US 3540764 A
Description (OCR text may contain errors)
United States Patent 2,836,806 5/1958 Show 287/127EX 3,187,089 6/1965 Cosbyetal. 287/127EX FOREIGN PATENTS 1,194,249 4/1957 France 287/127E Primary ExaminerDavid J. Williamowsky Assistant Examiner-Wayne L. Shedd Anorneys- Robert C. Cummings, Paul A. Rose, Frederick J.
McCarthy, Jr. and Cornelius F. OBrien ABSTRACT: An electrode joint in which expanded graphite is positioned between abutting end faces of the electrode sections. The expanded graphite is preferably primarily edge oriented and occupies a major portion of the available area. Flat oriented expanded graphite and a circumferential gap are also included adjacent the edge-oriented material. Electrical conductivity and thermal stress resistance of the joint are thereby greatly increased.
Patented Nov. 17, 1970 3,540,764
Sheet 1 of 2 ENTORS JO R PAUS SEPH F. REWLOCK ATTORNEY SPECIFICATION FIELD OF INVENTION This invention relates to arc furnace electrodes and more specifically to an improved joint between adjacent electrodes.
DESCRIPTION OF THE PRIOR ART It has for some time been the standard practice in arc furnaccs to join carbonaceous electrodes end to end to form a virtually continuous electrode. With this type of assembly, uninterrupted operation of the arc is maintained even though the electrodes are consumed as they are fed to the arc. A common way to join such electrodes is to provide the ends of each electrode with a threaded recess and to connect the end of the electrode with that of the adjacent electrode by means of a threaded nipple.
There are several important disadvantages associated with a connection of this type. For example, severe stresses due to thermal expansion of the joint members during operation sometimes cause rupture of the materials resulting in a breakdown of the arc entirely. In addition, the electrical resistance in such a joint is undesirably high because of gaps between threads and primarily because of the difficulty in maintaining mating front end faces of joined electrodes in direct contact.
Because of the high resistance joint, current flow and thus th are quality is adversely affected.
'A wide variety of joint modifications have been made in order to improvethe-performance ofarc electrode joints. U.S. Pat. No. 2,970,854 teaches that good results are achieved ifa fusible shim is placed between flanks of the threaded joint members. Other innovations such as tapered nipples, undercut threads and the like have also been somewhat successful in reducing the effects of thermal stresses. However, further improvements are still sought.
DESCRIPTION OF THE INVENTION material having a density of between about 2 to pounds per cubic foot, preferably in the form of an annular ring occupies a major portion of the area between end faces, while a second expanded graphite material having a density of greater than pounds per cubic foot is positioned adjacent the first material and is also preferably in the form of an annular ring. A recess is machined in the electrode end faces to accommodate the expanded graphite materials. In addition, a circumferential gap can be provided between end faces of the joined electrodes at the peripheral edge thereof, that is adjacent the second expanded graphite material, to provide further thermal stress reliefduring the operation ofthe electrodes. I
The expanded graphite of low density provides a resilient cushion which functions toabsorb the expansion of electrodes with increase in temperature. It is easily compressed and in the density range aforementioned it has an excellent resiliency factor thus enabling it to resume its s original size if required under the pressures normally experienced during the operation ofthe system. This first material also improves the electrical resistance of the joint throughout substantially all of the operating temperature range of the system. The second expanded graphite material is employed to provide structural stiffness ofthe electrode column in order to firmly support the column when it is tilted and to sustain lateral thrust imposed by movement of furnace charge materials. This material has approximately a 10 percent repeatable resilient "springback, that is, it can be repeatably deformed (compressedlup to IOpercent of its thickness and upon removal of the force, the material will return to its original thickness. The stress created 2 by changing thermal gradients and thermally incompatible joint components can thereby be further'absorbed during the operation of the electrodes. The circumferential gap at the periphery of the end faces, if it is used, is primarily to act as a positive stop" when the joint isunder a bending moment thrust load.
Graphites are characterized in their internal structure by layer planes of hexagonal networks of carbon atoms. These layer planes are substantially flat and are generally parallel to and equidistant from each other. They are bonded together by weak van der Waal forces which as it has been discovered, can be attacked with certain materials to further weaken the bond between layers; The result of such an attack is that the spacing between the superposed carbon layers can be increased so as to effect a marked expansion in the direction perpendicular to the layers, that is in the Cf direction. thereby forming expanded graphite particles. The particles may then be subsequently treated to form a useable product.
In U.S. Pat. Nos. l,l 37,373 and l.l9l.383 natural graphite particles are expanded by first subjecting the graphite purtic les for a suitable period of time to an oxidizing environment at a suitable temperature. Upon completion of the oxidizing treatment the soggy particles are washed with water and then heated to a temperature of between about 350C. and 600C. to expand the particles in the C direction. The particles are thus expanded up to 25 times their original C" direction dimension and are then combined with a phenolic resin and molded into desirable shapes.
.It has recently been discovered that particles which have been expanded at least times, and preferably 200 times in the "C direction can be. compressed together in the absence of a binder to form a cohesive sheet, paper strip, foam or the like. The product formed can be made to have a density of from 5 pounds per cubic foot or less to 137 pounds per cubic foot by varying the pressure during the forming process. This unique material has excellent flexibility, good strength and an appreciable degree of anisotropy. A full disclosure of this material and the method of makingit is set forth in copending U.S. Patf application Ser. No. 273,245, entitled Chemical Products and Processes? filed April 15, 1963.
For the purposes of this invention,'therefore, expanded graphite is intended to encompass within its meaning graphite which has been formed by expanding a graphite starting material which may be natural graphite, pyrolytic graphite, Kish flake graphite or any-other type, and then recompressing the expanded material. The preferred expanded graphite contains no binder as taught in the aforementioned copending application but may contain a binder if the amount of binder content is less than 10 percent by weight of the graphite product. Greater quantities of binder impart a rigidity to the graphite which lessens its flexibility. It is understood that other materials may be added to the expanded graphite either before or after it is molded to size provided that the important properties of resiliency, electrical and thermal conductivity are not seriously adversely affected. Metal powders or filaments, fibrous reinforcing metals such as fiber glass. clay and the like may well'be included to reinforce or strengthen the compressed product or to improve the electrical conductivity thereof. The expanded graphite as employed in this invention will generally be composed of a plurality of superimposed contiguous layers of thin sheets of expanded graphite which are held together by a binder material or simply by the inherent cohesive character of the material itself. This material may be positioned between electrode end faces with either of two orientations. It may be fedge oriented, that is, with the edges of the layers directed toward the end face surfaces and preferably in direct contact therewith or it may be "flat oriented", that is with the upper and lower layer surfaces placed between electrode end faces and preferably in direct contact therewith. It has been discovered that excellent results are achieved if a substantial amount of the expanded graphite between electrode end faces is edge oriented.
Although the terms edge oriented and flat oriented will generally be explained in terms of expanded graphite constructed of superimposed layers, they are equally applicable to expanded graphite which is formed by pressurizing expanded particles into an integral product. The particles will naturally be aligned such that the grain orientation is generally perpendicular to the applied force. The surface to which the force is applied during formation is the flat oriented surface while the surface 90 from this surface (across the grain) is edge oriented.
DESCRIPTION OF THE DRAWINGS Referring now to the drawings, FIG. 1 shows a joint com-' posed of a lower electrode section 10, and a nipple l2 placed therein connecting the lower section to an upper electrode section 14. A first expanded graphite material 16 is positioned adjacentto-and around the nipple 12. A second expanded graphite material 18 is placednext to the first material and adjacent a gap 20 in the end of the electrode sections. The expanded graphite materials contact the end faces 22, 24 on electrodes 10 and 14 respectively and thereby provide a low electrical resistance at the electrode faces.
FIG. 2 illustrates the expanded graphite more clearly. As shown, the first material 16 has a different orientation than the second material 18 in the preferred embodiment of the invention. The material 16 is edge oriented while material 18 is flat oriented as hereinbefore defined. The edge oriented graphite has a greater electrical conductivity in' a direction parallel to its layers and is able to withstand the pressures exerted by the expanding electrode sections with a greater degree of flexibility than the flat oriented material, the latter being the stronger material. The orientation of the materials is more clearly described by referring to FIG; 3 wherein the first expanded graphite material 16 is shown to be composed of a plurality of superimposed layers or tapes 26 of expanded graphite. The layers are secured to each other by simply applying pressure or by interposing a binder such as a phenolic resin between layers. The annular ring 16 is arranged about the nipple 12 such that the top surface 28 which contacts one end face of the electrode section is edge oriented, that is, the surface at which the edges of the layers are visible and to which little if any pressure was applied during the formation of the layer structure. However, expanded graphite 18 is shown to be composed of a plurality of layers of tapes 30 oriented perpendicularly to those shown in the first material 16. The top surface 32 which also contacts the end face of the electrode section is flat oriented, that is, that surface to which the pressure was applied during the formation of the'ring 18.
The improvement in the electrode joint between electrodes and therefore the improvement in the overall electrode system and arc quality can be demonstrated in several ways. For example, the electrical resistance at the end faces of two 14-inch diameter carbon electrodes joined by a standard 7-inch by 14- inch connecting nipple was measured by passing a current of approximately 20,000 amps therethrough and measuring the voltage drop. The test was carried out at an ambient temperature of 22C. and no expanded graphite was interposed between electrode end faces. The same test was also carried out at a second joint in the same electrode column except that expanded graphite was placed between end faces. Edgeoriented graphite having a density of 6 pounds per cubic foot was positioned between end faces in contact with substantially all of the area of the end faces without the use of a recess. The following table indicates the voltage data recorded at various joint temperatures:
TABLE Control joint Expanded graphite joint Tempera- Voltage 1 Tempera- Voltage 1 ture C.) (Volts) ture C.) (Volts) Measure from end face to end face between electrode sections.
graphite-filled electrode joint. The lower temperatures in the expanded graphite joint were experienced despite the same current asin the joint because of the lower resistance of the joint.
In another test, two carbon electrode sections l4 inches in diameter were joined by a 7-inch by l4-inch connecting nipple. The nipple was split into two segments and an electrically nonconductive material was placed between segments. In this manner, current flowing through the electrode sections was forced to flow around the nipple and thus. through the electrode end faces. A voltage source of approximately 6 volts was placed across the electrode sections and a current of about 20 amps passed through the joint. An electrical resistancebridge was used to measure the resistance across the end faces at various applied torques. Two sets of data were recorded with the same electrode joint; in the first run, no expanded graphite was employed while in the second run edge oriented expanded graphite was positioned between faces in the same manner as specified in the foregoing test above described.
FIG. 4 indicates the results of the test. The joint without expanded graphite is marked control while the joint employing the expanded graphite is designated by the letter A". An examination of FIG. 4 indicates that the highest resistance value on curve A is less than the lowest resistance value achieved with the control, even at the highest applied torques. Thus the incorporation of expanded graphite between end faces in an electrode joint has the added advantage of reducing the torques required to bring the electrode sections into electrical contact thereby minimizing the possibility of joint fracture due to excessive torque pressures. In addition, the very low resistance values achieved are important to the effective operation of the electrode system.
The following are examples of several electrode systems employing the joint of this invention:
EXAMPLE] Two semi-graphite electrodes each having a 68-inch diameter were joined by a graphite nipple 50 inches long and 29 /z inches in diameter. The end faces on the electrodes were recessed so that when electrode sections were joined a recess measuring 0.270 inch was formed. The recess extended about the entire circular end face area to within 2 inches of the outer edge of the electrode sections. A gap of 0.030 inches was formed in the outer 2-inch area between end faces in the shape of an annular ring adjacent the aforementioned recess. The recess was filled with edge-oriented expanded graphite having a density of 6 pounds per cubic foot except for the peripheral 2 inches of the recess adjacent the gap. This segment of the recess was filled with fiat-oriented expanded graphite of a density of 50 pounds per cubic foot and having a l00percent repeatable resiliency when cycling between 0 p.s.i. and 500 p.s.i.'The edge-oriented material had an initial thickness of 0.450 inches and was compressed in the assembly to 0.300 inch. This compression creates the desired initial tension which together with the 0.0l2-inch to 0.020-inch springback character ofthis material enables constant contact with the end faces to be maintained. The flat-oriented-expanded graphite had a thickness of 0.300 inch which was compressed during the operation of the electrodes to 0.270 inch at which level the gap was completely closed.
EXAMPLE 11 Two 24-inch diameter-electrode sections are joined by a l2 /2 -inch by 14-inch tapered nipple. A single recess measuring one-sixteenth inch is provided between end faces of the sections by machining the'same into the upper electrode section. An edge-oriented expanded graphite annular ring measuring one-eighth inch in thickness and havinga'n outside diameter of inches and an inside diameter of 13 inches is pressed into the recess between end'faecs. The outer 2 inches of the end face area has no clearance therebetween, the end faces being joined in direct contact in that area. The expanded graphite has a density of6 pounds per cubic foot.
As indicated in the examples, a variety of modifications can be employed in the joint of the invention, that is, the edge gap or the flat-oriented expanded graphite may be eliminated,
although in electrode joints employing large electrode sections, their presence is highly desirable. Furthermore, various combinations of flatand edge-oriented materials can be used. It will be appreciated that the dimensions of the expanded graphite material will differ in accordance with the demands of the system. Thus the size of electrodes, the electrode material, the electrical parameters and the structural loading forces are to be considered when a particular joint design is implemented. Furthermore, the expanded graphite can be positioned between the end faces of the electrode sections in a variety of mechanical connections.
Although the expanded graphite has been described as.
being in the form of annular rings, it may be in any suitable form. Segmented sections, properly spaced, will also provide the important improvements of the invention if a major portion ofthe end faces are contacted therewith.
We claim: 1. An electrode joint comprising:
a. two electrode sections having end faces joined by a threaded nipple withat least one of said end faces having a recess in a major portion thereof, said recess extending radially outward of the nipple cavity;
b. at least one compressible annular ring of edge-oriented expanded graphite positioned in said recess, said ring beingaxially thicker than the recess formed by the mated end faces so that when tightened said ring contacts said end faces so as to improve the electrical conductance thereat; and
c. at least one compressible annular ring of flat-oriented expanded graphite positioned in said recess adjacent to and radially outward from the edge-oriented expanded graphite, said ring being axially thicker than the recess formed by the muted end faces so that when tightened said ring contacts said end faces so as to improve structural stiffness at the electrode joint thereby enabling it to better withstand lateral thrust.
2. The electrode joint of claim I wherein said annular ring of edge-oriented expanded graphite has a density between about 2 pounds per cubic foot and about 15 pounds per cubic foot.
3. The electrode joint of claim 1 wherein said annular ring of flat-oriented expanded graphite has a density of greater than 25 pou nds per cubic foot.
4. The electrode joint of claim 1 wherein said annular ring of edge-oriented expanded graphite has a density between about Zdpounds per cubic foot and about 15 pounds er cubic foot an said annular ring of flat-oriented expande graphite gap is provided between the peripheral segments of the end faces adjacent to and radially outward from said annular ring of flat-oriented graphite, said gap providing a smaller clearance between said end faces than said recess to prevent movement of said electrode section when said joint is under a' bending movement thrust load."