INDUCTIVE HEATING DEVICE AND METHODS EMPLOYING A HEATING COIL AND WORKPIECES HEATED THEREBY

Abstract: An inductive heating device for uniformly heating a workpiece at rest has in one embodiment of the inven­ tion a plurality of heating coil units (31 to 36) disposed so as to surround the workpiece (21). The heating coil units (31 to 36) are connected in parallel to a power source, and a transformer may be used to vary the voltage across the terminals of the outermost units to increase the current flowing through these outermost units. The device, therefore, has a very short insuf­ ficient heating temperature zones attributable to end magnetic flux effects. In another embodiment using a non-divided coil, the end effect may be reduced by adjusting the rela­ tionship between the distance between the end of the coil and the end of the workpiece and the clearance between the coil and the workpiece.

Description: INDUCTIVE HEATING DEVICE AND METHODS EMPLOYING A HEATING COIL AND WORKPIECES HEATED THEREBY The present invention relates to inductive heating devices and methods for heating workpieces and is applieable to use for graphitizing carbon. Description of the Prior Art A conventional device for heating a workpiece at rest is shown in Fig. 1 of the accompanying drawings. In Fig. 1, electrodes 2 are placed on opposite sides of pieces of carbon 21. A power source 6 supplies electric current to the electrodes 2 and the pieces of carbon 21. The current is passed between the electrodes 2 and, therefore, the current is also passed through the pieces of carbon 21. A first thermal insulator 3, comprising carbon powder "A", is used to fill the spaces between the pieces of carbon 21, the carbon powder "A" facilitating the application of current to the pieces of carbon 21. A second thermal insulator 4, comprising carbon powder "B", is used to cover the top of the electrodes 2 and the pieces of carbon 2.1 to insulate thermally the top of the heating device. A refractory member 5 is placed below the electrodes 2, the pieces of carbon 21 and the carbon powder "A" to insulate thermally the bottom of the heating device. In the conventional heating device, in order to supply current uniformly to the pieces of carbon 21, the first insulator 3, comprising the carbon powder "A", is used to fill the gaps between the pieces of carbon 21. In this case, the spatial distribution of the current between the electrodes 2 varies, the amount of variation depending on how well the gaps between the pieces of carbon 21 are filled with the carbon powder "A". In addition, a long period of time is required for applying current to ade quately heat the pieces of carbon 21. Such a lengthy current application time results in excessive heat loss through the electrodes 2. For these reasons, the ratio of electric power applied by the power source 6 to the sum of the surface areas of the pieces of carbon 21 to be heated (hereinafter referred to as "a surface power density") is, in general, set to about 3.5 W/cm2. Summary of the Invention In view of the foregoing, an object of the present invention is to provide a more efficient heating device and method suitable for heating a workpiece. According to one aspect of the invention there is provided an inductive heating device for heating a workpiece characterized by a heating coil disposed so as to surround said workpiece, said heating coil being divided into a plurality of heating coil units. According to another aspect of the invention, there is provided a method of heating a workpiece characterized in that said workpiece is positioned within an electrical coil formed from a pluralitay of coil units connected in parallel to an alternating voltage source whereby induction takes place in said workpiece. According to a further aspect of the invention,-there is provided an inductive heating device for heating a workpiece characterized in that a heating coil is provided for surrounding said workpiece with the distance between an end of the coil and an end of the workpiece being at least approximately equal to the clearance between the coil and the workpiece. According to a further aspect of the invention, there is provided a method of heating a workpiece characterized in that the workpiece is placed within an electrical coil with the distance between an end of the coil and an end of the workpiece being at least approximately equal to the clearance between the coil and the workpiece and in that an alternating voltage source is connected to the coil whereby induction heating takes place in said workpiece. According to a yet further aspect of the invention, there is provided a workpiece heated in accordance with the devices and methods of the invention. As current flows into the heating coil, an eddy current is induced in the peripheral portion of the workpiece to be heated, thus heating the workpiece. The heat insulating layer reduces the dissipation of heat from the workpiece being heated and also prevents the surface of the workpiece from oxidizing. Where the heating coil comprises a plurality of heating coil units connected to a power source in parallel, in addition, the outermost heating coil units may be connected to a voltage varying means so that the current in these outermost units can be varied. Using the device so constructed, the workpiece can be heated to a temperature 0 of 2,200 C or higher, in a short period of time, with a high surface power density. In addition, the length of an insufficient heating temperature zone between the plurality of heating coil units, attributable to an end magnetic flux effect, is reduced. Brief Description of the Drawings Fig. 1 is a sectional view of a conventional inductive heating device for heating a workpiece at rest; Fig. 2 is a sectional view showing one embodiment of the present invention; and Fig. 3 is a sectional view showing a further embodiment of the present invention. Description of the Preferred Embodiments A first embodiment of the device of the invention will be described with reference to Fig. 2. As shown in Fig. 2, an inductive heating coil 22 is arranged around a workpiece 21 at rest to be heated, the workpiece being made of carbon. The space between the coil 22 and the workpiece 21 is filled with a heat insulating material 23, such as carbon powder "B". The heating coil 22 is then connected to an AC power source (not shown). In this embodiment, the inductive heating coil 22 surrounds the workpiece 21. As current flows into the heating coil 22, an eddy current is induced in the peripheral portion of the workpiece to be heated. The current permeation depth of the eddy current varies, depending on the geometry involved and the amount of current flowing in the heating coil 22. The core of the workpiece 21 is heated by the conduction of heat from the peripheral portion of the workpiece. In order to prevent the dissipation of heat from the peripheral portion of the workpiece and to prevent the peripheral portion of the workpiece 21 from oxidizing, the workpiece 21 is surrounded by the heat insulating material 23, such as carbon powder "B". The heating device and method as described is suitable for manufacturing silicon carbonate. In the device of Fig. 2, when the workpiece at rest is subjected to inductive heating, an end magnetic flow effect makes it difficult to heat the end portion of the workpiece 21 so that the workpiece 21 is heated uniformly as desired. In order to improve the heating of the end portions of the workpiece 21, the distance gc between the end of the heating coil 22 and the end of the workpiece 21, and the clearance gw between the heating coil 22 and the workpiece 21 are adjusted, with a particular relationship being maintained between the distance gc and the clearance gw. Through.experience, it is known that the particular relationship between the distance gc and the clearance gw is gw - gc. Therefore, if uniform heating of the workpiece at rest is to be achieved using this device, the relationship gw - gc should be maintained as described above. Therefore, if the clearance gw is large, then it is necessary to use a heating coil, the total length Xc of which is much larger than the total length Qw of the workpiece. Thus, in some instances, the heating zone of the heating device of the first embodiment is considerably long. Fig. 3 shows a second embodiment of the present invention wherein the length of the heating zone can be considerably reduced. Referring to Fig. 3, a heating coil 22' is disposed so as to surround a workpiece 21 to be heated uniformly, similar to the device of Fig. 1. However, in the device of Fig. 2, the heating coil 22' is divided into a plurality of smaller heating coil units 31, 32, 33, 34; 35 and 36, which are connected in parallel to an AC power source (not shown). The parallel-connected heating coil units 31 through 36, shown in Fig. 2, have the same voltage applied across their terminals. However, currents I31 through I36, flowing in heating coils 31 through 36, are determined -by the impedances Z31 through Z36 of the various heating coils, respectively. When the reactances (X31, X32 ... and X36) and the resistances (R31, R32, ... R36) of the various coil impedances are compared with each other, in general, the reactances are several times higher than the resistances in inductive heating. Therefore, the comparison of the coil impedances mentioned above can be replaced readily by a comparison of the coil reactances. The magnetic resistances R31 and R36 of the outermost heating coils are much higher than those of the remaining heating coils because their mean flux gath is longer. Since L oc1/R and X = wL are held by electromagnetic theory, the reactances of the outermost heating coils are lower than those of the remaining heating coils. Thus, the currents in the heating coils 31 and 36 at both ends of the coil 22' are higher than the currents in the other heating coils 22, 23 and 24. Accordingly, the magnetic flux is increased at both the end coils 31 through 36, thereby reducing the length of the insufficient heating temperature zone between coils 31 - 36 caused by the end magnetic flux effect. In order to further reduce the length of the insufficient heating zone, a voltage barying means may be connected to the two end coils 31 and 36, or a transformer may be connected thereto, so that the current flowing in these end coils 31 and 36 can be further varied by varying the voltages applied across the terminals of these end coils to further increase the magnetic created thereby. The above-described embodiments of the present invention for heating a workpiece at rest are applicable to a workpiece which should be subjected to heat-treatment at a high temperature (especially 1,5000 C or higher), while oxidation of the -workpiece due tosa high heating temperature is prevented. More specifically, the embodiments may be employed as a silicon carbide oven, a tungsten carbide oven, or a graphitization oven; i.e., a graphitization inductive heating device. Heretofore, when heating a workpiece at rest, the division type heating coil, in general, has not been employed because the workpiece is heated at different temperatures at the junctions between the heating coils. However, in the case where high temperatures must be used, as in the case of graphitization inductive heating, the clearance gw is set to a large value by taking the heat insulation characteristic and the refractory characteristic into consideration and, therefore, thejunctiongN can be maintained much smaller than the clearance gw. Then, the effect of the junctions gN of the heating coils can be disregarded. It should be noted that the invention is not limited only to devices used for graphitization inductive heating. The invention is applicable to inductive heating devices for heating workpieces other than carbon. According to the present invention, by providing a plurality of heating coils connected to a power source in parallel and incorporating voltage varying means for the outermost coils, the coil end magnetic flux effect is increased and, therefore, a workpiece can be heated uniformly while keeping the overall length of the device as small as possible.

Claims: CLAIMS 1. An inductive heating device for heating a workp ece (21) characterized by a heating coil (22) disposed so as to surround said workpiece (21), said heating coil (22) being divided into a plurality of heating coil units (31 to 36). 2. A device as claimed in Claim 1, further characterized by a power source, said plurality of heating coil units (31 to 36) being connected to said power source in parallel. 3. A device as claimed in Claim 1, further characterized by voltage varying means connected to outermost ones (31, 36) of said plurality of heating coil units (31 to 36) for varying a voltage applied across the ter minals of said outermost ones (31, 36) of said heat ing coil units (31 to 36). 4. A device as claimed in Claim 3, characterized in that said voltage varying means comprises a transformer. 5. A device as claimed in any one of the preceding Claims characterized in that the distance (9n) between ad jacent heating coil units (31 to 36) is maintained much smaller than the distance (9w) between said work piece (21) and said heating coil units (31 to 36). 6. A method of heating a workpiece (21) characterized in that said workpiece is positioned within an electrical coil (22) formed from a plurality of coil units (31 to 36) connected in parallel to an alternating voltage source whereby induction heating takes place in said workpiece (21). 7. An inductive heating device for heating a workpiece (21) characterized in that a heating coil (22) is pro vided for surrounding said workpiece (21) with the distance (9c) between an end of the coil (22) and an end of the workpiece (21) being at least approximately equal to the clearance (9w) between the coil (22) and the workpiece (21). 8. A method of heating a workpiece (21) characterized in that the workpiece (21) is placed within an electrical coil (22) with the distance (9c) between an end of the coil (22) and an end of the workpiece (21) being at least approximately equal to the clearance (9w) be tween the coil (22) and the workpiece (21) and in that an alternating voltage source is connected to the coil (22) whereby induction heating takes place in said workpiece (21). 9. A workpiece having been heated by the device of any one of claims 1 to 5 and 7 or by the method of claims 6 or 8.

Inventors: HOSHIKAWA, TORU C/O TOYO TANSO CO., LTD, OSAKI, YOSHIHIKO C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA, TOYO TANSO CO., LTD.
Application year: 1982
Main classification: C21D1
IPC codes: H05B6, C04B35