Principles of Induction Heating
Induction heating is a means of raising the temperature of metallic parts by the transfer of electrical energy from a high frequency current-carrying conductor, usually referred to as an induction heating coil. This induction heating coil sets up a field of magnetic flux that energizes the metal work piece in such a way that current is caused to flow around its surface. The resistance of the work to this flow, or its inability to carry the induced current, causes an immediate heating action to take place.
High frequency induction heating dates back many years. One of its earliest uses was for the melting of metals. But its use for other purposes didn’t advance for many years, because generators required to produce high frequency current were not commercially available. Within the past few years, however, this obstacle has been overcome and the induction heating of metal parts has advanced rapidly. Today, various types and sizes of generators are available to cover a broad field of induction heating applications.
As in any other process, induction heating has its field of applications as well as its limitations. It’s primarily used for localized heating, or zonal hardening of metal surfaces. It cannot be considered a basic substitute for other specific types of heating, but rather a selective means of applying heat to parts that lend themselves to this type of process. A full knowledge of the limiting factors should be analyzed by anyone contemplating the use of induction heating equipment.
For example if a machine spindle requires hardening all over, induction heating would not be the immediate choice. However, if only two or three localized surfaces actually require being hard and the specifications call for hardening all over and no other method of heat treatment is available, then high frequency induction heating would appear to be a practical means of heat treatment. The surfaces could be selectively hardened and, since heat is limited to the surface of a small area at one time, the part would not become warped or be subjected to metallurgical changes, and many of the economies of high frequency induction heating would be gained.
Induction heating as applied to industrial operations, such as soldering, razing, hardening, heat-treating, forging, and other forms of heat transfer, has so many practical applications that it cannot be overlooked by metalworking plants in search of new economies in manufacturing methods and improvements in the quality of their products. Already this art has developed to a point where operations hitherto impossible, or in which former methods proved difficult or inadequate, are being easily performed.
Many advantages of induction heating equipment have resulted from the broad experience gained through the diversified applications of induction heating. Outstanding are: (1) a comparatively low heat cost, especially in cases where only a localized surface requires heating as compared with an entire component in the past; (2) the application of heat at an exceptionally fast rate, which naturally increases output; and (3) uniformity for any given quantity of parts, with a reduction or elimination of spoilage.
Linked with these three economies are many other indirect advantages to induction heating equipment such as: 1) less deformation of heated parts, often making straightening unnecessary; 2) the hardening of surfaces without the formation of scale thus eliminating cleaning operations; 3) the substitution of a higher carbon steel as a means of eliminating carburizing; 4) the use of plain carbon steels in place of the more expensive alloys; 5) better bonds and stronger joints in brazing and joining applications; 6) and the use of lower melting brazing alloys in place of those requiring high temperatures and a controlled atmosphere.
How it Works
Since induction heating is a process by which the temperature of a metal part is raised by the electrical generation of heat within the material, the part being heated is in no way a part of any closed electrical circuit. To generate heat to a given part therefore requires a current-carrying conductor, usually referred to as the induction heating coil, which is made to surround the surface to be heated. The high frequency induction heating current enters at one terminal and passes around the coil and out of the other terminal. This current sets up a field of magnetic flux, which flows around the surface of the work in the opposite direction. Magnetic fields occur in the area surrounding the induction heating coil with their strength varying inversely as the square of the distance from the surface of the work. What takes place is a transfer of electrical energy from the coil into the material, where it’s converted into heat. For maximum heating, therefore, the current carrying coil should be arranged close to the work with the average coupling being from 3/32 to 3/16 inches away when fast induction heating is desired and increased proportionately for slower induction heating.
For induction heating of metallic parts by means of inductive currents, it’s necessary to take into account: (1) the output power of the generator; (2) the frequency of the power supply; (3) the design or shape of the induction heating coil and its relation to the surface of the work, and (4) the resistively of the metal being heated.
To begin with, there must be enough power to heat the surface or mass of the part to be treated. A generator with an output of 20 kW might cover a broad field, but there is a limit to the size of the part it can handle. For example, a gear blank 12 or 14 inches in diameter with a 2 or 3 inch face would be impractical with such a unit, because heat losses through convection and radiation would offset the heat-producing energy of the inductor; thus the outside would reach a certain temperature and go no higher. For such a part a generator of 50 to 100 kW output would be required.
In other words, with induction heating equipment it’s desirable to heat surfaces at relatively fast rates and to use a generator with enough power to overcome excessive heat losses through conduction or radiation. While no single rule can be applied to determine the power requirements for general induction heating applications, because of the wide variation in shapes and often because of certain induction heating coil limitations, a general guide is to provide about 2 kW output for each square inch of surface to be heated. This rate will vary with the size and shape of the part being heated but will serve as a reference to keep from going too far astray.
The energy required to heat a given part is expressed by:
W X S X T = B.t.u./min.
where W = the weight of material,
S = the coefficient of specific heat of the material, and
T = the temperature rise required.
This formula does not allow for normal heat losses, which may run from 25 to more the 50 per cent, but by comparing the approximate power needed with the available power of a generator, it is possible to determine quickly if the equipment is suitable.
The coefficient of the specific heat of a material is the number of B.t.u. required to raise 1 lb. of that material 1˚F. Water is assigned as a standard and is given in specific heat of 1.0, sot that it takes 1 B.t.u. to raise 1 lb. of water to 1˚F.
Relation of Frequency
Of great importance is the frequency of the power source, which of course has a direct influence on the depth of heat penetration. The higher the frequency, up to a certain range, the more pronounced the surface heating effect. A frequency of 2,000 cycles per second may heat a surface to a depth of 1/8 inches, whereas a frequency of 200,000 c.p.s. will produce a much shallower heat zone, on the order of 0.020 inch depth.
The speed at which a part can be heated and the depth of the heated layer are determined by the power output and the frequency of the generator. With proportionate power available, the speed at which heat may be applied can be made practically the same over a wide range of frequencies. However, there is usually a frequency best suited for the average range of work and, likewise, a generator which serves that range to best advantage. The problem therefore usually resolves itself to the matter of using available equipment to cover as broad a field as possible, rather than attempting to match a generator for each specific job.
For the heating of parts where a deep penetration is desired, frequencies of 2,00 to 10,000 c.p.s. usually are applied. Where only surface heating is required, a frequency range of 200,000 to 500,000 c.p.s., usually referred to as 200 to 500 kilocycles, is preferable and, as a rule, will cover most of the parts to which induction heating can be applied. For extremely thin heat layers, frequencies of 1 megacycle or more may be used. One kilocycle equals 1,000 cycles, and 1 megacycle equals 1,000,000 c.p.s.
The lower range of frequencies were produced by motor-generator sets, and the higher range mostly by vacuum-tube generators. Today these systems have been replaced by solid state medium and high frequency power supplies. High-frequency heating has two distinct fields induction heating for metals and dielectric heating for nonmetallic materials, up to 50 or more megacycles, is for nonmetallic materials, such as wood, plastics, rubber and ceramics.
With induction heating, an induction heating coil or inductor is made to surround the surface to be heated, whereas, with dielectric heating the charge is placed between two electrodes through which the high-frequency current passes to produce internal heat. The equipment used for these two type of heat is somewhat similar in design and principle, but the induction generator is suited only to metals and will not heat dielectrics which requires a much higher frequency and voltage across the electrodes. Likewise, a dielectric generator cannot be used for metallic parts, because of the voltage breakdown that would occur.
Resistance is the property of an electrical circuit that determines the rate at which electrical energy is converted into heat in relation to a given amount of current. The term is applied when the rate of conversion is proportional to the square of the current, in which case it is equal to the power conversion divided by the square of the current.
The power-output rating of a high-frequency generator is based on the formula for determining the kilowatt it will produce in a given mass to increase its temperature a specific number of degrees Fahrenheit, or:
kW. = 2.93 X wt. of mass X sp. heat X temp. rise X 10 ¯4
Electrical meters and measuring devices are available for determining the power and output of generators, such as an indicating kilowatt meter. If a check is wanted for reference where these are not available, a heating tank may be used. As water passes through the tank, it is heated by the induction heating coil, which surrounds it. Thermometers are placed at the inlet and outlet connections, and the differential in temperature for a given amount of water is calculated in B.t.u. or:
8.33 X gal. per min X temp. rise = B.t.u./min.
Graduated-flow meters with linear calibrations, employing a glass tube and a metering float, are often used for measuring the flow of water. The size of the heating tank and the type of coil used for heating may vary somewhat according to the characteristics of the generator, but the resulting calculations will give a fairly accurate rating of the actual output power produced.
Transfer of Heat
Induction heating equipment is based on established electrical formulas, and to a great extent, follows transformer principles. Linked with the electrical producing energy is the consideration of heat flow in metallic bodies. With a certain amount of current induced into a work piece which requires heating, the rate of absorption is determined by the nature of the work to absorb it. Magnetic materials heat more quickly than non-ferrous metals; therefore the heating rate varies with the coefficient of specific heat of the material. The relation of the induction heating coil, or current-carrying conductor, and the work piece to each other determines the amount of heat transfer in proportion to the power supply.
The induction heating coil theoretically becomes the primary and the work piece the secondary, and as in a transformer the closer the coil to the work surface the more intense the transfer of magnetic flux.
Since high-frequency induction heating of a metal part is the result of generating magnetic flux to the work's outer surface, which decreases in intensity toward the center, it is possible with proper timing to control the depth of heat so that, for operations such as surface hardening, a predetermined case can be obtained. The greatest part of the heat generated is on the surface, diminishing rapidly toward the center, where practically no heat is attained. At a distance of 1/8 inch below the surface, only about 25 per cent of the flux density is noted, whereas at approximately 1/32 inch depth below the surface, more than 80 per cent of the total heat is concentrated.
This curve will vary somewhat with a change of frequency. Using a lower frequency of 9,600 c.p.s., the depth of the heated area would be more near 1/8 inch, and with a much higher frequency, the penetration would be around 0.010 inch. These references are given merely to show the relative effect of frequency. The depth of the heated area can be increased, of course, by added heating time, if sufficient power is available.
When small steel rods and wires are to be heated by high frequency induction heating current, the relationship between the diameter and the minimum optimum frequency becomes more critical. Where hardening temperatures of 14500 to 1500˚F are required, the smaller the diameter, the higher the minimum frequency. However, the use of higher frequencies, particularly for larger sizes, is entirely practical, especially for surface-heating requirements. While theoretical calculations would show that a 1/8 inch diameter wire could be surface-heated at a frequency of about 500,000 c.p.s., allowances are usually overlooked for the rapid flow of heat by conducting below the surface. Usually small diameters heat through almost instantly, and surface-heating of such small sizes becomes impractical. The formula used for obtaining the frequency of such parts is:
F = 1.22 resistively (ohms-centimeters) 10
(radius of rod) ²
For nonferrous wires and rods of small diameters, a much greater frequency is required.
With all heating coils there are alternating magnetic lines of force that are perpendicular to the path of the current and which, in turn, follow a direction determined by the flow of current within the induction heating coil. This magnetic field surrounding the coil produces internal energy losses in the material located within the coil, causing rapid temperature rises.
Induction Heating Coil
Current-carrying conductors for induction heating equipment purposes are invariably made of copper. The conductivity of the copper should be high, at least 90 per cent or better. To be a good conductor, the material must afford a continuous passage of electrical current, even when subjected to a difference of electrical potential — the greater the density of current for a given potential, the more effective the conductor.
The conductivity of copper is calculated by dividing the resistively of the international annealed copper standard at 20˚C. Either mass resistively or volume resistively may be used.
Usually the copper used for standard tubing, bars, sheets, and flats will make good induction heating coils for induction heating purposes. When special cast-type coils are made, however, the material specifications should be carefully checked, because a copper containing alloys which will lower its conductivity, will detract from the heating efficiency of the coil. See the table of specific heats for comparative conductance values of various materials:
|Material||Coefficient of heat||Relative conduct %||Specific resistively|
|Steel||0.148||14||10 to 15|
Heat-producing losses are those internal energy losses that cause a temperature rise in the material linked by the magnetic lines of force. In magnetic material, these are divided into two classes-hysteretic losses and eddy-current losses. According to the popular conception of atomic structure, hysteresis losses are caused by the friction of adjacent molecules in a given material, when these molecules are caused to vibrate in an attempt to align to the frequency of the magnetic field. This loss is peculiar to magnetic material, and its probable cause is that molecules of magnetic materials are in themselves small magnets vibrating at the same frequency as the alternating magnetic field linking this material. Hence, a certain amount of heat energy is generated and this heat, commonly termed the hysteresis loss, is directly proportional to the frequency of the magnetic field causing the vibration and the magnetic-field strength.
Eddy-current losses are resistance losses resulting from small circulating currents within the material. These currents are caused to flow by virtue of differences in potential at various points in the material, these differences in potential being caused by the alternating magnetic field cutting the work. The loss is proportional to the square of the current flowing, and directly proportional to the electrical resistance of the material. Hence, it can be considered to be proportional to the square of the frequency and field strength, since the potential difference and the resultant currents losses are the only losses present; hence any induced heating must be accomplished through this source only.
When surface heating a steel part by high frequency induction heating current, the permeability of the material increases with the temperature up to the critical point, where magnetic properties no longer exist (about 1450˚F). Above that point, the permeability is equal to unity and the power absorbed at the surface of the work piece diminishes.
Continuing the application of high-frequency current, however, will result finally in additional layers becoming heated, until the entire cross-sectional area of the piece is heated through, provided the generator has sufficient power for this purpose. From this point on, the surface will begin to overheat and finally reach a melting temperature.
In high-frequency surface hardening, however, the purpose is to heat the surface only to a predetermined depth, then apply a suitable quench. In doing so, overheating usually does not take place because of the large unheated mass lying directly under the heated zone. Applying flame heating to a similar piece, the temperature rise is more constant during the heating period and there is a tendency to overheat the surface. This comparison is made merely to emphasize the advantage of induction heating, where power absorption diminishes above the critical point, usually eliminating overheating, over other methods where such heat control does not exist and burning of the surface is more likely to occur. Then, too, with high frequency induction heating, the heat producing current can be stopped at will, so that overheating can be easily controlled.
In as much as localized surface can thus be heated rapidly by means of high frequency induction heating current, new methods of manufacture and basic changes in product design are possible with this heating process. All these methods include modifications from normal procedures, which are but a few of the changes resulting from this process of heating. Example: a machine part in which three surfaces require hardening in different degrees. Normally it would be difficult to produce such a part, but by using high frequency, the undertaking is relatively easy. Each surface is heated separately, as an individual cycle. The eccentric or cam requires maximum hardness, since it operates a plunger and therefore is hardened to 62 Rockwell C. The gear teeth, because of their duty, need not be so hard and are treated to 56 to 58 Rockwell C. The clutch teeth, on the other hand, which are subject to impact only, are hardened to 50 to 52 Rockwell C. Many other machine parts requiring variations in hardness can be easily produced by the induction method.
Another example: a long shaft on which a spur gear is integrally machined, and on which only the teeth require hardening. Here, again, the normal procedure would be to subject the entire piece to heat, from which would result some deformation, requiring a straightening operation, and also the formation of scale, necessitating cleaning. With induction hardening, however, the gear teeth can be hardened individually without affecting the ductility of the remainder of the part, with an appreciable savings in manufacturing cost. The shaft is heat treated, but requires two hardened bearings, as indicated, one on each side of the short spine, on which needle bearings operate. Here, again, the normal procedure would be to heat the entire end of the shaft and quench it, which most likely would result in deformation and a difficult straightening operation. By the induction method, the journals can be individually hardened without metallurgically affecting the adjacent portion of the shaft.
Frequently, gears are hardened and fitted with sleeves or bushings, necessitating grinding of the hole by means of locating from the pitch diameter of the teeth. This is done to assure concentricity, but there is of course a chance also of certain misalignment. By means of high frequency heating, however, it’s possible to press the bushing into the gear blank before the teeth are cut, then cut the teeth from the bushing so that concentricity is assured. The final operation is the induction hardening of the teeth, which on a gear of this type can be carried out with practically no deformation whatever, and without the formation of usual scale.
Types of Induction Heating Coil
An induction heating coil for high frequency induction heating can be made in a wide variety of styles and shapes, depending upon the nature of the operation. To some extent, the design of a coil will be limited to certain types of generators. The most used general purpose type perhaps is wound or formed from copper tubing either symmetrical in contour or formed to suit the shape of the part. A solid type induction heating coil is also widely used and is particularly suitable for heating parts requiring a restricted zone, such as series design, which makes it possible to heat several pieces at one time. A flat strip coil can also be used in a variety of heating operations.
Regardless of the type of induction heating coil used, it’s necessary to provide cooling. In the case of the multi-turn coils, this is accomplished by circulating water through the coil itself. With single-turn inductors, a cooling tube is added to the outside (or other suitable internal cavities are provided for the passage of water).
A symmetrical induction heating coil can be used for a part with an irregular surface, provided, of course, that the outer surface or contour is not too irregular or pronounced in shape.
Since a high frequency induction heating current has a tendency to follow the outer surface of a piece placed within a coil, the irregular part will be heated fairly uniformly around its periphery even though its contour is irregular, resulting in a variation in coil coupling. If however, the contour of a certain part is especially pronounced it’s advisable to provide an induction heating coil that conforms to the approximate shape of the part.
An example: the principle of using induction heating coils for heating hexagon nuts. With the small nut, the variation in surface is not too pronounced, and a symmetrical induction heating coil may be used advantageously. With the much larger nut however, there’s more of a variation in the coupling, and if a symmetrical coil were used, there would be a tendency to produce more heat at the corners of the nut than at the flats. Therefore, the induction heating coil for this part is of hexagonal shape, which ensures a more even distribution of heat to the entire outer surface of the nut.
This heating condition, however, will vary with the height of the induction coil in relation to the contour of the work. With a wide induction heating coil, where the work is off center, a more uniform heat may be expected around the surface of the work than with a narrow induction coil under the same conditions. This shows that with narrow coils the need of a uniform coupling is more imperative than with wider coils. Where such a variation exists, owning to the shape of the work or to the impossibility of forming an induction heating coil to conform to the contour of the work's surface, the part should be rotated if possible so as to offset the one-sided heating condition.
The magnetic flux set up by the high frequency induction heating current of the type used for heating metals will pass through dielectric materials, or particularly those of a nonmetallic nature. For example, if an induction heating coil is placed under a sheet of plate glass with a piece of steel placed directly above, the portion closet to the induction coil will heat as readily as if the glass were not there.
Sometimes insulating material can be used to advantage by providing the correct spacing between the induction heating coil and the work. In this case, a pancake type induction coil is placed under a piece of plate glass and the part to be heated is located on top. When the induction heating coil is energized, the clutch teeth will heat immediately to the desired temperature. Such an operation might be controlled by a timer. The same condition is true when a glass tube is surrounded by a heating coil, and a tube is placed within the part requiring heating.
Dielectrics are widely used in connection with induction heating applications. Sometimes the heating coil itself can be covered, or coated with cement having a ceramic or porcelain base, and thus provide insulation so the work piece will not come in contact with the induction coil.
Rules governing the selection of an induction coil, whether it should be a single or multi-turn design, usually are based on the type of generator to be used and the nature of the operation to be performed. Usually the shape of the work and the area of the zone to be heated will determine the design most appropriate.
A few of the basic considerations are as follows:
- Single-turn induction heating coil is preferred when the heated area is narrow or restricted.
- Single-turn induction heating coil is more practical when the height does not exceed the diameter.
- Multi-turn induction heating coil is preferred for heating long areas.
- When the length of an induction coil exceeds eight times its diameter, uniform heating may become difficult.
- Long areas should be heated by progressive feed through a short induction coil.
- On a multi-turn induction coil, keep the space between windings at a minimum where uniform heat is wanted.