Industrial electric heating furnaces – Arc furnace device – Electrode
Reexamination Certificate
2003-11-17
2004-12-07
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Arc furnace device
Electrode
C439S087000
Reexamination Certificate
active
06829287
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an electrode connection including electrodes having end-side boxes with internal threads and nipples each connecting two electrodes, as well as to electrodes having a box located on one end side with an internal thread and an integrated nipple located on the other end side, and an electrode and nipple jointly constructed as a preset. The electrode connection is provided for an electrode string, operating at temperatures of substantially above 300° C., for use in an electric arc furnace for the production of high-melting metals.
The production of carbonized or graphitized carbon bodies is a technique which has been mastered for more than 100 years and is employed on an industrial scale and is therefore refined in many respects and optimized in terms of cost. One of the descriptions of that technique is found in ULLMANN'S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY, Vol. A5, VCH Verlagsgesellschaft mbH, Weinheim, Germany 1986, p. 103 to 113.
The applicability of electrodes, nipples and electrode strings in an electric arc furnace depends on the properties achieved during production, in particular surface properties. Those surface properties depend, for example, on the type of material (degree of graphitization), on the pore content, on the grain size, on the type of machining which determines the surface roughness, but also on the surrounding conditions. Electrodes are stored and handled in a steel mill and in that case are exposed to contamination caused, for example, by steel mill dust. The above-mentioned factors determine the coefficients of friction which play a part in the assembly of two bodies, for example an electrode and a nipple or two electrodes, and in the sliding of two surfaces on one another.
An electric arc furnace contains at least one electrode string. That string is held at the upper end of a carrying arm, through which the electrical current also passes into the electrode string. When the furnace is in operation, the arc passes from the lower tip of the string into the melt batch located in the furnace. The electrode string slowly burns away at its lower end due to the arc and to the high temperatures in the furnace. The shortening of the electrode string is compensated for in that the string is advanced piece by piece into the furnace and, if required, an additional electrode is screwed on at the upper end of the string. If required, a string which is partially burnt away is also extracted as a unit from the carrying arm and replaced by a fresh string of sufficient length.
The screwing of individual electrodes onto a string located in the furnace or the screwing together of electrodes to form a fresh string takes place by hand or through the use of a mechanical device. Particularly with regard to electrodes having a large diameter of 600 mm or above, considerable forces and torques have to be applied or screwing work performed in order to ensure that an electrode string is held together. It is critically important for the functioning of an electric arc furnace that a string be held together.
The holding together of a string is put at risk during transport, but primarily when a furnace is in operation. During the operation of a furnace, considerable bending moments due to the pivoting of the furnace vessel, including the string, act repeatedly on the electrode string, or the electrode string is exposed to persistent vibration, and impacts on the string caused by the charging material are also detrimental to the holding together of the string. All types of load, that is repeated bending moments, vibrations and impacts, may cause a loosening of the screw connection of electrodes. Loosening can be considered to be the result of unavoidable and/or undesirable actions.
The term “release torque” is appropriate for characterizing the holding together of an electrode string through the use of a measurement variable. The release torque for screwing on an electrode connection is determined through the use of a measuring apparatus. Below the range of mechanical damage to the threads involved, the loosening of a screw connection becomes more improbable and operation with the electrode string becomes more reliable, the higher the release torque of an electrode connection becomes.
In order to provide an understanding, the consequences of a loosening of the screw connections of an electrode string during furnace operation may be outlined as follows:
In the event of loosening, it is to be assumed that the bracing of the screw connection is reduced. Consequently, the pressure forces of the contact surfaces of adjacent string elements also decrease. Loosening can progress to such an extent that some of the contact surfaces separate from one another.
Consequently, the electrical resistance in the connection rises. The surfaces which have remained in contact are loaded with an increased current density. The increased current density leads to local thermal overheating.
During the loosening of a screw connection, as a rule, the nipple is exposed to a high thermal and mechanical load. Finally, the mechanical failure of the nipple due to overheating and mechanical load is exhibited. As a result, the tip of the electrode string falls off and drops into the steel melt, the arc breaks off and the melting operation is terminated.
The terms used in the following text are to be understood as follows:
The ends of an electrode are also designated by the term end side.
An electrode has a cylindrical surface area and, on each of the two sides, an end surface disposed perpendicularly to the electrode axis.
A box is a coaxially disposed depression in the end side of an electrode. Usually cylindrical or conical internal threads are incorporated into the coaxial inner walls of a box.
A nipple is a cylindrical or double-conical screw with an end surface disposed perpendicularly to the nipple axis, on each of the two sides. For the purpose of connecting two electrodes, a nipple is screwed, approximately halfway in each case, into a box of adjacent electrodes.
A preset is formed of an electrode and of a nipple screwed halfway into a box of the electrode.
There are electrodes which have a box on only one end side and have an outward-pointing coaxial thread on the other end side. Such an outward-pointing coaxial thread is designated as an integrated nipple.
It is not only an electrode and a nipple which have end surfaces, but rather the integrated nipple has an outer end surface disposed perpendicularly to the nipple axis as well.
Particulars as to the viscosity of the sliding layer relate to the delivery state of the electrodes and nipples, not to the state of the sliding layer at the time of production of that layer.
In order to deal as effectively as possible with the problems of the insufficient holding together and insufficient current transition of part of an electrode string, widely differing considerations have been adopted and the practice outline below is applied.
Swedish Patent No. 43352, filed Dec. 12, 1917, describes sheet-metal strips which have been inserted into the thread flights of electrodes with integrated nipples. Since electrodes for the melting of high-melting metals become very hot precisely in the vicinity of the electric arc, it is therefore to be expected that the sheet metal in the thread flights will melt and the intended effect will be lost. In the present-day practice of electric arc furnaces, insertion of sheet-metal strips into the contact surfaces between two elements of an electrode string is not employed.
In an article by J. K. Lancaster entitled “Transitions in the Friction and Wear of Carbons and Graphites Sliding Against Themselves” from ASLE TRANSACTIONS, Vol. 18, 3, p. 187 to 201, frictional conditions between carbon bodies, preferably in the case of different frictional velocities, are investigated. That publication does not reveal any teaching as to how two carbon bodies can be screwed as firmly as possible against one another, apart from the general recognition that, at very low rela
Baumann Stefan
Burkhart Georg
Richter Norbert
Greenberg Laurence A.
Hoang Tu Ba
Locher Ralph E.
SGL Carbon AG
Stemer Werner H.
LandOfFree
Electrode connection with coated contact surfaces does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electrode connection with coated contact surfaces, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electrode connection with coated contact surfaces will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3294452