US1710763A - Electrically-heated furnace - Google Patents

Description

' p 30, 1929. c. A. CADWELL 1,710,763

ELECTRICALLY HEATED FURNACE Filed April 16, 1924 2 Sheets-Sheet l IN VEN TOR.

(Ed Mes f: [Zdzue/Z ATTORNEY April 30, 1929. Q A, WE L 1,710,763

ELECTRI CALLY HEATED FURNACE Filed April 16, 1924 2 Sheets-Sheet 2 F w T4 I N VEN TOR.

mv-g uo ATTORNEYS Patented Apr. 30, 1929.

UNITED STATES PATENT OFFICE.

CHARLES A. CADWELL, F CLEVELAND, OHIO, ASSIGNOR TO THE ELECTRIC RAILWAY IMPROVEMENT COMPANY, OF CLEVELAND, OHIO, A CORPORATION OF OHIO.

ELECTBICALLY-HEATED TEURNAOE.

Application filed April 16, 1924. Serial- It'o. 706,848.

In the scientific heat treatment of steels, a heating medium of precise temperature definition, free from high intensity currents or radiation sources and of such capacity for heat that the article treated may be brought ening of carbon steel articles and of liquid baths for drawing and quenching bears witness to the value of heating by immersion.

Immersion baths in common use such as the lead or saltpeter bath of the ordinary hardening room are operated at relatively low temperature and no difficulty is experienced in maintainin a metallic container which is easily heated y means of a gas or oil flame. The hardening of high speed steel, however, requires a temperature which precludes the use of a metallic bath container and introduces difiiculties which have hitherto practically eliminated the bath as a method of hardening high speed steel. A bath of barium chloride with a container of graphite or carborundum is in limited use for articles that are manufactured exactly to size before the hardening operation is reached but the greatest care must be exercised in handling such a bath in order to prevent failure of the container by breaking either in the heating up or cooling down of the bath which isfiuid only above about 1800 F. Materials for the bath other than barium chloride, which may be used for the purpose present the same dithculty.

The present invention offers a solution of the high temperature bath problem based on the employment of a well in the center of a refractory mass as a container for the bath. and of an'electric heating element made of carbon plates placed in the bottom of the well with the bath material in direct contact there with. I have found that any crack or check in the refractory mass forming the containing walls becomes sealed by chilled bath material, and that by means of my new and unique resistor bed I am able to maintain a high tern perature bath suitable for the hardening 0 high sped steel continuously and with very little depreciation.

One general object of the present invention,

accordingly, is the provision of a simple and compact form of electric furnace for maintaining in molten state a bath of relatively high melting ingredients, whatever may be the particular use for which such bath is intended. A further object is to design a furnace of this type that will be simple and compact in construction and that will effect the conversion of the electricalcurrent into heat in an economical manner. Still another object is the provision of a resistor specially designed and suited for the use in hand.

To the accomplishmentof the foregoing and related ends, the invention, then, consists of the means hereinafter fully described and particularly pointed out in the claims, the annexed drawings and the following description setting forth in detail certain mechanism embodying the invention, such disclosed means constituting, however, but one of various mechanical forms in which the principle of the invention may be used.

In said annexed drawings 1F ig. l is a plan view of a furnace embodying my present improvements; Fig. 2 is a vertical longitudinal section of the same; Fig. 3 is an end view of the furnace proper, certain parts being shown in section on the plane indicated by the line 3-3, Fig. 2; Fig. 4 is a transverse section of such furnace taken on the plane indicated by the line 44, Fig. 9.; Fig. 5 is a perspective view showing a detail of the resistor that forms a part of the furnace; and Fig. 6 is a diagraimnatic plan view of a modified form.

The main body of the furnace, as shown in Figs. 1, 2 and l, comprisesan outer casing 1, preferably of sheet metal and of any desired shape and size, although a convenient shape will be the rectangular one illus trated. Since for the use specifically referred to above, namely as a bathfurnace in which articles or parts to be hardened will be placed, the bath should be suitably elevated above the floor, such casing 1 may be supported on legs 2,0ne on each corner, and in order that the bottom of the casing may be sufficiently rigid, l preferably form the same of a plurality of laterally juxtaposed channels 4, the flanges of which turn downwardly. The ends of the middle channel may furthermore'be extended beyond the ends of the casing 1 to provide rigid supports 5 for the electrodes, as will presently appear. The side walls of the casing, together with the channels thus composing the floor or bottom thereof, and the legs 2 may be welded or riveted together in any approved manner.

The sides and bottom of the casing are lined with-a layer 6 of suitable l'reat non-conducting material, such as infusorial earth, the inner furnace lining 7 being supported by such in sulating layer wholly out of contact with the casing, as will be readily understood.

The lining 7 is preferably formed by mixing refractory material to a ramming consistency and tamping the same in place about a collapsible form, which gives both the proper exterior shape and leaves the well or chamber hereinafter referred to. Such lining may,

however, be fashioned out of an integral bloclc of refractory material or it may be built up of brick so as to conform exteriorly in general with the shape of the casing 1, while leaving a centralchamber 8 of general rectangular form, which chamber is normally open above. Passages 9 furthermore lead from the respective ends of the lining 7 into such chamber 8. A removable cover (not shown) may be provided, if desired, to close the chamber 8. 1

Similar graphite electrodes 10 project inwardly through the passages 9, such electrodes being adjustable to press more or less forcibly'together an interposed series of resistance elements 11 that rest on the floor of floor of said chamber. .While the form and construction of these elements 11 may vary, they preferably consist of rectangular carbon plates, being flat on one face and formed with a series of corrugations 12 on the other, as shown in Fig. 5. The electric current in passing through the resistor composed of such plates will'accordingly tend to subdivide, the paths of least resistance being along the lines of contact or approximate contact between the corrugations on the successive plates with the fiat sides of the adjacent plates. The spaces between the corrugations are filled, this being necessary in the first place to seal the bottom of the chamber 8 and thus to form the working floor of said chamber; also I have found that by employing a filling material of the proper sort, the efiiciency of the resistor is measurably increased.

As a preferred filling material I use a mixture of carborundum element mixed with suitable other ingredients to form a paste which is applied. to the corrugated faces of the plates 11 preliminarily to their assembly in the bottom of chamber 8. Care is exercised that the quantity of paste and its cone sistency are such that the contact of plate against plate is not disturbed. After all of the plates arein place, pressure from the opposing electrodes is brought to bear by means hereinafter described to the end that the carbon plates are held tightly together. as a unit. The resistor element is then completed by tamping carborundum cement of ramming consistency in each corrugation separately, until the whole resistor element is a firm solid mass of uniform electrical conductivity and free from any danger of infiltration of the bath fluid.

Carborundum cement is an electrical conductor of the secondary class; that is, it becomes a high resistance conductor when hotbut is a conductor only of small conductivity when cold. The all carbon part of the car-- rying area of the resistor is therefore a starter for the carborundum part; while the electrical result of the combination of the two is a high electrical resistance within the short space of the furnace chamber. Upon initially heating up the resistor by passing an electric current therethrough, the resistance of the bed is at its maximum, due to the fact that only the all carbon portion of the bed will take current when cold; since the all carbon area of cross section of bed is small, and will remain small until the carborundum portion of the-resistor bed becomes heated. As heating progresses, the resistance of the bed decreases, and the flow of current increases until the dcsired rate of heatin is reached; when this point is reached the tow of current is secured from a transformer step which holds it constant.

Theouter projecting end of each electrode 10 is securely mounted in a steel shell 15 that is fitted with an internally threaded bushing 16 adapted to receive a correspondingly externally threaded shaft 17 to which the shell and thus the electrode are firmly attached. The outer portion of such shaft 17 is preferably left smooth and is reciprocably held in a tubular support 18 on a standard 19 that is fixedly attached to the corresponding projecting channel 5. The tubular support 18 is formed with a longitudinal slot 20 in which is guided a pin 21 that is fixed in the shaft 17 and mounted on the threaded portion of the shaft between such support and bushing 16 is a nut 22, a coiled compression spring 23 being interposed between the same and the adjacent end of the tubular support.

As a result of the foregoing construction, it will be seen that by rotation of nut 22 in the proper direction, the shaft 17 with the electrode carried thereby will be forced inwardly. Both electrodes are thus forced inwardly so as to place the resistor elements 11 under the proper degree of pressure. The interposed springs 23 will, however, allow for the necessary expansion of the resistor which occurs when the heating electric current is passed therethrough.

Copper bus bars are connected with the respective electrodes 10, preferably by being attached, as shown, to the corresponding images shaft 17 closely adjacent'its bushing 16 so that the current will require to travel only a relatively short distance through such shaft. In order to prevent undue heating and consequent variation of the exposed ends of the electrodes, as well as to prevent the liquid bath from seeping out along the electrodes (the latter being a very important consideration), I furthermore attach a cooling coil 26 on each shell 15, such coil being provided with necessary connections for circulating water 'or equivalent cooling fluid therethrough, as will be readily understood.

While, as shown, the portions of the electrodes 10 that extend through the passages 9 .of the furnace lining 7 are made of slightly tapered form in order to insure close fitting contact between such electrodes and passages, such tapered form is not essential, but the electrodes may have straight sides, the passages being of corresponding form, whether of rectangular or-circular cross-section, providing close fitting contact is maintained. The bath material, whatever its particular composition, upon being placed in the chamber 8, rests directly on the resistor 11 and may be readily reduced to a fused condition upon passing a heating current through the latter in the manner hereinbefore described. The size of the resistor will of course be properly calculated, not only to accommodate the current which is to be used, but to generate the proper amount of heat for use with the par ticular material in hand. A certain variation in the heating effect of the resistor may furthermore beobtained by varying the pressure exerted by the respective electrodes through adjustment of nuts 22 on the shafts 17. The latter also serve to take up any permanent contraction in the overall length of the resistor. The bath material, of course, protects the upper surface of the resistor against exposure to the air so that such resistor, as well as the electrodes 10, so far as they are contained within the passages 9 of the furnace lining, are wholly out of contact with the air and the permanence thereof assured. it simple, compact and durable furnace for the special use involved is accordingly provided. The plates comprising the resistor bed need not be arranged in a single series, as shown in Figs. 1 and 2, with the electrodes disposed to press against opposite ends of the series, but various other arrangements may be utilized with entirely satisfactory results. Thus, as shown in Fig. 6. such bed may comprise two parallel series 30 of plates, similar to those described above, with an intervening space 31 between the series filled with refractory material. Adjacent ends of the two series are bridged by a block 32 of graphite or equiva lent resistance material, while two electrodes 33 are pressed against the respective opposite ends of such series of plates. As a result, it will be seen that the electric current is caused to flow in series through the one set of plates, the block 32 and the other set of plates, thus completing the circuit.

Carbon plate resistor-elements have long been in common use. However, attempts to use plain rectangular carbon plates for the purpose at hand have led to practical dificulties in securing a uniform and permanent pressure contact between the plates, as is necessary, in order to insure uniform heating and to exclude the fluid material of the bath. lhe necessary careful surfacing of electrodes and plates and particularly the placing of the same in position under the conditions that obtain, lead to great difficulties. Even if a bed of plain plates were readily placed and were safe against infiltration of bath fluid, the electrical resistance of a bed of plain plates of size suficient to ren'derthe bed mechanically stable is so low that the required heating intensity of the bed can only be obtained by a current of very low potential and of great amperage. Such a heating current leads to large conductors, and to losses, that involve the square of the current, to a prohibitive extent.

it am able by means of the detail of resistor bed herein disclosed to install a carbon plate resistorbed of high electrical resistance, made with special plates, of size sutlicient for mechanical stability and of a uniformity of pressure over the Contact of plate against plate secured by my tamping process, such that a thorough uniformity of heating is secured as well as securing against any infiltration of the bath fluid.

Other modes of applying the principle of my invention may be employed instead of the one explained, change being made as regards the mechanism herein disclosed, provided the means stated by any'of the following claims or the equivalent of such stated means be employed.

it therefore particularly point out and distinctly claim as my invention 1. lln an electric furnace of the bath type, the combination of a body of refractory material formed to leave a chamber, such chamber being closed below and normally open above and passages extending through said body to such chamber; a resistor resting on the floor of such chamber, the upper surface of said resistor forming the working floor of the furnace; and electrodes fitted to and projecting inwardly through such passages respectively and into contact with said resistor.

2. ln an electric furnace of the bath type, the combination of a body of refractory material formed to leave a chamber, such chamber being closed below and normally open above and passages extending through said body to such chamber; a resistor resting on the door of such chamber, the upper surface of said resistor forming the working floor of the furnace; electrodes fitted to and projecting inwardly through such passages respectively and into contact with said resistor; and means for chilling the'outer portions of said electrodes. j

3. In an electric furnace of the bath type, the combination of a body of refractory material formed to leave a chamber, such chamher being closed below and normally open above and passages extending through said body to such chamber; a resistor resting on the floor of such chamber, the upper surface of said resistor forming thewworking floor of the furnace; electrodes fitted to and project-ing inwardly through such passages respectively and into contact with said resistor; and means adapted to adjust the pressure of said electrodes against said resistor.

4. In an electric furnace of the bath type, the combination of a; casing; a lining of refractory material therein formed to leave a chamber, said chamber being closed below and normally open above and passages extending through said lining from opposite ends of said chamber; a resistor resting on the floor of said chamber, the upper surface of said resistor forming the working floor of the furnace; and electrodes fitted to and projecting inwardly through such passages respectively and into contact with the adjacent ends of said resistor.

5. In an electric furnace of the bath type, the combination of a casing; a lining of refractory material therein formed to leave a chamber, said chamber being closed below and normally open above and passages extending through said lining from opposite ends of said chamber; a resistor resting on the floor of said chamber, the upper surface of said resistor forming the working floor of the furnace; electrodes fitted to and projecting in wardly through such passages respectively and intmcontact with the adjacent ends of said resistor; and means adapted to adjust the pressure of said electrodes against said resistor.

6. In an electric furnace of the bath type, the combination of a casing; a lining of refractory material therein formed to leave a chamber, said chamber being closed below and normally open above and passages extending through said lining from opposite ends of said chamber; a resistor resting on the floor of said chamber, the upper surface of said resistor forming the working floor of the furnace; electrodes fitted to and projecting inwardly.through such passages respectively and into contact with the adjacent ends of said resistor; and resiliently yieldable means adapted to press said electrodes against said resistor.

7. In an electric furnace of the bath type, the combination of a casing; a lining of refractory material therein formed to leave a chamber, said chamber being closed below and normally open above and passages extending through said lining from opposite ends of said chalnber; a resistor comprising a plurality of carbon plates assembled on the floor of said chamber, the upper surface of said resistor forming the working floor of the furnace; and electrodes fitted to and projecting inwardly through such passages respectively and into contact with the adjacent ends of said resistor.

8. In an electric furnace of the bath type, the combination of a casing; a lining of refractory material therein formed to leave a chamber, said chamber being closed below and normally open above and passages extending through said lining from opposite ends of said chamber; a resistor comprising a plurality of carbon plates assembled on the floor of said chamber, corresponding sides of said plates being ribbed; and electrodes fitted to and projecting inwardly through such passages respectively and into contact with the adjacent ends of said resistor.

. 9. In an electric furnace of the bath type, the combination of a body of refractory material formed to leave a chamber, such chamber being closed below and normally open above and passages extending. through said body to such chamber; a resistor comprising a plurality of carbon plates assembled on the floor of such chamber, corresponding sides of said plates being ribbed and the snares between the ribs in said plates being filled with a different resistance material; and electrodes fitted to and projecting inwardly through such passages respectively and into contact with said resistor.

10. In an electric furnace of the bath type, the combination of a body of refractory material formed to leave a chamber, such chamber being closed below and normally open above and passages extending through said body to such chamber; a resistor comprising a plurality of carbon plates assembled on the floor of such chamber, corresponding sides of said plates being ribbed and the spaces between the ribs in said plates being filled with a resistance material offering a greater electrical resistance than said plates when such material is cold and a decreasing resistance as said material becomes heated; and

electrodes fitted to and projecting inwardly through such passages respectively and into contact with said resistor.

11. In an electric furnace of the bath type, the combination of a body of refractory material formed to leave a chamber, such chamber being closed below and normally open above and passages extending through said body to such chamber; a resistor comprising a plurality of carbon plates assembled on the floor of such chamber, corresponding sides ,of said plates being ribbed and the spaces between the ribs in said plates being filled with carborundum cement; and electrodes fitted to and projecting inwardly through such passages respectively and into contact with said resistor.

12. In an electric furnace of the bath type, the combination of a body of refractory material formed to leave a chamber, such chamber being closed below and normally open above and passages extending through said body to such chamber; a resistor comprising a plurality of carbon plates assembled on the floor of such chamber, corresponding sides of said plates being ribbed and the spaces between the ribs in said plates being accessible from above for filling with different resistance material; and electrodes fitted to and projecting inwardly through such passages respectively and into contact with saidresistor.

13. In an electric furnace of the bath type, the combination of a casing; a lining of refractory material therein formed to leave a chamber, said chamber being normally open above and passages extending through said lining from opposite ends of said chamber; a resistor resting on the floor of said chamber, the upper surface of said resistor forming the working fioor of the furnace; electrodes fitted to and projecting inwardly through such passages respectively and into contact with the adjacent ends of said resistor; stationary supports adjacent the respective ends of said casing; a horizontally disposed shaft longitudinally guided in each of. said supports, the outer end of the corresponding electrode being aflixed to said shaft and a nut threaded on said shaft and co-operating with said support, whereby said shaft and thus said electrode may be longitudinally adjusted.

14. In an electric furnace of the bath type, the combination of a casing; a lining of refractory material therein formed to leave a chamber, said chamber being normally open above and passages extending through said lining from opposite ends of said chamber; a resistor restin on the floor of said chamber, the upper surface of said resistor forming the working floor of the furnace; electrodes fitted to and projecting inwardly through such passages respectively and into contact with the adjacent ends of said resistor; stationary supports adjacent the respective ends of said casing; a horizontally disposed shaft longi= tudinally guided in each of said supports, the outer end of the corres onding electrode being affixed to said sha t; a nut threaded on said shaft and co-operating with said support, whereby said shaft and thus said electrode may be longitudinally adjusted; and a compression spring interposed between said nut and support.

15. In an electric furnace of the bath type, the combination of a casing; a lining of refractory material therein formed to leave a chamber, said chamber being normally open above and passages extending through said lining from opposite ends of said chamber; a resistor resting on the floor of said chamber, the upper surface of said resistor forming the working floor of the furnace; electrodes fitted to and projecting inwardly through such passages respectively and into contact with the adjacent ends of said resistor; statioIiary supports adjacent the respective ends of said casing; a horizontally disposed shaft longitudinally guided in each of said supports; a shell secured to the inwardly directed end of said shaft, the outer end of the corresponding electrode being fixed in said shell; a cooling coil surrounding said shell; and a nut threaded on said shaft and cooperating with said support, whereby said shaft and thus said electrode may be longitudinally adjusted.

16. In an electric furnace, the combination of a casing; refractory material lining said casing and formed to leave a chamber; electrodes projecting through said lining into said chamber from opposite ends; and means independent of the furnace casing for supporting the outer end of each of said electrodes.

17. In an electric furnace, the combination of a casing; a lining of refractory material therein formed to leave a chamber; and electrodes projecting through said lining into said chamber from opposite ends, the bottom ofsaid casing comprising a plurality of channels secured together with flanges directed downwardly and one such channel projecting at each end to carry a support for the outer end of the corresponding electrode.

Signed by me this 7th day of April, 1924-,

CHARLES A. CADWELL.

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