US2820128A - Apparatus for induction heating - Google Patents

Description

I Jan. 14, 1958 I B. E. MO'ARTHUR 2,820,128

APPARATUS FOR INDUCTION HEATING Filed Jan. 10, 1955 I r ATTOPA/fKS INVENTOR.

United States Patent APPARATUS FOR INDUCTION HEATING Bruce E. McArthur, Youngstown, Ohio, assignor to Magnethermic Corporation, Youngstown, Ohio, :1 corporation of Ohio Application January 10, 1955, Serial No. 480,743

14 Claims. (Cl. 219-1067) This invention relates to a method and apparatus for the induction heating of billets and other metallic charges and particularly to a method and apparatus for the induction heating by means of low frequency alternating currents at relatively low voltages such, for example, as 25 to 60 cycle alternating currents at from about 50 to 460 volts, and of billets of high resistivity, such as steel or nickel of various lengths and diameters.

The induction heating of billets of ferrous and nonferrous metals with high frequency alternating current at correspondingly relatively high voltages is well known in the art. Due to the skin effect, also well known in the art of induction heating, the heat produced by currents induced in the billets by high frequency currents is created in the relatively thin surface stratum or skin of the billet. Therefore, if the interior of the billets is to be heated, as a result of the flow of the induced currents, the heating thereof must be by conduction of heat from the heated outer stratum or skin. Other factors being equal, the thickness of the stratum in which the heat is created decreases as the frequency increases and thus the temperature differential between the outer stratum and the interior of the billet is correspondingly greater for higher frequencies.

For a given power input, therefore, uniform heating throughout the entire cross section of a billet should require a lesser heating period when low frequncy is used than when high frequency is used, if low frequency can be properly applied.

Because of the skin heating effect which necessitates heating of the core of the billet by conduction, the time required for heating a billet throughout its cross section increases rapidly with an increase in the diameter of the billet.

Due to this temperature ditferential and the limitations on the maximum temperature to which the outer or surface stratum of the billet can be heated without deterioration, currents induced by high frequency current in the coil cannot be applied in some instances as nearly continuously as those induced by lower frequency current. Instead, to constrain the temperature differential from exceeding a practical maximum limit, the high frequency heating cycle often necessitates intermittent intervals of on-time during which current is applied to the coil separated by intervals of ofi-time during which the application of current to the coil is discontinued.

In accordance with the present invention some of the objections to the high frequency current methods are eliminated by the utilization of low frequency currents, preferably at the generally available commercial power distribution frequencies and voltages such as 25 to 60 cycle alternating current at from about 50 to 460 volts. These low frequencies cause the heat to be created in a thicker surface stratum or skin of the billet due to the deeper penetration of the current induced in the billet by the lower frequency current applied to the coil.

By the present method, billets or other charges of ferrous and non-ferrous metals can be heated with a much less temperature differential between the outer stratum, in which the heat is created, and the central core and consequently with a much less off-time interval and time lag in the heating cycle than in the case of high frequency current.

Furthermore, the lower frequency currents have the advantage that they do not require, for their use in induction heating, the expensive special motor-generator sets required as a source for high frequency current.

However, it is necessary for lower frequency induction heating to provide suitable practical apparatus. The present apparatus has the advantages of a low initial cost, low operational cost, low maintenance cost, and limited floor space requirements. Further, it is adapted for antomatic operation, has such versatility in temperature control that the resultant product is improved, permits improved working conditions for the workmen, and permits improved production planning.

One of the principal difficulties encountered in the heating of billets of high resistivity with low frequency current is the provision of apparatus which produces a heating pattern which is uniform over the entire length of the billet.

In the case of high frequency induction heating, uniform heating can be accomplished by varying the space between the turns of the induction coil. These so-called tapered turns generally employ relatively stiff conductors of uniform cross-section having their convolutions or turns spaced apart endwise of the coil, no insulating spacing material of limited compressibility being used to separate the convolutions from each other. Nor need provision be made to hold the turns firmly in position endwise of the coil as the high frequency currents do not cause the turns to vibrate strenuously.

In the case of low frequency heating, however, it is not practical to use coils having what are generally referred to as tapered turnsthat is, more turns in the coil per unit of axial length at one portion of the coil than at another due to different open spacing, axially of the coil, of turns of a conductor of uniform cross section. At the lower frequencies with their greater current as compared to higher frequency with the same power input, higher magnetic fields are caused between the turns of the coil and also between the billet and the turns. These intense fields cause strenuous vibrations of the turns or convolutions of the winding as a result of which the turns tend to pound against each other endwise of the coil. Accordingly the turns must be positively securely against movement relative to each other endwise of the coil.

In accordance with the present invention, a conductor of varying cross-section is wound into a helix to form an induction heating coil in which the conductor is of less dimension in a direction axially or endwise of the coil at some portions along the length of the coil than at other portions. The conductor preferably is suitably insulated and the insulated turns of the coil are drawn tightly against each other axially of the coil so that none of the turns can pound against the others due to magnetic fields caused by low frequency current flowing through the coil. If an uninsulated conductor is used, insulation must be interposed between the adjacent turns, at least at intervals along their length, and positively hold them out of contact with each other, thus providing, in effect, insulated turns. When the insulated turns are referred to hereinafter, turns of insulated conductor or turns of uninsulated conductor held apart by insulation are meant.

For the ordinary heating of billets the turns of less width endwise of the coil are disposed at the charging or a open end of the coil which is the end through which the billets are introduced into and removed from the coil.

Near its opposite or closed end, or at a portion beginning a distance from its open end and extending toward its closed end, the coil is provided with a number of take-off taps so that any given length of the coil, beginning at the open end and extending the desired distance toward the outlet end, can be used.

In addition, there is provided a water-cooled laminated core of high flux and heat conductivity which can be adjusted to different positions endwise of the coil as a re sult of which the flux introduced into the billet can be controlled so that the heating effect may be made substantially uniform throughout the length of the billet or patterned in various preselected manners.

Likewise, insulation is provided at the ends of the charge to reduce heat losses, and particularly between the core and charge so that an effective flux pattern can be obtained without destroying the resultant heating pattern by too great heat losses through the core used to improve the flux pattern.

Various objects and advantages of the present invention will become apparent from the following description wherein reference is made to the drawings, in which:

Figure l is a front elevation, partly in section, illustrating a preferred embodiment of the invention;

Figure 2 is a side elevation, partly in section, of the apparatus illustrated in Figure 1;

Figure 3 is a fragmentary cross sectional view of a laminated core forming part of the present invention and is taken on the line 33 of Figure 1;

Figure 4 is a fragmentary perspective exploded view of an assembly of a heat insulating pad, a gasket, charge supporting means, used in the present invention; and

Figure is a perspective view of a heat insulating pad used in the present invention.

Referring to the drawings, the invention comprises a coil 1 of electrically insulated conductor and having a plurality of take-off taps 2 distributed along its length. The coil 1 is mounted between rigid clamping plates 3 and 4, respectively, which engage the opposite ends of the coil and clamp the coil tightly therebetween with the insulated convolutions or turns of the coil in tight contact with each other in a direction endwise of the coil. For clamping the plates 3 and 4 in this relation, suitable tie rods 5 are provided, these rods extending through the plates 3 and 4 and being secured to the plate 3 for drawing it toward the plate 4.

Mounted on the rods 5, respectively, are sleeves 6 of electrical insulating material which prevents the rods 5 from coming into direct contact with the coil or other parts alongside the rods. The sleeves 6 are slightly less in length than the distance between the plates 3 and 4 so as not to interfere with the clamping relation between the plates and coil.

Mounted within the coil 1 and extending axially through the central passage of the coil is a sleeve 7 in which the billet or charge to be heated can be disposed. The sleeve 7 preferably is of material which is non-magnetic. If desired, it may be of a refractory material. Again, it may be a water cooled liner such as disclosed in application Serial No. 379,389, filed September 10, 1953, now Patent No. 2,781,437.

In the form illustrated, the charging or open end of the coil is indicated at 8, the opposite or closed end being indicated at 9. At the closed end 9 of the coil the rods 5 extend through the plate 4 and through discs 10 and 11. The sleeve 7 is slidably received in the discs 10 and 11.

Sleeves 12 mounted on the rods 5 extend from the upper or outer face of the disc 10 to suitable clamping nuts 13 on the rods 5, respectively, as a result of which the pressure from the nuts 13 may be transmitted through the sleeves 12 to the discs 10 and 11 and thence to the plate 4 for drawing the plate 3 toward the plate 4 and holding the plates 3 and 4 in endwise clamping relation to the coil, and with the insulatedconvolutions of the coil in sufficiently tight contact with each other endwise of the coil to prevent their pounding against each other.

Mounted on the sleeves 12 and guided thereby for movement axially of the coil is a carriage 14. In the form illustrated, the carriage 14 comprises a central sleeve 15 and annular discs 16 and 17 fixedly secured to the ends of the central sleeve 15 and held in fixedly axially spaced relation from each other by means of sleeves 18. The sleeves 18 are slidable axially on associated ones of the sleeves 12.

The carriage 14 is yieldably urged toward the clamping plate 4 and resiliently held against the disc 11 by means of compression springs 19 which are interposed operably between the disc 17 and the nuts 13 carried by the rod 5.

Mounted on the carriage 14 for adjustment to different positions axially thereof is a core support 20 which is in the form of a hollow sleeve telescopically fitting the sleeve 15 of the carriage 14. The sleeve 15 is slit along one side and is provided with a clamping latch 21 so that it can be clamped tightly about the sleeve or sup port 20 in different axially adjusted positions of the sleeve 20 relative to the sleeve 15. Thus, the core support 20 is adjustable axially of the coil 1 by adjustment of its position relative to the carriage 14 and it can move to different positions endwise of the coil while in fixed relation to the carriage 14 in any of its adjusted positions.

Mounted on the support 20 is a laminated core 22 which preferably is in the form of alternate layers of steel or ferrous metal, indicated at 23, and metal of high heat conductivity such as copper, indicated at 24. The laminations extend parallel to the path of the flux through the core.

Mounted on the upper end of the laminated core 22 is a means for conveying heat away from the core. This means may comprise a length of pipe having one or more loops, indicated at 25, which bear against the outer face of the core 22. The pipe has a water inlet portion 26 and an outlet portion 27 both of which extend to the outer end of the support 20 and are connected to the inlet and return line, respectively, of a suitable external cooling system. The inlet portion has an inlet fitting which is indicated at 28.

The outer end of the support 20 is closed by a suitable cap 29 and pipe portions 26 and 27 extend therethrough to the external fittings by which they are connected to an external cooling system.

For purposes to be described, the support 20 is hollow and is open at its lower end to permit the escape of gases therefrom into the interior of the sleeve 7 so that a reducing or non-oxidizing gas can be introduced through the support 20 into the sleeve 7 and a non-oxidizing or reducing atmosphere maintained in the sleeve 7 throughout the heating operation. Gas is introduced into the support 24) through a suitable port 31) in the cap 29.

In order to hold the sleeve 7 in coaxial relation to the coil while permitting its upper end. to travel endwise of the sleeve relative to the clamping plate 4 so as not to interfere with the clamping action, an annular sleeve 31 is mounted on the disc 16 and extends a short distance axially into the sleeve 7 at the end 9 of the coil. The sleeve 31 extends about the core and fits into the sleeve 7 telescopically with operating clearance.

The end 9 of the coil and sleeve 7 are thus arranged to prevent the escape therefrom of the gas and the entrance into the sleeve 7 of outside air. If the apparatus is operated in the upright position illustrated and the gas is helium or a gas lighter than air, then the bottom of the sleeve can be left open. If the apparatus is to be used in other than a vertical position or with a heavier gas, a gasket orseal of asbestos or other suitable material may be provided at the end 8 of the coil, as hereinafter described.

Thus the core 22 can be moved axially of the coil to different positions along the sleeve 7 and in all of said positions, through the medium of the carriage 14 and springs 19, can resist yieldably any force applied against it axially of the coil in a direction away from the charging end toward the end 9, can overcome such force without severe shocks, and can return itself and the billet to their original positions.

Accordingly, upon magnetic propulsion of any billet within the sleeve, endwise toward the core, the billet movement is yieldably arrested upon striking the core 22 so that shocks are absorbed and the ingot is moved back inwardly to its proper preselected position.

This is a safety precaution which is desirable particularly when the apparatus is operated with the coil disposed with its axis horizontally. Again, should an operator neglect to change the position of the core 22 for a longer billet than that for which the core had been set, the core can be moved by the longer billet fed into the sleeve 7, thus protecting both the coil and the feeding mechanism for severe shocks and stresses.

The seating of the carriage 16 by the springs 19 also assures an effective seal against the escape of gas from within the sleeve 7.

In order to feed the billets or ingots into the coil a suitable piston 32 having an axially adjustable piston head 33 is arranged to extend a predetermined distance into the charging end it of the coil.

Heat insulating pads, as indicated at 34 and 35, are interposed between the piston head 33 and its associated end of the billet and the core 22 and its associated end of the billet so as to reduce heat losses from the ends of the billet. eat induced in the core may be removed by means of the cooling water, if necessary to assure that the core operates at a sufficiently low temperature.

The pads 34 and 35 are secured to the core 22 and piston 33, respectively.

As best illustrated in Figure 4, the insulator 34 may be in the form of a disc 36 of thermal insulating material in a stainless steel container 37 having a top wall 38, a bottom Wall, and a side wall 39. The container 37 is slotted with small diametrically opposite radial slots 41 which extends through the end walls and from near the center of the insulator to and through the outer periphery. The slots substantially eliminate induced currents in the container 37.

The piston 33 also is provided with radial slots 42. if desired, as mentioned hereinbefore, a gasket 43 or asbestos pad may be interposed between the piston 33 and pad 34, the gasket being of insulating material and of sufiicient diameter to seal the lower end of the sleeve 7. The pad 34 is provided with lugs 43a for securing it to the piston 33.

As illustrated in Figure 5, the pad 35 is similarly constructed, having slots 44 and lugs 45 which are used to clamp together the laminations of the core 22.

Though the invention is particularly advantageous for use with alternating currents of from about 25 to 60 cycles and voltages of from about 50 to 460 volts, it is well adapted to all the standard commercial voltages and low frequency currents.

The turns of the coil should be increased in dimension in a direction endwise of the coil from the charging end 8 to the longitudinal center of the coil. Accordingly, beginning at the end S, the coil is started with a few turns of conductor of uniform cross section so that each of these first turns has the same dimension endwise of the coil. Considerable variation in the dimension of the cross section of the conductor radially of the coil is permissible. A little further along the coil a conductor of larger cross-section is used. This latter conductor may have the same dimension radially of the coil as the first portion, or may have a dilferent dimension radially, but

it is of greater dimension endwise of the coil. Addi= tional successive groups of turns of conductor are arranged so that, in a direction endwise of the coil, the dimension of each turn of that group is progressively greater than the dimension, endwise of the coil, of each turn of the group next adjacent to it at the side toward the charging end. This increase in dimension is continued from the end 8 to the longitudinal center of the coil. The end portions of the successive groups of turns may be electrically connected to the ends of the next adjacent groups by brazing or similar means. The condoctor may be of tubular Water-cooled type, if desired.

A modification of the coil which produces some of the eifects above described but which is not in many respects as satisfactory can be used in some instances. In the modified structure, the conductor is of uniform crosssection and, beginning at the charging end where the turns are relatively close together and continuing to the longitudinal center of the coil, the turns are spaced progressively farther apart the farther they are from the charging end, insulation material of limited compressibility being provided, and bridging entirely across the space between the turns.

The turns with their interposed insulation, are clamped firmly so that the turns, spaced by the insulating material, and the coil as a whole, are immobile endwise of the coil.

This modification is believed clear without illustration in the drawings.

As mentioned, the present apparatus is particularly desirable in the case of billets of different length and the provision of an apparatus to be used for heating billets of different lengths introduces problems which are not necessarily present in the case of heating billets of uniform length.

For example, if it were attempted to provide a flux pattern uniform throughout the length and cross section of the billet solely by the use of enough turns and taps disposed endwise of the coil beyond the upper end of the billet, the coil would be too long to effect the proper concentration of flux in the billet as the turns could not be concentrated at a number of dilferent places suitable for various lengths of billets. Accordingly, the core 22 must be used instead to obtain a proper flux pattern. But the core 22 conducts heat from the billet at the billet end. Further the core itself would be damaged by the heating and cooling. Accordingly the pad 35 is introduced to permit concentrating the flux with a core while eliminating the high heat losses that would occur due to the concentrating core 22.

In operation, assuming the apparatus is operating in the upright position as shown, the billet is placed upon the thermal insulating pad 34 on the top of the piston 33 and the piston 32 is then operated to raise the billet to the preselected position. Next, the latch 21 is loosened and the core support 20 is lowered in the carriage 14 until the lower face of the insulating pad 35 rests on or is spaced slightly above the upper end of the billet. Thereupon the latch 21 is operated to lock the carriage 14 and support 20 in fixed relation with each other. One side of a source of power is connected to one terminal of the coil. Depending upon the length of the billet and the position selected, the other side of the source is connected to a preselected one of the taps 2. The tap selected is usually such that the effective length of the coil extends 3 or 4 inches beyond the upper end of the billet in a direction toward the upper end 9 of the coil. Gas is introduced through the port 30, cooling water or fluid is circulated through the pipe 26, and the coil is then energized.

If the billet tends to shift its position axially due to magnetic propulsion, adjustment can be made in the pistion 33 and between the carriage 20 and support 14 accordingly so that the billet can readily be retained in exactly the desired position axially of the coil.

After the billet has been heated and removed, a' new billet is placed on the insulating pad 34 on top of the piston 33. The piston is then raised to position and the coil is energized. Any force that may result from the billet being propelled axially of the coil due to the magnetic flux and that may cause the billet to strike the core forcibly is relieved by the springs 19 through the medium of the support 20 and carriage 14 and the billet is returned to its final preselected heating position. Likewise, should a larger billet be introduced without readjustment of the position of the core, the shock which would be caused by the larger billet striking the core would be relieved.

Successive billets of the same length can be heated readily without further adjustment.

If a uniform flux penetration throughout the length of the billet is desired, the number of turns of the group adjacent the charging end 8 is greater than the number of turns of each of the groups spaced from the charging end 3 toward the opposite end 9. Preferably the dimension of each turn of the group nearest the end 8 in a direction endwise of the coil is about one-half of the dimension endwise of the coil. of each turn of the group nearest the end 9.

The flux density at the charging end 8 otherwise would be about half the flux density at the longitudinal center of the .coil, due to leakage paths.

The water cooled laminations at the end 9 concern trate the flux of a considerable portion of the coil which is beyond the upper end of the billet and thus raise the flux density at the upper end of the billet to oii-set the losses which would otherwise occur and thus make the flux density at the upper end more nearly equal to that at the longitudinal center of the coil. At the same time loss of heat is reduced by the pads 34 and 35. With such an arrangement a more nearly uniform flux and heating pattern throughout the length of the billet can be obtained.

With these principles in mind, it will be seen that specialized heating effects in which temperatures of the billet may be varied at different positions throughout the length of the billet to meet specific forging requirements can be obtained.

Having thus described my invention, I claim:

1. In a low frequency induction heating apparatus, a heating coil having a winding of insulated conductor and an axial passage adapted to receive thrrmgh one end, and to accommodate, a charge of metal to be heated. rigid clamping plates at opposite ends of the coil and bearing against the end turns of the coil, respectively, tie bolts connected to one plate and extending endwise of the coil through the other plate and having portions beyond the other plate in a direction away from the end of the coil, sleeves on the bolts operatively bearing against the said other plate, nuts on the bolts bearing against the sleeves and operative to draw the plates tightly against the opposite ends of the coil, respectively, a. carriage slidable axially of the coil on the sleeves, a core receivable through said other end of the coil into the coil passage, means connecting the core to the can riage for adjustment axially of the coil relative thereto and for movement therewith, and spring means urging the carriage toward the said other end of the coil.

2. In an induction heating apparatus, a heating low frequency coil including a winding with an axial passage which is adaptedto receive through one end of the coil, and to accommodate, a charge of metal to be heated, a laminated flux concentrating core having at least a portion accommodated in the said passage in spaced relation to one end of the coil, a support supporting the core for adjustment endwise of the passage to diiierent positions in which said portion of the core is within said passage, and a plurality of take-off taps on the coil distributed along that portion of the coil in which said core portion is adjustable.

3. A structure according to claim 2 characterized in that a thermal insulator is disposed in the passage in alignment with the innermost end of the core in a position so as to lie between said core and the adjacent end of a charge when a charge is accommodated in the passage.

4. In an induction heating apparatus, a heating coil including a winding with an axial passage which is adapted to receive through one end, and to accommodate, a charge of metal to be heated, a laminated flux concentrating core having at least a portion accommodated in said passage in spaced relation to said one end of the coil, and a thermal insulator disposed in the passage in alignment with the innermost end of the core in a position so as to lie between the said innermost end of the core and the adjacent end of the charge when the charge is accommodated in the passage.

5. In an induction heating apparatus, a heating coil arranged with its axis in upright position and including a low frequency winding with an axial passage which is adapted to receive through its lower end, and to accommodate, a charge of metal to be heated, a flux concentrating core having at least a portion accommodated in the passage at the upper portion of the coil, a support for supporting the core for vertical movement endwise of the passage to different adjusted positions in which said portion is within the passage, a plurality of take-off taps on the coil distributed along that portion of the coil in which said core is adjustable, a vertically movable support disposed at the lower end of the coil and adapted to support a billet of metal thereon in upright position and to move upwardly into the coil so as to dispose the billet in, and support the billet in, said coil with the lower end of the billet at least as high as the level of the lower end of the coil.

6. A structure according to claim 4 characterized in that cooling means employing cooling media for removal of heat are provided in heat exchange relation to the core for conducting heat away from that portion of the core accommodated in the coil passage.

7. A structure according to claim 4 characterized in that said coil is provided with a plurality of take-ofi taps distributing along that portion of the coil in which said portion of the core is accommodated.

8. In an induction heating apparatus, a low frequency heating coil including a winding with an axial passage which is adapted to receive, through one end of the coil, and to accommodate, a charge of metal to be heated, a laminated flux concentrating core having at least a portion accommodated in said passage in spaced relation to one end of the coil, a support for supporting the core for adjustment endwise of the passage to different positions in which said portion of the core is within the pas sage, a thermal insulator disposed within the passage in alignment with the innermost end of the core so as to lie between said innermost end of the core and the adjacent end of the charge when a charge is accommodated in the passage.

9. The structure according to claim 8 characterized further in that another thermal insulator is disposed in the passage in position to lie between the other end of the charge and the end of the coil adjacent to said other end of the charge.

10. The structure according to claim 8 characterized in that cooling means employing cooling media for re moval of heat and provided in heat exchange relation to the core for conducting heat away from said portion of the core.

1]. The structure according to claim 10 characterized in that said coil is provided with a plurality of take-off taps distributed along that portion of the coil along which said portion of the core is adjusted.

12. A structure according to claim 2 characterized in that means are provided to support the charge in the passage with one end of the charge adjacent one end of the coil and the other end of the charge spaced intions, endwise of the passage, and in some of said posiwardly axially of the coil from said other end of the coil. tions extends into the passage.

13. A low frequency heating apparatus comprising a low frequency induction heating coil, a carriage at one References Cited in the file of this patent end of the coil, means supporting the carriage for re 5 ciprocation endwise of the coil, means for holding the UNITED STATES PATENTS carriage in difierent adjusted positions endwise of the 1 937 453 Adams Jam 1935 coil, a core, means securing the core to the carriage for 2 319 093 Somes May 11 1943 movement of the core with the carriage, said core ex- 2 400 50 Strickland May 21 1945 tending into said one end of the coil when the carriage 10 2 434 5 Strickland o 1 1949 is in a near position relative to said one end of the coil, 2 5 3 3 C flifi et 1 Jam 5 1954 and feed means at the opposite end of the coil and ar- 2,676,234 Lackner et aL APR 20, 5 ranged to feed a charge of metal to be heated into, and 7 240 McAl-thm- May 9 195 to remove it from, the coil at said opposite end.

14. A structure according to claim 12 characterized in 15 FOREIGN PATENTS that the last-named means is adjustable to different posi- 710,426 Great Britain June 9, 1954

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