US2805879A

US2805879A - Electrode joints

Info

Publication number: US2805879A
Application number: US448323A
Authority: US
Inventors: Edward C Thomas
Original assignee: Great Lakes Carbon Corp
Current assignee: SGL Carbon Corp
Priority date: 1954-08-06
Filing date: 1954-08-06
Publication date: 1957-09-10
Anticipated expiration: 1974-09-10
Also published as: BE558765A

Description

Sept. 10, 195 E. c. THOMAS ELECTRODE JOINTS Filed Aug. 6, 1954 3 E. E M m F nited States Patent O M nrncrnona roiNrs Edward C. Thomas, Wilmette, 111., assignor to Great Lakes Carbon Corporation, New York, N. Y., a corporation of Delaware Application August 6, 1954, Serial No. 443,323 7 Claims. (Cl. 287-127) This invention relates to an improved joint for carbon electrodes. More particularly, this invention relates to an improved nipple for joining carbon and graphite electrodes used in electrothermal furnaces.

The use of carbon electrodes, which may be either amorphous or graphitic in nature, in electrothermal processes is well established. For example, electrodes consisting of amorphous carbon are widely used in the manufacture of calicum carbide as well as in the manufacture of ferroalloys and other alloys in which the carbon content is not critical. Graphite electrodes find particular utility in the manufacture of special alloy steels. In either event, the product to be manufactured is subjected to a high temperature in an electrothermal or metallurgical furnace, the heat in said furnaces being supplied by passing an electric current of high amperage through an air space between the two or more carbon electrodes or between the electrodes and the charge. The resultant high-temperature arc produces the melting, smelting or other electrothermal action desired.

Graphite electrodes for use in electric furnaces are commonly joined to form a continuous column as they are consumed, by a threaded, graphite, double male member, known in the art as a nipple and similar in function to that of a dowel pin. The nipple, of necessity, has a reduced cross section compared with the column of electrodes it supports. Accordingly, there is a stress concentration within the body of a nipple joining two electrodes, which demands that the mechanical strength of the nipple stock be greater than that of the material within the body of the electrodes it joins. As a consequence of this, nipple stock must be made of cokes known to produce stock of higher mechanical strength and density. This is done by select mix formulation and usually subsequent impregnation (after baking) with suitable impregnants. Either or all of these strengthening mechanisms have. been found to increase the transverse (radial) thermal expansion characteristics of the nipple stock over and above that found in electrode stocks. As a result, undue radial pressure is exerted on the threaded female electrode socket by the nipple when the assembled joint becomes heated to the temperatures experienced during normal use. On withdrawal from the heated furnace the thermally prestressed socket becomes chilled due to radiant loss of heat. it then shrinks, and since it is already prestressed in tension, splitting of the socket often occurs.

Attempts have been made to solve socket-splitting problems by cutting radial or diametrical kerfs or slots in aportion of the nipple paralel to its vertical axis, the purposebeing to allow radial or diametrical compression of the nipple into the slotted region and thus obviate undue thermal expansive forces on the electrode socket. However, since the expansion forces operate along every diameter throughout the length of the nipple such slot ting does not eliminate the problem of socket splits.

--' -Slotting procedures have also been applied to electrodes toreduce spalling (U. S. 2,527,294-B. L. Bailey;

P 2,805,879 Patented Sept. 10, 1957 and U. S. 2,603,669C. H. Chappell). Here, however, the problem involved is wholly different from that of socket splitting discussed above. Socket splits result from the differential radial thermal expansion characteristics in the electrode joint, i. e. between the nipple and the socket. For example, in allowing a heated electrodejoint assembly to cool between electrothermal operations, the socket walls which had become stressed in tension in the hot furnace by the aforementioned excessive diametrical expansion of the nipple start to shrink due to radiant cooling of the outer surface of the joint. As a result of this shrinkage which is aded to the tensile stress generated when the joint became heated in the furnace, tremendous hoop stresses are set up within the socket which often cause the socket wall to crack. These hoop stresses are much greater, and have an entirely different origin than the cooling stresses which result in the condition referred to as electrode spalling. Spalling in electrodes is not restricted to the region of the joint but rather is a surface phenomenon throughout the length of the electrode. ttempts have been made to reduce this by cutting internal and external radial, longitudinal slots into the electrodes. The function of the internal slots is to dissipate contractile forces in the outer surface of the electrode by permitting movement of the side walls of each slot toward each other. While such means may reduce crack propagation in electrode spalling, it would not relieve the substantially greater hoop stresses within the socket. Rather, it would appear that such slots would provide a starting point for the splitting of a socket subject to hoop stresses. It is further to be observed that such slots would reduce the physical cross section of the socket wall, which is already being over stressed, thereby weakening the socket mechanically.

It is an object of this invention to provide improved carbon nipples for use in joining amorphous carbon and graphite electrodes.

It is a further object of the invention to provide improved nipples which will not fracture the electrodes with which they are used when subjected to thermal and mechanical shock.

it is a further object of the invention to provide a continuous electrode system that is less subject to thermal and mechanical fracture at the electrode joints.

The above objects as well as others which will become apparent upon more complete understanding of the invention as subsequently herein described are accomplished by manufacturing a nipple having at least three nonradiaLlongitudinal kerfs or slots as set forth in the following description. 5

Reference should be made to the accompanying drawings which diagrammatically illustrate a number of embodiments of the invention.

Figure 1 is a perspective view of a nipple it), having three kerfs 12 intersecting the threads 11.

Figure 2 is a cross-sectional view of the nipple shown in Figure 1 taken along the line 2-4.

Figure 3. is a similar cross-sectional view of a further embodiment of the invention wherein four kerfs are used. 7 Figure 4 is a cross-sectional view of a further embodiment of the invention wherein curved kerfs 13 are used.

Figure 5 is a cross-sectional view of a further embodiment of the invention having a single kerf 1'4 of Archimedean form.

It has been found that by placing one or more nonradial, longitudinal kerfs or slots in a nipple used to join carbon or graphite electrodes, the resultant joint exhibits markedly improved resistance to mechanical and thermal shocks to which such joints are subjected in electrothermal processes. These kerfs may be placed in the graphite nipples by a sawingoperation takingcareto avoid seve'rin'g any substantial por'tion of the nipple (either a pcrtptreral 'Oi'bififa'l pbriiOifl';

A broad embodiment of the invention comprises preparing. a nipple bymixingtogether carbon aggregateof sn'chicarbon's or carbon producing materials as calcined petroleum coke, anthracite, graphite, electrode carbon scra orthefike' with a suflicienta'mount of pitch or'bitnnnnous materialto serve as a binder. The'mixing. operation is usually conducted abovethe melting point of the binder in order to insure uniform distribution of the binder throughou't'th'e carbon aggregate. Small amounts o'f viscou's petroleum oils may'be added as a lubricant, especially in the event that the nipple stock is to be prepared by extrusion througha' die. The extruded or' molded. greennip'ple stock is baked inorder to carb'onize' the binder after which the bakedcarbon body may be: impregnated with a suitable impregnant after which it may be heated to'higher temperatures in order to graphitize the carbon, the carbonized impregnant and the-carbonized binder which comprises the nipple stock. The nipple stock may then be machined into a threaded nipple having either; a'cylin'drical form or the tapered form illustrated in Figure 1. At least three non-radial slots-or kerfs are cut into the nipple in the longitudinal direction with respect to the nipple axis. The depth of the slots is more than 40% of the major diameter of the nipple but in any case substantially short of severing a portion of the nipple.

In a preferred embodiment of the invention, a threaded graphite nipple is prepared as described above. Three nonradial, longitudinal kerfs are then cut into the nipple in such a manner as to form a symmetrical linear pattern in any cross section, such as the pattern illustrated in Figure 2. The depth of the kerfs is more than 40% of the major diameter of the nipple, but in any case is substantially short of severing any portion of the nipple.

In a further'em'bodim'entof the invention the threaded nipple prepared as described above has four nonradial, longitudinal slots or kerfs cut into the nipple so as to form a symmetrical linear pattern in any transverse cross section of the nipple, each of the kerfs being in a plane perpendicular to the planes of the two adjacent kerfs. This embodiment is illustrated in Figure 3.

In a further embodiment of the invention at least three curvilinear kerfs arecut into a threaded nipple prepared as described above, so as to form curvilinear patterns in a transverse cross section of the nipple, the kerfs being of a depth more than 40% of the major diameter of the nipple but in-any case substantially short of severing a portion of the nipple. This embodiment is illustrated in Figure 4. V I

In a further embodiment of-the inventionacurvilinear nonradia l, longitudinal kerf or slot is cut into the nippleprepared as described above so as to form an Archimedean spiral in a transverse cross section of the nipple. Such kerfs will naturally have a depth more than 40% of the major diameter of the nipple and in any case will not sever a portion of the nipple. A spiral kerf such as this one, illustrated in Figure 5, has been found to be equivalent to the three or more kerfs described in the embodiments above.

Modification-s of the above embodiments, which are also considered a part of this invention, include variations in which the cross-sectional pattern formed is nonsymmetrical and/or combinations of linear (straight lines) and curvilinear patterns. I

In all of the embodiments described above, width of the kerfs may'vary from about 0.035 inch to 0.125 inch; In some cases it has been found advantageous to place a cardboard shim" or equivalent within the kerfs; Such shims will end greater rigidity to' the nipple-for purposes' ofassembly. Subsequent use of the nipple in an electrode joint during electrothe'rmal operations will result in shrink age -andcarbonizationof-the shim, so that there will be little remaining to prevent movement of the sidewalls of the kerfs toward each other to achieve their assigned flilfiiiii'rf of'stis'rlief. F illifi'g' the kerrswith' a carbons ceous binder material (which is essentially a solid at room temperature) prior to assembly will serve the same purpose as the cardboard shims.

In order to more fully illustrate the nature and character of the invention, but with no intention of being limited thereby, the following examples are set forth. Examples I through V are representative of the prior electrode art and reasonable modifications thereof.

Example I A threaded graphite nipple (approximately 11" diameter) similar to that illustrated in Figure 1, but not slotted, was used to join two graphite electrodes (20" diameter) having appropriately threaded sockets. The assembly was lowered into a Globar furnace and heated to a temperature of 1390" C. The assembly was then removed from the furnace and allowed to cool suspended in the open air. One electrode socket split after 3 /2 minutes of cooling.

Example II A-nipple-such as that described in Example I was'sawed so as to provide a single longitudinal radial kerf of a depth of 60% of the-major diameter and a width of 0.035 inch.- The' nipple was used to join two electrodes as described above and the assembly was similarly lowered'into the furnace'and heated to 1450 C. On removal to cool, as described in Example I, no splits developed. The assembled jointwas returned to the furnace and heated to 1505 C. and again removed to cool. After 4 minutes o'f cooling, a split developed in one socket;

Example III A nipple such as'th'at described in Example II but-having akerf width of 0.125 inch was tested as described above heating'to 1450" C. No socket split developed. On reheating to 1505" C. a split developed after'5 /z minutes of'cooling.

ExampleIV A nipple such as that described in Example I was sawed so as to provide three radial longitudinal kerfs having a depth /2 the major radius or about 2% inches anda kerf of 0.035- inch. When tested to 1480 C. as described above, a socket splitdeveloped after 5 /2 minutes of cooling.

Example V Anipple such as that described in Example I was sawed so as to provide three nonradial longitudinal kerfs of a depth -Va'the radius and having a kerf width of 0.035 inch. When-tested at 1480- C. as described above, no socket splitsneveleped. The assembly was then heated to -1520' C; On coolinga; socket split developedafter- 3 36P- minu't'es;

Example VI A nipple su'ch a's'that described in Example Twas-sawed so as" to' provide three nonradial longitudinal kerfs of a' depth 64% of the 'major diameter of the nipple and havingfa'kerf'width' of .035 inch. The cross-sectional pattern w'as'similar' to that'illustrated in Figure 2. Upon heating the' electrode assembly and cooling, as described above;

no socket splits developed.

2 Example VII A nipplesuch-as that described in Exampl'e Vl-ex'cept that the kerf width' was 0.052 inch, was used in' anelectrode'assembly which was heated to'1600" C. On" 0001 ing; nosock'et splits 'developed.

Example VIII A nipplesuch-asthat described in ExampleVI, except that the depth-ofthe kerf was 57% of the major diameter, was used in an-electrode assembly which was heated to-1550 C. Oncooling, no socket spli-ts developed Example 1X A nipple such as that described in Example VIII except that the kerf width was .052 inch, was used in an electrode assembly which was heated to 1550" C. On cooling, no socket splits developed.

From the results obtained in the above examples, it can be seen that electrode joints having nipples with at least three radial, longitudinal kerfs of a depth more than 40% of the major diameter of the nipple but in any case substantially short of severing a portion of the nipple, can be carried to considerably higher temperatures, such as those encountered in commercial electrothermal furnace operations, without danger of developing socket splits.

Various embodiments of the invention were represented by Examples VI, VII, VIII and IX. In each case, socket splits were avoided on cooling after heating to temperatures of 1550 C. and higher. The other examples, representing the prior electrode art, all developed socket splits on cooling after heating to temperatures ranging from 1390 to 1520 C.

The word carbon as used herein and in the appended claims is intended to include both the so-called gasbaked or amorphous carbon and also graphite.

Having thus described my invention, I desire it to be understood that my invention is not to be limited other than as defined by the claims.

I claim:

1. A carbon nipple for joining carbon electrodes, having at least three nonradial, longitudinal kerfs extending the entire length of the nipple and of a depth more than 40% of the major diameter of said nipple but in any case substantially short of severing a portion of said nipple.

2. A nipple according to claim 1 having three kerfs, said kerfs forming a linear pattern in a transverse cross section of said nipple.

3. A nipple according to claim 1 having three kerfs, said kerfs forming a symmetrical linear pattern in a transverse cross section of said nipple.

4. A nipple according to claim 1 having four kerfs, said kerfs forming a linear pattern in a transverse cross section of said nipple.

5. A nipple according to claim 1 having four kerfs, said kerfs forming a symmetrical linear pattern in a transverse cross section of said nipple, each of said kerfs being in a plane perpendicular to the planes of the two adjacent kerfs.

6. A nipple according to claim 1 in which said kerfs form curvilinear patterns in a transverse cross section of said nipple.

7. A carbon nipple for joining carbon electrodes, having a longitudinal kerf extending the entire length of the nipple and forming an Archimedean spiral in a transverse cross section of said nipple, said spiral having its terminus at the approximate center of said cross section.

References Cited in the file of this patent UNITED STATES PATENTS 1,287,678 Hall Dec. 17, 1918 2,125,018 Hamill July 26, 1938 2,482,176 Hamister Sept. 20, 1949 2,527,294 Bailey Oct. 24, 1950