Apparatus And Method For Plasma Spraying

Abstract

An extremely high-powered electrical plasma-jet spray torch is constructed economically, safely, and in a compact manner, by use of three coaxial body members held together by longitudinal, internal bolts. No external housing is necessary, the handle being screwed directly onto one of the body members. Two removable electrodes are provided and are separated from each other by a gas-injector ring of heat-resistant insulating material. The electrode ends adjacent the gas injector are water-cooled and are provided with O-ring seals. Powder is injected into the plasma by use of a rotatable anode adapted to permit selective use of different types of injection, thus vastly increasing the utility of the anode. In accordance with the method, supersonic flow is employed in combination with very high powers, and plural-port powder injection, to achieve extremely high spray rates.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATION:

This is a continuation of application Ser. No. 214,584, now abandoned filed Jan. 3, 1972, for Apparatus and Method for Plasma Spraying.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of high-pressure, high-power electrical plasma-jet torches and methods. The invention is particularly directed to the spray coating of substrates by means of such torches.

2. Description of Prior Art

Certain apparatus and methods set forth in my copending patent application Ser. No. 133,126, now U.S. Pat. No. 3,740,522 filed Apr. 12, 1971, for Plasma Torch, and Electrode Means Therefor, and in my copending patent application Ser. No. 143,956 now abandoned, filed May 17, 1971, for Method and Apparatus for Supersonic Plasma Spray, have been sold and/or in public use for more than one year prior to filing of the present application; however, the disclosures of said patent applications are not, per se, prior art against the present application.

Prior art includes the following U.S. Pat. Nos. 2,768,279; 2,943,182; 3,071,678; 3,114,826; 3,118,046; 3,140,421; 3,145,287; 3,179,782; 3,183,337; 3,197,605; 3,205,338; 3,246,114; 3,272,958 and 3,390,292.

SUMMARY OF THE INVENTION

Three coaxial annular body members, two formed of insulating material and one of metal, are held in closely nested relationship by a plurality of internal longitudinal bolts. Into such body members are inserted coaxial anode and cathode means, the cathode means including a stick (rod) electrode which extends into an arc chamber in the anode. The adjacent portions of the anode and cathode means abut an annular gas injector formed of heat-resistant insulating material, and which encompasses the stick electrode. Such adjacent anode and cathode portions are undercut to effect water cooling of O-ring seals which are provided between the electrodes and the intermediate body member. No external housing is required, since a handle means is screwed to the intermediate body member in enclosing relationship to conduit means which conduct water, electricity and powder to the anode means.

The anode means is so mounted in the forward body member that it may rotate about its longitudinal axis. Furthermore, the conduit means which supplies powder to the anode means is adapted to register selectively and sealingly with any one of a plurality of differently-constructed ports, depending on the rotated position of the anode means. Thus, the powder-injection characteristics may be quickly and economically altered as desired.

Extremely high spray rates are achieved by simultaneously employing very high electrical powers, supersonic gas flow, and plural-port powder injection. The present apparatus and method are thus such that a small, safe, compact, high-powered plasma torch achieves surprisingly high spray rates relative to the spray-coating of a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view illustrating an electrical plasma-jet spray torch constructed in accordance with the present invention;

FIG. 2 is a sectional view taken on line 2--2 of FIG. 1;

FIG. 3 is a section taken on line 3--3 of FIG. 2;

FIG. 4 is an enlarged fragmentary sectional view showing a lower-right portion of the torch of FIG. 1, and also illustrating in dashed lines various alternative positions for the powder port;

FIG. 5 is a transverse sectional view on line 5--5 of FIG. 1;

FIG. 6 is a transverse sectional view on line 6--6 of FIG. 1; and

FIG. 7 is a transverse sectional view illustrating schematically an embodiment wherein a plurality of powder sources are employed to inject powder into the torch.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1, the torch body is illustrated to comprise three annular body members 10, 11 and 12 which are mounted in closely nested relationship relative to each other. The rear body member, numbered 10, is formed of a suitable insulating plastic such as a phenolic. The intermediate body member 11 is also formed of insulating plastic but preferably one which is much stronger, such as a fiber glass-resin composition. Front body member 12 is formed of metal, such as brass.

Each of the annular body members defines an opening therethrough, and the three such openings combine to produce an opening or passage through the torch. Into such torch opening or passage are inserted the anode and cathode means, as described below.

The annular body members 10-12 are maintained in assembled relationship, despite the very high pressure contained therein, by means of three bolts 13-15 (FIGS. 1 and 2) which are oriented longitudinally to the common axis of the body members and are circumferentially spaced 120.degree. from each other. The bolts extend interiorly of the body members, instead of through external flanges or connectors, thereby greatly improving the compactness of the torch.

The head of each bolt 13-15 is recessed into a cylindrical cavity in the forward surface of front body member 12. The threaded rear end of each bolt is threaded into a nut 16 (FIG. 1) which is mounted in a recess in the rear member 10. An insulating plug 17 is cemented into the recess in member 10 to conceal the rear end of each bolt and to prevent the operator from making electrical contact therewith.

It is emphasized that all portions of the torch rearwardly of the metallic front body member 12 are insulating, except for the connection to the cathode means described below. This relationship makes the present apparatus relatively safe to operate.

As shown in FIGS. 1 and 3, the body members 10-12 receive in snug-fitting relationship an anode means 18 and a cathode means 19, the latter having a stick (rod) electrode portion 21 which extends coaxially into an arc chamber 22 in the anode means.

A gas-injector ring 23, formed of a heat-resistant insulating ceramic such as boron nitride, aluminum oxide, zirconium oxide, etc., is mounted between the adjacent or inner end portions of the anode means and the cathode means and in radially-outwardly spaced concentric relationship to the stick electrode portion 21. More specifically, the cylindrical inner surface of gas-injector ring 23 is flush with the cylindrical surface of the side wall of arc chamber 22, so that the gas-injector ring and the anode means cooperate to define a gas-vortex chamber 22a around the stick electrode portion.

Arc gas is introduced into such gas-vortex chamber 22a through a multiplicity of small-diameter gas-inlet passages 24 (FIGS. 1, 3 and 5) which are drilled through the gas-injector ring 23. The illustrated passages 24 are, in the illustrated embodiment, tangentially oriented relative to the gas-vortex chamber 22a and, furthermore, incline somewhat forwardly relative to a vertical plane which is perpendicular to the axis of the apparatus. In some instances, however, other types of gas injection may be employed, for example injection which is not adapted to effect vortical flow in the chamber around the stick electrode.

It is an important advantage of the present torch that the manner of arc gas injection may be readily changed, merely by substituting one ring 23 for another. The ring 23 also produces other important advantages, including (a) permitting the vortex chamber 22a to be small in diameter, and (b) effectively insulating the anode and cathode from each other.

Gas-injector ring 23 has a rectangular cross section except at the exterior surface thereof, which is provided with an annular groove 26 communicating with the various gas-inlet passages 24. The ring 23 is seated in a recess or counterbore which is formed in the forward side of intermediate body member 11. The wall of such recess or counterbore is undercut, at the region radially-outwardly of groove 26, to provide an annular manifold chamber 27 into which arc gas is introduced through a passage 28 (FIG. 3). Passage 28 communicates with a recess in intermediate body 11, and into which a tube 29 is sealingly inserted (there being an O-ring 31). Tube 29 is soldered to a fitting 32 adapted to be connected to a gas source which is schematically represented at 33.

It is emphasized that, in accordance with the present apparatus and method, a very large amount of power is "packaged" within a very small space, with consequent enormous generation of heat. For example, in a torch wherein the arc chamber 22 is only about two-thirds of an inch in diameter, the power input may be between 80 kilowatts and 100 kilowatts. The heat-resistant gas-injector ring 23, particularly since it is spaced rearwardly from the arcing tip of the electrode 21, is able to withstand the resulting extremely high temperatures. However, means are provided to cool the seals which prevent escape of gas from manifold chamber 27 except through the gas-inlet passages 24.

The inner end of anode means 18 has a radial flange the rear radial surface of which is abutted against the radial forward surface of gas-injector ring 23. The flange extends outwardly to a cylindrical element 34 the exterior surface of which abuts an O-ring 36 which is mounted in a groove formed in the interior wall of intermediate body 11. Furthermore, an undercut is formed between the element 34 and the main body of the rear portion of anode means 18, into which water flows to thereby maintain O-ring 36 sufficiently cool that it will not deteriorate.

In simiar manner, the forward end of the stick-holder or slug-holder portion (described hereinafter) of cathode means 19 is provided with a radial flange and with a cylindrical element 37, the latter contacting an O-ring 38 which is seated in the intermediate body. The resulting undercut receives water which maintains the O-ring 38 relatively cool. In addition, an O-ring 39 is mounted in the radial forward surface of the slug holder, in contact with the radial rear wall of the gas-injector ring 23, being cooled by water present in the undercut in the cathode means.

There will next be described the remainder of the cooling means which maintain the anode and cathode means sufficiently cool that they will not melt or deteriorate excessively, despite the great heat which is generated by the electric arc. Water from a suitable source 41 (FIG. 1) is caused to flow rapidly through a large-diameter conduit 42 and thence into a right-angle fitting 43 the upper end of which is brazed into a recess in front body 12. The fitting communicates with an annular groove 44 formed in the front body 12. From such groove 44, the water is forced rearwardly through a large number of saw cuts or notches 46 (FIG. 6) which is defined by teeth 47 extending outwardly from anode means 18 at the region around arc chamber 22. The teeth 47 are in surface engagement with an interior cylindrical surface 48 of the front body 12, so that the water is not merely caused to flow around the saw cuts or notches 46 but instead is forced rapidly therethrough in highly efficient cooling relationship to the anode means.

The interior surface 48 is formed on a neck portion of the front body 12, such neck portion extending rearwardly into a large counterbore in intermediate body 11. An O-ring 49 is provided to prevent leakage of water out of such counterbore.

The rear end of the neck portion of front body 12 is spaced forwardly from the opposed radial surface of intermediate body 11 at the indicated counterbore, whereby to form an annular chamber 50 into which the water flows after leaving the saw cuts or notches 46. It is emphasized that the water entering the chamber 50 impinges against the above-described cylindrical element 34 to aid in cooling the O-ring 36 adjacent thereto.

From chamber 50, the water flows rearwardly through a large number of circumferentially-spaced longitudinal bores or passages 51 in intermediate body 11 (FIG. 5), such bores having rear portions which incline inwardly and rearwardly to the chamber 52 which is defined in intermediate body 11 around cathode means 19. The chamber 52 communicates with a plurality (for example, six) of passages 53 which are formed through the cathode means 19 and which communicate with a central passage 54 therein and thus with a fitting 56 leading to a suitable drain 57.

An O-ring 58 is provided around the cathode means 19 to prevent leakage of water from chamber 52. An additional O-ring, numbered 59, is formed around the front portion of the anode means 18 between a cylindrical external surface thereof and an interior cylindrical surface of the front body 12, forwardly of annular groove 44. Heating of the last-mentioned O-ring 59 is prevented by water present in an undercut region 60 of the anode means, such region being located radially-inwardly of a cylindrical portion 61 of the anode means and which abuts the O-ring 59.

Anode means 18 is a single element made of copper, and which is machined or otherwise formed to contain the various cooling portions described above. The anode means also contains the arc chamber 22 as described, which arc chamber 22 communicates coaxially with a smaller-sized arc chamber or counterbore 62 located forwardly of the rounded tip of the cathode stick or slug 21. The forward regions of the arc chamber 22 and of the smaller arc chamber 62 are generally rounded or spherical.

The smaller arc chamber 62 communicates coaxially with a nozzle passage 63 having a cylindrical rear portion and a somewhat flared or conical forward portion. The illustrated nozzle passage is of the supersonic type, as is greatly preferred for reasons stated hereinafter, but it may also be subsonic if desired for certain applications. Nozzle passage 63 will not be described in detail since counterparts thereof are set forth in my copending patent application Ser. No. 143,956 now abandoned, cited above.

The present spray torch is of the non-transferred arc variety, wherein the entire arc is contained within the torch. Thus, a D.C. power source 64 (FIG. 1) has the positive terminal thereof connected to conduit 42 (which is formed of copper) to thereby supply D.C. power of positive polarity to the fitting 43 and thus to the front body 12 and to the anode means 18 in contact therewith. The negative terminal of power source 64 is connected to fitting 56 and thus to the cathode means 19. An electric arc is thus maintained between the tip of the cathode means and the wall of the arc chamber 62.

Because gas is introduced at high pressure from source 33 (FIG. 3) through the passage described above, the gas pressure in chambers 22 and 62 is high (for example, 120 psi gauge when the electric arc is present). This high pressure cooperates with the high electric power contained in the torch, and with the characteristics of nozzle passage 63, in such manner that the flow through the nozzle passage is caused to be supersonic, for example between Mach 1 and Mach 3 (preferably about Mach 2 when spraying is being effected in the atmosphere, as distinguished from being effected in a vacuum chamber).

The present torch may also be employed for cutting purposes, for example by causing the positive terminal of D.C. power source 64 to connect to an electrically conductive workpiece (such as a steel plate to be cut) instead of to the conduit 42. For cutting purposes, the nozzle passage 63 is preferably shortened, and the electric arc extends clear through the nozzle passage from the stick electrode 21 to the workpiece.

As previously indicated, the cathode means 19 comprises (in addition to the thoriated tungsten slug, stick or rod 21) a slug-holder or stick-holder 66 having a radial flange 67. Such flange is seated between the bottom of a recess in rear body 10 and the rear end of a neck 88 on intermediate body 11. The slug holder 66 is preferably formed of copper.

Referring next to FIGS. 1, 2 and 6 in particular, the torch further comprises handle means which are screwed directly to the torch body by means of the screws 70. The screws project into inserts in the intermediate body 11, which (being formed of fiber glass) is very strong. Screws 70 project respectively through the upper ends of first and second handle portions 71 and 72 which are mirror images of each other and abut at the central plane of the torch. Portions 71 and 72 are secured together by bolts 73 and 74 shown in FIG. 1.

The handle portions 71 and 72 have grooves therein which cooperate to form passages through which the above-described conduit 42 passes, as does an additional tube or conduit 76 adapted to supply spray powder to nozzle passage 63 as described below. Both the conduit 42 and the conduit 76 extend upwardly through the handle and then bend forwardly to a position in advance of the handle, whereupon they bend upwardly into forward body 12 as shown in FIG. 1.

Powder tube 76 is supplied by a source 75 (FIG. 1) with spray powder entrained in gas. Such a source is shown in U.S. Pat. No. 3,517,861.

DESCRIPTION OF THE SELECTIVE-PORT POWDER SUPPLY MEANS

The upper end of powder conduit 76 projects slidably through a corresponding radial bore in the lower region of front body member 12. Furthermore, as best shown in FIG. 4, the extreme upper end of the powder conduit or tube 76 is beveled at 78 to abut the conical wall of a recess 79 formed in anode means 18. A cross-member or fitting 80 (FIG. 2) is rigidly secured to the powder tube or conduit 76 (as by brazing), and is fastened by screws 81 (FIG. 2) to the front body 12.

Loosening of the screws 81 permits the operator to shift the upper end of powder tube 76 downwardly and out of the recess 79 (FIG. 4), thereby permitting rotation of the anode means 18 as described below. Correspondingly, tightening of the screws 81 forces the beveled upper end of tube 76 into recess 79 and effects a seal with the wall of such recess, so that all gas and powder which flow upwardly through the tube 76 pass into the nozzle passage 63.

Anode means 18 is provided with a plurality, for example eight in the illustration, of such recesses 79 (FIG. 4), in circumferentially-spaced relationship. Each recess 79 communicates with a port or passage which extends inwardly to the nozzle passage 63. The various ports or passages are numbered 82-89 in FIG. 2, and each has at least one characteristic different from that of all the others. Thus, for example, the passages may have different diameters, different inclinations, etc.

As an example, the passage 85 (FIG. 2) is shown as being tangentially related to the nozzle passage, the relationship being such that the powder is introduced in a clockwise manner as viewed from the front of the torch. This is opposite to the direction of introduction of arc gas through injector ring 23, this being counterclockwise as shown in FIG. 5.

Referring to FIG. 4, the passages 82a and 82b correspond to passage 82 except that they are inclined at different angles relative to a plane which is perpendicular to the nozzle passage 63 and contains the powder tube 76. It is pointed out that the three passages 82, 82a and 82b (FIG. 4) are not present in the same torch (there preferably being only one port 82, etc., which communicates with tube 76 at any one time). The passages 82a and 82b are illustrated herein as alternative angles of powder introduction. Various angles and types of powder introduction are described in my copending application Ser. No. 143,956 now abandoned, cited above.

To cause a selected one of passages 82-89 to register with powder conduit 76, the upper end of such conduit is lowered by loosening the screws 81 (FIG. 2) as described above. Thereafter, a front retainer ring 92 (which normally locks the anode means 18 in position) is removed from the front of the torch by removing mounting screws 93 (FIG. 3) therefor. After the front ring is removed, a threaded tool is inserted into an internally-threaded bore 94 (FIG. 1) in the forward face of the anode means 18.

The threaded relationship between the tool and the threads in bore 94 permits the operator to pull the entire anode means 18 forwardly for a fraction of an inch, until there is no longer any engagement with an indexing pin 96 (FIG. 1) which is permanently and fixedly mounted in the forward face of front body 12 in parallel relationship to the axis of the torch. The pin 96 is selectively received in any one of eight circumferentially-spaced bores 97 which are provided in a flange 98 in anode means 18. Each bore 97 corresponds in position to one of the conical recesses 79 described relative to FIG. 4. The flange 98 seats in a counterbore in the face of front body 12.

Since the indexing pin 96 is no longer inserted in one of the bores 97 after the anode means 18 is pulled forwardly as stated above, the operator may rotate the anode in order to cause the desired one of passages 82-89 to register with the upper end of powder conduit 76. Thereafter, the anode 18 is pushed rearwardly until indexing pin 96 is again inserted in one of the bores 97, following which the front ring 92 is mounted in position by means of screws 93, and following which the screws 81 (FIG. 2) are tightened to elevate the powder tube 76 and effect a seal at bevel 78 (FIG. 4) as stated above.

The described rotation of the anode 18 permits a single anode to have a much greater utility than in the prior art, so that various types of powders, various settings of the torch, etc., may be employed with a single anode as necessary in order to achieve maximum spray rates.

DESCRIPTION OF THE METHOD, EMBODIMENT OF FIG. 7

Th method which will next be described, in connection with FIG. 7, relates to the discovery that the combination of supersonic plasma flow, simultaneous plural-port injection of spray powder, and very high arc power produces spray rates which are surprisingly high. It is emphasized, however, that the arc power must be related to the size of the torch, since the larger torches normally generate arc powers much higher than do the smaller torches.

The torch and method described in my copending patent application Ser. No. 143,956 now abandoned, cited above, discloses supersonic flow but does not disclose simultaneous plural-port powder injection. such prior-art U.S. Pats. Nos. as 3,144,826; 3,183,337; and 3,197,605 teach plural-port powder injection, but only at subsonic flows and relatively low arc powers. U.S. Pats. Nos. 3,179,782 and 3,246,114 purport to teach supersonic flow, and appear ambiguous relative to whether or not there are plural powder ports. It has now been ascertained that if the arc power is extremely high compared to the size of the torch, if plural-port powder injection is employed, and if supersonic flow is employed, then the spray rates increase extremely rapidly.

The method will be described in connection with the present torch, which has (as mentioned above) an arc chamber diameter of about 2/3 inch (chamber 22). The torch has a nozzle passage diameter of 0.234 inch at the smallest portion thereof, and a nozzle passage length of 0.812 inch.

For a torch of the indicated size, the arc power is in excess of 50 kilowatts, and is preferably in the range of 80 kilowatts to 100 kilowatts. As one illustrative condition, the arc current is 900 amperes and the arc voltage 90 volts.

The pressure of the arc gas (for example, argon) introduced from source 33 (FIG. 3) is in excess of 50 psi gauge, which produces in arc chamber 22 a gas pressure of 120 psi gauge after the arc is initiated. The combination of the high gas pressure and the high arc power combine with the characteristics of the supersonic nozzle passage 63 to create a supersonic flow through the nozzle and out the torch. The velocity of the plasma emanating from the torch may be about 10,000 feet per second when spraying occurs in the atmosphere. Stated otherwise, the plasma emanating from the torch is in the range of Mach 1 to Mach 3, being preferably about Mach 2 (when spraying is in the atmosphere as distinguished from in a vacuum chamber).

By plural-port powder injection it is meant that separate powder sources are employed simultaneously to feed powder and gas through separate conduits to the nozzle passage 63, as described below relative to FIG. 7. It is not preferred to use a single powder source which feeds powder and gas to a plurality of ports.

Referring to FIG. 7, there is schematically represented a powder source 101 and a powder source 102 which are connected, respectively, to the torch by means of powder tubes 103 and 104. Such sources 101 and 102 combine with powder from the first source 75 and which is connected through the tube 76 as described relative to FIGS. 1 and 4.

The powder tubes 103, 104 and 76 communicate, respectively, with powder ports 106, 107 and 82a which is provided in the anode means 18a.

In the present example, all of the ports 106, 107 and 82a correspond to each other in diameter, angle, etc., and all are constructed as shown in FIG. 4 relative to port 82. It is to be understood, however, that different longitudinal or circumferential positionings may be employed, as well as different angular relationships, etc.

In the present example, and with the torch of the size described above, each port 106, 107 and 82a has a diameter (for example) of 1/16 inch. The rate of gas flow from each source 101, 102 and 75 is 50 scfh. The spray coating is thus deposited on a substrate (not shown) at a rate of, for example, 40 pounds per hour.

Except as specifically stated above, the torch of FIG. 7 is identical to the one previously described in detail relative to FIGS. 1-6, inclusive.

It is pointed out that the anode means 18 may also be regarded as a "nozzle means," since it has a nozzle passage 63 therethrough. Also, the cathode means 19 may be regarded as a "rear electrode means." The front end of slug holder 66 is a "shoulder portion" which extends radially outwardly from stick electrode 21.

The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.

Claims

I claim:

1. A high-power electrical plasma-jet torch, which comprises:

first body means formed of insulating plastic,

said first body means having a large-diameter central passage therethrough,

second body means formed of metal,

said second body means also having a large-diameter central passage therethrough,

means to mount said first and second body means adjacent each other and with said central passages in mutual alignment to thereby form a continuous passage,

a slug holder formed of metal and inserted into one end of said continuous passage radially-inwardly of said first body means,

an elongated electrode slug having its base end mounted in said slug holder at the inner end thereof,

all portions of at least the forward end of said slug being formed of tungsten,

said slug extending longitudinally of said passage a substantial distance forwardly from the inner end of said slug holder,

said slug having a diameter smaller than that of said inner end of said slug holder,

nozzle means formed of metal and inserted into the other

end of said continuous passage radially-inwardly of said second body means,

the inner end of said nozzle means being spaced longitudinally from said inner end of said slug holder,

the inner end of said nozzle means encompassing said forward end of said slug,

a gas-injector ring formed of heat resistant and electrically-insulating material,

said gas-injector ring being disposed concentrially around said slug between said inner ends of said slug holder and nozzle means,

means to maintain a very high-current electric arc at least part of which is disposed in said nozzle means,

the rear terminus of said arc being at said forward end of said slug,

means to conduct arc gas to and through said gas-injector ring for inward flow around said slug and then forwardly through said nozzle means,

said gas being heated by said arc and discharging from said torch at a high temperature and velocity, and

means to effect cooling of said nozzle means and of said slug holder.

2. The invention as claimed in claim 1, in which the nozzle passage through said nozzle means is supersonic.

3. The invention as claimed in claim 1, in which said arc-maintaining means is a D.C. source the negative terminal of which is connected to said slug holder and the positive terminal of which is connected to said nozzle means.

4. The invention as claimed in claim 1, in which said means to conduct arc gas to said gas-injector ring includes an annular space defined between the outer surface of said ring and a part of said first body means, and further includes circumferentially-spaced passages extending through said ring and communicating with said annular space and also with the space between the inner surface of said ring and the outer surface of said slug.

5. The invention as claimed in claim 1, in which said first body means comprises two annular components each formed of insulating plastic, and in which said mounting means mounts said two components of said first body means adjacent each other and also adjacent said second body means.

6. The invention as claimed in claim 5, in which said mounting means comprises a plurality of bolts which extend longitudinally of the torch through said two components of said first body means and also through said second body means, and which maintain the same in closely stacked relationship relative to each other.

7. The invention as claimed in claim 5, in which said slug holder has a radially-outwardly extending flange which seats between adjacent portions of said two components of said first body means, whereby to maintain said slug holder in a predetermined desired position in said continuous passage.

8. The invention as claimed in claim 1, in which said inner end of said slug holder has a face which seats against a face at the rear side of said gas-injector ring, and in which said inner end of said nozzle means has a face which seats against a face at the forward side of said gas-injector ring.

9. The invention as claimed in claim 1, in which an O-ring is provided concentrically around said slug and is seated between said inner end of said slug holder and a portion of said gas-injector ring.

10. The invention as claimed in claim 1, in which said nozzle means defines a relatively large-diameter arc chamber around said forward end of said slug, and defines a smaller-diameter chamber forwardly of said forward end of said slug, said smaller-diameter chamber receiving the forward terminus of said arc when said arc-maintaining means applies a voltage between said nozzle means and said slug, and in which said nozzle means further comprises a relatively small-diameter nozzle passage extending forwardly from said smaller-diameter chamber.

11. The invention as claimed in claim 10, in which said last-named nozzle passage is a supersonic passage.

12. The invention as claimed in claim 1, in which said gas-injector ring is formed of ceramic, and in which said means to conduct arc gas to and through said gas-injector ring comprises a channel defined circumferentially around said gas-injector ring between said ring and an opposed portion of said first body means, and further comprises passages through said gas-injector ring.

13. The invention as claimed in claim 1, in which said first body means comprises two annular components each formed of insulating plastic and stacked relative to each other, and in which said means to conduct arc gas to and through said gas-injector ring comprises a tube which extends through the outer one of said two components and into a passage in the inner one of said two components, and in which an O-ring is provided around the portion of said tube which extends into said inner one of said two components.

14. The invention as claimed in claim 1, in which said means to maintain said electric arc and said means to effect cooling of said nozzle means and slug holder comprise means to conduct both water and electricity to said slug holder, means to conduct both water and electricity to said second body means and around said nozzle means, and means to conduct water from said second body means through said first body means to said slug holder.

15. The invention as claimed in claim 1, in which means are provided to removably mount said nozzle means in said continuous passage, whereby to facilitate replacement of said nozzle means.

16. The invention as claimed in claim 1, in which said gas-injector ring is formed of a ceramic.

17. The invention as claimed in claim 1, in which a handle is secured by screws to at least one of said first and second body means, and in which no portion of said handle encompasses either of said body means.

18. The invention as claimed in claim 1, in which said means to effect cooling of said nozzle means and slug holder comprises means to conduct water to said second body means circumferentially around said nozzle means, an O-ring disposed circumferentially around said inner end of said nozzle means between said nozzle means and said first body means, passage means extending through said first body means to conduct water from said second body means to an annular recess formed externally in said slug holder, a second O-ring mounted circumferentially around said inner end of said slug holder and seated between said inner end and a portion of said first body means, and means to pass water through said slug holder from said annular recess therein.

19. The invention as claimed in claim 18, in which said inner end of said nozzle means is undercut to receive water from said second body means, whereby to interpose water between said first-mentioned O-ring and said slug, and in which said inner end of said slug holder is undercut to receive water from said annular recess in said slug holder and thereby interpose water between said second-mentioned O-ring and said slug.

20. The invention as claimed in claim 19, in which a third O-ring is provided between a rear surface of said gas-injector ring and said inner end of said slug holder, and third O-ring being cooled by water contained in said undercut in said slug holder.

21. The invention as claimed in claim 1, in which the means to cool said nozzle means comprises a plurality of circumferentially-spaced teeth extending radially-outwardly from said nozzle means and seated against an interior wall of said second means, said teeth being disposed radially-outwardly of the portion of said nozzle means forwardly adjacent said forward end of said slug, means to conduct water to an annular space formed forwardly of said teeth, whereby said water flows rearwardly through the gaps between said teeth, and in which means are provided to conduct water from the rear portion of said second body means to a drain.

22. Th invention as claimed in claim 21, in which an O-ring is provided between said nozzle means and said second body means at the forward portion of said second body means, and in which an undercut is provided in said nozzle means to interpose water between said O-ring and the nozzle passage through said nozzle means.

23. The invention as claimed in claim 1, in which said gas-injector ring includes gas passages so directed as to effect vortical gas flow around said slug.

24. The invention as claimed in claim 23, in which there are a multiplicity of such gas passages, each of small diameter.

25. The invention as claimed in claim 24, in which each of said passages is inclined forwardly relative to a radial plane perpendicular to said slug.

26. The invention as claimed in claim 1, in which said gas-injector ring includes a multiplicity of small-diameter gas flow passages circumferentially spaced therearound.