Industrial electric heating furnaces – Arc furnace device – Power supply system
Patent
1987-08-10
1989-01-10
Envall, Jr., Roy N.
Industrial electric heating furnaces
Arc furnace device
Power supply system
H05B 7148
Patent
active
047978970
DESCRIPTION:
BRIEF SUMMARY
The invention relates to apparatus for controlling the distance of a melting electrode from the surface of the melted material in a vacuum arc furnace.
BACKGROUND OF THE INVENTION
For producing high-grade metals and metal alloys, which have the least possible inclusions of foreign objects and are homogeneous in structure, various methods are known. One of the best-known methods is electric-arc melting, in which an electrode extends towards a crucible and, by applying an electric potential between the electrode and the crucible, the tip of the electrode melts away and falls as a liquid material into the crucible. As a rule, the so-called melting electrode is connected to one pole of a direct-current voltage and the crucible to the other pole of this direct-current voltage. However, superpositions of alternating-current voltages are also possible to achieve particular effects.
A major problem in operating arc melting furnaces of the above-mentioned type lies in the control of the arc length, that is, of the distance between the lower end of the electrode and the surface of the melted material that is already in the crucible. If the arc is too long, the electrode and/or the melted material could be heated wrongly, so that the quality of the melted material is greatly reduced. Since, on the one hand, the level of the melted material in the crucible is constantly rising and, on the other, the distance between the end of the electrode and the surface of the crucible cannot be observed directly, special measures must be taken to control this distance.
In arc furnaces which operate at or only slightly below atmospheric pressure, the arc length is controlled by maintaining a given arc voltage. At atmospheric pressure, the plasma is characterized by the fact that it has a particular voltage gradient, for example, 20 volts per 2.5 cm. The voltage drops at the cathode and anode surfaces together amount to an additional 20 volts, so that, if an arc length of, for example, 1.25 cm is to be maintained, the electrodes will then have to be brought into such a position that the arc voltage is 30 V. This can be realized by means of conventional equipment which measures and controls the arc voltage.
In arc melting furnaces which operate under vacuum, the above-described method can, however, not always be employed. Such arc melting furnaces are used especially for melting so-called refractory active metals, such as titanium or zirconium, as well as for preparing stainless steels and high-temperature alloys. When the gas pressure, which surrounds the arc, decreases, the voltage gradient of the arc plasma also decreases and, at very low pressures, the voltage gradient of the arc plasma may, for example, by only one volt per 2.5 cm. Since the anode and cathode voltage drops for steel, for example, are approximately 20 volts, the voltage drop at the arc is very small in comparison with the remaining voltage drops. Changes in the gas content and the alloy composition affect the anode and cathode voltage drops in the order of magnitude of the voltage drop in the "arc column". Consequently, the method of controlling the arc by keeping the arc voltage constant is not very effective, especially in the case of steels, since the actual length of the arc will usually deviate greatly from the desired length.
An arc melting furnace has meanwhile become known which, for controlling the distance between the electrode and the surface of the melted material, makes use of the knowledge that, even in normal operation, the voltage breaks down briefly at certain time intervals (U.S. Pat. No. 2,942,045). This effect is caused by short-circuits, which originate from liquid metal droplets, which drop from the electrode into the crucible and briefly connect the electrode electrically with the melted material in the crucible. As long as the duration and frequency of these short-circuits are not very large, the arc operates almost at full power, so that there is no substantial effect on the heating of the melted material. If the arc becomes shorter, t
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Stenzel Otto
Thomas Friedrich-Werner
Envall Jr. Roy N.
Leybold Aktiengesellschaft
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