US2291007A

US2291007A - Electric furnace

Info

Publication number: US2291007A
Application number: US377888A
Authority: US
Inventors: Lee R Titcomb
Original assignee: Individual
Current assignee: Individual
Priority date: 1941-02-07
Filing date: 1941-02-07
Publication date: 1942-07-28
Anticipated expiration: 1959-07-28

Description

July 28 1942 "L. R. 'rrrcoMB 2,291,007

' ELECTRIC FURN-ACE Filed Feb, 7, 1941 2 sheets-sheet -1 July 28, 1942 l.. R. TwcOMB 2,291,007

` v ELECTRIC FURNACE Fi 1ed Feb. '7, 1941 2 Sheets-Sheet 2 .ya y J4! mmm/.J7

Patented July 28, 1942 UNITED STATES PATENT OFFICE ELECTRIC FURNACE Lec B. Titcomb, Chicago, lll. y Application February 7, 1941, Serial No. 317,888

(Cl. 13Z3) g 2 Claims.

The present invention relates to electric furnaces adapted for heating salt baths used in the heat treatment of steels and other metallurgical processes.

It is well known in the art that steel articles, such as tools and dies, may be tempered by heating the articles in a furnace and subsequently dipping them in a `quenching bath. This method is not entirely satisfactory since in many instances corners of articles, such as tools will be overheated and the larger masses thereof underheated. As a result of this objection, it has become customary in the art today to heat treat articles by immersing them in a hot salt bath, which is maintained at a desired temperature, so that by leaving the articles in the bath a suflicient lengthof time they will be brought to the temperature of the bath and will be uniformly heated throughout their entire masses.

These salt baths, 'of which sodium cyanide is an example, are generally maintained at the desired temperature by disposing electrodes in the bath and passing an electric current between them and through the bath so that the bath itself constitutes the principal resistance element of the circuit for heating the bath. While this principle is not new, the electrical resistances at room temperatures,'of nearly all salts which are useful in metallurgical processes, are so high that for all practical purposes the salts are substantially non-conductive. In order to pass current through such asalt bath the salt must first be brought to a fairly high temperature. Prior to my invention this has been accomplished by melting a 'portion of the salt'bath by some external means,

such as a gas flame or carbon bridge disposed across the electrodes at the surface of the bath. Whena sufiiclently large puddle has been formed, a current capable of continuing and maintaining the melting process will flow between the electrodes and after the expiration of considerable time the bath will be brought to operating temperature.

This method of starting prior furnaces-is an inconvenient and time consuming process and therefore objectionable.

It is the primary object of my invention to provide an electric furnace having means for bringing the salt bath to proper operating temperature in a relatively short time.

A further object is the provision of means for heating the salt bath to a temperature sufficient to permit the flow of normal load current therethrough after which the means referred to is automatically rendered inoperative.

Inorder to attain the above objects, I propose to incorporate an electrical resistance element within one or more of the electrodes disposed within the salt bath and cause an electric current to flow through the resistance element which in turn will heat the electrode and consequently the salt bath which is in contact with the electrode. The resistance element is maintained in circuit with a source of current supply until the bath has been heated to a temperature suiiicient to permit a predetermined current to flow through the bath and between the electrodes disposed therein, after which, the circuit through the resistance element is opened automatically. Cur- 'rent from the source ofv supply will continue to flow through the bath between the electrodes in a known manner for maintaining the bath at any desired operating temperature.

A preferred structural feature is a resistance element which extends substantially the entire length of that portion of the electrode which is disposed in the salt bath so that a substantial portion of the bath is heated and thus the bath is caused to be quickly brought to operating temperature.

Now in order to acquaint those skilled in the Y art with the manner of constructing and operating my invention, I shall describe in conjunction with the accompanying drawings specific embodiments of the invention.

In the drawings:

Figure l is a schematic diagram of my invention incorporated in a single-phase circuit; and

Figure 2 is a schematic diagram of a'modiflcation of the invention incorporated in a threephase circuit.

' Referring now to Figure l, I have shown a `single phase transformer i having a variable primary winding 2 and a secondary winding 3. The primary winding is connected to a current supply line through the instrumentality of a suitable outlet box 4 subject to control by a switch 5 for energizing ordeenergizing the transformer. The secondary winding is electrically connected by the pair of leads E and 1 extending from opposite terminals of the secondary winding to suitablev electrodes 8 and 9, respectively, which are immersed within a salt bath I0.

Nearly all salt baths employed in metallurgical` processes have the characteristic of being substantially non-conductive electrically at normal or room temperatures and which when heated to a fairly high temperature are rendered conductive. For example,`sodium cyanide and potassium cyanide which are used in case hardening have the characteristic noted.

It will be seen, therefore, that when the bath i0 is at room temperature no current will ow between the electrodes 8 and 9.

In order to preheat the salt bath I0 to a temperature at which current will flow between electrodes l and 9. I provide suitable resistance elements l2 and I3 disposed within longitudinally extending bores ll-ll in the electrodes 8 and 9. respectively. The resistance elements i2 and I3 are embedded in a suitable` refractory I5, such as aluminum oxide or magnesium oxide within the bores I4-I4 of the electrodes for spacing the resistance elements from the inside walls of the tubular electrodes. 'I'he resistance elements I2 and I3 are electrically connected to the lower ends of the electrodes 8 and 9 and extend substantially the-entire length of the portion of the electrodes which are immersed in the bath to provide a substantial heating surface for rapidly bringing the salt bath between the electrodes to a temperature at which a current will pass between the electrodes.

The resistance elements I2 and I3 are connected to the secondary winding 3 by leads AI1 and I8, respectively, through a switch I9 and leads 20 and 2I, respectively. When the bath is at room temperature and the switch 5 is closed to energize the transformer I, current will flow through leads 6 and I9, the switch I9, which is also normally closed, the lead 2I, resistance element I3, electrode 9 and lead 1 to complete the circuit. A circuit through resistance element I2 each of the secondary windings '35, 36, and 31 in will also be established through the leads 1 and I1, switch I9, lead 20, resistance element I 2, electrode 8, and the lead 6. The resistance elements will heat the electrodes which in turn raise the temperature of the bath between the electrodes. When the bath between the electrodes has been raised a sufficient extent, the main operating circuit will be established between the electrodes 8 and 9 and leads 6 and 1. When the latter condition has been established, a current transformer winding 23 will induce a current in the relay winding 24 which will attract an armature 25 and open the switch I9 and consequently the circuits of the heating elements I2 and I3. The bath between the electrodes then becomes the main resistance element and the bath will be quickly brought to operating temperature. The heating elements I2 and I3 are disconnected from the source of current supply after they have performed their function of heating the bath to a sumcient temperature to complete the main operating circuit and are thereby prevented from becoming overheated.

The salt bath I9 is retained in a suitable receptacle 28 which is surrounded by any suitable heat insulating material 21 for preventing loss of heat.

A pyrometer 28 may be 'disposed in the bath and connected to suitable meter 29 which is also connected to the line. for indicating the temperature of the bath.

It will be obvious that if desired only one heating element need be employed, for example, resistance element I2 and its circuit, but it is preferable to provide both electrodes with heating means, as shown, to rapidly pre-heat the bath and establish the main operating circuit.

In Figure 2, I have shown my invention as utilized with a source of three-phase current. A three-phase transformer 3l having

primary windings

32, 33, and 34 and

secondary windings

35, 38, and 31, respectively, is connected to the line in any suitable manner. In this embodiment of the invention three electrodes 38, 39, and 40 are immersed in a salt bath III'. The electrodes 38 and 48 are connected by leads 4I and 42 to corresponding terminals of the secondary windings 38 and 31, respectively. The electrode 39 is identical with electrodes 8 and 9 of the previous circuit. Resistance element 48 of the electrode 39 is connected to the secondary winding 3B by lead 41, through switch I9', shown in its open' position, lead 48, electrode 39, and lead 43. The resistance element 46 with the switch I9' in its closed position will heat the bath between the electrodes until the main operating circuit is -established between the electrodes 38, 39, and 40 after which a current transformer winding disposed about the lead 43 will energize the relay winding 5I and open switch I9 and consequently open the circuit of the resistance element 46 with the secondary winding 36.

In Figure l, the salt bath is shown with the salt bath between the electrodes heated to the point where current will pass between electrodes 8 and 9 and with the switch I9 about to be opened. In Figure 2, the salt bath is at its operating temperature and switch and resistance element 46 out of circuit. In the latter embodiment the electrodes 38 and 40 are simply bar electrodes.

While I have shown what I consider to be the preferred embodiments of my invention, it will be readily understood that various circuits may be readily devised for carrying out the objects of my invention. For example, in the latter embodiment, three electrodes each having a resistance element for preheating the bath may -be provided or if desired a greater number of electrodes may be employed. It will be understood.

therefore, that the present embodiments are merely illustrative and not limiting since the advantages of my invention may be incorporated in an electric furnace in various ways.

I claim:

1. In a salt bath furnace the combination of electrodes adapted to be immersed in a saltbath, one or more of said electrodes being a heater electrode consisting of a hollow tubularI body, a resistor longitudinally disposed in said tubular body, said resistor being electrically insulated from the walls of said tubular. body but in thermally conductive relation thereto, current supply means including connections for impressing electrical potential lsimultaneously upon the terminals of the resistor to energizel the same for melting the salt bath about the heater elec- -trode and upon the electrodes to pass heating current through the-salt bath as soon as lt is molten between the electrodes, a switch in said resistor circuit, and means responsive to a predetermined iiow of heating current through said electrodes in series with the molten salt bath, said switch being opened to cut off the heater due to the response of said means.

2. An electrode for an electric furnace or the like comprising a tubular metallic member closed at one end, a resistance element disposed in said tubular member and electrically connected to the closed end thereof, land refractory material in the tubular member in which said resistance element is embedded to space the latter from the inside wall of said tubular member, the resistance element being adapted to heat said refractory material which in turn heats said tubular member by conduction.

- LEE R. TITCOMB.'

I9' opened l,