US2862748A

US2862748A - Joint for carbon electrodes

Info

Publication number: US2862748A
Application number: US567113A
Authority: US
Inventors: Bruce L Bailey; Leslie H Juel
Original assignee: SGL Carbon Corp
Current assignee: SGL Carbon Corp
Priority date: 1956-02-23
Filing date: 1956-02-23
Publication date: 1958-12-02
Anticipated expiration: 1975-12-02
Also published as: FR1172404A; BE555170A; GB827763A; LU34949A1; DE1029959B; ES233838A1

Description

Dec. 2, 1958 B. L. BAILEY ET AL 2,

JOINT FOR CARBON ELECTRODES Filed Feb. 23. 1956 fizd 7zfor'ax ,ZSZi J7 Jzge grace Z.3azley J'oI'NT FOR CARBON ELECTRODES Bruce L. Bailey and Leslie H. Juel, Lewiston, N. Y., assignors to Great Lakes Carbon Corporation, New York, N. Y., a corporation of Delaware Application February 23, 1956, Serial No. 567,113

8 Claims. (Cl. 287-127) This invention pertains to joints for carbon electrodes of the type currently employed in electric steel-melting furnaces and those used in the manufacture of ferro alloy. More particularly this invention pertains to a method for forming joints in a column or string of carbon electrodes and to the means whereby these joints are formed.

In the operation of electrothermic furnaces such as are used for the melting of alloy steels and for the manufacture of calcium carbide, ferro alloys and other products, a gas-baked carbon and graphite electrodes are employed to strike a high temperature are between the end of the electrode and the material being melted. Since the amount of current carried by these electrodes is very high, often in excess of 40,000 amperes for a diameter electrode, severe conditions due to excessive temperatures and thermal shock are encountered. An electrode joint, being a discontinuous system, offers high resistance to the flow of electrical energy. The joints in an electrode column therefor tend to run hotter than the electrode members. This condition, together with the structural weakness of the machined electrode socket, makes the joint vulnerable to splitting. Also during the charging of many of these furnaces and the pouring of the metal or other material therefrom the furnace and/ or electrode column will be tilted thereby placing the latter under vibrational and mechanical strain and shock in addition to the aforementioned thermal shock.

It has been standard practice to provide a column or string of carbon or graphite electrodes in which the electrode sections are joined together by means of a threaded nipple. A centrally located socket is machined in each end of the electrode sections and a nipple connecting member having a contour similar to that of the electrode socket is provided. At the present time both socket and nipple portions are threaded, for example with an Acme thread in the case of a graphite electrodes. This joint is often the source of high electrical resistance because of discontinuous contact between the thread faces of the socket and nipple, and between the end faces of the electrode sections. Furthermore the electrode column is often weak at the joint because of the stresses set up as previously mentioned. In addition there is usually a difference in the coefficients of thermal expansion, in both the longitudinal and transverse directions, between the carbon of the socket and the carbon of the nipple which often results in the splitting of the sockets and, as the electrode column is consumed during furnace operation, the split socket ends occasionally drop into the metal or cause dropping of substantial portions of the electrode into the metal. Vibration of the electrode column also often causes loosening of the electrode joint which results in unwinding of the electrode sections from the connecting nipple.

Attempts have been made in the past to obviate the above mentioned joint difiiculties. For example there were early attempts at stabilizing the joint such as are atent O M 2,362,748 Patented Dec. 2, 1958 unwinding of the electrodes from the nipple, such as.

the filling of the spaces between the contact surfaces with a joint compound composed of a carbonaceous material such as carbon or graphite with or without the addition of tar or pitch (see U. S. 1,534,269B. G. Klugh), and also the use of liquid binders such as glucose, molasses ,etc. These joints have not proven to be satisfactory in commercial practice because many of these compositions lack sufficient cementing properties when carbonized during furnace operation or else proved unworkable because the materials were difficult to handle by employees operating the furnaces. More recently it has been proposed to provide a carbon or graphite joint in which a reservoir for a carbonizable material, such as coal-tar pitch is provided at the bottom of the socket of the electrode member or in a coaxial bore in the nipple which is additionally provided with machined internal pasages whereby the pitch may flow from the reservoir to between the interfaces of the threaded portions of the socket and nipple. This type of joint has not achieved commercial success for varying reasons.

Therefore, one object of the present invention is to provide an improved column of carbon electrodes with joints having greater mechanical strength and lower electrical resistance and which are more resistant to breakage and unwinding than electrode columns heretofore employed.

Another object of this invention is to provide a method for making such joints whereby a liquefiable, carbonizable material is placed in the pores of a carbon or graphite connecting member or nipple and which becomes available as a cementing agent during electric-furnace operation.

It is a further object of the invention to provide a carbon electrode joint which will .prevent or minimize electrode unwinding and the effects of socket splitting.

It is a further object to provide an electrode joint having lower electrical resistance and an electrode column having a more uniform electric-current density.

An additional object of the invention is to provide an electrode column which in service will bind or bond the electrodes together into an improved joint which will better distribute the mechanical and thermal stresses which are normally set up during furnace operation.

Other objects and features of the invention will be apparent from the following description, andin connection with the accompanying drawing, in which- Figure 1 is an elevation of a graphite nipple according to this invention; and Figure 2 is an elevation of a carbon electrode according to this invention.

According to the present invention joints for a column of carbon or graphite electrodes are formed by machining a coaxial socket at each end of each electrode member of the column. In the case of large graphite electrodes, the socket tapers inwardly from the face of the electrode. A correspondingly contoured graphite nipple is provided and both the socket and the nipple are threaded, usually with an Acme thread. In the case of amorphous or gasbaked electrodes the socket and nipple are generally in the shape of a solid cylinder and the nipple has a round thread in contrast to the Acme thread of a graphite nipple.

According to our invention we provide a joint having the afore-mentioned improved properties by impregnating the nipple member with a substantial quantity of a liquefiable carbonaceous composition .in order to fill the available pores over the entire surface of the nipple and to fill a portion of the available voids in its interior structure while retaining a sufficient amount of air or other gas entrapped in the porous internal structure of the nipple. Upon heating of the joint in service the entrapped air or other gas will expand and force the liquefied composition out of the nipple and onto the surface thereof and in between the thread faces of the nipple and the socket and to a lesser extent, between the end faces of the electrodes. The liquid composition will then carbonize as the joint heats up and there is thereby established a firm bond between the nipple and socket thread faces and between the end faces of the electrode members. As will be illustrated later herein, the torque required to break the resulting bond is four times that required to break a joint which has been heated without utilizing the inventive concept set forth herein.

It is known that in all carbon and graphite bodies which are made by baking and graphitizing formed pieces consisting of carbon aggregate and coal-tar-pitch binder, a certain degree of internal porosity will be inherent due to gasification of the binder during baking. The degree of porosity may be somewhat regulated by repeated impregnation with pitch followed by baking and graphitization but there will always remain some internal porosity. According to our invention we take advantage of the residual internal porosity by employing a portion of the available pores in defined areas of the nipple as a reservoir for a liquefiable carbonizable composition, preferably coal-tar pitch. It is important, however, that not all of the internal available pores of the nipple member be saturated or filled with the liquefiable carbonizable composition because in such event the desirable joint properties which we have discovered are not realized and may even be detrimental. It is also important that a sutficient quantity of the liquefiable carbonizable composition be forced into the pores to subsequently provide adequate bonding of the joint. We have found it to be essential to impregnate the carbon nipples to fill the available pores thereof from the entire surface inwardly for a depth equivalent to or represented by from about to 75% of the total available porosity while retaining entrapped air or other gas in the remaining available pores of the interior nipple structure.

In a preferred embodiment of our invention a carbon or graphite solid member is machined to the general contour of the electrode socket which it will engage. The premachined carbon body is then immersed in a coal-tar pitch having a softening point of between about 60 and 120 C. and maintained at a temperature of about 250 I C. under a pressure of 50 lbs., or until about 10 to about 75 of the total available pores inwardly from the entire surface have become filled with pitch. This can otherwise be expressed as the percent weight increase of the carbon member which will be anywhere from 1 to depending upon the internal porosity (apparent density) of the carbon member. The impregnated member is then removed from the liquid pitch, cooled and accurately machined. This presents a clean, nontacky surface to furnace operators. We have found that when a nipple prepared in this manner is used in an electrode joint and the joint is heated to 700 C. for 24 hours, the torque required for breaking the joint at room temperature will be 3 to 4 times that used in breaking a joint which contains no cementing agent and which has been similarly thermally cycled.

By the term liquefiable carbonizable composition as used herein and in the appended claims we contemplate the use of materials which are solid or only slightly plastic at room temperature such as natural tars and pitches including rosin pitch, wood pitch, gilsonite, coal-tar pitch and pitches of petroleum origin. We also contemplate the use of synthetic resins such as those produced by preliminary condensation of phenol and formaldehyde and similar materials, resins produced by condensation of 4 furfural and furfuryl alcohol by means of acid or alkaline catalysts, resins produced by polymerization of materials such as styrene and butadiene, acrylic resins, etc. Preferably we employ a coal-tar pitch having a softening point of from 60 C. to 120 C. for reasons of economy and because this material has a favorable coke residue.

In Fig. 1 there is illustrated a graphite nipple 11 of the type employed in our invention which is provided with an Acme thread 12 designed to engage a correspondingly threaded electrode socket. The interior portion 15 of nipple is illustrated as having a porous structure and there is indicated at 14 and 16 an impregnant such as coal-tar pitch which has penetrated and filled the pores of the walls and end faces of the nipple for a distance of F half the radius thereof. Similarly in Fig. 2 there is shown an amorphous carbon nipple 11 with a rounded thread 13 designed to join an amorphous carbon electrode having correspondingly threaded cylindrical sockets. The gas-filled pores are illustrated at 15 and the pores filled with impregnant are illustrated at 14 and 16.

In each embodiment the impregnation operation preferentially and necessarily fills the available pores of the nipple from the entire surface thereof inwardly for a certain predetermined depth which can be best expressed or represented in terms of the overall available porosity of the nipple, i. e. from about 10 to 75% of the total available porosity. This leaves a rather well defined core in the nipple wherein the available pores are not filled with impregnant (15 in Figs. 1 and 2) but rather contain entrapped air or other gas which is utilized when the nipple is placed in service.

In practicing our invention we contemplate impregnation not only of a premachined nipple blank such as disclosed in the above preferred embodiment, but also the impregnation of a finished machine nipple in which case the surface is cleaned of excess pitch. This cleaning operation is automatically accomplished when a premachined blank is impregnated followed by a final machining operation. In both cases the final nipple has a clean surface which is not contaminated with a tacky material. This is in contrast to the use of pitchy coating materials formerly employed to cement electrode joints which are always considered to be objectionable by plant employees.

In order to further illustrate our invention the following specific examples are set forth. In this series, tapered, graphite nipple blanks 8 /2 high and 12 /2" in diameter were used. These were immersed in a commercial, impregnating coal-tar pitch which was maintained in an autoclave at a temperature of 220 C. under 50 lbs. per square inch pressure. After removal from the autoclave the nipples were finally machine-threaded and were assembled in 14" diameter electrode sockets. The joint was tightened with a torque wrench to 600 ft. lbs. The electrode joint was then placed in a furnace at 700 C. for 24 hours after which it was cooled toroom temperature and the joint broken with the torque wrench.

The results appear in Table I:

Pitch Percent Torque Joint pickup, voids (ft. lbs.) weight impregto break percent mated joint 1 (blank) 0 0 620 2. 45 33. 6 l, 800 l. 83 25. 1 2, 500 l. 26. 0 2, 000 7. 3 (i) 1 Failure in furnace.

Other carbon and graphite joints have been similarly assembled employing petroleum pitch (softening point 100 C.) and rosin pitch as the impregnants for the connecting nipples. In all cases it was found that the torque required to break the joint after terminal cycling was at least three times the torque required to break a similarly cycled joint in which no cementing agent had been used. Further it was found that the cementing agent had been uniformly distributed over the interfaces of the threaded portions of the socket and nipple. The resulting cemented joint minimized the effects of socket splitting which is so often encountered in the usual practice. No evidence of unwinding of the electrode members from the nipple was observed.

The electrode joints herein described may be formed by processes similar or equivalent to those specifically illustrated and other liquefiable carbonaceous compositions may be used which are equivalent to those set forth herein without departing from the scope of the invention.

Having thus described the novel features of our invention but with no intention of being limited to the specific disclosures made herein, we claim:

1. In a continuous-type electric-furnace electrode comprising at least two coaxially aligned carbon electrodes having opposed end faces, a central threaded recess in each of said faces and a carbon connecting member threaded into each of said recesses, the improvement comprising said connecting member containing a liquefiable carbonizable composition in an amount to fill the available pores of the member from the entire surface inwardly for a depth represented by from about to 75% of the total available porosity, the unfilled interior pores containing a gas.

2. A device according to claim 1 wherein the carbonizable composition is a coal-tar pitch having a softening point between about 60 and about 120 C. i

3. A threaded carbon member for connecting correspondingly threaded carbon electrodes wherein the available pores of said member from the entire surface inwardly are filled with a liquefiable carbonizable composition for a depth represented by from about 10 to 75 of the total available porosity of said member, the unfilled interior pores containing a gas.

4. A device according to claim 3 wherein the carbonizable composition is a coal-tar pitch having a softening point of between about 60 and 120 C.

5. A device according to claim 3 wherein the carbonizable composition is a coal-tar pitch having a softening point of between about and about 120 C. and the gas in the unfilled interior pores is air.

6. A device according to claim 3 wherein the carbonizable composition is a coal-tar pitch having a softening point of between about 60 and about 120 C. and the gas in the unfilled pores is air, the surface of the resulting impregnated carbon being substantially free from excess pitch.

7. A method of manufacturing a carbon connecting member for carbon electric-furnace electrodes having central threaded recesses at each end which comprises impregnating a carbon blank whose configuration conforms generally to the electrode recesses, solely by means of pressure with a liquefiable carbonizable composition in an amount to fill the available pores of said blank from the entire surface inwardly for a depth represented by from about 10 to of the total available porosity while retaining entrapped gas in the interior unfilled pores of the carbon blank, and machining the resultant impregnated blank to conform to the threads of the electrode recesses.

8. A method according to claim 7 in which about 10 to about 50% of the volume of available pores, including all the surface pores, contain a liquefiable carbonizable composition and the remaining available interior pores contain a gas.

References Cited in the file of this patent UNITED STATES PATENTS 1,510,134 Broadwell Sept. 30, 1924 2,093,390 Wyckoff Sept. 14, 1937 2,461,365 Bennett et al Feb. 8, 1949 2,510,230 Johnson et al June 6, 1950 2,637,072 Greaves et al May 5, 1953 2,735,705 Johnson et al Feb. 21, 1956 2,810,117 Abbott Oct. 15, 1957

Claims

1. IN A CONTINUOUS-TYPE ELECTRIC-FURNACE ELECTRODE COMPRISING AT LEAST TWO COAXIALLY ALIGNED CARBON ELECTRODES HAVING OPPOSED END FACES, A CENTRAL THREADED RECESS IN EACH OF SAID FACES AND A CARBON CONNECTING MEMBER THREADED INTO EACH OF SAID RECESSES, THE IMPROVEMENT COMPRISING SAID CONNECTING MEMBER CONTAINING A LIQUEFIABLE CARBONIZABLE COMPOSITION IN AN AMOUNT TO FILL THE AVAILABLE PORES OF THE MEMBER FROM THE ENTIRE SURFACE INWARDLY FOR A DEPTH REPRESENTED BY FROM ABOUT 10 TO 75% OF THE TOTAL AVAILABLE POROSITY, THE UNFILLED INTERIORR PORES CONTAINING A GAS.

7. A METHOD OF MANUFACTURING A CARBON CONNECTING MEMBER FOR CARBON ELECTRIC-FURNACE ELECTRODES HAVING CENTRAL THREADED RECESSES AT EACH END WHICH COMPRISES IMPREGNATING A CARBON BLANK WHOSE CONFIGURATION CONFORMS GENERALLY TO THE ELECTRODE RECESSES, SOLELY BY MEANS OF PRESSURE WITH A LIQUEFIABLE CARBONIZABLE COMPOSITION IN AN AMOUNT TO FILL THE AVAILABLE PORES OF SAID BLANK FROM THE ENTIRE SURFACE INWARDLY FOR A DEPTH REPRESENTED BY FROM ABOUT 10 TO 75% OF THE TOTAL AVAILABLE POROSITY WHILE RETAINING ENTRAPPED GAS IN THE INTERIOR UNFILLED PORES OF THE CARBON BLANK, AND MACHINING THE RESULTANT IMPREGNATED BLANK TO CONFORM TO THE THREADS OF THE ELECTRODE RECESSES.