US2090693A - Method of graphitizing carbon bodies - Google Patents

Description

Aug. 24, 1937. R. MELTON METHOD OF GRAPHITIZING CARBON BODIES Filed Dec. 28, 1934 INVENTOR. ROMIE I... MELTON ATTORNEY.

Patented Aug. 24 1937 UNITED STATES PATENT OFFICE METHOD OF GRAPHITIZING CARBON BODIES laware Application December 28,1934, Serial No. 759,577

1 Claim.

This invention relates to a process of graphitizing carbonaceous bodies, such as electrodes and the like, and particularly to a method of measuring and controlling the degree of graphitization.

5 More specifically this invention relates to a method of graphitiz'ing carbonaceous bodies by heat treating in' a suitable furnace and measuring the specific resistance of the. said body while within the furnace whereby the degree of graphitization of the said body may be indicated and controlled to any desired degree. The invention is of particular use in connection with induction furnaces.

Graphitized articles, such as electrodes, tubes, crucibles and the like are manufactured by mixing carbon material with a suitable binder, molding, heating to a sufficient temperature to drive off the volatile matter and thereafter heating to the high temperature required for graphitizatlon. Graphitization has a marked eflect on the electrical resistance of the bonded carbonaceous material. In well-graphitized electrodes the electrical resistance is generaly reduced to approximately 20 to per cent of that of a corresponding ungraphitized article. The greater the con- 25 version to graphite the lower the resistivity.

" The electrical resistance of certain carbon and graphitized articles, such as furnace electrodes is of particular importance and it is desirable to reduce the resistance by graphitization to a minlmum in order to reduce the power loss due to the FR. loss in the electrode itself. It is possible. however, to overheat or overgraphitize the material and produce an electrode which is of low resistivity but too soft and lacking in mechanical strength to withstand the mechanical stresses present in normal furnace operation. Consequently the requirements both electrically and mechanically are rather stringent.

Heretofore in the manufacture of such carbonaceous articles, the practice has been to pack the articles in a furnace in a prescribed way, apply heat thereto according to an arbitrary schedule, determined by experience, and to measure the electrical resistance after removal fromthe heat treating furnace. Should the electrical resistance of the graphitlzed body be higher than that permissible it was necessary to repack in the furnaee and reburn. Overheated articles however cannot be salvaged and must of course be scrapped. Due to the number. of variables involved in packing and heat treating such carbonaceous articles it has been diillcult to control the process of sraphitizing and impossible to pre- 56 determine the electrical properties. Such operation therefore has been a hit or miss method and not entirely satisfactory.

1 The above described diili'culties are overcome in the herein described method of graphitlzation in which the electrical resistance of a carbonaceous article can be measured while the said article is still within the heat treating furnace, and the temperature of the furnace or the time of heat treatment so controlled that the article is graphitized to a desired degree.

It is well known that the step of graphitization is a time-temperature function and that the rate of graphitization increases with temperature. According to the present practice carbonaceous articles are heat treated for considerable periods of time though it is possible to graphitize carbonaceous articles in a very short time, for example 15 minutes by heating to a temperature above 2600 C. It is also known that a carbonaceous article changes drasticailyin' specific resistance as it is converted from the raw or arm; phous state into a well-graphitized article. Gen erally the electrical resistance is reduced to about one fourth of the original value. Thus the electrical current flowing in such a body at a. given potential would increase with the extent of graphitization. The present invention utilizesthis factor in providing an accurate method of indicating and controlling the extent of graphitization to any desired degree.

The nature of my invention will be more fully evident from a consideration of the accompanying drawing which shows a section of a portion of a graphitizing furnace together with a schematic wiring diagram of the control circuit and induction heating circuit.

Referring to the drawing in detail there is shown one form of apparatus which may be used to carry out the present method of measuring the resistance of a carbonaceous body and thereby indicating and controlling the degree of graphitization of said body. The carbonaceous body 9 to be heat treated or graphitized is placed in a suitable furnace in which the temperature is raised to the required value by any convenient means such as for example, induced currents from the induction coil 3.

The furnace illustrated in the drawing is composed of an outer shell I of a thermal and electrical insulating material and protected by a refractory lining 5. The graphitlzing zone of the furnace is further insulated with a high temperature insulating material 4, such as pulverized carbon or lampblack. An inner wall 6 of rigid high temperature thermally insulating material retains the lampblack 4 in place. This cylinder is preferably carbon and is split longitudinally so as not to shield the body 9 from the magnetic field set up by the furnace coil 3 or the measuring coil i0. 5 The carbonaceous body 9 forms a single turn secondary of a transformer coupled circuit of which the induction furnace coil 3 is the primary. Thus alternating current of a suitable frequency, supplied by the alternator I, is induced into the 10 body 9 from the coil 3 and produces heating of said body. A condenser 8 is connected across the coil 3 to counteract the high inductive reactance oi the said coil 3 and thus maintain substantially unity power factor in the supply circuit. A second coil i0 is inductively coupled with the heat treated portion of the body 9 and connected to a source of alternating current H which supplies a constant voltage at a suitable frequency. Means, not shown, are provided to vary the frequency of this alternator or oscillator for purposes to be described later. The power in this measuring circuit may be of relatively small value since it is necessary only to induce suflicient current into the body 9 to cause a change in de fiection of the instrument I2. A condenser 13 is connected across the coil ID to correct the power factor and increase the sensitivity of the circuit by operating at or near the resonance point of the circuit.

' The coil I 0 forming part of the measuring circuit is preferably located near the outlet end of the graphitizing zone so that the electrical resistance of the said body may be measured immediately priorto leaving, said zone. Thus the specific resistance and degreeof graphitization is measured and the temperature of the furnace or the rate of passage of said body through the furnace regulated so as to produce any desired and predetermined degree of graphitization. The drawing shows means for varying the power input to the furnace in order to control the graphitization though it is apparent that the rates of travel of the botiy may be regulated instead.

With certain arrangements of the various elemerits, and particularly with close coupling of the heating or furnace coil 3 and the measuring coil ill, a change in current through the said heating coil will by inductive action produce a change in the. current flowing in the measuring coil and upset the control circuit. Such a disturbing in:

fluence, caused by the action of these two magnetic fields, may be eliminated by applying to the measuring circuit coil I0 an alternating current which is double or of some even multiple of the frequency of the alternating current supplied to the heating coil 3, and which passes through the zero portion of its cycle at the same instant as the heating coil current.

The manner in carrying out the present method of controlling the degree of graphitization may be described briefly as follows: The body 9 to be graphitized, as for example a bonded mixture of carbonaceous material, is moved through the furnace and heated to'a suitable temperature as it moves through the heating zone within the induction furnace coil 3. 'The indicating instrument l2, which may be a suitably calibrated ammeter, wattmeter or admittance meter, is set at a point corresponding to the desired specific resistance of the graphitized product. Such electrical resistance is indicated by the pointer 22 which is also provided with suitable contacts and adapted to make and break an electrical circuit at definite points above and below the desired resistance value. Should the resistance of any portion of the body 9 be above the upper limit the current induced from the coil I. would decrease causing a decrease in deflection of the meter I 2.

The change in deflection causes an electrical contact to be made from the pointer 22 to the contact 2|, completing an electrical circuit to the reversing motor l5 which operates to drive the power alternator rheostat Ii by a belt l1 so as to increase the current through the field winding ll of the alternator I. This change in field current increases the output from the alternator and the power supplied to the furnace, thereby increasing the temperature of said body and the extent of graphitization thereof.

Conversely any portion of the body 9 having a resistance slightly lower than the set value causes a corresponding increase in deflection of the instrument l2. An'electrical contact is made from the pointer 22 to the contact II which causes themotor l5 to change the setting of the field rheostat I6 so as to decrease the field current of the alternator I and-decrease the power input to the furnace to such a value that the body 9 will not be overheated.

While I have described the method of controlling the power input to the furnace in order to control the degree of graphitization it isunderstood that with a continuous type of furnace such as that illustrated the power input may be held at any desired value and the rate of movement of the body controlled.

By varying the frequency of the alternating current source II, I am able to determine the uniformity of the graphitized article and detect any variation in electrical resistance from the outside toward the center of said article. This is possible because of the fact that the depth of penetration of an alternating magnetic flux into the graphitized body is inversely proportional to the square root of the frequency. With a frequency of 1000 cycles per-second the depth of penetration of the induced currents is approximately 3.2 centimeters and the resistance of a ring of graphitized material 3.2 centimeters thick will be indicated by the meter l2.- By, lowering the frequency of the source II to cycles per second the ,depth of penetration of the induced currents is increased to approximately 10.0 centimeters and further reduction in frequency increases the depth of penetration and the cross section of the body measured. These successive determinations may be readily converted to specific resistance, which is the resistance per cubic centimeter, a comparison of which shows whether or not the graphitized body is of uniform resistance throughout.

By the above described method the degree of graphitization of carbonaceous bodies'may be indicated and controlled automatically regardless of variations in the raw material, furnace charge, or of radiation losses- I claim:

A process of graphitizing carbonaceous bodies which comprises subjecting such bodies to heat treatment at a temperature sufficiently high, to produce graphitizationand measuring the electrical resistance of said body to an induced electrical current while at such elevated temperature.

MIELTON.

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