US2775811A - Methods for forming contact surfaces - Google Patents

Description

1957 J. cs. WILSON METHODS FOR FORMING CONTACT SURFACES Filed May 19, 1955 INVENIOR James M50 6 MW. {-f

ATTORNEYS United States Patent O METHODS FOR FORMING CONTACT SURFACES James G. Wilson, Huntington, W. Va.

Application May 19, 1955, Serial No. 509,519

3 Claims. (Cl. 29401) This invention relates generally to methods for forming electrical contact surfaces and more particularly to methods for forming and reconditioning the copper or copper alloy contact surfaces of electrode holders, bus tubes and the like.

For purposes of disclosure the methods of the present invention will be described and illustrated in connection with the restoration of the worn contact surface of massive electrode holders used in electric arc furnaces since it is in this application that the methods presently have primary utility and in which the advantages of the invention are most fully realized. However it is to be understood that the methods of the invention are of more general utility and may be applied with excellent results to the formation and restoration of other copper or copper alloy structures.

The electrode holders to which the methods of the invention have been applied with particular success are used to support carbon electrodes in electric arc melting and smelting furnaces which are operated at high temperatures for extended periods and are thus subject to relatively rapid deterioration despite water cooling and other measures which are taken to extend their useful life.

Such holders are of large and massive construction weighing from 400 pounds to 3500 pounds and must be capable of rigidly supporting and supplying high amperage current to electrodes varying in diameter from to 24 inches. While electrode holders of this type would ideally be fabricated from substantially pure copper, nevertheless because of cost considerations bronze alloys of high copper content are generally used. The initial cost of the larger size holders may run as high as $7500.

Ordinarily such holders in active service have a useful life slightly over a year. Since from three to six of these electrode holders are required in a furnace it will be appreciated that their cost is considerable in a mill of substantial size.

Deterioration of the holder contact surfaces is caused 1 by general or localized overheating which may result from inadequate cooling or from increased resistance in the metal at the contact area caused oxidation of the contact surface or by the deposition or infusion of ferrous compounds and other impurities in the bronze parent metal which inevitably results from normal smelting operations.

Once pitting of the contact area is initiated deterioration is rapid and arcing at the pitted areas may deepen the pits to several inches. Complete deterioration may follow if the holders are not removed from service. Ordinarily, however, loss of efficiency dictates removal of the electrodes long before the final stages of deterioration are reached.

Despite the fact that electric arc furnaces have been in less of their state of deterioration at a cost substantially below the cost of a new holder.

Patented Jan. 1, 1957 Briefly the method as applied to the reconditioning and restoration of unserviceable electrode holders, comprises broadly the steps of cleaning the surface to be restored, machining the contact area to predetermined dimensions, preheating the holder generally or locally, building up deeply eroded areas to a predetermined contour, depositing substantially pure copper on the contact areas by fusion welding in one or more passes, mechanically Working the deposited metal and finally machining the contact area to desired dimensions. While several or perhaps all of these steps may be found individually in prior methods, no known method involves the practice of the steps in the manner set forth in detail hereinafter to produce results which have heretofore been unattainable.

It is, accordingly, the primary purpose and object of the present invention to provide novel methods which effectively solve this long standing problem in the art of forming electrical contact surfaces and reconditioning electrode holders and similar copper or copper alloy surfaces.

It is also an object of the present invention to provide novel methods of forming electrical contact surfaces which do not require elaborate equipment or skills beyond that demanded by good welding and machine practice.

It is a further object of the present invention to provide novel means for reconditioning the contact areas of electrode holders which produce contact surfaces having physical and electrical properties superior to those present in the original surface.

Additional objects and advantages will become apparent as the description proceeds in connection with the accompanying drawings in which:

Figures 1 and 2 are perspective views of two forms of typical electrode holders to which the novel methods herein disclosed have been applied with particular success; and

Figures 3 through 5 are plan views of Figure 2 showing the configuration of the contact surface at various stages of the method.

The electrode holder, shown in Figure 1, comprises a substantially rectangular mounting section 10 formed integrally with a cylindrical section 12 which is provided with a substantially cylindrical bore 14 adapted to receive a carbon electrode. Electrode holders of this type usually are solid bronze castings which may weigh as much as 3500 pounds. Normally such holders are provided with a ram (not shown) which extends through the mounting portion 10 into the bore 14, or other mechanical wedging or gripping member and is hydraulically or mechanically actuated to clamp the electrode firmly against the contact area 18 shown shaded in Figure 1. Also in this type of electrode holder, a cooling coil having an inlet 20, an outlet 22 and an intermediate portion 24 encircling the electrode is formed integrally in the casting and is supplied with a constant flow of water or other coolant during operation to protect the holder and the electrode against the effects of excessive temperatures. The contact surface 18 in a conventional holder extends along an arc of approximately throughout the length of the bore 14. The high amperage current required during operation is supplied to the holder through a pad 26 usually formed as an integral extension of the mount portion 10 of the holder. The pad has a semi-cylindrical contact surface 28 which supports a bus tube 30 held in place by a semi-cylindrical clamp 32 bolted or otherwise firmly secured to the pad 26.

A second form of conventional electrode holder is shown in Figure 2. This type of holder has a mount portion 34 of substantially rectangular shape formed integrally with a contact portion 36 having a semi-cy-'- "lindrical bore 38 which is adapted to receive the electrode,

not shown. A heavy cooling coil indicated generally at 40-havinganinlet 42 and an outlet 44 is embedded in the holder when cast and has projecting coil portions 46 which in effect form a continuation of the semicylindrical bore 38. However it will be noted that the diameter of the space enclosed by the coils 46 is slightly larger than the diameter of the bore 38. In accordance with conventional practice the electrode, which has substantially the same diameter as the bore 38, is held in high pressure engagement with substantially the full area of the bore 38 by one or more wedges inserted between the projecting coil portions 46 and the adjacent surface of the electrode. The holder of Figure 2 is also provided with a pad assembly 50 which may be identical in the corresponding assembly 26 described in connection with Figure 1.

As indicated above, after the electrode holders have been in service for some time, the formation of oxides and the penetration of ferrous compounds and other impurities into the contact area of the holder produces increased resistance at the contact area with a consequent loss of efficiency and the production of local or general overheating. As this action progresses the contact area becomes pitted and the arcing which results at the pitted areas may rapidly increase the depth of the pit to several inches. This accelerated deterioration is allowed to progress ordinarily until the electrical efficiency of the holder reaches a predetermined minimum figure at which time it was necessary, prior to the present invention, to scrap the holder.

Figure 3 is a typical horizontal section through an electrode holder of the type shown in Figure 2 showing the condition of the contact area of the holder when it is removed from service. The original contour of the contact area of the holder is indicated by the dotted line 60 which forms a continuation of the bore 14. Several deep pits formed in the contact area by severe arcing are indicated at 62.

In accordance with the methods of the present invention the first step in re-conditioning the unserviceable holder shown in Figure 3 is the thorough cleaning of the entire bore 14 to remove the oxides and other impurities formed on the contact area. This cleaning operation which must be thorough to permit the proper accomplishment of the remaining steps of the process may be effected by sand blasting, pneumatic chipping, electric grinding or by immersion of the contact surface of the holder in an acid solution which effectively dissolves the undesired oxides. In many cases it may be desirable to use more than one of these methods to assure complete cleaning. An air blast may be used to complete the cleaning process to assure the removal of all oxides and foreign particles.

After the preliminary cleaning operation is completed the contact surface is machined to a uniform depth usually approximately A2" below the original surface. For electrode holders the original contour of the bore 14 is undercut approximately /s" on a radius to increase the bore diameter by A, the resulting contour being shown in Figure 3 by the dash line 64.

The pits 62 are then chipped out to facilitate the subsequent build-up of metal in the pits to, approximately the rebored contour 64.

The build-up of the pitted areas is accomplished by depositing substantially pure copper by welding. It has been found that excellent results may be obtained by using the metal inert gas fusion welding process disclosed in Patent No. 2,504,868 and the Welding rod disclosed in Patent No. 2,220,464. It has been discovered however that the deposition of metal can be accomplished only after the effected areas have been properly preheated; Satisfactory preheating may be obtained by the application of an open flame although the heating may alsobe effected by induction or by soaking the holder ina furnace. In accordance withrthe present invention it has. been found unnecessary to maintain a reducing atmosphere during preheating and accordingly the pre heating may be accomplished in atmosphere. Proper preheating is of critical importance to the success of the present method. If too little preheating is employed an unsatisfactory weld will result from incomplete fusion. If the preheating is excessive the formation of excessive amounts of oxides will produce an unsatisfactory porous weld. The preheating procedure will vary considerably depending upon the size and copper content of the holder as illustrated by the following examples.

In the case of a small holder, that is one which is from 400 to 500 pounds in total weight and is adapted to support an electrode from five to ten inches diameter the holder is preferably put in a preheated furnace until the temperature of the entire mass is raised to 350 F. to 450 F. The temperature variation will depend upon the copper content of the holder, the higher temperatures being used for holders having higher copper'content. As soon as the holder has reached the desired temperature it is removed from the furnace and the welding process is initiated immediately. It has been found that further heat to maintain the desired temperature is unnecessary in holders of this size, the desired heat level being maintained by the welding process. In holders of the smallest size, localized preheating may be employed to raise the temperature of the area to be welded to the levels indicated above. In the case of such holders it is not necessary to maintain preheat during welding.

In the case of larger holders, for example, ranging in size from 2200 pounds to 3500 pounds, and adapted to support electrodes from twenty to twenty-four inches in diameter, it has been found necessary to employ preheat temperatures ranging from 380 F. to 500 F. to assure maintenance of the proper temperature throughout the welding process. Again the exact preheat temperature employed will depend upon the copper content of the holder with the higher temperatures being used for holders having higher copper content. The larger holders may be raised to this temperature by any convenient preheat method. Because of the relatively large mass of such holders it has been found that the heat generated during the welding process is ordinarily not suflicient to maintain the desired temperature. Accordingly the application of heat preferably by blow torches or other external gas flames is continued during the entire welding process as required to maintain the desired preheat temperature.

In building up the pitted area the weld metal will be deposited in a number of passes each pass resulting in the addition of a mass of metal approximately one-eighth inch thick in the pitted area. Each welding pass is thoroughly cleaned before the next pass is begun. This cleaning which may be effected by sand blasting, wire buffing or other conventional methods, assures the removal of oxides and assures the formation of a dense non-porous homogenous mass of deposited metal.

The deposition of the weld metal in the pitted areas is continued until the bored contour 64 is reached. Any excess metal may be chipped off so that the resulting bore is substantially cylindrical on the radius of the bore 64, as shown in Figure 4. After the bore 64 is thoroughly cleaned the holder is again preheated in the manner previously described. A substantially uniform layer of copper is then deposited over the entire surface of the bore 64 to a depth of approximately one-eighth inch using the aforementioned Welding rod of Patent 2,220,464 and the metal inert gas welding process of Patent 2,504,868. This layer is shown in exaggerated size at 66 in Figure 5.

The surface of the layer 66 is then thoroughly cleaned by sand blasting or wire buffing and a second layer of weld metal 68 is then preferably applied over the surface of the layer 66. The additional layer 68is deposited in the same manner and under the, same conditions a the layer 66 and is also approximately one-eighth of an inch thick. Often one weld pass will produce the deposition of suflicient weld metal to produce a bore of the desired size in the holder. Additional weld passes may be required for this purpose. A second pass may substantially improve the quality of the contact surface. This is due to the fact that the first pass may in some cases be objectionably porous because of the presence of oxides and other foreign matter on the undercut bore 64 of the original metal. The second pass is of dense and homogeneous structure and covers the porous structure of the first pass. While additional passes may result in an increase in the density of the final pass nevertheless such additional passes are not ordinarily economically justified and oifer only a slight improvement over the use of two passes. After the final pass, which is usually the second pass, has been completed the holder is allowed to cool to room temperature.

The deposited weld metal is then mechanically peened with a pneumatic hammer or other conventional apparatus or is mechanically worked by apparatus such as a revolving pressure roll on a boring bar. Either of these operations further increases the density of the weld which in turn results in increased hardness and toughness of the metal at the contact surface. The peening or other mechanical working process is continued until the thickness of the deposited layers is reduced by from one-sixteenth to three-thirty-seconds of an inch at which time the desired hardness and toughness will have been produced.

It will be noted that mechanical working after the first welding pass is not essential since the metal of the underlying pass i annealed by the application of the subsequent pass and many of the advantages of work hardening are thus lost.

After the completion of the peening operation the holder is placed in a lathe or boring mill and the bore of the holder is machined to final dimensions.

Actual tests have shown that the holder after being reconditioned in accordance with the method described above has 30% better electrical conductivity than the original holder because of the deposit of substantially pure copper on the contact surface. Further, the weld is of substantially greater density than the cast parent metal and this factor further improves the performance of the reconditioned electrode holder and increases its service life as compared with the original holder.

It will be apparent that the above-described process lend itself admirably to the formation of contact surfaces in the original manufacture of electrode holders. In such a case the holder lwould originally be cast with a bore approximately one-eighth inch oversize and the method would then be performed substantially as described above except for the building-up and restoration of the pitted areas which would not be present in the newly manufactured electrode.

It will be understood that the presently described method has equal application to the restoration or formation of other electrical contact areas for example the mating contact areas of the pad 16 and the bus tube 30 or the contact areas of large size electrical switches.

From the foregoing it will also be apparent that the above stated objects of the invention have been accomplished. The methods of the present invention as applied to the restoration or reconditioning of electrode holders provide in such a holder :a contact area having greater density, less porosity, better electrical and thermal conductivity and a longer service life than a newly cast electrode holder. The same advantages can be derived from the use of the method in the initial manufacture of electrode holders or other electrical contact surfaces.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. The method of restoring the worn and pitted arcuate contact surface of a copper or copper alloy electrode holder comprising the step of cutting said surface away on a substantially uniform radius to a depth of approximately /8 or more, removing damaged or oxidized metal from said pits, cleaning said contact surface, preheating said holder to a temperature of at least 350 F., fusion welding a mass of metal having substantially the same copper content as said electrode holder in said pits in one or more passes in an inert atmosphere to build said pits up generally to the contour of said cutaway surface, shaping said welded metal substantially to the contour of said cutaway surface, fusion Welding a mass of metal having substantially the same copper content as said electrode holder to said surface in one or more passes in an inert atmosphere while the temperature of said holder is above 350 F. until the radius of the surface formed by said weld metal is less than the radius of the original surface, and machining said welded metal to the desired dimension.

2. The mehod of restoring the worn and pitted arcuate contact surface of a copper or copper alloy electrode holder comprising the steps of cutting said surface away on a substantially uniform radius toa depth of approximately of an inch or more, removing damaged or oxidized metal from said pits, cleaning said contact surface, pro-heating said holder to a temperature of at least 350 F., fusion welding a mass of metal having substantially the same copper content as said electrode holder in said pits in one or more passes in an inert atmosphere to build said pits up generally to the contour of said cutaway surface, shaping said weld metal substantially to the contour of said cutaway surface, fusion welding a mass of metal having substantially the same copper content as said electrode holder to said surface in one or more passes in an inert atmosphere while the temperature of said holder is above 350 F. until the radius of the surface formed by said weld metal is less than the radius of the original surface, mechanically work hardening said weld metal after the final pass at a temperature substantially below the pro-heating temperature until the desired hardness and toughness of said metal is obtained, and machining said weld metal to the desired dimension.

3. The method of restoring the worn and pitted arcuate contact surface of a copper or copper alloy electrode holder comprising the steps of cutting said surface away on a substantially uniform radius to a depth of-approximately A3 of an inch, removing damaged or oxidized metal from said pits, cleaning said contact surface, preheating said holder to a temperature of at least 350 F., fusion welding a mass of metal having substantially the same copper content as said electrode holder in said pits in at least two passes in an inert atmosphere to build said pits up generally to the contour of said cutaway surface, shaping said weld metal substantially to the contour of said cutaway surface, fusion Welding a mass of metal having substantially the same copper content as said electrode holder to said surface in two passes each of said passes comprising a layer approximately A; of an inch thick, mechanically work hardening said weld metal after the final pass at a temperature substantially below the preheating temperature until the desired hardness and toughness of said metal is obtained, and machining said weld metal to the desired dimensions.

References Cited in the file of this patent UNITED STATES PATENTS 1,994,479 Lamborn Mar. 19, 1935 2,134,366 Hardy Oct. 25, 1938 2,187,348 Hodson Jan. 16, 1940 2,267,342 Schwartz Dec. 23, 1941

Claims (1)

1. THE METHOD OF RESTORING THE WORN AND PITTED ARCUATE CONTACT SURFACE OF A COPPEER OR COPPER ALLOY ELECTRODE HOLDER COMPRISING THE STEPS OF CUTTING SAID SURFACE AWAY ON A SUBSTANTIALLY UNIFORM RADIUS TO A DEPTH OF APPROXIMATELY 1/8" OR MORE, REMOVING DAMAGED OR OXIDIZED METAL FROM SAID PITS, CLEANING SAID CONTACT SURFACE, PREHEATING SAID HOLDER TO A TEMPERATURE OF AT LEAST 350*F., FUSION WELDING A MASS OF METAL HAVING SUBSTANTIALLY THE SAME COPPER CONTENT AS SAID ELECTRODE HOLDER IN SAID PITS IN ONE OR MORE PASSES IN AN INERT ATMOSPHERE TO BUILD SAID PITS UP GENERALLY TO THE CONTENT OF SAID CUTAWAY SURFACE, SHAPING SAUD WELDED METAL SUBSTANTIALLY TO THE CONTOUR OF SAID CUTAWAY SURFACE, FUSION WELDING A MASS OF METAL HAVING SUBSTANTIALLY THE SAME COPPER CONTENT AS SAID ELECTRODE HOLDER TO SAID SURFACE IN ONE OR MORE PASSES IN AN INERT ATMOSPHERE WHILE THE TEMPERATURE OF SAID HOLDER IS ABOVE 350*F. UNTIL THE RADIUS OF THE SURFACE FORMED BY SAID WELD METAL IS LESS THAN THE RADIUS OF THE ORIGINAL SURFACE, AND MACHINING SAID WELDED METAL TO THE DESIRED DIMENSION

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