Patent Application
20070084834
I. Field of the Invention
This invention relates generally to the field of plasma arc torches, and more particularly to methods and apparatus for treating the collimator employed in the plasma arc torch to reduce the effects of corrosion and thereby extend the service life of the collimator.
II. Discussion of the Prior Art
Plasma arc torches, as known in the prior art, are capable of efficiently converting electrical energy to heat energy producing extremely high temperatures. For example, a plasma arc torch may typically operate in a range as high as from 6000° C. to 7000° C.
Plasma arc torches are known which use water-cooled, reverse polarity, hollow copper electrodes. A gas, such as argon, nitrogen, helium, hydrogen, air, methane or oxygen, is injected through the hollow electrode, ionized and rendered plasma by an electric arc and injected into or integrated with a heating chamber or process.
As is explained in the Hanus, et al. U.S. Pat. No. 5,362,939, plasma arc torches can be made to operate in either of two modes. In a first mode, termed “transferred arc”, a water-cooled rear electrode (anode) applies a high voltage and current to the gas injected into the torch. The material to be heat-treated is made the opposite polarity electrode. As such, the plasma gas passes through a gas vortex generator contained within the torch and out through the central bore of a conductive copper collimator and is made to impinge onto the material serving as the cathode electrode. In the non-transfer arc mode, the arc emanates first from the anode within the torch and reattaches to the cathode at the outlet of the torch. In jumping from the first electrode to the second electrode, the arc extends out beyond the tip of the torch and can be made to impinge upon a workpiece that does not form part of the electrical circuit. Thus, in the non-transferred arc mode, the torch can be used to effectively heat/melt/volatilize non-conductive workpiece materials.
In the case of transfer arc mode torches, the collimator generally comprises a copper holder that screws into the working end of a generally cylindrical torch body in which is contained a rear anode electrode that is electrically-isolated from the collimator. The cylindrical body further contains flow passages for receiving cooling water, routing it through the collimator, and then back through the body of the torch to an outlet port. Likewise, the torch gas has its own passageway to a vortex generator disposed adjacent the central bore of the collimator.
Those readers interested in details of construction of a typical plasma torch are referred to the Hanus, et al. U.S. Pat. No. 5,362,939, the teachings of which are hereby incorporated by reference as if fully set forth herein.
In certain applications of plasma torch technology, the collimator portion of the torch is exposed to corrosive materials. For example, when used in solid waste disposal furnaces to solidify bottom ash and fly ash mixtures into a glass-like mass, chlorine gas is produced from the thermal destruction of plastics. The chlorine can combine with hydrogen to form hydrochloric acid, which can rather rapidly corrode copper surfaces exposed to the acid. It is imperative that the collimator not be corroded to the point where a cooling water channel within the collimator assembly is breached. A stream of water impinging on super-heated surfaces in the furnace can be a serious safety problem and must be avoided. This necessitates frequent shut-down and replacement of the collimators before corrosion reaches the point where the leaking can occur.
The collimator used in transferred arc plasma torches may also experience secondary arcing. In such an arrangement, the collimator is floating in potential and, if the voltage gradient between it and the local plasma potential becomes great enough, a branch of the plasma arc may strike the collimator, pitting and eroding its surface.
It is accordingly a principal object of the present invention to provide a corrosion-resistant barrier on exposed surfaces of the collimator used on plasma torches.
It is a further object of the invention to provide a corrosion barrier that is less subject to cracking due to thermal stresses and/or secondary arcing.
The present invention provides an improved plasma arc torch having a collimating nozzle at its distal where the exposed face surface and substantial portion of the inner exit bore of the collimating nozzle includes an anti-corrosive covering thereon.
In accordance with a first embodiment of the invention, the anti-corrosive covering comprises a relatively thin electroless nickel coating, an alumina coating or a nickel chromium coating. In accordance with an alternative embodiment, the exposed face surface and substantial portion of the inner exit bore of the collimating nozzle is clad to a predetermined thickness with a suitable anti-corrosive alloy applied in a number of different ways, including a plasma transferred arc welding process, a flame spray process, a plasma spray process, an explosion bonding process, a hot isostatic pressing (HIP) and laser cladding process.
The foregoing features, objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of a preferred embodiment, especially when considered in conjunction with the accompanying drawings in which like numerals in the several views refer to corresponding parts.
a is a perspective view from the side of the collimator holder used in the design of
b is a perspective view from the top of the collimator holder of
Certain terminology will be used in the following description for convenience in reference only and will not be limiting. The words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the device and associated parts thereof. Said terminology will include the words above specifically mentioned, derivatives thereof and words of similar import.
Referring first to
The gas injected into port 28 becomes ionized and is rendered plasma by the arc 32 and is injected onto the work piece 30. The collimator 22 includes a longitudinal bore 34 having a frustoconical taper 34 and serves to concentrate the plasma into a beam, focusing intense heat that speeds up melting and chemical reaction in a furnace in which the plasma torch is installed.
The exposed toroidal face 36 of the collimator 22 is exposed to corrosive chemicals given off from the melting/gasification of the work material 30, resulting in erosion and pitting of the collimator. Also, the collimator is subject to secondary arcs, especially in the tapered zone 34 of the collimator.
It is imperative that the collimator not be allowed to deteriorate to the point where cooling water can escape the normal channels provided in the torch and flow out onto the work piece that may be at a temperature of 2000° F. or more. Resulting superheated steam can create an explosive force within the confines of a plasma arc heated furnace. To avoid such an event, it becomes necessary to shut down the process and replace the collimator at relatively frequent intervals. The purpose of the present invention is to prolong the useful life of the collimator, thereby reducing the down-time of the process in which the plasma arc torch is used.
Referring next to
Located directly below the threaded zone 42 on the holder member is a plurality of bores, as at 44, that is regularly spaced circumferentially about the periphery of the holder member. An integrally formed annular collar 46 is provided at the proximal end of the collimator.
The collimator assembly 22 further includes a tubular insert 54 machined from a copper billet and having a central lumen 56 and an outer wall 58 whose diameter is dimensioned to fit within the central bore 48 of the holder member with a predetermined clearance space between the wall defining the central bore of the holder member and the outer diameter of the tubular insert. The insert is also formed with a circular flange 60 at its distal end and that surrounds the lumen 56. Further, the cross-sectional view of
In the prior art collimator assembly shown in
As is explained in the Hanus, et al. '939 patent, supra, cooling water is made to flow through a first annular passageway, through the radial bores 44 and through the clearance space between the bore 48 and the outer tubular wall 58 of the insert 54 and from there, out through an annular port to another passageway contained within the shroud 12 and leading to the water outlet port 26 (
In that the tubular insert 54 is also preferably formed from copper, it is subject to corrosion due to exposure to chemical substances produced during thermal destruction of target materials being heated/melted in a plasma torch heated furnace. The face surfaces 52 and 64 of the holder member and the insert, respectively, will lose material due to corrosion and erosion due to secondary arc strikes. The e-beam weld in the joint between the flange 60 and the counterbore 50 is also particularly vulnerable and should a leak occur in this joint, cooling water under high pressure may leak from the aforementioned cooling water passages in the collimator as a jet-like stream only to impinge on the work piece 30, which may be at a temperature in excess of 3000° F.
The present invention provides methods for prolonging the life of the collimator used in plasma arc torch constructions. Specifically, by providing an anti-corrosive covering on the exposed face surface and substantial portion of the inner exit bores of the holder member and the insert, the useful life of the collimator can be extended.
In accordance with a first method for reducing the effects of corrosion on the life of the collimator, the exposed face surfaces 64 and 52 of the design of
The aforementioned plating/thin coating operations have proven effective in extending the time-to-replacement by a factor of three. Coating failure ultimately tended to occur at the location of any sharp edges, especially where the tapered bore 62 intersects with the somewhat planar forceps of the insert's flange.
In an attempt to gain even further improvement, various changes were made to the collimator geometry itself prior to the plating/coating operations. More particularly, sharp edges at the intersection of the tapered portion of the insert's lumen with the exposed face surface were smoothly radiused, as were the peripheral edges. This reduces cracking of the coating and exposure of the underlying copper. Generally speaking, the thin plating of anti-corrosion coatings and sprayed on anti-corrosive coatings proved effective until cracks or deep craters due to secondary arcing developed that exposed the underlying copper. The smoother edges proximate the tapered portion of the inserts lumen, plus the plated and/or plasma-sprayed collimators resulted in a 20 times useful life extension over the prior art bare copper collimators. The coatings remained effective until deep craters due to secondary arcing ultimately ate through the coating layers to expose the underlying copper.
Still further improvement in the useful life of collimators used in plasma arc torches has been achieved by covering the exposed face surface and substantial portion of the inner exit bore of the copper collimating nozzle with a cladding layer of a predetermined thickness. Cladding materials that have proven successful include Hastelloy (C-22), Iconel-617, and Inconel-625 materials.
Referring to
Two key areas that affect coating quality are surface preparation and spraying parameters. Surface preparation is important for adhesion of the coating 94 and can affect the corrosion performance of the coating. The main factors are grit-blast profile and surface contamination. Spraying parameters are more likely to affect the coating microstructure and will also influence coating performance. Important parameters include gun-to-substrate orientation and distance, gas flow rates and powder feed rates.
The bond of a thermally sprayed coating is mainly mechanical. However, this does not allow the bond strength to remain independent of the substrate material. All thermal spray coating maintains a degree of internal stress. This stress gets larger as the coating gets thicker. Therefore, there is a limit to how thick a coating can be applied. In some cases, a thinner coating will have higher bond strength.
Turning next to
Plasma spraying has the advantage in that it can spray very high melting point materials, such as refractory materials, including ceramics, unlike combustion processes. Plasma-sprayed coatings are generally much denser, stronger and cleaner than other thermal spray processes.
It is possible to regulate process conditions so that the whole amount of powder and only a thin film on the workpiece are melted. As a result, a metallurgical bond between the cladding layer and the billet is provided with the minimum dilution of the detailed materials. Argon is basically used for arc plasma supply, powder transport and molten material shielding. Plasma transferred arc cladding affords high deposition rates up to 10 kilograms per hour. Deposits between 0.5 and 5 mm in thickness and 3 to 5 mm in diameter can be produced rapidly.
Still another method for cladding the billet is illustrated in
Due to its use of explosive energy, the process occurs extremely fast; unlike conventional welding processes, parameters cannot be fine-tuned during the bonding operation. The bonded product quality is assured through collection of proper process parameters, which can be well controlled. These include metal surface preparation, plate separation distance prior to bonding, an explosive load, velocity and detonation energy. Selection of parameters is based upon the mechanical properties, mass, an acoustic velocity of each component metal being bonded. Optimal bonding parameters, which result in consistent product quality, have been established for most metals combinations. Parameters for other systems can be determined by calculation using established formulas.
The first step in explosion cladding is to prepare the two surfaces that are to be bonded together. The cladding layer comprises a plate 126 of a selected, anti-corrosive alloy. Its surfaces are ground or polished to achieve a uniform surface finish. The cladding plate 126 is positioned and fixtured so as to be positioned parallel to and above the surface of the copper billet 80 to be clad. The distance, d, between the cladding plate and the billet surface is referred to as “the standoff distance”, which must be predetermined for the specific metal combinations being bonded. The distance is selected to assure that the cladding plate collides with the billet after accelerating to a specific collision velocity. The standoff distance typically varies from 0.5 to four times the thickness of the cladding plate, dependent upon the choice of impact parameters as described below. The limited tolerance in collision velocity results in a similar tolerance control of the standoff distance.
An explosive containment frame (not shown) is placed around the edges of the cladding metal plate. The height of the frame is set to contain a specific amount of explosive 128, providing a specific energy release per unit area. The explosive, which is generally granular or uniformly distributed on the cladding plate surface, fills the containment frame. It is ignited at a predetermined point on the plate surface using a high velocity explosive booster. The detonation travels away from the initiation point and across the plate surface at the specific detonation rate. The gas expansion of the explosive detonation 130 accelerates the cladding plate across the standoff gap, resulting in an angular collision at the specific collision velocity. The resultant impact creates very high-localized pressures at the collision point. These pressures travel away from the collision point at the acoustic velocity of the metals. Since the collision is moving forward at a subsonic rate, pressures are created at the immediately approaching adjacent surfaces, which are sufficient to spall a thin layer of metal from each surface and eject it away in a jet. The surface contaminants, oxides and impurities are stripped away in the jet. At the collision point, the newly created clean metal surfaces impact at a high pressure of several hundred atmospheres. Although there is much heat generated in the explosive detonation, there is no time for heat transfer to the metals. The result is an ideal metal-to-metal bond without melting or diffusion.
a and 5b illustrate the holder member after the billet 80 and its cladding layer 82 have been machined. Likewise,
Once the insert is placed into the holder member, electron beam welding may be used to form a continuous weld along a joint between the periphery of the flange on the insert and the wall in the holder member defining the counterbore. Although plating showed about a three times improvement in collimator life compared to an untreated copper collimator, with cladding, the improvement was about ten times.
As illustrated schematically in
Rather than starting with a solid disk 132 of anti-corrosive alloy, the copper billet 132 may also be clad in a HIP process by first machining the billet 130 as shown in
This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.
