applications include fitting shafts to rotors in small motors, fitting steel shafts into aluminum housings in the manufacture of vehicle water pumps, fitting bearing assemblies to shafts and fitting pins into connecting rods. Induction coils are arranged to heat both the outside and the inside of components. Typical induction units used for this operation range from 5 to 100 kW at frequencies from 1 to 450 kHz.
Brazing and soldering
Induction brazing is a fast, easy and accurately controlled process. Both single and multiple components are simultaneously brazed. Components range from simple tube-to-fitting joints to complex items having multiple braze joints, such as musical instrument valves.
As with conventional brazing, the joint areas need to be clean and fluxed. Filler material often is cut to suit the joint, which regulates the amount of material used with a resultant reduction in cost. The process can be conducted in air or in a protective atmosphere depending on the materials being brazed.
Metals that can be induction brazed include steel, brass, copper and aluminum. Power requirements depend on the materials being joined.
Induction heat also is used to cure heat-setting bonding materials used at the interface between adjoining materials in the manufacture of auto doors, deck lids and trunks, and the fabrication of office furniture.
Induction heating is used in the manufacture of prestressed-concrete wire, tire cord, wire rope, fencing wire and electrical cable. The replacement of lead baths with induction heating to process prestressed-concrete wire results in an improved working environment, eliminating the issues related to the use and disposal of lead. The wire is continuously hardened and tempered using induction equipment. Induction has replaced resistive heating to diffuse or melt surface coatings in the manufacture of tire cord, in many cases heating multiple wires simultaneously. Speeds of close to 1000 ft/min (305 m/min) have been achieved. Wire rope undergoes low relaxation, fencing wire is heated prior to plastic coating and electrical cable is heated prior to coating or to cure a heat sensitive coating. One of the largest growth areas in this market is heating small-diameter wires using high frequency solid state induction power supplies.
Equipment used in these applications is rated from 5 to 2000 kW at frequencies from 3 to 400 kHz. The efficiency of heating wire is related to the material being heated, the temperature to which it is being heated and to the wire diameter relative to the frequency of the induction power source.
Degassing of vacuum tubes, particularly expensive x-ray tubes, is carried out by positioning an induction coil around the outside periphery of the glass envelope to heat the plate (anode) of the tube to remove impurities (degass) via a vacuum pump. Reducing impurities increases tube life. This arrangement replaces other techniques such as applying a high voltage to the anode to burn off the impurities. Power units are rated at up to 100 kW at frequencies of up to 50 kHz.
Gettering is the removal of excess gas within a cathode-ray tube by firing the getter within the neck of the tube during manufacture. A flat, or pancake, style induction heating coil is positioned on the outside of the tube neck; upon initiating the coil, the getter slowly burns away, absorbing the residual gas. Equipment typically operates at high frequencies of 50 to 450 kHz at power levels of 2 to 5 kW.
In this process, a seed of germanium crystal on the end of a rod is drawn slowly out of molten germanium in an induction-heated carbon crucible. The temperature and, therefore, the power level, are maintained within close limits. A typical system comprises a combined vacuum and gas crystal-pulling unit capable of pulling up to 1 lb (0.5 kg) of silicon or 2 lb (1 kg) of germanium. Equipment typically is sized from 15 to 100 kW and operates at frequencies from 10 to 400 kHz.
In the manufacture of semiconductors, epitaxial growth is the decomposition of a gas on the surface of a substrate, which is heated to a temperature of 2100ÝF (1150ÝC) via conduction from a graphite susceptor.
The induction coil is wound around a heat-resistant glass or similar enclosure through which the doping gases are fed. Multiple substrate slices are simultaneously processed on the graphite susceptor. Close control of the gases and susceptor temperature and, therefore, the induction heating power level are essential to obtain a quality product. Typical power sources are rated up to 150 kW at output frequencies of 400 kHz.
For more information: Kelvin Spain is Vice President of Sales, Radyne Corp., 211 W. Boden St., Milwaukee, WI 53207-6277; tel: 414-481-8360; fax: 414-481-8303; e-mail: email@example.com.