US3470347A - Method for shielding a gas effluent - Google Patents

Description

p 30, 1969 J. E. JACKSON METHOD FOR SHTELDING A GAS EFFLUENT 5 Sheets8heet 1 Filed Jan. 16, 1968 FIG.

FIG. #1

INVEN'IOR. JOHN E. JACKSON 5:9 9

ATTORNEY Sept. 30, 1969 Filed Jan. 16, 1968 PERCENT OXYGEN J. E. JACKSON METHOD FOR SHIELDING A GAS EFF'LUENT S Sheets-Sheet 2 mo MR E] ARGON 0.8m ANNULUS I0 --O ARGON 3.3m ANNULUS HELIUM 0.8 m ANNULUS FLOW RATE, CFH

INFLUENCE OF PROCESS PARAMETERS I ON OXYGEN CONCENTRATION.

FIG. 2

. INVENI'OR. JOHN E. JACKSON BYMM ATTORNEY p 30, 9 J. E. JACKSON 3,470,347

METHOD. FOR SHIELDING A GAS EFFLUENT Filed Jan. 1a, 1958 5 Sheets-Sheet :5

E] ARGON 0.8 m ANNULUS O ARGON 3.3m ANNULUS A HELIUM 'O.B|N2 ANNULUS 'Q'FLOW RATE ftlsec.

. A AREA n. 0.!

PERCENT OXYGEN P DENSITY abs/n.

O. Ol 9.00!

o l L L 1 0 2 CORRELATION OF OXIGEN CONTENT IN 'EFFLUENT 0F COAXIAL JET TORCH WITH PROCESS PARAMETERS.

' v INVENTOR.

JOHN E. JACKSON A 4 TTORNEY Sept. 30, 1969 PERCENT Filed Jan. 16, 1968 J. EvJACKSON METHOD FOR SHIELDING A GAS EFFLUENT' 5 Sheets-Sheet 4 CONDITIONS! v8" DIA. NOZZLE 45o CFH ARC TORCH GAS 15o AMPERES I INCH STANDOFF I000 CFH 2000 CFH 4 I I I I I I I I I I I I I 0.3 0.4 0.5 0.6 0.8 .l.0 L5 2.0 3.0 4.0 W/o, INCH IEFFECTIANNULUS WIDTH PARAMETER AND SHIELDING FLOW RATE 0N smsgpme IPERFORMANCE- WITH A I.O INCH.STANDOF F.

' 4 INVENTOR.

JOHN E. JACKSON ATTORNEY Sept. 30, 1969 J. E. JACKSON METHOD FOR SHIELDING A GAS EFFLUENT 5 Sheets-Sheet Filed Jan. 16, 1968 CONDITIONS! a" DIA. NOZZLE 450 CFH ARC TORCH GAS I50 AMPERES I/ INCH STANDOFF 2000 CFH V p INCH 2 r 50o CFH AIR Ewumwm ZQZEZWQZOQ 5856 563mm INFLUENCE OF SHIELDING FLOW RATE AND ANNULUS WIDTH PARAMETER 0N SHlELDlNG PERFORMANCE WITH A 0.5 lNCH STANDOFF.

lNVENTOR. JOHN E. .mcxsou United States Patent 3,470,347 METHOD FOR SHIELDING A GAS EFFLUENT John E. Jackson, Indianapolis, Ind., assignor to Union Carbide Corporation, a corporation of New York Filed Jan. 16, 1968, Ser. No. 698,268 Int. Cl. B23k 9/04 US. Cl. 219-76 2 Claims ABSTRACT OF THE DISCLOSURE A method is disclosed for producing a substantially oxygen-free coating on a substrate by the use of a plasma arc coating torch. The torch produces an arc plasma through a constricting nozzle orifice so as to provide a high velocity, high energy are eflipent which carrys the coating material to be deposited onto the substrate. The gas efiluent is protected from contamination by its surrounding environment by surrounding such effluent with a coaxial annular stream of gas having a width which is in the range of from 0.25 to about 4.0 times the square root of the diameter of the nozzle orifice, and which has a flow rate value for the square root of the longitudinal momentum flux that is greater than 2.0 lb. per sec. ft. as given by the equation (Q/A) wherein Q is the flow rate in ft. sec. of the annular gas stream; A is the annular area in ft. and p is gas density in lb./ft.

This invention relates to a method for shielding a gas effluent from the environment around such gas effluent and more particularly to a method wherein such gas efiluent is an arc efiiuent.

Gas flowing out of a nozzle, hereinafter referred to as gas effluent, has been used in many processes particularly in metal fabrication, welding, and coating processes. For example, oxy-fuel gas efiiuents with and without powder entrainment have been used to cut and treat metals. Gases have been used to shield electric arcs. Gases have also been introduced into electric arcs so that at least some of the gas becomes part of the arc and becomes an arc effluent. In many cases it is desirable to protect the gas or are efiiuent from contamination by the natural environment around the gas or are effiuent.

This invention in its broadest aspects relates to a novel method for preventing contamination of a gas efiluent by the natural environment around such gas effluent. Therefore, it should be understood that the teachings hereinafter set forth are applicable to all processes typified by the above referred to examples, while for sake of simplicity of describing those teachings and presenting a preferred embodiment of the invention the greater part of this specification will refer hereinafter to are coating processes wherein arc effluents are utilized.

One of the most useful of the arc coating processes is a process wherein an arc is established between two electrodes and a gas in introduced into such are and passed through a nozzle having a constricting orifice. The material to be deposited as a coating, usually in powder form, is introduced into the arc and carried to the work-to-becoated by the arc effiuent. This process is described in greater detail in US. Patent 3,016,447 issued to R. M. Gage et al.

While this process is eminently useful, there are some undesirable shortcomings. Arc coating or plating, as it is sometimes referred to, suffers from contamination of the arc efiluent by the surrounding air. As a result, the coating material becomes oxidized and produces an oxidized coating. When deposits of pure metal are made, the metallic coatings are porous, possess low ductility and are difficult to machine to accurate dimensions with good surface finishes. In addition, the chemical changes which 3,470,347 Patented Sept. 30, 1969 the hot entraned coating material undergo in the presence of the contaminates may alter the desirable physical properties of the coating material.

Accordingly, it is a main object of this invention to provide a method for minimizing contamination of a gas efiiuent with the surrounding environment.

It is another object to prevent the encironment surrounding an arc efiluent from being aspirated into said are efiiuent.

Another object is to provide a method for are coating wherein oxidation of the material entrained in an arc eifiuent is minimized.

A further object is to provide a method for producing substantially oxygen-free arc coatings on a substrate.

Yet another object is to prevent oxidation of powdered material entrained in an arc efiluent.

These and other objects will either become apparent or will be pointed out when referring to the following description and drawings wherein:

FIGURE 1 is a schematic diagram illustrating the concept of the invention;

FIGURE 1A is a partial cross-section view of the front and of typical torch apparatus for practicing the inven- FIGURE 2 is a curve illustrating the effect of the annular shielding gas stream flow rate on oxygen concentration in the gas efiluent being shielded for various annulus areas and shielding gases;

FIGURE 3 is a curve illustrating the effect of the parameters of the annular shielding gas stream on oxygen concentration;

FIGURES 4 and 5 are curves illustrating the effect of the ratio W/D on oxygen concentration at different standoff distances.

The objects of the invention are accomplished in a general way by a method wherein the gas effluent, which is to be protected from contamination by its natural environment, is surrounded with an annular shielding gas stream the width of which, measured in inches, should be in the range of from 0.25 to about 4.0 times the square root of the diameter of the orifice (measured in inches) in the nozzle through which said effluent passes. The gas flowing in the annular shielding gas stream is coaxial with the gas efiluent and the square root of the momentum flux of the annular stream is at least 2.0 lbs. /sec. ft as given by the equation (Q/A) p wherein Q is the flow rate in ft. /sec. of the annular gas stream; A is the annular area in ft. and p is gas density in lb./ft.

This invention is predicated on the discovery that an annular shielding gas stream having a width and forward momentum flux within limits herein defined and which has uniform turbulent flow will remarkably shield a gas effluent so that the oxygen concentration in the gas eflluent is essentially equal to that of the annular shielding gas stream. When the width of the annular gas stream is between 0.25 and 4.00 times the square root of the diameter of the orifice in the nozzle from which the gas effluent emerges, both measured in inches, and when the square root of the forward momentum flux of such gas stream is greater than 2.0 lb. /sec. ft. as defined by the equation (Q/A) 2 the Reynolds number (Re) of the gas flow is usually greater than 2000. While it is possible to practice the invention with a Re less than 2000, the corresponding marginal shielding performance achieved is improved by increasing Re while keeping the ratio W/D within the limits defined.

This invention is admirably suited to and useful for the shielding of an arc efiluent wherein a powdered coating material is entrained and carried in a heated state to a workpiece which is to be coated with such material.

In the process of arc coating an arc is usually established between a first and second electrode contained in an arc device. An arc gas is introduced into the arc to create an arc efiluent. The coating material is introduced into the arc and arc efiluent and is carried thereby to the work. Up until now the coatings achieved by such a process were usually oxidized. When the present invention was used with the arc coating processes described, the oxygen contamination was substantially eliminated so that deposits essentially as pure as the starting coating material were achieved. The following table clearly illustrates the capabilities of the invention:

TABLE I.-OXYGEN CONTENT OF ARC TORCH DEPOSITS Oxygen content (percent) As will be noted from the table, the oxygen content of deposits made with the invention is substantially and remarkably lower than that obtained with conventional coating, i.e., without coaxial jet shielding, and in most cases is equal to or even less than the oxygen content in the starting powder.

A schematic representation of typical equipment for carrying out the invention is shown in FIG. 1. Attached to a conventional arc torch device 1 of the type mentioned above and shown in US. Patent 3,016,447 is a coaxial jet shielding device 3 so that the arc efiiuent from the nozzle orifice 5 having a diameter D in torch 1 passes through the center of the device 3. The coaxial jet shielding device 3 is selected so that the width measured in inches of the annular shielding gas stream is between 0.25 and 4.00 times the square root of the dimension D measured in inches. In this case the width W of the device 3 is the width of the annular shielding gas stream. In FIGURE 1A the diameter D of device 3A is selected and correlated with the diameter D of the nozzle A orifice so that the width of the annular gas stream which in this case is falls within the range given below. It should be noted that in this embodiment of the invention, the inner surface 7A of device 3A does not restrict the jet discharging from nozzle orifice 5A but merely provides a passage through which the arc efiluent emerges from device 3A. However, if surface 7A were made sufiiciently small so that substantial constriction of the eflluent jet resulted, then the inner diameter of this constriction becomes the orifice diameter for computation of the width parameter.

FIGURE 4 indicates that at a 1 inch standoff distance, that is distance of the arc torch from the work, the effectiveness of the annular shielding gas stream increases for a given how rate up to a maximum or preferred ratio of about 1.2 in." and then begins to decrease until a ratio of about 4.0 inf is reached at which point the effectiveness is marginal. FIGURE 5 is a curve similar to FIGURE 4 indicating that the effects illustrated in FIGURE 4 are more pronounced when the standoff distance is /2 inch. It has been found that the invention herein described is useful in proceses where the standoff distance, measured from the end of the device, is up to about 40 to 50 times the nozzle orifice diameter.

The curve of FIGURE 2 illustrates that the molecular weight of the gas used has an effect on the resultant oxygen contamination of the arc effluent. It is evident that argon is more eifective than helium and that at the same flow rate, it is more desirable to use a smaller annulus and achieve higher velocities.

In addition to being dependent on the width of the annular shielding gas stream, successful shielding performance according to the invention is dependent on what is called herein the square root of the momentum flux and which I chose to define by the equation (Q/A)p The derivation of the term (Q/A)p is as follows:

If the term (Q/A)p were written as (Q /A W or as t ee- 1 one ends up with momentum flux to the half power since (Q is the weight flow rate, (Q/A) is the velocity and (l/A) is the area to the minus 1 power. In English units the weight density is divided by 32.2 to give a mass density. Under these conditions (Q/A) of 2.0 lb."*/ sec. ft. is equal to 0.352 slugs"/sec. ftf".

In FIGURE 1 the area A is the cross sectional area of the annulus through which the shielding gas passes. In FIGURE 1A the area is computed by 1r/4 [DJ-D FIGURE 3 illustrates that in order to achieve a significant reduction in oxygen content in the arc effluent the value of (Q/A) should be greater than 2 1b sec. ft.

The composition of the gas, of course, effects the momentum flux. FIGURE 2 indicates that argon gas for example is a better shielding gas for purposes of this invention than is helium.

In a preferred embodiment of the invention, an arc torch having a tungsten electrode and nozzle with an /8 in. orifice diameter was adapted to receive a coaxial jet shielding device of the type shown in FIGURE 1A having a diameter at D of 1 inch. The annulus in the shielding device is preferably covered with porous metal, screens or other material to insure uniform flow around the cross section without velocity disturbances caused by the gas inlets. The invention is most useful when uniform turbulent flow is achieved as opposed to smooth laminar flow. An arc is established between the tungsten electrode and a second electrode in the arc torch. Powdered coating material is introduced into the arc and carried to a workpiece in the arc effluent. Shielding gas, preferably argon, is introduced into the coaxial jet shielding device 3 or 3A to produce a flow rate capable of providing the necessary momentum flux.

If the coaxial jet shielding device has an annulus which is too wide, very high flow rates are needed to achieve the desired momentum flux and the process becomes less attractive commercially. On the other hand, if the annulus is too narrow, the annular shielding gas stream becomes too thin and unstables and does not perform its function of reducing the amount of gas entrained by the central gas efiluent. I have also found that the are conditions such as arc amperage and are gas flow rate have a negligible effect on the effectiveness of the annular shielding gas stream. For example, the arc gas was varied from 200 c.f.h. to 600 c.f.h. with very little change in the oxygen content in the arc efiluent.

Having described the invention, the following examples are illustrative of conditions falling within the scope of the invention and are provided to assist those skilled in the art in understanding how to practice the inventive concept.

Example I An arc torch having a tungsten electrode surrounded by a nozzle having /8 in. diameter and in. length was used as the coating device. Argon gas was introduced into the arc torch as an arc gas at a flow of 450 c.f.h. The are current was amperes at 78 volts. The torch standoff distance was /2 in. Nickel powder having an oxygen analysis of .172% was introduced into the arc torch at the rate of 24 g.p.m. (grams per minute). The rate of deposition of the powder on the substrate was 15 g.p.m. A coaxial jet shielding device similar to that shown in FIG. 1A was attached to the arc torch. The annular coaxial gas stream surrounding the arc eflluent had a width of 0.312 in. Argon gas was introduced into the device at a flow rate of 2000 c.f.h. to produce a value for the square root of the momentum flux of 60 lb. sec. ft. The oxygen analysis of the coating formed was .150%, a reduction of 022%. The coating was very clean with excellent machinability.

Example II Apparatus similar to that used in Example I was used in this test with the exception that the nozzle length was V8 in. Argon gas was introduced into the arc torch at a flow rate of 450 c.f.h. The arc current was 100 amperes at 80 volts. The standoff distance was /2 in. Titanium powder having an oxygen analysis of .651% was introduced into the arc torch at the rate of 20 g.p.m. The rate of deposition of the powder on the substrate was 13 g.p.m. The coaxial jet had a width of 0.312 in. Argon was introduced into the coaxial jet at the ilow rate of 2000 c.f.h. to produce a value for the square root of the momentum flux of 60 lbfi /sec. ft. The oxygen analysis of the coating formed was .730%. The machinering characteristics and ability to take a high surface finish was outstanding.

Example III All apparatus was the same in this example as in Example H. The nozzle length in this example was in. The are conditions were the same as in Example II. Molybdenum powder having an oxygen analysis of .419% was introduced at the rate of 33 g.p.m. into the arc torch. The rate of deposition was 20 g.p.m. The coaxial jet had a width of 0.312 in. Argon was introduced into the coaxial jet at the [How rate of 2000 to produce a value for the square root of the momentum flux of 60 lb. /sec. ft.". The oxygen analysis of the coating was .155. The reduction in oxygen was a remarkable 264%.

Example IV The conditions here were the same as in Example III. The arc current was 150 amperes at 80 volts. Tungsten powder having an analysis of .027 oxygen was introduced at the rate of .68 g.p.m. into the arch torch. The deposition rate was 51 g.p.m. The width of the coaxial jet was 0.312 in. and its square root of the momentum flux was 60 lb. ft. sec. ft). The oxygen analysis of the coating was .030%. The coating retained substantially the low oxygen level of the original tungsten powder making it more suitable for applications requiring high strength at room and high temperatures.

While the invention has been described with reference to certain preferred embodiments, it should be understood that certain modifications can be made thereto without departing from the spirit and scope of the invention.

What is claimed is:

1. A method for producing substantially oxygen-free coating on a substrate which comprises:

establishing an arc between a first and second electrode;

introducing an arc gas stream into said arc to create an arc plasma;

passing said arc plasma through a constricting nozzle orifice so as to provide a high velocity, high energy are eflluent; carrying coating material in said arc efiluent; surrounding said coating material carrying arc efliuent with an annular coaxial gas stream the width of which measured in inches is the range of from 0.25 to about 4.0 times the square root of the diameter of the orifice measured in inches of said nozzle;

introducing gas into said annular coaxial gas stream at a flow rate to cause said annular coaxial gas stream to have a value for the square root of the longitudinal momentum flux that is greater than 2.0 lb.' /sec. ft. as given by the equation (Q/A)p wherein Q is the flow rate in ft. sec. of the annular gas stream; A is the annular area in ft. and p is gas density in 1b./ ft

2. Process according to claim 1 wherein the width of the annular coaxial gas stream is about 1.2 times the square root of the diameter of the orifice in said nozzle when both dimensions are measured in inches.

References Cited UNITED STATES PATENTS 3,016,447 1/ 1962 Gage et al. 11793.1 3,071,678 1/1963 Neely et al. 117--93.1 3,075,066 1/1963 Yenni et al. 117-93.1 3,246,114 4/1966 Matvay 117--93.1 3,312,566 4/ 1967 Winzeler et al 117-93.l 3,313,908 4/1967 Unger et al. 21976 3,358,114 12/1967 Inque 219--76 3,387,110 6/1968 Wendler et al. 2l9--76 JOSEPH V. TRUHE, Primary Examiner W. D. BROOKS, Assistant Examiner US. 01. X.R. 111-934

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