Electric heating – Metal heating – By arc
Utility Patent
1998-07-16
2001-01-02
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121480
Utility Patent
active
06169265
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to plasma arc generators of both the transferable and non-transferable types, and more specifically to plasma apparatus of the kind generating a plasma arc that circulates in a closed path. The invention further relates to an electrode for use in plasma generators of the kind specified.
Plasma arc generators are used for the heat treatment of various objects in numerous technological processes, for example in metallurgical processes for so-called plasma remelting, plasma casting, plasma cleaning, etc. By one of its aspects, the invention relates to a process for heating with a circulating plasma arc a liquid metal chilling and crystallizing within a mold, with the object of eliminating typical casting defects, such as the formation of blowholes and porosity, segregation, formation of contraction cavities, inhomogeneity of chemical composition and crystal structure across the ingot, etc.
BACKGROUND OF THE INVENTION
Plasma generators including plasma arc torches are known in the art, and general descriptions of their design and of their use for various metallurgical applications, can be found in numerous technical monographs or handbooks, e.g. the chapter “Plasma Melting and Casting” in Metals Handbook, Ninth Edition, Vol. 15, Metals Park, Ohio, and the monograph “Plasma Metallurgy, The Principles” by V. Dembovsky, Elsevier, 1985, p.314-315.
Basically, plasma generators can be divided into two groups: those in which both cathode and anode form part of the apparatus which are known as plasma generators with non-transferable arcs or non-transferable plasma arc generators; and those which include only one electrode while the counter electrode is an electricity conducting substrate, which are known as plasma generators with transferable arcs or transferable plasma arc generators.
GB 1268843 describes a non-transferable plasma arc generator comprising a water cooled cathode and two annular anodes, one for ignition and the other for regular operation, connected to a power supply. The cathode tip is protected by injection of an inert gas such as argon, helium or nitrogen.
U.S. Pat. No. 4,958,057 describes a typical transferable plasma arc generator for use to heat metal in a continuous casting process. It comprises a cylindrical cathode-holding member with water cooling arrangements, an ignition anode and a ring-shaped cathode, having an inner channel for the injection of an inert protecting gas. An electric discharge is effected between the cathode and substrate to be treated, which is set as the anode.
It is an intrinsic disadvantage of the conventional plasma generators of both the non-transferable and transferable types, that for proper functioning the injection of a protecting gas or water cooling are required. Where gas cooling is employed, so-called plasma torches are used which comprise a plasma delivery nozzle. Injection of a pressurized inert gas into the torch is associated with the formation of an elongated plasma jet ejected at high velocity from the plasma delivery nozzle which in case of treatment of a solidifying cast metal causes the exertion of localized pressure on the surface of the still solidifying metal, leading to the formation of large cavities during chilling.
The presence of cooling water is dangerous because any leaking water that reaches the hot liquid metal may cause an explosion.
There are also known plasma generators in which a plasma arc is controllably displaced with respect to a treated substrate in an open, e.g. straight, or closed, e.g. circular fashion along a correspondingly shaped electrode. Such displacement of the arc avoids overheating, provides for a more uniform treatment of the substrate and reduces erosion of the electrodes, thereby prolonging the life span of the apparatus. Thus U.S. Pat. No. 5,132,511 discloses a non-transferable plasma torch having two coaxial tubular electrodes axially spaced from each other and provided with an electromagnetic coil for rotating the arc. The coil is mounted in a sealed cylindrical chamber positioned between the two electrodes.
U.S. Pat. No. 5,393,954 describes a non-transferable plasma torch which comprises two coaxial tubular electrodes at least one of which is surrounded by a magnetic field associated with electronic control means, whereby the plasma arc foot is displaced in a controlled fashion. When a plasma-generating gas is injected into a chamber separating said electrodes, an arc is ignited.
It is known that the arc in a plasma generator may be displaced by the action of a ponderomotive force known as the Lorentz force. A Lorentz force arises when an electric charge moves in a magnetic field and is proportional to the magnetic induction of the field, the electric charge, its velocity and also depends on the angle between the vectors of magnetic induction and velocity of the moving charge. It is known that a Lorentz force is created in a plasma generator as a result of interaction between the arc (being an intensive electric discharge), its magnetic field, and the magnetic field created in the generator by the electric current flowing through the electrodes. When the electrodes form a so-called two-rail structure the Lorentz force accelerates and displaces the electric arc.
The term “two-rail structure” used herein with reference to the electrodes in plasma generators should be understood as meaning two parallel current conducting objects (so-called rails) spaced from one another, and connected each to one of the electric power supply poles. When an electric arc is initiated between the electrodes, it moves along the rails away from the site of electric contact thereof with the power supply.
In accordance with prior art terminology plasma arc generators in which the arc discharge is accelerated by a ponderomotive force within a space between two parallel electrodes are sometimes referred to as electromagnetic rail accelerators or plasma accelerators with rail geometry.
The phenomenon, by which the Lorentz force accelerates and displaces the plasma arc in a plasma arc generator with a two-rail structure, is known as the principle of electromagnetic acceleration. It is mentioned in the literature with reference to plasma accelerators or magnetic hydrodynamic generators, e.g. in “
Impulse Plasma Accelerators
” by Alexandrov et al., Charkov, 1983, pp. 192, 194 and in “
Electroslag Welding and Melting
” by J. Kompan and E. Sherbinin, Machinostroenie, 1989, pp. 191, 192. A specific application of the Lorentz force is described in “
Scaling Laws for Plasma Armatures in Railguns
” by Lindsey D. Tornhill and Others,
Transactions of Plasma Science
, Vol. 21, No. 3, June 1993, 289-290.
An example of a non-transferable plasma arc generator with magnetic rail acceleration is described in SU 890567. In that generator, the electrodes are in form of two coaxial elliptical tubes and the space between the electrodes holds a dielectric material. A wall of each of the tubes is axially slotted such that the slot in one tube faces a non-slotted wall portion of the other tube. Adjacent to each slot there is one electric contact and in this way a two-rail structure is achieved. For uninterrupted circulation of the plasma arc it must be capable of crossing the slots and to this end the width of each slot must be less than the thickness of the arc. However, when crossing any of the slots the arc arrives exactly at the zone of the adjacent electric contact, where direction of its further movement is indefinite, and consequently the speed at which the arc moves near the slots is reduced and the discharge is occasionally even interrupted, which is an obvious disadvantage.
SU 847533 describes a transferable plasma arc generator for treating an electrically conductive substrate. It comprises a main electrode forming part of the generator and the electrically conductive substrate is set as the counter electrode. The main electrode is in form of a spirally wound hollow longitudinal body having one winding whose partially overlapping ends are angularly displaced relative eac
Dvoskin Pavel
Rosen Ran
Zlochevsky Valery
Browdy and Neimark
Netanya Plasmatec Ltd.
Paschall Mark
Van Quang
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