AWS C5.1-73 Plasma Arc Welding.pdf

COPYRIGHT American Welding Society, Inc.Licensed by Information Handling ServicesCOPYRIGHT American Welding Society, Inc.Licensed by Information Handling Services

I AWS C5.1 7 3 m 07842b5 0002' i23 5 m

.- ~-~

AWS C5.1-73

RECOMMENDED PRACTICES

FOR PLASMA-ARC

WELDING

Prepared by AWS Arc Welding and Arc Cutting Committee

Under the Direction of AWS TechnicaI Activities Committee

Jay Bland Technical Director

AMERICAN WELDING SOCIETY, INC. 2501 N.W. 7th Street, Miami, Florida 33125

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Library of Congress Number: 73-88838 International Standard Book Number: 0-87 17 I - 107-9

American Welding Society, 2501 N.W. 7th Street, Miami, FL 33125

@ 1973 by American Welding Society. All rights reserved.

Note: By publication of these recommended practices, the American Welding Society does not insure anyone utilizing these recommended practices against liability arising from the use of such recommended practices. A publication of a code. standard, or recommended practices by the American Welding Society does not carry with it the right to make, use, or sell patented items. Each prospective user should make an independent investigation.

Printed in the United States of America

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AWS C5.1 73 M 0784265 0002425 _ 7 ~- W __

Contents

Foreri*osd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Pei-soiincl . . . . . . . . . . . . . . . . . . . , . . . . . . . , . . . . . . . . . . . . . . . . . . . . , . . . , vi

1 . Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Definitions of T e r m s , , . . , . , . . . , . , . . . . . . , . . . . . . . . . . . . . . . . . . . . . . 1

3. Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 6

1 1 11 11 13 14

3.1 3.2

Process Description . . . , . . . . . . . , . . . . . . . . . . . . . . . . , . . . . . . . . . Principles of Operation . . . . . , , . . . . , . . , . . . . . . . , . . . . , . . . . . . .

3. Equipment and Apparatus Requirements . . . . . . . . , . . . . . , . , . , , . . . . . Manual Welding . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . Mechanized Welding . , . . , , . , . . . . , . . . . . . . . . . . .‘. . . . . . . . . . . Powder Surfacing . . . . . , . . . . . , , . . . . . . . . . . . , . . , . . . . . . , . . , Hot Wire Surfacing . . . . . . . . . . . . . . . . . . . . , . . . . . . . , . . . , . . . .

4.1 4.2 4 . 3 3.4

5 . Application of the Plasma-Arc WeIding Process to Metal Joining , . . . . , . . , , . , . , . . . . . . I . . . . . . . . . . . . . . . . 15 5.1 General Areas of Application . , . , . . . . . , . , . . . . . . . . . . . . . . . , . 15 5.2 Base Metals . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . 16 5.3 Filler Metal Addition. , . . . . . , . , . . , . , . . . . , . . . . . . . . . . . . . . . . 16 5.3 Gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.5 Auxiliary Weld Shielding . . . . . . . , . . . . , . . , . . . . , . . . . . . . . . . . 18 5.6 Joint Des ign , . . , . . . . . . . :. . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . 19 5.7 Tooling Practices,. . . . , . . . . . . . . . . , . , , . , . . , . , . , , . . . . . , . , . 20 5.8 Manual Welding . . . . , . . , . . . . . . . . . . . . . , . . . . . . , . . . . . . . . . . Li-

5.9 Mechanized Welding, . , . , . . . . . , . . . . , . . . . . . . . . . . . . . . . . . . . 22 5.10 Multipass Welding.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . 42 5.1 I Reverse Polarity Welding . . . , , . . . . . , . , . . . . . . . . . . . . . . . . . , . 42 5.12 Reconmended Practices , . , . . , , . . . . . , . , . . . .’. . . . . . . . . . . . . . 42 5 .13 Advantages and Liinitations . . . . , . . . . . . . , . . . . . . , . . . . . . . . , . 42

39

6. Application of the Plasma-Arc Welding Process to Surfacing . , . . , . . . . . . . . . . . . . . . . , . . . , . . . . , . . . , . . . . . . 6. I General Considerations . . . . . . . . . . , . , . . . . . . . . . , . . , . . . . . . . . 6.2 Powder Surfacing . . . . . . . . , . . . . . . , . . . . . . . . . . . . . . . . . . . . . . 6.3 Hot-wire Surfacing , . . . . . , . , . . . . . . . . . , . , . . . . . . . . , . . . . . . .

46 46 46 52

7. Process Control . , . . . . , . , . . . . , . . . . , . . . . , . . . . , . . . . . . . . . . . . . . . . 7.1 7 .2 Joint Preparatiw and

54 . . . 54

54

General. . . . . . , . . . . . , . . . . . . . . , . . . . . . . . . . Tolerances . . . . . . . . , . . . . . . . . . . . . . . . . . . . . .


!

CON TENTS

7.3 Considerations for Welding Thin Metal Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

7.4 Controlling the Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 7.5 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 7.6 Inspection and Testing Methods, . . . . . . . . . . . . . . . . . . . . . . . . . . 61 7.7 Design Data and Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 7.8 Applicable Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

8. Training and Qualification of Welders and Welding Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

9. Safety Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

10. Practical Applications.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 10.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 10.2 Manual and Low-Current Plasma-Arc

Welding Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 10.3 Mechanized High-Current Plasma-Arc

Welding Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 10.4 Surfacing with the Plasma-.4rc

Welding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Appendix: Occupational Noise Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

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Foreword

Plasma-arc welding was introduced as a practical fabricating process approximately fifteen years ago. Early applications involved melting, cutting, and spray-coating metallic materials. In the past several years, rapid advances have been made in the development of this technique, and plasma-arc welding has now achieved acceptance as an efficient metal joining process.

During the recent period of progress, sufficient data have been gathered and organized to yield an authoritative source of technical information on plasma-arc welding. Accordingly, the AWS Arc Welding and Arc Cutting Committee has prepared these recommended practices through the work of the Subcommittee on Plasma-Arc Welding. These recommended practices are based on a survey of plasma-arc welding as used in the metal fabricating industry.

The description of plasma-arc welding and its salient features is presented here as clearly and concisely as possible. Because the plasma-arc process is similar to gas tungsten-arc welding, similarities in the processes are not described in detail, to keep the text as brief as possible. The plasma-arc welding process, however, does have unique features in operation and equipment that are advantageous for a certain range of metal thicknesses. These features are fully explored in the text.

The Committee developed these guidelines in the hope that they would lead to further development of the plasma-arc welding process and thus to higher quality and performance standards. Comments on this publication will be most welcome. They should be addressed to the Secretary, AWS Arc Welding and Arc Cutting Committee, American Welding Society, 2501 N. W. 7th Street, Miami. Florida 3312.5.

L -E


Person ne1

AWS Arc Welding and Arc Cutting Committee

R . D . Ilîciriri D . H . Mtrr-liri J . MI. Mir(-hell R . L. O'ßriívi E . R . P icwc L . J . Priiwmik

Hobart Brothers Company A mer ican We Id in g Society Kaiser Aluminum & Chemical Sales. Incorporated Coluinbin Gas System Service Corporation Detroit Diesel Allison Division. General Motors Corporation Army Materials and Mechanics Research Center Electric Boat Division. General Dynamics Corporation Arcos Corporation Penn Central Company General Electric Company Consolidated Edison Company ' Pittsburgh-Des Moines Steel Company Massachusetts Institute of Technology Esso Research and Engineering Conipan. The Budd Company Boe in g Co ni pan y International Harvester Company Sun Shipbuilding &( Drydock Company Westinghouse Electric Company Teledyne McKay Company Welding Products. Chemetron Corporation New pon N e w s S h i p bu i 1 ding and Drydock Company Airco, Inc. Dravo Corporation - Ford Motor Company Linde Division. Union Carbide Corporation hliller Electric Company S te ani Di vis i on . Westinghouse Electric Corpordtion A . O. Smith Corporation Airco. Inc. U . S. Kaval Ship Engineering Center Sun Oil Company U . S . Steel Corporation

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M. D. S t c p i t h E . P . Vilktrs R . Wtilkcv- D . V . Wi1c-o.r G. K . Wil lr t4c R . A . Wilsort F . J . Winsor.

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Arcair Company Astro- Arc Company Babcock & Wilcox Conipany Reynolds Metals Company Miller Electric Manufacturing Company Lincoln Electric Company Foster Wheeler Corporation

Subcommittee VI11 on Plasma-Arc Welding

G. A . LrCltrir. H . R . Mil lc~~* L . J . Prìiwzriik

1

Personnel

Detroit Diesel Allison Division. General Motors Corporation A nier i c an We 1 ding Soc i et y Aerojet General Corporation Thermal Dynamics Corporation Convair Division, General Dynamics Corporation Foster Wheeler Corporation Linde Division. Union Carbide Corporation Steam Division. Westinghouse Electric Corporation TAFA Division, Humphreys Corporation

vii


RECOMMENDED PRACTICES FOR PLASMA-ARC WELDING

1. Scope 1.1 These recommended practices present a description of the plasma-arc welding process and practical procedures as applied to joining parts and surfacing. These

. discussions apply to a wide variety of metals and represent methods used in industry.

2. Definitions of Terms* 2.1 General. Some of the terms used in describing plasma-arc welding are the same as those used in gas tungsten-arc welding. Other terms listed are peculiar to the plasma-arc welding process. Many of these terms are shown in Fig. 1.

2.2 Plasma arc. A plasma-a gas heated to a condition of at least partial ionization that is capable of conducting electric current-exists during any arc occurence. In nature, the gas ionized by a lightning bolt constitutes a plasma. The sanie ionization phenomenon occurs in welding arcs, carbon-arc lights. and arc furnaces. In recent years, however, the expression "plasma arc" has become associated with those processes eniploying a constricted arc. Arc constriction is brought about by forcing the arc to pass through a small nozzle or opening as it passes from the electrode t o the workpiece.

2.3 Plasma-arc welding. An arc welding process wherein coalescence is produced by heating Lvith a constricted arc between an electrode and the workpiece (transferred arc) or the electrode and the constricting nozzle (nontransferred arc). Shielding is obtained from the hot. ionized gas issuing from the orifice which may be suppleinonted by an auxiliary source of shielding gas. The shielding gas may be inrn or a mixture of gases. Pressure niay or may not be used, and filler metal may or may not be supplied.

2.4 Constricted arc. An arc column shaped by a constricting nozzle orifice.

2.5 Constricting nozzle. A water-cooled copper nozzle surrounding the electrode and containing the constricting orifice.

2.6 Constricting orifice. The hole in the nozzle through which the arc passes.

2.7 Double arcing. A condition in which the main arc does not pass through the constricting orifice hut transfers to the inside surface of the nozzle. A secondary arc is siniultaneously established between the outside surface of the nozzle and the workpiece. Double arcing usualIy damages the nozzle.

*For AH'S ternis iiiid definitions. refer to the latest edition of publication AWS A3.0. Terms and Definitions. Please note that sonie of the temi5 and definition5 used in this publication are not included i i i AWS A3.0. They are either new teniis defined after the last revision of A3.0 or they are used to clarify this pUb~ic;iI¡oiT.

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2 / PLASMA-ARC WELDING PRACTICES

E l e c t r o d e ,

S h i e l d i n

O u t e r Gas

T h r o a t L e n g t h

T o r c h S t a n d o f f

g Gas P1 enum

CUP Chamber E l e c t r o S e t b a c k

7 - 4 - n m ì c i r V I I I l b

F i g . I - Plusaiu-arc torch rerniitiokogy.

d e

e D iam

2.8 Electrode setback. The distance the electrode is recessed behind the constricting orifice measured from the outer face of the nozzle.

2.9 Keyhole. A condition in which the plasma column penetrates completely through the workpiece at the leading edge of the weld puddle. As the torch progresses, the molten metal, supported by surface tension, flows in behind the keyhole to form the weld bead.

2.10 Lack of fill. Slight and blending reduction of thickness at the toe(s) of the weld generally associated with keyhole-type welds.

2.11 Multiport nozzle. A constricting nozzle containing two or more orifices located in a configuration to achieve a degree of control over the arc shape.

2.12 Nontransferred arc. An arc established between the electrode and the constricting nozzle. The workpiece is not in the electrical circuit.

2.13 Orifice gas. The gas directed through the plenum chamber and constricting orifice to form the plasma column.

2.14 Pilot arc. A low-current arc established between the electrode and the nozzle to ionize the orifice gas and facilitate starting the main welding arc.

2.15 Plenum or plenum chamber. The space between the inside wall of the constricting nozzle and the electrode.

2.16 Shielding gas. A protective gas provided at the outer periphery of the arc to prevent air contamination of the weld and/or base metal.

2.17 Single port nozzle. A constricting nozzle containing one orifice, located below and concentric with the electrode.

2.18 Throat length. The length of the constricting orifice.


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AWS C5.L 73 m 0784265 0002432 b W

Fundainetituls I 3

f

2.19 Torch standoff. The distance from the bottom of the constricting nozzle to the workpiece.

2.20 Transferred arc. An arc established between the electrode and the workpiece.

3. , Fundamentals 3.1 Process Description

3.1 1 General Description 3.1.1.1 The distinguishing feature of the plasma-arc welding process lies in

the use of a constricting orifice. Arc constriction by a nozzle brings about several changes in arc characteristics. The most important of these is that the arc can be projected as a stream of ionized gas similar to a water stream from a hose nozzle, with strong directional stability.

3.1.2 Arc-Constricting Nozzle 3.1.2.1 A wide variety of nozzles has been made and evaluated. These

include the single port nozzles and multiport nozzles with holes arranged in

S i n g l e Port Nozzle

i f i c

G a s

Multiport N o z z l e

e-


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AWS C 5 - L 73 W 0784265 0002433 8 ~~~ ~- - -~


circles. rows, and other geometric patterns. The single port nozzles are most widely used. Among the multiport nozzles, the most widely used design is the one in which the center orifice is bracketed by two smaller ports, with a common centerline for all three openings. These more common nozzle types are shown in Fig., 2.

3.1.2.2 The electrode i n the plasma-arc torch is recessed in the arc-constricting nozzle. As the arc passes through the nozzle, it is collimated and focused so that the arc heat is concentrated on a relatively small area of the workpiece. This increased heat concentration. coupled with the characteristically more forceful plasma stream, can produce a narrower weld fusion zone in a certain range of metal thicknesses.

3.1.2.3 With the single port nozzles, the arc and all of the orifice gas pass through the single orifice. With the multiport nozzle shown in Fig. 2. the arc and some of the orifice gas pass through the larser center orifice, while the remainder of the orifice gas is discharged through the two smaller ports that bracket the center orifice. The effect of the gas flou from the side ports is to squeeze the cross section of the circular plasma-arc column into an oval or elongated shape. This is particularly desirable in keyhole-mode Lvelding.

3.1.3 Keyhole-Mode Welding 3.1.3.1 In plasma-arc ivelding of certain nietal thicknesses special

combinations of plasma-gas flou.. arc current. and \veld travel speed will produce a relatively small weld puddle with a hole penetrating completely through the base metal at the leading edge of the \veld puddle (called the keyhole). The plasma-arc process is the only gas-shielded ivelding process with this unusual characteristic.

3.1.3.2 In a stable keyhole-mode operation. molten metal is displaced to the top bead surface by the plasma stream (in penetrating the plate) to form the characteristic keyhole. As the plasma-arc torch is mechanically inoved along the weld joint. metal melted by the arc is forced to tlow around the plasma stream, along the molten side surfaces of the keyhole and t o the rear where the weld puddle is formed and solidified. This niotion of molten metal. and the complete penetration of the metal thickness by the keyhole allow gases and impurities to flow to the surface or be expelled more readily before solidification. This action is similar to "magnetic stirring" developed for gas tungsten-arc welding. The maximum iveld puddle voluine and the resultant root surface profile are largely determined by the effects of a force balance between the molten weld metal surface tension and the plasma stream velocity characteristics. The bead appearance for a butt weld made in the keyhole mode is pictured in F'ig. 3.

3.1.3.3 With appropriately designed weld joints, the multiport nozzle shown in Fig. 2 can be used to advantage. When the multiport nozzle is aligned to place the common centerline of the side ports perpendicular to the weld groove. the arc is elongated in line with the joint. This allo~vs an increase of keyhole-mode welding speeds of from 30 to 50% over those obtained with single port nozzles, without undercutting and with welds having narrower fusion and heat-affected zones.

3.1.4 Comparison of Nonconstricted and Constricted Arcs 3.1.4.1 Typical electrical and thermal differences between constricted and

nonconstricted arcs are shown in Fig. 4. The schematic representation of a


AWS C 5 - L 73 87892b5 0002434 T -- -

Fundamentals I 5

Fì,q- 2<i - Picroricrl rcywesetitcitioti of the keyhole effect.

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~~~ ~~~~ AWS C5.1 ~~- 73 m 0784265 0002435 1 m - 6 / PLASMA-ARC WELDING PRACTICES

nonconstricted gas tungsten-arc is shown on the left side of the figure. Arc conditions are 200 amperes (A) at 15 volts (V) with 40 cfh of argon. The right side of Fig. 4 is a schematic view of a tungsten-arc that has been constricted by passing it through a 3/16 in. diameter orifice. Note that, with the same current and gas flow; arc voltage doubles to 30 volts. Note also that the very high temperature zones of the arc have been projected downward where they can be directly applied to the workpiece. Constricting the arc tends to concentrate the arc energy over a smaller area of the workpiece because the constricted arc does not have the angle of divergence of the nonconstricted arc,

[Ca thode ( - 1

N o n c o n s t r i c t e d Arc C o n s t r i c t e d Arc

40 c f h , Ar 200 A 1 5 V

3/16 i n . diam o r i f i c e

- 14,000

- 18 ,000

- 24 ,000

- U P

3.1.5 .Arc Modes 3.1.5.1 Two arc modes are used in plasma-arc welding: transferred arc and

nontransferred arc. With a transferred arc, the arc is established between the workpiece and the electrode within the torch. With a nontransferred arc, the workpiece is not in the arc circuit; the arc is established between the constricting orífice and the electrode inside the torch. Transferred arcs have the advantage of greater energy transfer to work, but they require an electrically conductive workpiece. Nontransferred arcs are useful for cutting and joining nonconductive workpieces or when lower energy concentration is desirable. Figure 5 illustrates the two modes of arc transfer.

3.2 Principles of Operation 3.2.1 Manual Plasma-Arc Welding

3.2.1.1 Manual plasma-arc welding was developed to obtain a stable, controllable arc for welding thin gage metal. It combines a continuously operating pilot arc within the torch for positive welding current initiation, and transferred arc


1 Tran

,-Orifice Gas

9 o n t r a n s f e r r ed Ar c .red Arc

Fi.?. 5 - .Ilo</c.c ojph is i i t i i p v t m i t i o i t : It$. t t w t i s j ~ v w d LII 'C; right. riortt~iiii.\fi.rr.r</ ciri..

constriction t o provide a stable arc at currents as low as 0. 1 ampere. The manual process is uell adapted t o welding metal thicknesses up to 0.125 in. The lwv-current volt-ampere (VA) characteristics of the plasma arc operating in argon are shoivn i n Fig. 6 . Note that the slope ofthe lokv-current plasnin-arc VA curve is essentially tlat in this range.

3.2.1.2 A needle-like plrisnia jet is obtained by passing a lmv-current arc through a small diameter orifice. The lon.-current plasina jet has good directional properties and heat concentration. exkllent arc stability. and reduced sensitivity to variat ions in torch-to-ivork spacing.

3.2.1.3 A typical electrical circuit for lo\+.-current manual plasma-arc Lvelding is shoun in Fig. 7. Note that the sj'sterii utilizes two separate power supplies-one for the pilot arc and one for the transferred arc.

3.2.2 Mechanized Plasnia-Arc Welding 3.2.2.1 Mechanized plasma-arc Lvelding utilizes a transferred. constricted

arc and is generally operated Lvith high current. Figure 8 illustrates the general characteristics of a typical high current plasma-arc Lvelding system. as kvell a s the electrical circuit used. A direct current (de) poiver supply is used to furnish the welding current. and a high frequency generator is used to initiate a pilot arc betu-een the electrode and the constricting nozzle. A current-limiting resistor and condenser netv,di conipletes the electrical circuit to the power supply.

3.2.2.2 As the orifice gas passes through the torch to the work, i t is heated by the arc. expands and passes through the arc-constricting nozzle at an accelerated rate. The plasiiia-gas flow range usable for welding is liiiiited because too forcefuf a plasma stream can cause plasma cutting. This orifice-gas tiou. alone is not adequate to protect the molten puddle froiii atniospheric contamination. Auxiliary


AWS C5.L ~~

8 I PLASMA-ARC WELDING PRACTICES

shielding gas is needed and is provided through an outer gas nozzle on the torch. See 5.12 for typical orifice- and shielding-gas flow rates.

3.2.3 Powder Surfacing 3.2.3.1 This fully mechanized application of the plasma-arc welding process

utilizes a constricted arc torch and filler nietal in powder form. The application is shown schematically in Fig. 9. In addition to the orifice- and shielding-gas supplies used in plasma-arc welding. the surfacing application requires a third gas flow to convey the powdered filler metal from a storage hopper to the torch. Argon is generally uskd for all three gas systems.

3.2.3.2 The gas-borne powder is introduced into an annular powder chamber in the torch that uniformly distributes the powder into the arc immediately beneath the arc-constricting orifice. After entering the arc. the powder particles are heated and deposited on tho workpiece. These particles are completely melted upon contacting the u.eld puddle. thus forming a hmiogeneous deposit, fusion-bonded to the workpiece.

.

50

4 0

30

>

2 0

10

Plasma-arc l e n g t h = 0 . 2 5 i n .

( 0 . 0 3 0 ' i n . diam n o z z l e )

Gas t u n g s t e n - a r c l e n g t h = 0 . 0 5 i n .

- 0 . 0 4 0 i n . diam e l e c t r o d e

O 2 4 6 8 10

.+:


1-

Frrtidutzrerituls 1 9

I I I I I I l l I I I l I I I 1 I I I I l 1 I l l

Cu C o

r n

r e n t t r o l

I I l 4 C o n t a c t o r I

I I I E l e c t r o d e l I I I I O r i f i c e r

S h i e l d

lûu t e r ! N o z z l e I I I

I I N o z z l e I

( - )

Ga s

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Cool i n 9 Water

S h i e l d i n g Gas

O u t e r Gas Nozzle


AWS C5.L 73 W ~- 0789265 0002437 7 W


I G T u n g s t e n E l e c t r o d e

I 111 O r i f i c e Gas

3.2.4 Hot-wire Surfacing 3.2.4.1 In the application of the plasma-arc welding process to hot-wire

surfacing. filler nieta1 in wire form is resistance heated and deposited on the workpiece in the puddle formed by a pIasnia-arc torch. This application is shown schematically in Fig, 10.

3.2.4.2 The electrical circuit and apparatus used for plasma-arc surfacing are similar to the requirements for plasma-arc welding.

D - C Plasma-Arc Power S u p p l y

1 Plasmó-arc F i 11 er, Metal

m a - A r c P P l Y


4. Equipment and Apparatus Requirements 4.1 Manual Welding

4.1.1 General Requirements 4.1.1.1 A complete system for manual plasma-arc welding consists of a

torch, control. power supply, orifice- and shielding-gas supplies, source of cooling water. and accessories such as foot switch. rheostat, timers, and remote current controls. Apparatus is presently available for operation with low welding current of O. i and high of 100 amperes. Development of higher range torches and power supplies has begun.

4.1.2 Power Supply 4.1.2.1 Transferred Arc Power Supply. Rectifier type power supplies

ivith a drooping VA characteristic are generally used for plasma-arc welding. 4.1.2.2 Pilot Arc Power Supply. Pilot arc supplies are preset to deliver

approximately 5 A continuously to provide a stream of ionized gas for instant ignition of the main welding current.

4.1.3 Control Unit 4.1,3.1 Incorporated in the control unit are the main power supply, pilot arc

supply. flowmeters, solenoid valves for controlling orifice and shielding gases, and \vater flow. Indicating current meters. control switches, and connections for re mot e current control are ge neral 1 y a vai 1 a bl e.

4.1.4 Torches 4.1.4.1 Low current plasma-arc torches are lightweight and designed

priixirily for manual tvelding. A supply of cooling water at a flow rate of about 1 qt/inin and 30 psi input pressure is necessary to dissipate heat generated in the constricting nozzle b! the pilot and main welding arcs. The tungsten electrode is automatically centered in relation to the constricting orifice by a ceramic bushing. Auxiliary shieldiiig gas is supplied through a separate gas system, Sonie manual plasma-;u-c yelding torches are available for operation on direct current straight pciltirity (dcsp) at cuirents up to 225 amperes. Standard torch holders can be used to inount the torch for iiiechanized use.

4.1.5 Filler Rletal 4.1 .S.'1 Filler inetnl can be added during nianual Lvelding in the same manner

;I> during gas tungsten-arc \veldiiig. \Vire diaineters should be of suitable size for thc cLIn'en~s used. Sce 5.3.

3.1.6 Gases 4.1.6.1 Scc 5.3.

4.2 Xîechanized M'elding 4.2.1 General Requirements

4.2. I . I Mecha -¡zed equipment must be used to achieve the welding speed and penetration iidvaiitagcs itssociated \vith high current plasma-arc welding. A mechanized insiullution consists of a power supply, control unit, welding torch. torch stand or carriugc. cooling \vater pump. high-frequency generator, and a


supply of orifice and shielding gas. Accessory units such as un arc-voltage control lind filler metal feed systeiii iiiiiy be used as required.

4.2.2 Power Supply 4.2.2. I Direct current pouw supplies with a drooping V A chnracteristic ure

used for plasma-urc Lveldiiig. Rectifier-type units arc preferred over motor generators for this service because rectifier output is less sensitive to variations in ope rii t i n g te i n pe rat LI re . ,4 i so . au to ni at i c c urren t s 1 op i ng is s i nip I e \vit h rec t i fi e rs . Rectifiers \vitIl ;in open circuit voltage i n the range of 65 to 80 V ( ¡ .e . . those used for gas tungsten-arc uelding) arc satisftictory for plasma-arc welding Lvith argon or ;i11 itrgoii-h!-drogen gas iiii.rture containing up to 7% hydrogen. Hon.ever. if helitiiii 01: ;in argon-hydrogen gus iiii'rturc coiitiìiïiiiig inore than 7 % hydrogen i s used. additionul open circuit voltage i s required for reliuble arc ignition. This nia! be obtained b!- connecting t \vo power supplies in series. An tilternate :ipprouch requiring the use ofonl!. one pou.cr suppi! is t o strike the arc in pure argon and then ~iutoiiiatically sxitch over to the desired iirgoii-hFdrogcn inirture o r heliiini for the \\.elding operutiim. P o u w supplies with current slope control ;ire required for \\.elding circuiiifereiitial joints where the keyhole niiist be initiated iinci closed out g r d u~i l l j . . Recent dc \'e I op nicn t \\-orl\ i ii pi a m ;i- arc \{.el ci i ng al u ni i n uni an ci iiiagnesiuiii alloj s indicate3 that the use of iin ~iltcrnatiiig-curreiit (u-ci p o \ w suuppl!. uith a continuous high fi-eqiienc!- iirc can produce satisfactor! v. c.lcl.4.

4.2.3 Control Systeni 4.2.3. I A typical control s"steiii for iiiechanized high current plnsiiiu-urc

\velding consists o f n iiiain console control. iijunction box for gas and ivatcr hoses. u u-elding operator's penduiit control box. and a high frequencj- generator.

4.2.3.2 The mniii control uni t sequences the pou.er supply. high frc.qiic'nc*! generator. orifice. shielding Lind backing gases. torch travel. ausili:ir! nire fecding. and cooling water for the torch. Travel speed and ivire feed rates are CO n t r o I led b j- e I ec t ro II ic go ve rno rs ..

4.2.3.3 Flo\vmeters ;ire provided for nietering the orifice. shielding. aiicl backing gases. in addition. timers and ;i gas-tapering device are provided for up- and doi\m-slopiiig of the orifice gas tlou. An adequate supp1~- of cooling vat ter for the torch electrode and constriciting nozzle is provided by a piinip ivith about 6 qt/iiiiri capacitJ-. A tlou s\\.itch and interlocking circuitry assure that cooling \vater i s tlrn+iiig to the torch bcfore arc ignition can take place.

3.2.3 Torches 3.2.4. I M ec h tin i zed p I asni a- arc u'c I ci i n g torches are a va i 1 a b I e co ni nie rc i ;i I I !

for operation on either dcsp o r dcrp (direct current reverse polarity) ut currents up to 500 anipercs. Straight polarit!. pou-er is used u.ith a tungsten electrode for most u.clding applications. Reverse polarity is used to a liniited extent \vit11 \~-atei--cooled copper o r tungsten electrodes for \veldhg riluminuin. Reverse polarity is :dso used \ \ - ¡ th specinll!- designed torches and copper electrodes for joining titaniiini and zirconium sponge compacts u.here freedom froiii tungsten o r copper contaniinat ion is a prime consideration.

4.2.4.2 A \vater-cooled power cable is used to bring pnver and cooliiog \valer into the torch: other hoses arc provided for orifice-gas input. shielding-gas

\-


input. and water return. The electrode holder in a plasma-arc welding torch is designed to center the electrode very accurately with respect to the central port in the nozzle. Misaligninent of the electrode with the central port tends to cause iiielting of the copper nozzle near the orifice and to shorten its life.

4.2.4.3 The reíatively low orifice-gas flows used for welding do not provide dequate protection for the iiiolten weld puddle. Accordingly, auxiliary shielding gas is supplied through un outer gas nozzle assembly on the torch. For some ap p I i c a t io ii s, add it ion al trai I in g -gas sh ie 1 d s ni ay be rey u i red.

4.2.4.4 A variety of nozzle designs exists for different welding applications. The dianieter of-the nozzle's orifice used in a particular application depends on the \vcId¡iig current to be used. Higher currents require larger dianieter orifices.

.

4.2.5 Wire Feeders 42.5.1 Conventional auxiliary wire feeding and hot-wire systems can be

used Liith the plusina-arc welding process. 4.2.6 Gases 4.2.6.1 See S.4.

4.3 Powder Surfacing 4.3.1 General Requirements 4.3.1.1 All installntic~iis consist of a dual power supply. surfacing torch.

control box. pondci--dispenser uni t , plumbing box. high-frequency generator. torch oscillator. cooling-tvater puinp. and reinote-control pendant. 4.3.2 Power Supply 4.3.2.1 The diiol po\r-er supplj- used for surfacing houses both the

iioiitïntisfcrrcd tind transferred iirc p v e r sources (see Fig. 9). Both sections have drooping V A cliaractcristics siniilar to those used in gas tungsten-arc welding. The riontransferrcd arc section is connected betu-een the tungsten electrode and the ;irc-coiisti'ictiiig orificc. Its function is to niuintain the 41-75 A nontransferred pilot arc usecl to initiate thc iliain ;ire and. in some applications. to supplement the heat of the i l inin transfc'n'ccl arc. The pilot arc is started by a high-frequency spark. The transferred arc po\\-ci* source is connected bet\vcen the tungsten electrode and the i t orkpiece to supplj iiinin arc currents betiveen 2 0 and SOO aiiiperes. Both pouer siippl ics ;ire coiiiic'ctcd ïor dcsp operat ion. 4.3.3 Control Unit 4.3.3,l The control unit contains relays and timers for proper sequencing of

clcctricul func-tians aiid electronic governors t o control the powder-dispensing rate ;incl ciirrkige trnvcl spccd. Flou,iiieters ;ire provided for nietering orifice-. shiclding-. m d p('~vder-dispeiising gas tlo\v rittes. and a water-flow sLvitch is prcnkied for regulating the tloti- of cooling itxter to the torch. A reimte pendant hou is provided s o thc n-elding operator ciin control starting and stoppins functions i n close proziiiritj to thc u w k . 4.3.4 Torches 4.3.4.1 The torch incorporates LI standard tungsten electrode, an

m-constricting orifice. ¿i nie;ins of dircctiiig gas-borne surfiicing powders into the arc. ii nieans of directing shielding gas iiround the \veld puddle. and provisions for


\\-atei' coaling the cciiisti-ictiiig cirificc aiici other critical i\re;is. The arc-constricting orifice is l u r~c r than that used for ivclding and ranges b e t ~ ~ e n 1/8 in. iintl 3/16 in. i11 dia i i i~ t~ i ' . A htLì\.! -tl~t!. 1 1 0 ~ 1 ~ ;issciiibl>- is tiïiìilLibit foi- SOIIE iìpplic~ìtitiiis. 4.3.5 Powder Feeders

4.3.5.1 Po\\.dcrs arc stored :incl nietered fro111 il dispensi11g un i t and Iraiisportccl to tlic siirtiiciiis torch i n ;i strcuiii of argon. Poudcr fio\\. rates arc ~l~l. i~l~tetl b! YN! iiig t l i ~ s p ~ ~ d of ;I i.Otiititig clispetising druiii and the height tif the LI i s p ~ i i ~ i tig i i ~ z t l ~ ;I boïc t hc c l r ~ 111 .

4.3.6 Gases 4.3.6.1 Argon is gc.ncrnli>- used for ali three systems.

4.4 Hot-W'ire Surfacing 4.4.1 General Requirements

3.A.1.1 The surt'acing system consists of a control panel assembly. plasiiia-urc po\ver suppi!-. hot-wire power supply, plasma-arc torch, two hot-wire guns with a trailing shield. an oscillator. and a voltage control head.

4.4.2 Power Supply 4.4.2.1 Plasma-Arc Power Supply. The plasma-arc power suppl!- is a

conventional direct current unit with provisions for remote current adjustment. Control of the plasma-arc current is provided by a "raise-lower" toggle slvitch on the pendant box.

4.4.2.2 Hot-wire Power Supply. Pomw for the hot wires is provided bl- a special a-c constant potential supply. Its operating voltage is belou. the arc threshold \\.hich prevents an inadvertant sustained arc between the wires.

4.3.3 Control Unit 4.4.3.1 The control cabinet for hot-\i.ire surfacing with the plasma-arc

welding process ccintains the preset control elements that generally do not need to be monitored during a surfacing operation. These controls include the preflow and postflow timers. the \vire dela> tiniers. the \veld finish sequence tinier. the tlou.nicters for the plasma-torch orifice :tiid shielding-gas flou-, the trailing shield-gas flou.. Lind the crater-CiII gas flow. The governor controls for the oscilintoi' atid wire f e d iiiotors. the arc voltagc control. and an arc time meter are d s o n~ciuiited i n the control cabinet. A high-freqiienc!. generator for starting the p1;isni:i arc. go5 and [vatci' solenoids. metcr shunts, and service connections are housed in ii separate junction bus. A pendant box is provided so the welding operator can position the w i d i n s head aiiú start or stop the ueld c!rlc from a re riicitc locut ion. 4.4.3 ' Torches

4.4.4.1 The plusnia-arc- torch. hot-\virc guns. oscillator. arc-voltage control heaù. and \vire feeders arc assembled as a self-contained unit Lvhich can be niounted o n it boOiii or other type of fixturing.

4.4.4.2 A single hot-uirc fccding system may be used. The plasiiia-arc torch provides heut to fuse the deposit to the base nietal. The hot-ivire guns supply power for resistance heating of the fillcr tiietal and also serve as guides to direct the \vires


Appiicwriori t o Metal Joining I 15

into the puctdlc. Running adjustinents are provided to control the position of the \\.ires relative to the puddlc.

4.4.4.3 A irailiiig shield provides inert gas coverage for the weld puddle and scrvcs ;is ;i iiir>unting bracket for the hot-nire guns. The assembly is designed to perinit removing the torches for servicing without disturbing ivire adjustments.

4.4.4.4 Cooling ~ . o t c r is required for the plasnia-arc welding torch. the hot-ivire gins- iiiicl the trailing shield. The cooling water requirements are tivo gallons per iniiiutc at 100 psi. A recirculating cooling uni t or any other suitable water ~ i p p l y niay be used,

4.4.4.5 Oscillator Assenibly. The oscillator assembly serves as a mount for the torcfi assenibly and provides the transverse motion required for surfacing. The oscilluticin frequency is udjustable up t o 80 cycles per minute, and the stroke is ad-justable bctLveen 3/3 in. and 2 1/2 in.

4.4.4.6 Arc-Voltage Control-Head Assembly. This component is a rigid, niotorized slide controlled by a servo-system to niaintain constant plasma-arc voltage. hence constant torch-to-work distance. This system assures uniformity of deposit despite variations in vessel roundness, etc. Manual jogging of the slide provides inotorized positioning of the torches in the vertical direction.

4.4.5 Wire Feeders 4.4.5.1 The two wires are driven by a dual-wire feeder through standard wire

feed ;iccsssories. Wire supply iiiountinp accommodate either wire spools or c o k Tu-o rotary. motor-driven straighteners are used to remove helix and cast from the \\.ires. These wire straighteners are necessary to niaintain a constant relationship in the \vire entry into the ueld puddle and assure uniformity of deposit over long cIperuting cycles.

4.4.6 Gases 4.4.6.1 Three separate gas fi0143 are used for hot-wire surfacing with the

plasnia-urc wlciiiig proccss. A 55 cth flou- of ;t 75% heIiurti-3% argon mixture pnsses thrciugh the plasmu-arc torch orifice t o support the transferred arc. Thirty to Sixt!. c i l of iirgon prisses through the torch shield to protect the arc zone. Another í l o ~ of 30 to 60 cth of iirgrin is used in the trailing shield assembly t o protect the surfucing. deposits.

5. Application of the Plasma-Arc Welding Process to Metal Joining

5.1 General Areas of Application 5.1.1 The plasnici-arc welding process is applicable to nietals weldable by the

gus tungsten-arc \vclcling process. 5.1.2 For platc thichncsses i n the 114 in . range. square-groove welds are made

i n butt joints without filler nietal addition at up to tkvice the travel speed of the gas tungstcn-arc Lveldiiig process. tvith the keyhole inode of.the plasnia-arc welding prc~cess. Table 1 illustrates the mdding cpecds typical o n stainless steel tubing. At thicknesses î'roiii 1/16 i n . donm to 0.002 in., the low-current nonkeyhole niode of


I6 / PLASMA-ARC WELDING PRACTICES

Table 1 -Typical plasma-arc welding speeds for welding stainless steel tubing

Wall T h i c k n e s s ( i n . ) Plasma-Arc Welding Speed ( i p r n l

0.109 36

0 . 1 2 5 36

0 . 1 5 4 36

0 . 2 1 6 1 5

O. 237 1 4

the plasma-arc welding process is applicable. Speed is no longer an advantage for these thicknesses but the other advantages associated with the process remain.

5.2 Base Metais

5.2.1 The plasma-arc welding process is applicable to the wide range of metals considered weldable by other processes. The fully-killed carbon and low-alloy steels, including AIS1 4130 and 2 1/4 Cr-i Mo, are readily joined as are most austenitic, martensitic, and ferritic stainless steels. The weldable aerospace superalloys in the Hastelloy* series and Inconel** 7 18, special application steel alloys such as DóAC, the 9 Ni-4Co series, the 18% Ni maraging group, and titanium and its weldable alloys such as 6 AL4V are also weldable by the plasma-arc welding process.

The metallurgical effects the process has on metals are not different from other processes, and appropriate procedures that preserve metallurgical integrity must be followed. Preheat, postheat, shielding, and gas selection are similar to those used with the gas tungsten-arc welding process. Metals joined may determine the gas mixtures used, particularly in the case of hydrogen additions to orifice and shielding gas, where the quality of the welds in titanium and low-alloy steels would be severely degraded. (The tables under 5.12.3 list welding conditions for a variety of metals.) There is limited information on the application of the plasma-arc weldin. process to aluminum and magnesium alloys. Some success has been achieved in producing satisfactory welds by special system setups using an a-c power supply and continuous high frequency superimposed upon the arc.

5.3 Filler Metal Addition 5.3.1 For nonkeyhole mode welding, filler metal can be added to the leading

edge of a plasma-arc weld puddle in the same manner as with the gas tungsten-arc welding process. Hot-wire systems feed wire into the trailing edge of the weid puddle for filler passes. Wire height adjustments are not so critical with plasma-arc welding because the wire can be lifted off the plate and melted into the plasma stream without contaminating the electrode.

E

*Tradernark of Stellite Division. Cabot Corporation. **Tradematk of International Nickel Company.


~~

000244b b -

Applicution to Metal Joining / 11

5.3.2 For the keyholing method, filler metal can be added to the leading edge of the puddle formed by the keyhole. The molten weld metal will flow around the keyhole to form a reinforced weld bead. Depending on fitup and bead contour requirements, this technique may be used on single pass welds in butt joints with a square-groove joint preparation in metals up to about 114 in. thick. On heavier sections, a joint preparation is selected that will alIow the plasma jet to melt the maxiinum amount of base metal supportable by surface tension. For this reason, filler metal is generally not added in making the root pass of a multipass weld.

5.3.3 Keyhole-mode circumferential welds in butt joints with square-groove joint preparation require close control of timing and slope rates for arc current and orifice-gas flow during keyhole initiation and withdrawal, The addition of filler metal in making such welds may complicate the keyhole withdrawal operation and niay be undesirable for such applications.

5.3.4 The type of filler metal used is determined by the metallurgical factors involved and is based on recommendations and uses developed for other processes.

5.4 Gases 5.4.1 Argon is the preferred orifice gas for low-current plasma-arc welding

because its low ionization potential assures reliable starting and a dependable pilot arc. Since the pilot arc is used only to maintain ionization in the plenum chamber, pilot arc current is not critical and can be fixed for a wide variety of operating conditions. In commercial units, the recommended orifice-gas flow rates are less than 1 cfh, and the pilot arc current is fixed at five amperes.

5.4.2 The shielding gas provided through the gas nozzle can be argon, an argon-hydrogen mixture, or an argon-helium mixture, depending on the welding application. Shielding gas flow rates are usually in the range of 20 to 30 cfh for low curren t application s.

5.4.3 Various percentages of argon-hydrogen mixtures can be obtained by blending argon and cylinder premixes of 5% hydrogen in argon (H-5) or 15% hydrogen in arson (H-IS). The required ratios can be obtained by using the flowmeter settings shown in Table 2.

5.4.4 The choice of gas to be used for plasma-arc welding depends on the metal to be welded. In all but a few cases. the shielding gas is the same as the orifice gas because variations in the consistency of the arc effluent would be inevitable if two different types of gas were used. Although argon is suitable as the orifice and shielding gas for welding all metals, it does not necessarily produce optimum welding results, As in gas tungsten-arc welding, additions of hydrogen to argon produce a hotter arc and efficient heat transfer to the workpiece. In this way, higher welding speeds are obtained with a given arc current. The amount of hydrogen that can be used in the mixture is limited because excessive hydrogen additions tend to cause porosity or cracking in the weld bead. With the plasma-arc keyhole technique, a given metal thickness can be welded with higher percentages of hydrogen than are possible in the gas tungsten-arc welding process. The ability to use higher percentages of hydrogen without inducing porosity may be associated with the keyhole effect and the different solidification pattern it produces.

F


~ AWS C 5 - L - 73 0784265 0 0 0 2 4 4 7 8 W

18 / PI.AS!ilA-ARC WEL.I>ING PRAC'I'IC'ES

Table 2 - Flowmeter settings for argon-hydrogen shielding gas mixtures - - _

F ì o w n i e t e r S e t t i n g s ( 1 S c a l e S c a l e A r g o n ( A r - H ) M i x

c f h c f h

2 0

i 6

1 2

1 0

8

4

o

O

4

8

1 0

1 2

1 6

2 0

P e r c e n t a g e o f H y d r o g e n ( 2 )

F o r F o r H-5 M i x H-15 M i x

il

1

2

2 1 / 2

3

4

5

O

3

6

7 1 / 2

9

1 2

1 5

Note : ( 1 ) 2 0 c f h t o t a l s h i e l d i n g g a s f l o w .

p r e m i x o f H-5 o r H-15 . ( 2 ) P e r c e n t a g e o f h y d r o g e n b a s e d o n b l e n d i n g w i t h

5 .45 Argon is used for welding carbon steel, high-strength steels, and reactive metals such as titanium and zirconium alloys. Even minute quantities of hydrogen in the gas used to weld these metals may result in porosity. cracking, or reduced niechanical properties.

5.4.6 Argon-hydrogen mixtures are used as the orifice and shielding gases for making ke!.hole-riiode welds in stainless steel, nickel-base, and copper-nickel alloys.

5.4.7 Permissible hydrogen percentages vary from the 5% used on 1/4 in. thick stainless steel to the 15% used for highest welding speeds on O. i SO in. and thinner wall stainless tubing in tube mills. In general. the thinner the workpiece, the higher the permissible percentage of hydrogen in the gas mixture up to a maximum of I5 percent.

5.4.8 Use of helium as an orifice gas increases the heat load on the torch nozzle and reduces its service life and current capacity. Because of the lower mass of helium, it is difficult. at reasonable flow rates. to obtain a keyhole condition with this gas. Therefore, helium is used only for making nonkeyhole-mode welds.

5.4.9 Helium additions to argon produce a hotter arc for a given arc current. A mixture must contain at least 50% helium before a significant change can be detected; mixtures-containing over 75% helium behave about the same as pure helium. Argon-helium mixtures containing between 50 and 75% helium are generally used for making keyhole-mode welds in heavier titanium sections and for filler passes on all metals when the additional heat and wider heat pattern obtained from these mixtures are desirable.

5.5 Auxiliary Weld Shielding 5.5.1 I t is useful to supply auxiliary weld shielding in welding titanium and

other metals that react with air and may suffer degradation of metallurgical


I

Application to Metal Jok ing I 19

properties as a result. in addition, air can substantially alter the fluidity of the molten weld meia1 in several base metals. This can change the keyholing characteristics. For such applications an auxiliary shield is beneficial in reducing variations in weldability. This auxiliary shielding is also necessary when welding more comnion and less reactive metals at high travel speeds. An important part of the auxiliary shield is the insulator between the torch and the shield; pyrex glass provides excellent electrical insulation and the temperature stability and stiffness necessary to transfer dragging forces from the torch to the shield.

5.6 Joint Design 5.6.1 In nieta1 thicknesses of 0.002 to 0.010 in., edge-flange welds can be

made with the plasma-arc welding process in the nonkeyhole-mode operation. Flanging of the panel edges can be accomplished with a rolling flanger. Typical flange heights are shown in Fig. 1 1 .

M e t a l t h i c k n e s s , t ( i n . ) F l a n g e h e i g h t , h ( i n . )

0 .002 0.010 t o 0 .020

0.005 0 .020 t o 0.025

0 . 0 1 0 0.030 t o 0.040

5,6.2 in 0.010 to 0.060 in. thick metal sections. butt joints with square-groove design are coinnionlu used for plasma-arc welding. In this thickness range, the nonkeyhole mode of plasma-arc welding is used. Tee, edge. and comer joints are crisily welded with or tvithout filler metal addition.

5.6.3 Metal thicknesses of 0.060 to 114 i n . are usually welded with square-groove u.clds in butt joints utilizing the keyhole mode of the plasma-arc welding process.. These welds are inade in one pass. usually without filler metal addition. The sides of the fusion zone are more parallel and narrower than those nixie u.ith the gas tungsten-arc welding process. This results in reduced joint shrinkuge and distortion. Joint gap and niisniatch are not so critical as in gas tungsten-arc welding because of the stiffness of the plasma-arc stream and its insensitivity to variations in voltage. Tee and edge joints can be welded in this

’


range of t h i c lin e s se s us i ng non ke!. ho le - mode ope rat i on . Fi 1 let ive Ids, u' i t h fi 1 I e r iiictol addition. teiid to be decpl!- penetrated with H concave fillet surface.

5.6.4 hletal thicknesses over 1/4 t o I in . require a U or Vec preparation for butt \veldiiig. However. a siiiiplc squm-groove weld can be niade in nietnl thicknesses up to Y 8 i n . Lvhen \veldhg froiii both sides. Beyond.S/8 in . . U- o r Vee-type grooves-single or double-iiiust be u d . Thew groove configurations can incrirpor;ite substantiall!. \vider root faces (up to 1/3 in . ) than those used in gas tungstcn-;irc \\.elding (sec Fig. 12). I n this bvay. plate thicknesses approaching I i n . can be \veldcd ivith the root pass iiiadc in the keyhole niode and subsequent filler passes in the ncinkexholc iiiode.

i I 4

1 / 1 6 i n .

- _ _ - - - _ - _ _ _ _ - - - J o i n t geometry f o r g a s t u n g s t e n - a r c w e l d i n g

J o i n t geometry f o r p l a sma-a rc w e T d i n g

F.'i.l,'. /? - ~ ' i l l l l / J ~ l l ~ ~ \ i J l l f!/' r\./>ii.<l/ , j r J j l l l ~ C ~ l l l l < ~ / i ~ ~ ~ ~ \ $ I / . .i!</.\ / l i l l ~ ~ . \ / < ' l l - ~ l ï ~ ~ i l l l ~ l

~>l<l. \ l l l<l-<l ï<' \\.<*Ill.\ Oll JI< i l l . / I I i C ~ A .\/<*<*I.

5.6.5 The coniiiion tj.pcs of \\.elds successfui1~- inadc with the plasnia-arc \veldhg process are: square-groovc. single- and double-U-groove. and single- and double- Vec-grciove, Thcsc are generally uscci for \veldiiig butt joints fro1i.i one o r both sides ot'the joint and ivith single- or multiple-pass Lvelding. Fillet u ~ l d s can be innde using a technique siiiiilar to gas tungsten-arc welding. Tee joints can be ~ ~ I d e d \\i-ith the plnsiiiu arc by penetrating the u.eb niember froin the flange-iiienibcr sidc of the tcc. thuh siiiiultaneriusly producing a fillet o n both sides of the \vcb niciiibcr. The fillet is generated from the portion of the ivcb members i li at pro t rude s t h ro iig h t he :i bu t t i n g tl an ge - iiie ni ber co ni po ne n t s . iii et a 1 ni ay ;I I s o be added t o [he u ~ l c l piiddlc to provide the fillet.

5.7 Tooling Practices 5.7.1 In general, tooling requirements for plasma-arc welding are simpler and

therefore less expensive than for sas tungsten-arc or gas metal-arc welding. The keyholing technique essentially equalizes contraction stresses at the top and


bottom of the weld. Therefore. tooling has a smaller distortion tendency to counteract. and can be simpler when using the keyhole mode.

5.7.2 Keyholing eliminates the need for an underbead heat sink such as a copper backup bar. Instead, a gap is necessary for accommodating the .ioint-penctrating pl os ni;^. in this mode of operation. a backing bar with a simple rectangiiliir groove (as shown in Fig. 13) is sufficient. Because the weld puddle is supported by the surface tension of the molten metal and does not contact the hacking bar. underbead gas shielding is a1waj.s required to protect the molten underbead froni atniospheric contamination. Hence, the use of such a backing bar servcs to support the weldrnent. to contain the underhead shielding gas, and to provide a vent space for the plasma jet. Groove dimensions are generally about 1/2 in. \vide and 3/3 in. deep. Argon or helium can be used as the underbead shielding gas. i n some applications uhere the purge volunie is in the form of a relatively closed chamber. it i5 important to use helium as the underbead shielding gas.

B a c k i n g Bar

END V I E W

5.7.3 Whcre a truiling shield is necessary. shield length must be much longcr - SO to 100% - than for gas tungsten-arc welding because welding speeds are considerably faster for plrisnia-arc welding. The faster travel speed also diminishes the tolerance for filler metal inlct or trailing shield attachment openings at the torch. Such openings more easily scoop in air that contaniinates (embrittles) the weld bead or discolors (oxidizes) the weld surface.



i

f

5.7.4 The high plasma velocity due to plasma-arc constriction makes the arc much more resistant to deflection than the gas tungsten-arc. Therefore, the arc is quite stable in the vicinity of sharp or protruding edges such as joint mismatch, and can more readily accommodate them than the gas tungsten-arc. This is another reason why tooling may be simpler and more economical.

5.8 Manual Welding 5.8.1 Manual plasma-arc welding is generally limited to applications up to

100 A with apparatus designed to be hand-heid, and where contour welding is necessary. The recently developed apparatus uses a pilot-arc system and a foot contactor to transfer welding current through the plasma stream. The pilot plasma arc visible to the welder wearing protective lens, facilitates accurate positioning of the torch for weld starting. Transfer to weld current is positive and instantaneous. not subject to the difficulties inherent in the gas tungsten-arc welding process I

starting at low welding currents. 5.8.2 The pilot arc is started by moving the electrode forward until i t touches

the nozzle and then retracting i t . The pilot-arc circuit is always energized when the unit is in use. The transferred-arc circuit is then energized by closing a contactor in the lead to the workpiece. Another system utilizes high frequency to start the pilot arc, and a single power supply.

5.8.3 Like manual gas tungsten-arc welding, manual plasma-arc welding is bet ter adapted to n o n k e y hol e , fus ion- type we 1 ding . H ig her-c u rre n t (approximately 100 A and above) plasma-arc welding is usually mechanized to obtain the benefits of the keyhole technique; low-current plasma-arc welding can be performed manually as \vel1 as with niechanization. Manual plasma-arc welding can be used in all positions common to gas tungsten-arc welding.

5.9. Mechanized Welding 5.9.1 Mechanized Lvelding is required for high-current plasma-arc applications

such as makins keyhole-mode welds or high-current filler passes. Mechanization is required because of the high travel speeds, the need for accurate joint alignment, narrow plasnia-arc weld fusion zones, and the types of apparatus available. For repetitive operations, mechanized welding is applicable, as it would be for other processes. Mechanized welding is applied to low-current applications where advantageous.

5.9.2 The mechanized plasma-arc torch utilizes high frequency to initiate a pilot arc between the electrode and the constricting nozzle. The ionized gas generated by the low-current pilot arc flows through the constricting orifice.to the workpiece and completes the circuit for the main welding arc. In most systems, the high-frequency generator and pilot arc are turned off after the main arc is initiated.

5.9.3 The high-current mechanized plasma-arc welding system is capable of utilizing the keyhole mode in approximate thicknesses of 1/16 to 1/4 in. Recent tests indicate that, when joining titanium alloys. straight butt joints can be welded with the keyhole technique up to 1/2 in. thick. Low-current, mechanized. plasma-arc welding can utilize the keyhole mode down to an approximate thickness of 1/32 in. Application to thicknesses above 1/4 in. in a butt joint may


Appiicution to Metal Joining I 33

require joint preparation, but can use a root face up to 1/4 in. wide. Addition of filler metal to f i l l the prepared joint using multiple-pass welding procedures is commonly done in plasma-arc welding. The practical limit of application is approxiniately a I in. thick Vee groove, where the plasma-torch nozzle size prevents accessibility to the root of the joint. Further development is necessary in and above this thickness range.

5.9.4 When welding metal thicknesses under i/8 in . , keyhole-mode welds for straight seani and circumferential welding can be started at full operating current, travel speed. and orifice-gas tlow. In this thickness range. the keyhole is developed with little disturbance in the weld puddle, and the weld surface and underbead are reasonably sniooth.

5.9.5 I t is iniportant to differentiate between straight seam welds, where runoff tabs can be utilized to isolate the keyhole initiation and withdrawal areas, and circumferential o r girth type welds where the keyhole initiation and withdrawal zones must be included within the inspectable weld fusion zone. The operating currents and orifice gas velocity required for keyhole-mode welds in thicknesses greater than iipproxiinately 1/8 in. generally produce a plasma stream that tends to gouge or tunnel underneath molten metal during keyhole initiation. Because the gouging action may cause plnsnia sas-entrapment voids and severe surface irregularities. runoff tabs are usually used to make straight seain welds in the keyhole niode in these thicknesses. When welding circumferential joints, runoff tubs cannot be used: 11 suitable keuhole initiation zone can be achieved using a prcigraiiiiiied increase in welding current and orifice-gas flow rate with travel speed set ut the \{.elding rate. An!. gas entrapment voids that may form in the keyhole initiation area are removed bv the overlapping keyhole and are not geiierally a quality problem. These iiiipoiiant sloping functions for welding current and orifice-gas flou. rate are normally produced automatically by coinniercial \vcldiiig eqiiipiiient and controls. See 5 . 12. Figs. 14. 16. 17 and 20.

5.9.6 If the ke!.hoiing plusnia arc is abruptl> interrupted at the end of a circunifereiitinl \veld. a variet!. of \veld defects inal occur. Although abrupt keyhole terniiiiation niay be desirriblc in straight seani ~ d d s , it is necessary to em p I o y a ve ry d i ffc.i.cn t lie!. ho le t crin i nat io n proced Ure for ci rc uinfe ren t i al welds in the various nictals and thicknesses. The principal problem area in keyhole-mode circumferential ivclds is the keyhole u.ithdra\val zone.

5.9.7 Proper terinintition of thc keyhole in a circumferential weld requires doun-sloping the arc current aiid orifice-gas tlow rate. The net effect of this procedurc is to graduail!. reduce the keyholing force of the plasma arc Lvhile retaining the arc heat necessary to niaintain puddle fiuidity as the keyhole is filling in . Travel speed is generally maintained at the welding rate. Specific metals generally require variations in down-slope time phasing and duration using this procedure. and in sonie cases i t may be beneficial to simultaneouslx reduce the travel speed to help in maintaining puddle fluidity during the time the keyhole is being filled. Sec 5.12, Figs. 14, 16, 17 and 20.

5.9.8 Mechanized plasma-arc welding is usually performed in the tlat position. Limited work in horizontal and vertical welding has been satisfactorily accomplished.


AWS C5.3 73 W 07842b5 0002453 3 W


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Application to Me fa1 Joining I 27

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AWS C 5 - L 73 0784265 0002457 O -~ - ^ _ _ - - ~ ~ - ~


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~~ AUS C5.1 73 W ~- - 0784265 ~ 0002458 _ _ _ _ _ ~ ~ E' W- ~

Applimtion to Metal Joining 1 29

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AWS C 5 - L ~- 73 07842b5 - 0002459 - - 4 30 / PLASMA-ARC WELDING PRACTICES

Table 8 -Typical plasma-arc welding conditions for welding butt joints in thin gage metals

M e t a l T r a v e l C u r r e n t Gasayb

T h i c k n e s s ; y ; ; ; di;; S h i e l d i n g Remarks ( i n . )

S t a i n l e s s S t e e l 0 .001 5 0 . 3 992 A r + l % H E d g e - f l a n g e

.II 'I O . 003 6 1 . 6 99% A r t l x H E d g e - f l a n g e

II 2.0 99% A r t l X H

II 8 6 .0 99% A r + l Z H

II 1 0 . 0 9 9 1 A r + l % H

we1 d

we1 d

II O . 006 5

0 . 0 1 0

'I 0.030 5

T i t a n i um 0.003 6 3.0 5 0 % Ar+50% He E d g e - f l a n g e we1 d

II O . 008 5 5.0 A r

0 .015 5 5.8 A r

o . 022 7

II

II 1 0 . 0 8 5 % H1+25% A r

I n c o n e l 718 0.012 15 . 6 .0 7 5 1 He+25% A r

H a s t e l l o y X 0.005 1 0 4 . 8 A r

0 .010 8 5 . 8 A r

0 .020 . 1 0 10 .0 A r

11 Il

I t II

Copper 0 .003 6 1 0 . 0 75% He+25% A r E d g e - f l a n g e w e l d

a O r i f i c e gas - 0.5 c f h a r g o n f o r a l l w e l d s .

b S h i e l d i n g gas f l o w r a t e - 20 c f h - t o t a l .

' O r i f i c e d i a m - 0 .030 i n .


Table 9 -Typical plasma-arc welding conditions for welding edge joints in thin gage metals

M e t a l T r a v e l C u r r e n t Gasa ,b ,c

T h i c k n e s s f;;;! d;;y S h i e l d i n g ( i n . )

S t a i n l e s s S t e e l 0 ,001 5 0 . 3 9 9 % A r t l X H

O. 005 1 5 1 .6 99% A r t l % H II II

II II 0.010 5 4 . 0 99% A r + l % H

T i t a n i u m 0 .003 5 1 . 6 A r

O . 008 5 3 .0 A r II

H a s t e l l o ] X O . 005 1 0 1 . 5 99% A r + l % H

I l II 0.010 3 3 . 0 A r

0.020 -7 6 . 5 A r II II

K o v a r d 0 .011 20 9 . 0 95% A r t 5 % H

a O r i f i c e gas - 0 .5 c f h a r g o n f o r a l l w e l d s .

b S h i e l d i n g gas f l o w r a t e - 20 c f h - t o t a l .

O r i f i c e d i a m - 0.030 i n . C

d T r a d e m a r k o f S t u p a k o f f Ceramic & M f g . C o .

,


! 32 / PLASMA-ARC WELDING PRACTICES

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Application to Metal Joining I 33

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Mo. 1 No. 2 tic. 3 t:o. 4

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Application to Metal Joining 41

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P o s i t i o n o f Welding

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C F i l l e r me ta l - 0 . 0 4 5 i n . d i a m w i r e .

I



5.10 Multipass Welding 5.10.1 A multipass plasma-arc weld involves a keyhole-mode root pass and

one or more nonkeyhole-niode filler passes, with or without filler metal addition. The nonkeyhole-mode plasma arc uses a plasma in which the force of the plasma stream has been substantially reduced, Total orifice-gas flow rate is reduced, torch standoff is increased. Varying percentages of helium or hydrogen gas may be mixed with argon in the orifice- and shielding-gas circuits to cause the arc heat to dissipate over a larger surface area on the weld joint. Helium can be used, and is favored, for some applications because it provides a broader heat-input pattern and produces a .flatter filler pass weld bead. The very wide range of weld penetration characteristics attainable, using these techniques with the high-current plasma arc, provides substantially greater process flexibility than is available with other gas-shielded arc-welding processes.

5.11 Reverse Polarity Welding 5.11.1 Tungsten electrodes exhibit reduced current capacity when connected

for reverse polarity operation - whether they are used for plasma-arc or gas tungsten-arc welding. Therefore, a water-cooled electrode is preferable for reverse polarity welding. Compared to the arcs encountered in reverse polarity gas tungsten-arc operation, the plasma jet produces an inherently stable arc. A single port nozzle is generally used for reverse polarity work because multiport nozzles do not provide the advantages realized with straight polarity welding.

5.12 Recommended Practices 5.12.1 Metals to be joined. joint configuration. and thickness are the major

considerations for plasma-arc welding. 5.12.2 The requirements for metal cleanliness, edge alignment, fitup, and the

tolerance for mismatch for plasma-arc welding are commensurate with the quality required and the general welding results. Preheat and postweld thermal treatments and heating are similar to other processes and dependent upon metallurgical considerations for the base metals.

5.12.3 Tables 3 through 10 and Figs. 14 through 21 are grouped by metals. The plasma-arc welding conditions listed are recommended as nominal for the thicknesses and applications noted. These data are a summation of conditions used in production and process development for both low-current manual and high-current mechanized applications.

5.13 Advantages and Limitations

.

5.13.1 Sensitivity to Changes in Arc Length 5.13.1.1 The collimated shape of the low-current plasma jet is chiefly

responsible for the lack of process sensitivity to changes in arc length. Figure 22 shows a low-current plasma arc operating at ten amperes. The plasma arc emerges from a 0.030 in. diameter nozzle and is 1/4 in. long. The total included angle of divergency is 6 deg in the low-current plasma arc.

5.13.1.2 Arc cross-sectional area and total arc current establish the average current density at any location in the arc. Current density establishes the rate at

!

r

i



which heat is transferred to a unit area of weld puddle or work surface when the work is placed at a given location. Thus a well-collimated arc can tolerate relatively large variations in arc length before its melting capability is affected seriously. This usually obviates the need for sensing and maintaining a constant length. The longer permissible torch-to-work distance affords the.welder better visibility,

5.13.1.3 Returning to the arcs in Fig. 22 and assuming that a change in arc cross-sectional area of 20% can be tolerated, measurements and calculations show that the low-current plasma-arc length can be varied by I0 .05 in. without significantly affecting weld width and penetration,

Fig. 22 - Cottipmisoti oflo '4 piusttiu arc arid gus ttttigsteti-arc ut urc lerigths cottittiotily itsed for welilitig ven. t h i t i nietu1 sectioris: leftn low-citrretit plosnici arc' is 0.25 iri. lorig: right. gus turigsieri-urc is 0.025 i l l . lot1g.

5.13.2 Arc-Voltage Controls 5.13.2.1 The relative insensitivity of the plasma-arc weldins process to arc

length variations usually obviates the need for arc-voltage equipment for many conimercial applications. However, arc-voltage control has been used with plasma-arc welding to follow contoured weldnients. It is necessary to lock out the height control when current or orifice-gas sloping is employed because varying these settings causes corresponding changes in arc voltage. Thus, due to the sloping of these functions, marked changes in arc voltage can occur even though the torch-to-work distance remains constant. Figure 23 illustrates the slope similarity in arc length/arc voltage curves for plasma-arc and gas tungsten-arc torches.

5.13.2.2 The use of fixed reference voltage settings in applications of automatic arc-voltage control equipment with commercial high-current plasma-arc torches may be complicated by voltage repeatability considerations. Small variations in electrode point configuration, setback and centering, orifice-gas flow rate, and tolerances of orifices may cause variations in arc voltage.

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N o t e : ( a ) The o r i f i c e g a s f l o w r a t e s f o r a l l p l a s m a - a r c t o r c h e s w e r e h e l d a t v a l u e s i n d i c a t e d a b o v e o n a f l o w m e t e r c a l i b r a t e d f o r p u r e a r g o n g a s .

t o r c h w e r e f o r t h e s h i e l d i n g g a s . ( b ) T h e g a s f l o w r a t e s shown a b o v e f o r t h e GTAW

Fig. 23 - Arc voltuge churucreristics of three torches jbr three dgerent urclplastnu sicpport guses. using arc conditions for welling 0.25 iti . thick piute.



5.13.3 Electrode Contamination 5.13.3.1 Because the electrode in the plasma-arc torch is recessed within the

arc-constricting nozzle, it is not possible to touch the electrode to the workpiece. This property greatly reduces the possibility of tungsten inclusions in the weld and can substantially extend the period between electrode dressings. The life of the electrode is also improved by a constant flow of inert orifice gas which minimizes erosion of the electrode.

5.13.4 Keyhole Effect 5.13.4.1 One of the chief differences between the plasma-arc and gas

tungsten-arc welding processes is the “keyhole” effect. It can be obtained with the plasma arc when making square-groove welds in butt joints in certain thicknesses of most metals. The keyhole is a positive indication of complete penetration and weld uniformity. For details, see 3. I .3 .

5.13.4.2 Many metallurgical advantages are possible with the keyhole mode. The smaller heat-affected zone reduces strength loss at the joint for heat-treated metals, and promotes less grain growth which results in better ductility. The reduced welding time interval results in less embrittlement by carbides and complex intermetallic compounds for stainless steels and superalloys. Further, the equalizing of distortion stresses - top to bottom of the weld cross section - results in less residual stress. The surface condition in prepared joints and on filler metais is a major factor in causing porosity. Because the plasma-arc welding process requires less filler metal in the keyhole mode, porosity is significantly reduced when making simple square-groove butt weIds by this method,

5.13.4.3 The greater penetrating power of the plasma jet, as compared to gas tungsten-arc welding, can be used to produce a higher depth-to-width ratio in the weld.

5.13.5 Fabrication Advantages .

5.13.5.1 Various fabrication advantages are possible with Lhe plasma-arc welding process. They include fewer weld passes, reduced cost for filler metal. and decreased possibility of human error due ro less manipulation in general. Another advantage is that need for interpass cleaning, back-gouging. and temperature maintenance, if needed, are needed less. For keyholing applications, machining is reduced in making a square-groove type joint preparation instead of a Vee- or a U-type. Tooling can be simpler and therefore less costly because distortion tendencies are reduced.

5.13.6 Process Limitations 5.13.6.1 The plasma-arc welding process is limited in application to a

iiiaxitnuni metal thickness of one inch in making butt welds. Further development is needed to extend its application to thicker sections. Mechanized plasma-arc welding is generally used in the flat and horizontal positions although development in vertical welding has been satisfactory. Manual plasma-arc welding can be used in all positions.

- . - i ;


I


6. Application of the Plasma-Arc Welding Process to Surfacing 6.1 General Considerations

6.1.1 Surfacing is applied to metals for improved resistance to abrasion. corrosion, or for improved impact properties not ordinarily displayed by the base nietal. I t can also be used for replacement of worn or corroded metal.

-Consideration must be given to the filler nietal. deposition rate required. niaxiiiiuni diIution permitted with the base nietal. and the thickness or quantity of the deposited metal. Table i 1 illustrates results obtained with various welding processes in their applicaion to surfacing.

6.1.2 Since plasnia-arc weld surfacing is a fusion process. it is necessarj. to follow the sanie preheating. postheating. and slow cooling procedures specified for ivelditig certain types of base metals. In general. the higher the alloy content and the larger the workpiece. the niore heating and other precautions are necessary. Also. large differences in thermal expansion coefficients betu-een the brise metal and the filler metal should be avoided to minimize the possibility of cracking.

.

i

6.2 Powder Surfacing 6.2.1 Operational Characteristics

6.2.1.1 A standard powder surfacing torch is used to apply overlays from 1/64 in. to l/S in. thick at currents up to 250 amperes. Stringer beads as narrow as 3/64 in. can be made, and beads as wide a s i 3/4 in. can be produced by oscillating the torch. A scheiiiatic o f a standard pouder surfacing torch is shwvn in Fig. 9. By replacing the lower section of the torch with a heavy-duty nozzle assembi) . deposits up to ¡/-I in. thick can be made at currents up to 350 amperes. The heavy-duty assembly also provides means for separately depositing large-size wear-resistant particles such as tungsten carbide into a weld deposit to produce heterogeneous deposits. An additional tungsten carbide dispensing uni t is required for this application. A relatively low f l o ~ v of arson (bebveen 3 and 10 cfh) directed through the constricting orifice t o niinimize weld penetration and dilution. Heating of the puddle on the workpiece. which determines the extent of weld dilution, is controlled b>. adjusting the transferred-arc current from the transfered-power supply.

6.2.2 Effects on the Base Metal 6.2.2.1 Plasma-arc deposits solidify from the molten state into a dense cast

structure. Consequently. the nietallurgical properties of these deposits are similar to those of gas tungsten-arc overlays. The depth of the heat-affected zone is a function of the rate of heat input and the chill produced by the base inetal. On nonpreheated base nietal l/4 in. thick and heavier, the depth of the heat-affected zone is approximately equivalent to the thickness of the deposit. On thinner sections, the base metal structure will be affected throughout its thickness unless cooling means are used.

6.2.3 Filler Metal 6.2.3'1 A wide variety of cobalt-, nickel-, and iron-base alloys is availabic

in powder form for powder surfacing with the plasnia-arc welding process. They


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arc listed in Table I3 along with typical deposit hardness values. The powders are classed a s high-alloy types and have varying degrees of abrasion, impact. and CO rros io ti resist an ce .

6.2.4 Recommended Praciices 6.2.4.1 The relationship between \velding conditions can be seen in Fig. 23.

These churacteristic curves for a surfixing allo>- consisting of 64% cobalt, 30% chroiiiiuni. and 4% tungsten indicate that. for a given deposit thickness, weld dilution incrcaws as the deposition rate increases. This is logical because the increased current required t o increase the deposition rate causes an increase in n d d depth of fusion. The curves d s o shou. that. for a given deposition rate. weld dilution cIcc~*c;iscs a s the dcposit thickness increases. The heavier deposit. with its larger \wld puddle. acts a s a buffer to reduce arc penetration and thus decreases d i l LI t i o n .

6.2.3.2 The curves in Fig. 24 ;ire useful froiii the practical viewpoint because the! s h o ~ . that the iiictal deposition rate is liiiiitcd by the required deposit thickness iiiicl the tolerable dilution kvel . Far example. i f t h c desired deposit thickness of the Co-Cr-M' allo!. is 1/8 in. and the dilution cannot exceed 10%. the maxinium deposition rutc is autoiiiatically fixed at about 3 Ib/h. Hov..ever, if a dilution level 01' 7Oct is iicceptublc. a deposition riitc of niorc than 6 lb/h can be used. Typical pi~occs\ settings for depositing a variety of H idths and thicknesses are shown in Table 17.

6.2.4.3 The effect of alloy composition o n the relationship betueen ~iiifaciiig conditions c m be seen by conipuring the curves in Fig. 24a to those in Fig. 74b. Tlic latter set o ï c u r v c ~ .cs.as obtained using ;i filler iiietal similar to that u ~ c d to ohtoiii the curves i n Fig. 74a - except ;i siiiall percentage of boron \vas ;iilrlcrl to the p o ~ der. Boron rcduccs the melting point ofthc alloy and improves its \setting ch:iriictei:istics. t1itircb'- cftcc*tiiig ;I dcsirnblc donmward shift in the fiimily 01' cur\'cs. This nic;ins that ivith the boron-bearing uIlo!-. thinner deposits can be niade :it lo\vcr dilution levels than u i t h thc Co-Cr-lï alloy. Note that for a 1/8 in, th ich dcposit ;it 10% dilution. the allo\vablc deposition rate is 9 Ib/h for the Co-Cr-M--H allo>- coiiipnrcd to 3 Ib/h for the Co-Cr-W allo!-.

6.2.5 Ad va nt ages 6.2.5.1 Pou der surfacing u i t h the plosmu-arc uelding process can be Used

to achicvc high iiictal-depc~siti~)t~ rates. I O N biisc-iiictrtl dilution. and unusurilly thin deposits. Deposition rntcs con be controlled frniii less than 1 Ib/h to niore than 12 Ihlh :it dilution Itvcls of 5 to 30 pcrcent. A significant advantage of pouder sut-l~tciiig. however. is its ability to use iiictal po\vrlers. This is important because dl iiictala ciin be pro\%lcd i n poMder forni. but thcy cnnnot all bo provided in rcxior u-ire foriii suitable for continuous operation. Pou-dered cobalt-base alloys, for e i; ;i ni p I c . c ;i n be ci e pos i t cd co n t i n uo us I y and u n i fonii I y wit ho ut the I i ni i t at ion s iiiipiiscd b> the use of thc iiictiil in rod forni. Also. by tailoring the poLvder li)riiiulution. iiiiiiij speciiIl iilloya ciin bc prepared for depositing: For satisfactor' results. the filler nietal must have thc s;inic or Iwver melting point than the base n1ctol.

/- Y


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Fig. 14 - Charurrrristic curws Sr>r two types .f alloys deposited with a plasnio arc f¿)r powder m j k i t i g . (a ) Co-Cr-W ulhy . íh) Co-Cr- IV-B ulloy.


i

AWS C5.L 73 W 0184265 0 0 0 2 4 & 0 b W - __ _-___

Application to Surfacing I 51

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,

52 / PLASs1.4-AKC WELDING PRACTICES

6.3 Hot-N'ire Surfacing 6.3.1 Operational Characteristics

6.3.1.1 In hot-wire surfacing with the plasma-arc welding process, the two \vires that are fed into the puddle are resistance heated with a current tlowing in series through theni. A scheiiiatic is shown in Fig. I O. This circuit is used so that the magnetic fields set up about the two \vires, which might otherwise detlect the arc id cause irregularities in melting. tend to cancel each other. A constant potential ;i-c pouw source provides the resistance ( FR) heating of the filler metal uires. The function of the hot-wire systeiii is to supply molten filler nietal t o the arc zone. A single hot-\vire systeiii niay be used to obtain narrou deposits; hoLvever. deposition rates are limited to about 15 Iblh.

6.3.1.2 The hot-Lvire system is independent of the plasnia arc. This allows for indepeiideiit adjustnients in wire speed and melting rate t o suit requii-eiiients of deposit thickness and deposition rate. Use of the plasma-arc torch permits control over the arc current. orifice-gas tlow. and orifice size. This provides ;i measure of CO tit ro 1 o ver d i I ut ion.

6.3.1.3 The spucing between the hot-wire guns and the Lvork is critical bccause the resistance heating of the filler metal kvires depends o n the extension of thc nires betiveen the guns and the nwkpiece. If the distance is too short. the \vires \vil1 not be heated sufficiently. If the distance is too long. the voltage drop across the nire estension inay iiicrease enough for an arc to occur. This \vil1 cause irregularities in the deposit. For these reasons. :in arc-voltage control is usually uscd to iiiuiiitain a constant standoff distniicc for the hot-wire guns.

6.3.1.3 An argon-heliuiii misture is used a s the orifice gas in the plasma-arc torch. mid argon is used as the shielding gas. Since the filler nietal uii-es are brought to their melting point in 1111 argon atiiiosphere. the!. tend to outgas and thus drive off an! surface contaiiiinants kvhich might be present. This eliniinutes a potciitial source oí' porosity in the overlay and produces a higher quality overlay than 111 i g h t be o bt ;i i necl \i. i t li other sur fric i iig iiie t hod s u t i 1 i zing fi I ler nie t al add it ion in nire form.

6.3.2 Filler Metal 6.3.2.1 hlan!. surfacing filler iiietals have been successfully applied with the

plnsiiia-urc u.elding process. The!. include austenitic and niartensitic stainless steels. nickel and nickcl-bpc dloys. ancl copper aiid copper-basc alloys.

6.3.3 Recommended Practices 6.3.3.1 Hot-Lvire surf:icing \vith the plasma-arc Lveldiiig process is relatively

siiiiple because iiiuny of the variables reiiiaiii esseiitiall!- the sanie regardless of the deposition objectives or the nietal being ;ipplicd. The operation is mechanized. iiinkiiig surfiicing conditions highlj reproducible. Shielding-gas tlokv rates for the s!.steiii. i n nio.;t cases. remain the sanie. The plasriin-arc torch standoff is adjusted for 7/X in. b j . presetting the voltage control head. The oscillation Ividth and travel specd :ire selected for ;i desired ma-surfacing rate so that the volunie of filler nietal to be iiiclted \vil1 produce the desired deposit thickness. For example. a 15 sq in./niin ( 1 . S in. width x I O ipni) area-surfacing rate \vil1 produce a 3/16 in. thick dcposit when depositing filler metal at 50 Ib/h. The plasma-arc and hot-wire


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current levels are selected to meet the deposition objectives. Typical deposition rates and welding conditions for a variety of surfacing filler metals are shown in Table 14.

6.3.3.2 As a rule of thumb, the diameter of the hot wire should not exceed one-half the desired deposit thickness. Accordingly, solid 1/16 in. diameter wire is recommended for applying 1/8 in. to 3/16 in , thick overlays. Solid 3/32 in. diameter wire may be used for applying 3/16 in. thick and heavier deposits. Composite stainless steel wire of i / I 6 or 3/32 in. diameter can be used to produce 3/16 in. thick and heavier deposits at a deposition rate approximately 70% of that obtainable with solid wire, Composite surfacing wires can similarly be used if available in II1 6 or 3/32 in. diameters. Flux cored stainress steel or surfacing wires are not recommended because flux interference in the weld puddle precludes satisfactory deposit formation.

.

6.3.4 Advantages 6.3.4.1 Using the plasma-arc welding process for hot-wire surfacing

exhibits significant advantages, particularly with high-nickel alloys. The principal advantages are minimum deposit dilution and freedom from voids and inclusions. Practically any metal available as a solid or composite wire can be applied. Because the filler metal melting system is independent of the base metal heating system, dilution is lower and can be better cntrolled than in other surfacing methods where filler metal melting and arc heat are interrelated. With most metals. single layer deposits can be produced at deposition rates up to 60 Ib/h with dilution levels as IOW as 5 to I O percent. Surfacing costs are iower’tor certain metais because thinner deposits which meet the required chemical analysis are possible in a single layer. Good deposit quality can be achieved with little likelihood of voids in the overlap region between adjacent weld passes.

7. Process Control 7.1 General

7.1.1 The elements necessary for adequate process control of the plasma-arc welding process are similar to the gas tungsten-arc welding process. While such elements as arc voltage or standoff distance are not too critical, other factors, such as orifice-gas mixture and orifice wear are important.

7.2 Joint Preparation and Tolerances 7.2.1 For most metals with section thicknesses above 1/4 in., a 1/4 in. 3t 1/32

in. root face with a bevel of either 30 or 45 deg is generally acceptable. On some metais, satisfactory square-groove welds can be made in butt joints in thicknesses as great as 318 in . with single-pass welding using the keyhole technique. This, however, has to be established by trial for the specific metal being welded.

7.2.2 Dimensional tolerances are comparable to gas tungsten-arc welding. Sheared edges up to 1/4 in. thick may be satisfactory. but machined joints are preferable. Metal-to-metal fit is preferred, although a gap up to 0.020 in. is permissable on 1/4 in . thick or heavier metal sections. On thinner thicknesses, a


AWS ~~ C C - 4 73 07B42bC 0002484 3 = _ _

Process Control I 55

proportionately smaller gap is permitted. Mismatch up to i / i 6 in. is permissible on thicknesses 114 in. or greater. Proportionately less mismatch to approximately 25% of thickness is permissible on thinner sections.

7.2.3 For metal thicknesses of 0.030 in, and less, illustrations of tolerances for joint fitup and fixturing are shown in Fig. 25 and Fig. 26. Figure 25 shows the tolerances for joint gap and mismatch as well as the fixture dimensions for hold-down clamps and backup grooves. The tolerances are listed in relation to the metal thickness. Thus, for a square-groove weld in a butt joint, a maximum joint gap of 0.2 x t = 0.006 in. (where t = 0.030 in.) is permitted. This is a very small gap indeed. I t decreases to only 0.001 in. for a foil thickness of 0.005 in. The fixturing for such weldments must be tooled with precision.

7,3 Considerations for Welding Thin Metal Sections 7.3.1 It should be noted that the weld puddle in thin metal sections and foils

behaves quite differently from the weld puddle in thicker sections. This is due to surface tension, which has become the dominant force. For example, as thinner sections are used, say below 0.030 in., the effect of weight or gravity diminishes until, at 0.015 in., it disappears. The strong effect of surface tension then determines the shape of the molten metal, regardless of the welding position, i.e., whether overhead. vertical, or in the flat (downhand) position.

7.3.2 Good shielding and fixturing are especially important. Weld defects leading to nonrepairable damase and part rejection are largely related to shielding and fixturing due to their effects on the surface tension of melted joint edges. Common oversights in foil welding procedures include:

( I ) E.wcssive joirtt gcips which cannot be bridged by the melted edges. ( 2 ) Weld osidciriori or base r)ietcil osides which prevent good wetting and

(3) Utibalarrcedptir-t geometries (i:e., the free end of a butt joint) which allow

(4) Inucleyuote cltrnipitig which prevents joint warpage during welding.

7.3.3 The basic requirement in welding thin sections is to make sure, by whatever means possible, that both joint edges are in continuous contact and that both edges melt simultaneously to form a single weld puddle. Separation between the joint edges before or during welding will cause the edges to melt separately and remain separate.

7.3.4 Increased latitude in butt joint fixturing tolerances can be obtained by modifying the joint, Le., flanging the edges. The turned-up edges act as preplaced filler metal to fill the gap and assure melt contact with both sides of the joint. They also stiffen the joint edges to minimize warpage from heat buildup during welding.

7.3.5 Figue 26 shows the fitup and fixturing tolerances foredge joints. Note that on every count, the tolerances are much greater than those for butt joints. Because of this wide tolerance, the edge joint is the easiest and most reliable joint for welding section of foil thickness. Therefore, successful welding of foil-thickness assemblies is greatly assured by converting the joint into some form of edge joint whenever possible.

attachment of the molten films.

puddle contraction in only one direction.


- - ~

AWS ~ C5.L 73 m ~~- 07842b5 0002485 5 - -~


A

un- i

Butt Clamp Backupa J o i n t

Type o f Weld ma x ma x m in ma x 'min ma x

Square-Groove 0 . 2 t 0 . 4 t i o t 2 0 t 4 t 1 6 t

F langedb 0 . 6 t I t 1 5 t 3 0 t 4 t 1 6 t

Mismatch spnc;ng Groove " B " C " D " !?;o -

aGas underbead s h i e l d i n g , e i t h e r a rgon o r h e l i u m , i s r e q u i r e d .

bEdge f l a n g e - w e l d i s recommended f o r b u t t j o i n t s i n t h i c k n e s s e s be low 0.010 i n .


It-' max

3 X t max

-7-

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* W r i n k l i n g , o f j p i n t e d g e s b e g i n s a t 8 t

C 1 anip d i s t a n c e



7.4 Controlling the Operation 7.4.1 For thin inetal sections, weld current has a pritnary influence over weld

quality. Figure 27 illustrates u range of operating currents that will produce acceptable welds.

7.4.2 Figure 28 indicates. relative to an established optimum. the variations in arc length, travel speed. and percent hydrogen in the shielding gas within which satisfactory \velds can still be produced, Available apparatus will easily operate within these limits to yield acceptable joint quality.

7.4.3 For the high-current keyhole technique for metal thicknesses of 1/16 in. and above. Lvelding conditions are critical and it is recommended that the following tolerances be applied:

( 1 ) Weld current. +-5 A. (2) Travel speed. 2 1/8 i n h i n . ( 3 ) Orifice-pas flo~v rate, 4 1/4 cfh. (3) Shielding-gas flow rate, + S cfh. ( 5 ) Torch standoff, k l / i 6 in.

7.4.4 For the filler passes where filler metal is added, the control of the welding conditions is not quite so critical as for the root pass using the keyhole technique. The follo\ving tolerances for filler passes are recommended:

( 1 ) Weld current, + 10 A. ( 7 ) Travel speed, + IL2 ipm. ( 3 ) Orifice-gas tiow rate. 4 i /? cfh. (4) Shielding-gas tlow rate. _t5 cfh. ( 5 ) Torch standoff, 1/8 in. to 3/8 in.

7.4.5 Pure argon is generally used for the keyhole-mode root pass. Either helium or mixtures of helium-argon can be used for filler passes. Where pure helium is specified. high flow rates are required, particularly for shielding. The use of a trailing shield in addition to the normal torch shielding is desirable for making multipass welds on some metals.

7.5 Maintenance 7.5.1 Maintenance is relatively simple. The greatest maintenance problem will

be with regard to the orifice nozzle which will elongate with use and will require periodic replacenient. The replacement cycle will depend on the current density used and the weld cycle. Replacement will vary from an estimated one hour for high-current, high-duty cycle applications to an indefinite period for low-current, low-duty cycle applications. On equipment that utilizes a pointed tungsten electrode. orifice nozzle life will still depend on the accuracy of centering the tungsten electrode Lvith respect to the orifice.

7.5.2 The tungsten electrode life will be quite long and will depend on such factors as current density. duty cycle, purity of the orifice gas, and on whether the electrode is water cooled, Under normal operating conditions, tungsten electrode life between dressings should be on the order of 8 to 40 hours.

7.5.3 Maintenance of equipment such as power supply, controls, meters, etc., will be comparable to the gas tungsten-arc welding process and should be done


10 .0 - 9 . 0 - 8.0 - 7.0 -

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Ultimate Tens i le Strength a n d Yield Strength ( k s i )


Process Control 1 61

periodically. The frequency of maintenance is determined by the specific application and duty cycle.

7.6 Inspection and Testing Methods 7.6.1 Nondestructive testing methods applicable to other welding processes are

used for plasma-arc welded joints. Visual, liquid penetrant, and radiographic techniques are adequate to determine weld quality. There are no unique effects exclusive to the process which cannot be evaluated by these methods. Quality levels common to those attainable with the gas tungsten-arc welding process can be anticipated, with the usual considerations being given to cleanliness of parts and material, proper inert-gas shielding at root and face of welds, and metallurgical characteristics of the alloy being joined.

7.7 Design Data and Test Results 7.7.1 Uniaxial tensile strength, bend-test limits, and notch toughness of

plasma-arc welds are dependent upon the metallurgical characteristics of the metal welded, in combination with filler metal additions and thermal treatments.

7.7.2 Because many plasma-arc butt welds up to 1/4 in. are made without filler metal, their properties are unaffected by dissimilar filler metal additions. The low depth-to-width ratio of plasma-arc welds has not indicated advantages in properties, but has indicated reduced defects based upon the quantity of metal melted in the weld. 7.7.3 The typical properties shown in Fig. 29 illustrate that the uniaxial tensile

test properties for Type 310 stainless steel are equivalent to those for the gas tungsten-arc welding process.

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Base We1 d Weld metal panels panels

I Fig. 79 - Typical average uniariai tensile properties of loir amperage plasma-arc welds for Type 310 stainless steel, 0.030 in. thick.

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.



7.7.4 Tensile properties of plasma-arc welds in solution-treated and aged 6A1-4V titanium plate 0.25 in. thick exhibit ultimate tensile strengths ranging from 157 to 162 ksi at elongations greater than 6.5% in 1 in. Pre-cracked Charpy impact (PCI) values are significantly higher in titanium weld metal and heat-affected zones than in the base metal for plasma-arc welds. Specimens from several plasma-arc welds evaluated by tensile and PCI testing have been analyzed for interstitial gas content; these analyses all suggest that the keyhole-mode plasma arc actually extracts oxygen from the weld metal. The weld fusion zone in these weldments exhibited oxygen contents from 25 to 40% less than the base metal. The PCI values for weld areas and base metal are in close correlation with gas analyses.

7.8 Applicable Specifications 7.8.1 The plasma-arc welding process has been individually qualified for

several critical government fabrication contracts and accepted as an alternate to the gas tungsten-arc welding process. It will undoubtedly be incIuded when specification revisions are made. The AWS Committee on Welding Symbols has adopted PAW as the process designation for the welding symbols.

7.8.2 Welds that meet the existing standards for aerospace and nuclear joint quality have been produced by contractors.

7.8.3 Hot-wire surfacing deposits made with the plasma-arc welding process have been shown to be capable of complying with all the requirements specified in the ASME Boiler and Pressure Vessel Code, Sections III and VIII, and in NAVSHIPS 250- 1500- I ,

8. Training and Qualification of Welders

8.1 Welders and welding operators having previous gas tunosten-arc welding experience are readily trained as plasma-arc welders and welding operators,

8.2 The major difference in training for plasma-arc welding as compared to gas tungsten-arc welding lies in understanding the significance of the orifice-gas flow and in recognizing the lesser effect of arc voltage as a welding variable. The length and extent of the training will depend on the trainee's former experience, intelligence, attitude, and mechanical aptitude.

8.3 In manual welding. the welder needs to learn that it is not necessary to maintain a close arc as in other processes, and that the long arc is usually the chief advantage. Along with this, he should be cautioned that amperage decreases as arc length increases, but this, within limits, is not usually a significant factor in the end result. Insofar as torch manipulation is concerned, an inexperienced trainee will generally adapt more rapidly with manual plasma-arc welding than with gas tungsten-arc welding.

8.4 In mechanized welding, welding operators must be trained to recognize, generate, and maintain keyholing conditions. Torch-side monitoring of keyholing is not adequate until the welding operator has considerable experience. Morever, for some metals, such as titanium, the required oxidation protection shields at the

and Welding Operators

=.


Safe8 Recommendations I 63

torch prevent observing keyholing from the torch side. An underbead viewing system should be arranged whenever possible for maximlltr. reliability.

8.5 For mechanized welding. the equi2ment should be qudified to assure that it can be controlled within the tolerances outlined to produce a weld or an overlay of quality comparable to similar work standards €or other welding equipment. Testing of the weld should parallel the methods and procedures employed for other welding processes.

8.6 Specific qualification requirenznts will be dictated by customer requirements and controlling agency specifications. A typical example is MIL-T-502 1 C, Tests; Aircraft and' Missile Welding iiperator's Qualification.

8.7 Because there are several manufacturers of plasma-arc welding equipment, it is recommended that instructions and assistance be obtained from the equipment supplier in establishing a training program.

9. Safety Recommendations 9.1 For detailed safety information, refer to the manufacturer's instructions and the Intsst editions of the following publications: Safety in Welding and Cutting. ANSI 239. I : Recommended Safe Practices for Gas-Shielded Arc Welding, AWS A6, I : and the AWS WELDING HANDBOOK (Sixth Edition), Section 1 I Chapter

,9, "Safe Practices in Welding and Cutting." For mandatory federal safety regulations established by the U.S. Labor Department's Occupational Safety and Health Administration. refer to the latest edition of OSHA Standards, Code of Federal Regulations. Title 29 Part 1910 available from the Superintendent of Documents. U.S. Printing Office. Washington, D.C., 20402.

9.2 When welding with transferred arc currents up to 5 A. spectacles with side shields or other types of eye protection with a No. 6 filter are recommended. Although face protection is not nornially required for this current range, its use depends on personal preference. When welding with transferred arc currents betLveen 5 and IS A , a fulI-face light green plastic shield is recommended, in addition t o eye protection w.ith a No. 6 filter. At current levels over 15 A, a standard welder's helmet with the proper shade of filter glass for the current being used is required. 9.3 Wear suitable clothing t a protect exposed skin from arc radiation.

9.4 Turn off welding power before adjusting or replacing electrodes. Use iidequate eye protection when observation of a high frequency discharge is required to center the electrode.

9.5 Turn off the main pou.er switch before opening the control cabinet.

9.6 Accessory equipment such as wire feeders, arc-voltage heads, oscillators. etc., should be grounded. If they are not grounded, insulation breakdown might cause these uni ts t o become electrically "hot" with respect to ground.

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64 / Pi.ASM.A-AKC U't l .DING PRACTICES

9.7 Usc adequate ventilation particularly when welding metals with high copper, lead. zinc. or beryllium contents.

9.8 If chlorinated solvents have been used to degrease or clean the workpiece, be sure the solvent has been removed before welding. Do not weld near degreasing I ;ink s .

9.9 Hydrogen is ;i combustible gus and can be an explosion hazard. Keep open tlaiiies ;i\! ;I!. froiii h> drcyen cylinders and hoses. Keep open flames away from the torch Lvhcn purging. iV:i.i*cr I I S C I , 2, or ci% c i rgo~ i -osyg~n gus inixtures ki'itti I i ~ * d i * o , q c i i . TIic n i i . u i r r - 1 . i s i~.vplo.sii~c.

9.10 Turn off the welding power and shut off any shielding gas supply at the supply source uhen leaving the work or stopping the work for any appreciable tinic. or u.hc.n moving the machine.

9.11 When a pilot arc is run continuously, use normal precautions to protect against arc tlash and heat burns from the effluent.

9.12 See Appendix for OSHA ear protection equirements.

10. Practical Applications 10.1 General

10.1.1 The plasma-arc Lvelding process is accepted in the aerospace, hydrospace. nuclear. electronic. shipbuilding, and many other commercial industries. I t offers process fiibricrttion latitude and economy while maintaining high quality and reliability. Most metals weldable with the gas tungsten-arc u.clding process can be satisfactorily welded with the plasma-arc welding process. Accordinglj . no exceptions are required in the established gas tungsten-arc ;tcceptonce specifications for weldinents.

10.2 Manual Low-Current Plasma-Arc Welding Applications

10.2.1 As previously stated, manual operations do not use the keyhole technique and are therefore similar to gas tungsten-arc welding. However. the welding of very thin metal sections can be accomplished with greater ease and relirtbility by plasma-arc welding using the nonkeyhole mode. There are exceptions where skilled welding operators have found it advantageous to use the low-current keyhole niode to achieve complete and uniform weld penetration in pipe fabrication. I n this regard, the plasma-arc welding process does offer more versatility and is well suited for welding many and varied complicated structures made of thin metal sections.

10.2.2 A nuinber of proven manual and low-current plasma-arc welding applications are listed below.

( 1 ) Thin wire mesh screen filters. (2) Thin wire butt welds. (3) Relay cases. (4) Bellows assemblies. ( 5 ) Exhaust chambers.


Practical Applications 1 65

Air ducting. Therinal shields. Vanes and blades. Thin wall pressure vessels. Vacuum tube components. Microcapsules. Thermocouples. Filament assemblies.

10.3 Mechanized High-Current Piasma-Arc Welding Applications 10.3.1 Mechanized high-current plasma-arc welding has proven commercially

important in several areas of application due mainly to the keyhole characteristics and the increased welding speeds which result in high quality joints. Many applications are being developed for production. Typical applications are listed below:

( i ) Stainless steel and titanium tubing (longitudinal welds). (2) Girth joints in pipe fabrication. (3) Missile tankage. (4) Turbine engine components. ( 5 ) Furnace electrodes - compacted reactive metals. (6) Tee joints for structural members.

10.4 Surfacing with the Plasma-Arc Welding Process 10.41 Plasma-arc weld surfacing is a fully mechanized operation and shows

the ?ratest economic advantage with high-duty cycle in high-production-type applications. Both metal deposition methods, powder and hot-wire, are highly efficient. The service requirement and the available form of surfacing filler metal. dete riil ine each specific applic at ion.

10.4.2 Typical applications of powder surfacing are listed below: ( I ) Wear rings. (2) Corrosion resisting components. (3) Valve cores and faces. (4) Bearing surfaces. ( 5 ) Abrasion- and erosion-resistant surfaces. (6) Buildup for worn or corroded surfaces.

( I ) Heavy wall chemical and nuclear reactor vessels. (2) Dished heads and flange rings. (3) Cover plates and tube sheets. ( 3 ) Watcr-cooled electrodes.

10.4.3 Typical applications of hot-wire surfacing are listed below:



Appendix: Occupational Noise Exposure*

"Protection against the effects of noise exposure shall be provided when the sound levels exceed those shown in Table [A] when measured on the A scale of a standard sound level meter at slow response. When noise levels are determined by octave band analysis, the equivalent A-weighted sound level may be determined as shown [in Fig. A].

"When employees are subjected to sound exceeding those listed in Table [A], feasible administrative or engineering controls shall be utilized. If such controls fail to reduce sound levels within the levels of Table [A], personal protective equipment shall be provided and used to reduce sound levels within the levels of the table.

"If the variations in noise level involve maxima at intervals of one second or less, it is to be considered continuous.

"In all cases where the sound levels exceed the values shown herein, a continuing, effective hearing conservation program shall be administered."

*OSHA Standards. Code o f Federal Regulations. Title 79, Part 1910.

Table A - Permissible noise exposuresn

Dura t ion p e r day , hours

Sound l e v e l dBA s low re sponse

90 92 95 97

1 O0 102 105 110 115

'When t h e d a i l y n o i s e exposure i s composed of two o r more p e r i o d s of n o i s e exposure of d i f f e r e n t l e v e l s , t h e i r combined e f f e c t shou ld be c o n s i d e r e d , r a t h e r t h a n t h e i n d i v i d u a l e f f e c t o f each . I f t h e s u m of t h e f o l l o w i n g f r a c t i o n s : Cl /Tl + C 2 / T 2 C n / T n exceeds u n i t y , t h e n , t h e mixed exposure shou ld be c o n s i d e r e d t o exceed t h e l i m i t v a l u e . Cn i n d i c a t e s t h e t o t a l t ime of exposure a t a s p e c i f i e d n o i s e l e v e l , and T n i n d i c a t e s t h e t o t a l t i m e of exposure p e r m i t t e d a t t h a t l e v e l .

Exposure t o impu l s ive o r impact n o i s e shou ld n o t exceed 140 dB peak sound p r e s s u r e l e v e l .


Occupational Noise Exposure / 67

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1 2 0

110

1 O0

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1 2 0 - 115 J

W > W

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Band C e n t e r Frequency (Hz)

Equivalent sound level contours. Octave band sound pressure levers may be converted to the equivalent A-weighted sound level by plotting theni on this graph and noting the A-weishied sound level corresponding io the point of highest penetration into the sound level csntours. This equivalent A-weighted sound level. which iiiay differ froni the actual A-weighted sound level of the noise is used IO determine exposure limits froiii Table [A].

1 i



i

Bibliography

I , Cooper, G . , Palerino, J. . and Browning. .I.A.. "Recent Developments in Plasma Welding." Welrlitig Jourricil. 44 (4). Apr. 1965, pp. 768-376.

2. Filipski, S. P. "Plasma Arc Welding," Welditig Jourticil. 43 ( 1 1). Nov. 1964. pp. 937-943

3. Gage. R. M., "Principles of the Modem Arc Torch." Hélrlit~g Jourrid, 38 (101. Oct. 1959. pp. 959-963.

4. Garrabrant, E. C. and Zuchowski. R. S . , "Plasma Arc-Hot Wire Surfacing-A New High Deposition Process." Weldilig Joirrtml. 48 (S). May 1969. pp. 385-395.

5 . Gaw. W. D. and Starr, G. L., "Plasnia Arc Welding Process Development Program," Technical Repon AFML-TR-68, Vols. 1 , 2. and 3. March 1969.

6. Gorman. E. F.. Skinner. G. M.. and Tenni, D. hl.. "Plasma Needle Arc for Very LOH. Current Work." Weldiiig Jourtiul. 45 ( I l ) , Nov. 1966, pp. 899-908.

7. Hackman. R. L.. "Plasma Arc Techniques." ASTME Technical Paper No. SP60-135.

8. Hackman. R. L., "Putting Plasnia Jets to Work." The Tool ri t id Muritfiicrrrritig Etigiwers. ASTME. March 1961.

9. Langford. G. J.. "Plasma Arc Welding of Structural Titanium Joints." Welditig Jourticil. 47 (31. Feb. 1968. pp. 102-1 13.

10. MacAbee, P. T.. Dyar. J. R-. and Bratkovich. N. F. . "Plailma Arc \Velding ofThin Materials." Technical Report AFML-TR-66- 177. June 1967.

I I , Miller, H. R. and Filipski. S. P.. "Automated Plasma Arc Welding for Aerospace and Cryogenic Fabrications," U'clrlitig Joi~rtid, 4.5 (69, June 1966. pp. 493-501.

12, O'Brien. R. L.. "Applications of the Plasma Arc." ASTbIE Technical Paper No. SP63-56.

13. O'Brien. R. L. , "Arc Plasmas fix Joining. Cutting. and Surfacing." WRC Bulletin No. I3 I . Juty. 1968.

14. "Plasma-Arc Welding." WELDING HANDBOOK. 6th ed.. Section 3B. American Welding Society, Miami. 1971, pp. 54.1-54.33.

15. Privoznik, L. J. and Miller, H . R., "Evaluation of Plasnia Arc Welding for 120 In. Diameter Rocket Motor Cases." Weltlitig Joitrrid. 45 (9). Sept. 1966. pp. 717-775.

16. Zuchowski. R. S. and Garrahrant, E. C.. "New Developments in Plasma Arc Welding Surfacing," Wrlding l o u r d . 43 ( I 1. Jan. 1964. pp. 13-16.


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