AWS WHB 9 FCAW

CHAPTER 5

FLUX CORED ARC WELDING

Prepared by the Welding Handbook Chapter Committee on Flux Cored Arc Welding:

D. B. Arthur, Chair J . W Harris Company B. A. Morrett ITVErlHobart Brothers

Company

J. E. Beckham Thermo King-Zngersoll

Rand D. Sprenkel Consultant

Welding Handbook Committee Member:

C. E. Pepper ENGlobal Engineering, Inc.

Contents

Introduction

Fundamentals

Applications

Equipment

Materials

Process Control

Joint Designs and Welding Procedures

Weld Quality

Troubleshooting

Economics

Safe Practices

Conclusion

Bibliography

Supplementary Reading List

210

210

211

215

219

237

241

247

247

247

250

252

252

253

210 CHAPTER5 FLUX CORED ARC WELDING

CHAPTER 5

FLUX CORED ARC WELDING INTRODUCTION

Flux cored arc welding (FCAW) is a welding process that uses an arc between a continuous filler metal electrode and the weld pool. The process is used with shielding from a flux contained within the tubular electrode, with or without additional shielding from an externally supplied gas, and without the application of pressure.1,2

The remarkable operating characteristics and weld properties that distinguish FCAW from other arc welding processes are attributable to the continuously fed flux cored electrode. The tubular electrode is a filler metal composite consisting of a metal sheath and a core of various powdered materials manufactured in the form of wire. During welding, an extensive protective slag cover is produced on the face of the weld bead.

Flux cored arc welding offers two major variations, self-shielded (FCAW-S) and gas-shielded (FCAW-G), which add great flexibility to the process. These variations differ in the method of shielding the arc and weld pool from atmospheric contamination (oxygen and nitrogen).

Flux cored arc welding is an efficient welding process readily adaptable to semiautomatic or automatic welding operations and capable of producing high-quality weld metal at a high deposition rate. Many industries rely on flux cored arc welding to produce high-integrity welds. Users of the process include manufacturers or

1. American Welding Society (AWS) Committee on Definitions and Symbols, 2001, Standard Welding Terns and Definitiom, A3.0:2001, Miami: American Welding Society. 2. At the time of preparation of this chapter, the referenced codes and other standards were valid. If a code or other standard is cited without a date of publication, it is understood that the latest edition of the document referred to applies. If a code or other standard is cited with the date of publication, the citation refers to that edition only, and it is understood that any future revisions or amendments to the code or standard are not included. As codes and standards undergo frequent revision, the reader is advised to consult the most recent edition.

builders of pressure vessels, submarines, aircraft carri- ers, earth-moving equipment, and buildings and other structures.

This chapter covers the fundamental operating prin- ciples of the flux cored arc welding process and describes the necessary equipment and materials. Signif- icant information is included for a variety of flux cored electrodes used in the major applications of FCAW. Welding procedures, process control, and weld quality are discussed. The chapter ends with comments on the economics of the process and important information on safe practices.

FUNDAMENTALS

The history of gas-shielded arc welding provides the background for the technology and evolution of flux cored arc welding. Gas-shielded metal arc welding processes have been in use since the early 1920s, when it was demonstrated that a significant improvement of weld metal properties could be produced if the arc and molten weld metal were protected from atmospheric contamination. However, the development of covered electrodes in the late 1920s diminished interest in gas- shielding methods. Interest was renewed in the early 1940s with the introduction and commercial accep- tance of gas tungsten arc welding (GTAW) and, later in the same decade, gas metal arc welding (GMAW). Argon and helium were the two primary shielding gases used at that time.

Research conducted on manual welds made with covered electrodes focused on the analysis of the gas produced in the disintegration of electrode coverings. Results confirmed that carbon dioxide (CO2) was the predominant gas given off by electrode coverings. This


discovery quickly led to the use of CO;? as a shielding gas for welds made on carbon steels with GMAW. Although early experiments with COZ were unsuccess- ful, techniques were eventually developed which per- mitted its use. Gas metal arc welding using CO;? became available in the mid-1950s.

In concurrent research, CO;? shielding was combined with a flux-containing tubular electrode, which over- came many of the problems previously encountered. Operating characteristics were improved by the addition of the core materials, and weld quality was improved by eliminating atmospheric contamination. These experiments resulted in the development of flux cored arc welding. This new process was introduced at the American Welding Society (AWS) Exposition in Buffalo, New York, in 1954. By 1957 the electrodes and equipment were refined, and the process was introduced commercially in essentially its present form.

During the 1990s significant improvements were made in both gas-shielded and self-shielded electrode arc stability that resulted in much less spatter than the earlier electrodes produced. The impact resistance of FCAW electrodes was also significantly improved. The development and production of alloy electrodes and small-diameter electrodes, down to 0.8 millimeters (mm) (0.030 inches [in.]), were other advances.

Improvements continue to be made to the FCAW process. Modern power sources and electrode (wire) feeders are greatly simplified and more dependable than their predecessors. Welding guns are lightweight and rugged, and electrodes undergo continuous improvement.

CHAPTER5 211

PROCESS VARIATIONS The two major variations of the FCAW process, the

self-shielded and the gas-shielded versions, are shown in Figure 5.1. Both illustrations in Figure 5.1 emphasize the melting and deposition of filler metal and flux and show the formation of a slag covering the weld metal. Cross sections of examples of FCAW electrodes also are shown in Figure 5.1.

In the gas-shielded method, the shielding gas (CO;? or a mixture of argon and COz) protects the molten metal from the oxygen and nitrogen present in air by forming an envelope of gas around the arc and over the weld pool. Little need exists for denitrification of the weld metal because air is mostly excluded, along with the nitrogen it contains. However, some oxygen may be generated from the dissociation of COZY which forms carbon monoxide and oxygen. The compositions of the electrodes are formulated to provide deoxidizers that combine with small amounts of oxygen in the gas shield.

Self-shielded flux cored arc welding is often the process of choice for field welding because it can tolerate stronger air currents than the gas-shielded variation. The main reason for this distinction is that some shield-

ing is provided by the high-temperature decomposition of some of the electrode core ingredients. The vaporiza- tion of these ingredients displaces air from the area immediately surrounding the arc. In addition, the wire contains a large proportion of scavengers (deoxidizers and denitrifiers) that combine with undesirable elements that might contaminate the weld pool. A slag cover protects the metal from the air surrounding the weld.

AP P LI CATION S

Both self-shielded and gas-shielded flux cored arc welding can be used in most welding applications. However, the specific characteristics of each method make each suitable for different operating conditions. The process is used to weld carbon- and low-alloy steels, stainless steels, cast irons, and nickel and cobalt alloys. It is also used for the arc spot welding of lap joints in sheet and plate, as well as for cladding and hardfacing.

Flux cored arc welding is widely used in fabrication shops, for maintenance applications, and in field erection work. An example of field erection work is shown in Figure 5.2, in which both self-shielded and gas- shielded FCAW are used in the fabrication of an off- shore oil drilling structure.

Flux cored arc welding can be used to produce weld- ments that conform to the A S M E Boiler and Pressure Vessel CodeY3 the rules of the American Bureau of Ship- ping: and Structural Welding Code-Steel, AWS Dl.l.5 The process is given prequalified status in AWS D1.l. Stainless steel, self-shielded, and gas-shielded flux cored electrodes are used in general fabrication, surfacing, joining dissimilar metals, and maintenance and repair.

Figure 5.3, which shows the fabrication of a suction filter used in the pulp and paper industry, illustrates the versatility of the FCAW process. The base material in this application was ST-360-C; E308LT1 electrodes were used.

The self-shielded method can often be used for applications that are normally welded with the shielded metal arc welding (SMAW) process. Gas-shielded FCAW can also be used for some applications that are welded by the GMAW process. The selection of self- shielded or gas-shielded FCAW depends on the type of electrodes available, the type of welding equipment available, the environment in which the welding is to be

3 . American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Committee, 1998, Boiler and Pressure Vessel Code. New York: American Society of Mechanical Engineers. 4. American Bureau of Shipping (ABS) Group, ABS Plaza, 166855 Northchase Drive, Houston, TX 77060-6008. 5. American Welding Society (AWS) Committee on Structural Weld- ing, 2002, Structural Welding Code-Steel, AWS Dl.l/Dl.lM:2002, Miami: American Welding Society.

FLUX CORED ARC WELDING 212 CHAPTER5

EXAMPLES OF CROSS FCAW WITH SECTIONS OF FLUX FCAW WITH

ELECTRODE ELECTRODE ELECTRODE SELF-SHIELDED CORED ARC WELDING GAS-SHIELDED

FLUX CORED ELECTRODE

CONTACT TIP

SELF-GENERATED GAS SHIELDING

I DIRECTION OF TRAVEL 7 INSULATED DIRECT1 TORCH BODY OF TRAVEL 7 SHIEL CA.C

"C-Lrj Y. .Y I J /LAG WELD POOL 1 / /

III. 1 FLUXCORED / ELECTRODE

WELD POOL ~

I CONTACTTIP

Figure 5.1 -Self-shielded and Gas.Shielded Flux Cored Arc Welding

done, the mechanical property requirements of the welded joints, and the joint design and fitup. The advantages and disadvantages of FCAW should be compared to those of other processes when it is evaluated for a specific application.

ADWANTAGES When compared to SMAW, higher productivity is the

chief advantage of flux cored arc welding for many applications. This generally translates into lower overall

costs per pound of metal deposited in joints that permit continuous welding and easy FCAW gun and equipment accessibility. The advantages are higher deposition rates, higher operating factors, and higher deposition efficiency (no stub loss).

In addition to the advantages of FCAW over the manual SMAW process, FCAW also provides certain advantages over submerged arc welding (SAW) and GMAW. In many applications, FCAW produces high- quality weld metal at lower cost with less effort on the part of the welder than SMAW. Flux cored arc welding

FLUX CORED ARC WELDING CHAPTER5 213

Figure 5.2-Off shore Drilling Structure Fabricated with Self-Shielded and GasmShielded Flux Cored Arc Welding

is more forgiving of minor disparities in procedures and differences in welder skill than GMAW, and it is more flexible and adaptable than SAW. Among the benefits offered by FCAW are the following:

1. 2. 3. 4. 5.

High-quality weld metal deposit, Excellent weld appearance, Welds many steels in a wide thickness range, High operating factor and easily mechanized, High deposition rate (up to four times greater than SMAW) and high current density,

6. 7. 8. 9 .

10.

11.

12.

13.

Relatively high electrode deposit efficiency, Allows economical engineering of joint designs, Visible arc contributes to easy use, Requires less precleaning than GMAW, Often results in less distortion compared to SMAW, Exceptionally good fusion when used with shielding gas compared to GMAW-S, High tolerance for contaminants that may cause weld cracking, Resistance to underbead cracking,


14.

15.

Photograph courtesy of Bohler Thyssen Welding USA, Inc.

Figure 5.3-Flux Cored Arc Welding of a Suction Filter Used in the Pulp and Paper Industry

Self-shielding characteristic of electrodes elimi- nates the need for flux handling and gas appara- tus, and Self-shielding tolerates windy conditions in out- door applications. (See No, 6 relative to gas shields under Limitations in the next section),

suitable exhaust equipment, except in field work. Com- pared to the slag-free GMAW process, the need for removing slag between passes is an added labor cost when using FCAW. This is especially true in making root pass welds.However, in most cases, slag is easily removed and cleanup time is minimized, as shown in Figure 5.5. The limitations of FCAW are summarized as

An example of the good sidewall fusion, deep penetration and smooth weld profile that can be obtained with gas-shielded FCAW is shown in Figure 5.4

LI M lTATl0 N S Compared to the SMAW process, the major limita-

tions of FCAW are the higher cost of the equipment, the relative complexity of setup and control of the equipment, and the restriction on operating distance from the electrode wire feeder. Flux cored arc welding may generate large volumes of welding fumes and requires

foiiows:

1. FCAW is limited to welding ferrous metals and nickel-base alloys;

2. The process produces a slag covering that must be removed;

3 . FCAW electrode wire is more expensive on a weight basis than solid electrode wires, except for some high-alloy steels;

4. The equipment is more expensive and complex than that required for SMAW, however, increased productivity usually compensates for this;


Photograph courtesy of Bohler Thyssen Welding USA, Inc.

Figure 5.4-Flux Cored Arc Weld Profile

PhotoQraph courtesy of Bohler Thyssen Welding USA, Inc.

Figure 5.5-Self-Peeling Slag Reveals a Clean Flux Cored Arc Weld

5. The wire feeder and power source must be fairly close to the point of welding;

6. For gas-shielded FCAW, the external shield may be adversely affected by breezes and drafts;

7 . Equipment is more complex than that used for SMAW, so more maintenance is required; and

8. More smoke and fumes are generated by FCAW than by GMAW and SAW.

It should be noted that self-shielded FCAW is not adversely affected by windy conditions, except in very high winds, because the shield is generated at the end of the electrode exactly where it is required.

EQUIPMENT

The basic equipment setup for flux cored arc welding is shown in Figure 5.6. Equipment consists of a power source, electrode feed and current controls, a shielding gas source, a wire electrode feeding system, a welding gun, and the associated cables and gas hoses. In addition, appropriate fume extraction equipment may be needed. Proper ventilation or some means of fume removal is necessary for FCAW.

SEMIAUTOMATIC EQUIPMENT Control equipment for semiautomatic self-shielded

and gas-shielded flux cored arc welding is similar. The major difference between the shielding variations is the provision for supplying and metering gas to the arc of the electrode in the gas-shielded method.

Power Source The recommended power source is the direct current

(dc) constant-voltage type, similar to power sources used for GMAW. The power source should be capable of operating at the maximum current required for the specific application. Most semiautomatic applications use less than 500 amperes (A). The voltage control should be capable of adjustments in increments of one volt or less. Constant-current dc power sources of adequate capacity with appropriate controls and wire feeders are sometimes used, but applications are rare.

Electrode Feed Control The purpose of the electrode (wire) feed control is to

supply the continuous electrode to the welding arc at a constant preset rate. The rate at which the electrode is fed into the arc determines the welding amperage supplied by a constant-voltage power source. If the


DIRECT-CURRENT CON STANT-VOLTAG E

POWER SOURCE 1

Li I I

TO SOLENOID I VOLTAGE CONTROL

(74 VALVE \I c -

W VOLTMETER AND CONTACTOR CONTROL

I 1 I AAMMETER

115 V SUPPLY

- I 1 I I I

WIRE DRIVE MOTOR

I / ELECTRODE POWER CABLE

WORKPIECE JORKPIECE CABLE

NOTE: GAS SHIELDING IS USED ONLY WITH FLUX CORED ELECTRODES THAT REQUIRE IT.

SHIELDING 3AS SOURCE

Figure 5.6-vpical Equipment for Semiautomatic Flux Cored Arc Welding

electrode feed rate is changed, the welding machine automatically adjusts to maintain the preset arc voltage. The electrode feed rate can be controlled by mechanical or electronic means.

Semiautomatic flux cored arc welding requires the use of drive rolls that will not flatten or otherwise dis- tort the tubular electrode. Various grooved and knurled feed roll surfaces are used to advance the electrode. Some wire feeders have a single pair of drive rolls; others have two pairs of rolls with at least one roll of each pair being driven. When all rolls are driven, the wire can be advanced with less pressure on the rolls.

Welding Guns Typical guns for semiautomatic welding are shown in

Figure 5.7 and Figure 5.8. They are designed for handling comfort, easy manipulation, and durability. The guns provide internal contact with the electrode to con- duct the welding current. The welding current and electrode feed are actuated by a switch mounted on the gun.

Welding guns may be either gas-cooled or water- cooled. Gas-cooled (including air-cooled) guns are favored because a water delivery system is not required; however, water-cooled guns are more compact, lighter

FLUX CORED ARC WELDING CHAPTER 5 21 7

CONDUIT POWER

- - - - - - -

CONTACT0 R LEADS

PROTECTIVE HAND SHIELD

CONTACT TUBE

EXTENSION GUIDE

in weight, and require less maintenance than gas-cooled guns. Water-cooled guns generally have higher current ratings (up to 700 A, continuous duty). Current ratings for gas-cooled guns are based on using COZ. If argon- based gas is used, the gun current rating should be decreased 30%. Guns have either straight or curved nozzles. The curved nozzle may vary from 10" to 90". In some applications, the curved nozzle enhances flexibility and ease of electrode manipulation.

Some self-shielded flux cored electrodes require a specific minimum electrode extension to develop proper shielding. Welding guns for these electrodes generally have guide tubes with an insulated extension guide to support the electrode and assure a minimum electrode extension. Details of a self-shielded electrode nozzle showing the insulated guide tube are illustrated in Figure 5.9.

Figure 5.7-Gun for Semiautomatic Self-shielded Flux Cored Arc Welding

AUTOMATIC EQUIPMENT Figure 5.10 shows the equipment layout for an auto-

matic flux cored arc welding installation. A direct- current power source with constant-voltage designed for 100% duty cycle is recommended for automatic operation. The size of the power source is determined by the current required for the work to be performed. Because large electrodes, high electrode feed rates, and long welding times may be required, electrode feeders for automatic operation necessarily have higher- capacity drive motors and heavier-duty components than similar equipment for semiautomatic operation.

Two typical nozzle assemblies for automatic gas- shielded flux cored arc welding are shown in Figure 5.11. Nozzle assemblies are designed for side shielding or for concentric shielding of the electrode. Side shielding


POWER CABLE

GAS COOLED CHAMBER

CONTACT TUBE

(A) Gas-Cooled

P CONTACT TUBE WATER COOLED CHAMBER

L GAS NOZZLE

ARROWS INDICATE .C-- WATER IN - WATEROUT

0 GAS SWITCH -

HAND SHIELD I+

(B) Water-cooled

POWER CABLE, GAS INLET, WATER IN AND OUT

Figure 5.8-Wpical Guns for Gas=Shielded Flux Cored Arc Welding

permits welding in deep, narrow grooves and minimizes spatter buildup in the nozzle. Nozzle assemblies are air- cooled or water-cooled. In general, air-cooled nozzle assemblies are preferred for operation with welding currnts up to 600 A. Water-cooled nozzle assemblies are recommended for currents above 600 A. For higher deposition rates with gas-shielded electrodes, tandem welding guns can be used, as shown in Figure 5.12.

For large-scale surfacing applications, automatic multiple-electrode oscillating equipment can be used to increase productivity. The equipment for these installa- tions may include a track-mounted manipulator sup- porting a multiple-electrode oscillating welding head with individual electrode feeders and a track-mounted, power-driven turning roll, in addition to the power source, electronic controls, and an electrode supply sys-


WORKPIECE

Figure 5.9-Self=Shielded Electrode Nozzle

tem. Figure 5.13 illustrates the operating details of a six-electrode oscillating system for the self-shielded surfacing of a vessel shell with stainless steel.

The dew point of shielding gases should be below -40F (-40C).

The fumes generated during flux cored arc welding can be hazardous. To assure adequate ventilation, por- table fume extraction systems and welding guns with integrated fume extractors are available. A welding gun fume extractor usually consists of an exhaust nozzle that encircles the gun nozzle. It can be adapted to gas- shielded and self-shielded guns. The nozzle is ducted to a filter canister and an exhaust pump. The aperture of the fume-extracting nozzle is located at a sufficient distance behind the top of the gun nozzle to draw in the fumes rising from the arc without disturbing the shielding gas flow. The chief advantage of this fume extraction system is that it remains close to the fume source wherever the welding gun is used. In contrast, a porta- ble fume exhaust system generally cannot be positioned as closely to the fume source and requires repositioning the exhaust hood for each significant change in welding location.

FUME EXTRACTION One disadvantage of the welding gun fume extractor

system is that the added weight and bulk make semiautomatic welding more cumbersome for the welder. If not properly installed and maintained, fume extractors may cause welding problems by disturbing the gas shielding. In a well-ventilated welding area, a fume- extractor and welding gun combination may not be necessary. Additional information on proper ventilation is presented in the Safe Practices section of this chapter.

MATERIALS

MATERIALS Like GMAW electrodes, gas-shielded FCAW elec-

trodes require gas shielding in addition to the shielding provided by the internal flux. Gas shielding equipment includes a gas source, a pressure regulator, a flow metering device, and the necessary hoses and connec- tors. Shielding gases are dispensed from cylinders, cylin- der manifolds, or bulk tanks from which gases are piped to individual welding stations. Regulators and flow meters are used to control pressure and flow rates. Because regulators can freeze during rapid withdrawal of COz from storage tanks, heaters are available to prevent that complication. Welding-grade gas purity is required because small amounts of moisture can result in porosity or hydrogen absorption in the weld metal.

The base metals commonly welded with flux cored arc welding, the shielding gases used, and electrodes appropriate for various applications are described in this section.

BASE METALS Most of the commonly used types of ferrous plate,

pipe, and castings and many nickel alloys can be welded using the FCAW process. The categories of base metals generally welded with FCAW are mild steels, high- strength steels, chrome-molybdenum steels, stainless steels, abrasive-resistant steels, cast steels, and nickel alloys.


-

DI RECT-CU RRENT CONSTANT- VOLTAGE

--------- I \GAS IN

A

SHIELDING

L

Y I

WIRE WHEEL

ELECTRODE POWER CABLE

i I 1 I I I I I I I I I 1 I

I I I I I I > GAS OUT I

WELDING GUIDE TUBE AND CONTACT TUBE

NOTE: GAS SHIELDING IS USED ONLY WITH ELECTRODES THAT REQUIRE IT.

CABLE

Figure 5.1 O-Typical Automatic Flux Cored Arc Welding Equipment

Mild Steels Structural and pressure-vessel grades of mild steel,

such as A36, A515, and A516 are the steels most often welded with FCAW. Pipe and castings of similar composition also are welded using this process. These steels are relatively easy to weld with FCAW using minimal precautions except under extreme environmental conditions. Potential moisture pick-up must be considered in very humid environments. (See Protection from Mois- ture in the Electrodes section of this chapter. When

the base metal is very cold or welding is being done on thick sections, preheating of the base metal may be necessary.

H ig h-Strength Steels This category includes high-strength low-alloy

(HSLA) steels, such as ASTM A441, A572, and A588. The high yield strength, quench-and-tempered (Q&T) steels ASTM A514 and A517 are also welded with the FCAW process. Welding these classes of steel is increas-


AIR-COOLED SIDE-SHIELDED NOZZLE ASSEMBLY

- ELECTRODE

PO LEAD I

WATER-COOLED CONCENTRIC-SHIELDED NOZZLE ASSEMBLY

ELECTRODE

Figure 5.1 l--'Fypical Nozzle Assemblies for Automatic Gas-Shielded FCAW

TRAIL

1 * J WELDING DIRECTION

'RODE

Figure 5.1 2-Automatic Tandem Arc Welding with Two Gas=Shielded Flux Cored Electrodes


ASSEMBLY

A. CONTACT TUBE B. PNEUMATIC CONTROL PANEL

A. CABINET B. OPERATOR STATION

2. CONTACT TUBE ASSEMBLY

3. ELECTRONIC CONTROL SYSTEM

4 WELDING POWER SOURCE 5 . AUXILIARY ELECTRODE HANDLING SYSTEM 6. WELDING HEAD MANIPULATOR 7. WORKTURNING ROLLS

Figure 5.1 3-Typical Multiple-Weave Surfacing Installation

ing as manufacturers produce steels with increasingly higher strength-to-weight ratios.

Precautions recommended by the base metal and filler metal manufacturers must be followed when welding high-strength steels. The rapid cooling rates associated with welding alter the metallurgical structure and properties of the heat-affected zone (HAZ) of the base metal in the weld joint. Generally, as alloying content increases (especially carbon), there is increased need for precautions such as preheat and postweld heat treatment. The change in properties in the HAZ must be anticipated during weld- ment design. Further information on these precautions is presented in the AWS Welding Handbook, Materials and Applications-Part 2, Volume 4, 8th edition.6

6 . American Welding Society (AWS) Welding Handbook Committee, W. R. Oates and A. M. Saitta, eds., 1998, Materials and Application+Part 2, Vol. 4 of Welding Handbook, 8th ed. Miami: American Welding Society.

Chromium-Molybdenum Steels Chrome-molybdenum (Cr-Mo) steels such as 1-1/4 %

Cr-1/2% Mo and 2-1/4%0 Cr-1% Mo and 9% Cr-1% Mo (Grade 91) are welded with the FCAW process. As with high-strength steels, precautions must be taken to allow for hardenability.

Stainless Steels Most corrosion-resistant stainless steels such as AISI

types 304, 309, 316, 409, 410, and 17-4 PH are weld- able with the FCAW process. Stainless steel castings are also welded using flux cored electrodes. Discussion of the metallurgy and weldability of these steels is beyond the scope of this chapter. More detailed information is


presented in the AWS Welding Handbook, Materials and Applications-Part 2, Volume 4, 8th edition.'

CHAPTER5 223

Thus, the arc atmosphere contains a considerable amount of oxygen that reacts with elements in the molten metal. The oxidizing tendency of COZ shielding gas is recognized in the formulation of flux cored electrodes. Deoxidizing materials are added to the core of the electrode to compensate for the oxidizing effect of the COz.

In addition, molten iron reacts with COZ and produces iron oxide and carbon monoxide in a reversible reaction:

Abrasion-Resistant Steels Abrasion-resistant steels are often welded with the

FCAW process. These steels have very high hardness and high tensile strength. The welding consumables (electrodes and fluxes) used to weld abrasion-resisting steels usually have neither the structural strength nor the abrasion resistance of the base metal. The lack of strength is generally not a great concern because these steels are not normally used for structural applications. The intended function of the weld metal is mainly to hold the plates in position rather than to provide structural strength. If the weld requires abrasion resistance equal to the base plate, a hardsurfacing electrode should be used on the surface of the weld after the plates are welded in position.

Nickel Alloys While nickel alloys are welded using the FCAW pro-

cess, their metallurgy and weldability are beyond the scope of this chapter. More detailed information is presented in the AWS Welding Handbook, Materials and Applications-Part 1, Volume 3, 8th edition.*

SHIELDING GASES Carbon dioxide (CO,) and mixtures of argon and

COZ are the preferred shielding gases for flux cored arc welding.

Carbon Dioxide Carbon dioxide is widely used as a shielding gas for

flux cored arc welding. This gas usually provides a globular metal transfer, although some flux formula- tions produce a spray-like metal transfer in COZ. It pro- motes deep weld penetration and is lower in cost than mixed gases.

Carbon dioxide is relatively inactive at room temperature. When heated to high temperature by the welding arc, COZ dissociates to form carbon monoxide (CO) and oxygen (OZ), as indicated by the following chemical equation:

7. See Reference 6. 8. American Welding Society (AWS) Welding Handbook Committee, W. R. Oates, ed., 1996, Materials and Applications-Part 1 , Vol. 3 of Welding Handbook. 8th ed. Miami: American Weldine Societv.

Fe + COz @ FeO + CO (5.2)

At red heat temperatures, some of the carbon monoxide dissociates to carbon and oxygen, as follows:

2 C 0 @ 2 C + 0 2 (5.3)

The effect of COZ shielding on the carbon content of mild and low-alloy steel weld metal is unique. Depend- ing on the original carbon content of the base metal and the electrode, the COz atmosphere can behave either as a carburizing or decarburizing medium. Whether the carbon content of the weld metal will be increased or decreased depends on the carbon present in the electrode and the base metal. If the carbon content of the weld metal is below approximately 0.05%, the weld pool will tend to pick up carbon from the COz shielding atmosphere. Conversely, if the carbon content of the weld metal is greater than approximately 0.10% the weld pool may lose carbon. The loss of carbon is attrib- uted to the formation of carbon monoxide caused by the oxidizing characteristics of COZ when used as shielding gas at high temperatures.

When this reaction occurs, the carbon monoxide can be trapped in the weld metal and will create porosity. This tendency can be minimized by using an electrode that provides an adequate level of deoxidizing elements in the core. Oxygen reacts with the deoxidizing elements rather than the carbon in the steel. That reaction results in the formation of solid oxide compounds that float to the surface of the weld pool where they form part of the slag covering.

Gas Mixtures Gas mixtures used in flux cored arc welding may

combine the separate advantages of two or more gases, including carbon dioxide, oxygen and argon. The higher the percentage of inert gas in mixtures with COz or oxygen, the higher the transfer efficiencies of the deoxidizers contained in the core will be. Argon is capable of protecting! the weld pool at all welding! tempera-

" " tures. ?he presence of argon in sufficient quantities in a


a closing roll rounds the filled shape and closes it. The round tube is pulled through drawing dies or rolls that reduce the diameter and compress the core. The electrode is drawn to final size, and then wound (as wire) on spools or in coils. Other manufacturing methods are also used.

Manufacturers generally consider the precise composition of their cored electrodes to be proprietary information. By proper development and selection of the core ingredients (in combination with the composition of the sheath), manufacturers have achieved the following:

shielding gas mixture results in less oxidation than occurs with 100% C 0 2 shielding.

The mixture commonly used in gas-shielded FCAW is 75% argon and 25% carbon dioxide. When welding with this mixture, a spray-transfer arc is achieved. The 75% argon/25% C 0 2 mixture provides better arc characteristics than 100% C02, resulting in greater operator appeal.

Weld metal deposited with this mixture generally has higher tensile strength and yield strength than weld metal deposited with 100% COZ shielding because of high transfer efficiencies. Manganese and silicon are transferred into the weld pool and remain as alloying elements instead of combining with oxygen. When shielding gas mixtures with high percentages of inert gases are used with electrodes designed for COZY the shielding gas mixture may cause an excessive buildup of manganese, silicon, and other deoxidizing elements in the weld metal. The resulting higher alloy content of the weld metal changes the mechanical properties. There- fore, the electrode manufacturer should be consulted to ascertain the mechanical properties of weld metal obtained with specific shielding gas mixtures. If data are not available, tests should be made to determine the mechanical properties for the particular application.

ELECTRODES As previously described, the flux cored electrode is a

composite tubular filler-metal electrode consisting of a metal sheath and a core of various powdered materials. Both the sheath and core contain ingredients that con- tribute to the highly desirable operating characteristics and weld properties of the process. The use of this electrode differentiates the FCAW process from other arc welding proces~es.~

Flux cored arc welding owes much of its versatility to the wide variety of ingredients that can be included in the core of the tubular electrode. For ferrous alloys, the electrode usually consists of a low-carbon steel or an alloy-steel sheath surrounding a core of fluxing and alloying materials. The composition of the flux core varies according to the electrode classification and the particular manufacturer of the electrode.

Most flux cored electrodes are made by passing a steel strip through rolls that form it into a U-shaped cross section. The formed strip is filled with a measured amount of granular core material (alloys and flux); then

9. The electrogas welding (EGW) process uses a flux cored electrode to make single-pass welds in the vertical position. See Chapter 8, Electrogas Welding. Flux cored electrodes are also used in gas tungsten arc welding (Chapter 3) for some applications. It should be noted that metal-cored electrodes do not match the definition of flux cored electrodes and are not discussed in this chapter. Metal-cored electrodes are described in Chapter 4, Gas Metal Arc Welding.

1. Electrodes with welding characteristics ranging from high deposition rates in the flat position to proper fusion and bead shape in the overhead position,

2. Electrodes for various gas shielding mixtures and for self shielding, and

3 . Variations in the alloy content of the weld metal from mild steel for certain electrodes to high- alloy stainless steel for others.

The primary functions of the flux core ingredients are to accomplish the following:

1. Provide the mechanical, metallurgical, and corrosion-resistant properties of the weld metal by adjusting the chemical composition;

2. Promote weld metal soundness by shielding the molten metal from oxygen and nitrogen in the air or, in the case of self-shielded FCAW, to react with nitrogen or oxygen, or both, in the air and render it harmless;

3. Scavenge impurities from the molten metal through the use of fluxing reactions;

4. Produce a slag cover to protect the solidifying weld metal from the air.

5. Control the shape and appearance of the bead in the different welding positions for which the electrode is suited; and

6. Stabilize the arc by providing a smooth electrical path to reduce spatter and facilitate the deposition of uniformly smooth, properly sized beads.

Table 5.1 lists most of the elements commonly found in the flux core, the form in which they are integrated, and the purposes for which they are used.

In mild steel and low-alloy steel electrodes, a proper balance of deoxidizers and denitrifiers (in the case of self-shielded electrodes) must be maintained to provide a sound weld deposit with adequate ductility and toughness. Deoxidizers, such as silicon and manganese, combine with oxygen and form stable oxides. This helps control the loss of alloying elements through oxidation and the formation of carbon monoxide, which otherwise could cause porosity. The denitrifiers, such as


Table 5.1 Common Core Elements in Flux Cored Electrodes

Element Usually Present As Purpose in Weld Aluminum Metal powder Deoxidize and denitrify Boron Ferroboron Grain refinement Calcium Minerals such as fluorspar (CaF2) and limestone

(CaC03) Provide shielding and form slag

Carbon Element in ferroalloys such as ferromanganese Increase hardness and strength Chromium

Iron

Manganese

Molybdenum

Nickel

Potassi urn

Silicon

Sodium

Ferroalloy or metal powder

Ferroalloys and iron powder, sheath

Alloying to improve creep resistance, hardness, strength, and corrosion resistance Alloy matrix in iron-base deposits, alloy in nickel-base and other nonferrous deposits

Ferroalloy such as ferromanganese or as metal powder

Deoxidize; prevent hot shortness by combining with sulfur to form manganese sulfide; increase hardness and strength; form slag

Ferroalloy

Metal powder

Alloying to increase hardness and strength; in austenitic stainless steels to increase resistance to pitting-type corrosion Alloying to improve hardness, strength, toughness and corrosion resistance

Minerals such as potassium-bearing feldspars and silicates in frits Ferroalloy such as ferrosilicon, or silicomanganese; mineral silicates such as feldspar Minerals such as sodium-bearing feldspars and silicates in frits

Stabilize the arc and form slag

Deoxidize and form slag

Stabilize the arc and form slag

Vanadium Oxide or metal powder Increase strength Titan i u m Ferroalloy such as ferrotitanium; in mineral, rutile

(titanium dioxide) some stainless steels Deoxidize and denitrify; form slag; stabilize carbon in

Zirconium Oxide or metal powder Deoxidize and denitrify; form slag

aluminum, combine with nitrogen and tie it up as stable nitrides. This prevents nitrogen porosity and the formation of other nitrides that might be harmful.

Electrode Classifications The American Welding Society has developed a sys-

tem of electrode classifications for the various welding processes and the most commonly used metals and materials. The Society maintains the classifications with current information and publishes specifications for the various electrode classes. The descriptions of the various electrodes in this section are contributed by the American Welding Society Committee on Filler Metals and Allied Materials.

Mild Steel Electrodes Most mild steel FCAW electrodes are classified

according to the requirements of the latest edition of

ANSUAWS A5.20, Specification for Carbon Steel Elec- trodes for Flux Cored Arc Welding.lo The identification system follows the general pattern for electrode classification and is illustrated in Figure 5.14.

The classification system can be explained by consid- ering a typical designation, E70T-1. The prefix E indicates an electrode, as in other electrode classification systems. The first number refers to the minimum as-welded tensile strength in 10,000 pounds per square inch (psi) units. In this example, the number 7 indicates that the electrode has a minimum tensile strength of 70,000 psi. The second number indicates the welding positions for which the electrode is designed. Here, the 0 means that the electrode is designed for flat groove and fillet welds, and horizontal groove and fillet welds.

10. American Welding Society (AWS) Committee on Filler Metals and Allied Materials, Specification for Carbon Steel Electrodes for Flux Cored Arc Welding, ANSUAWS A5.20, Miami: American Welding Society.


Mandatory Classification Designators*

Designates an electrode.

This designator is either 6 or 7. It indicates the minimum tensile strength (in psi x 10 000) of the weld metal when the weld is made in the manner prescribed by this specification.

This designator is either 0 or 1. It indicates the positions of welding for which the electrode is intended.

0 is for flat and horizontal position only.

1 is for all positions.

This designator indicates that the electrode is a flux cored electrode.

This designator is some number from 1 through 14 or the letter G with or without an S following. The number refers to the usability of the electrode. The G indicates that the external shielding, polarity, and impact properties are not specified. The S indicates that the electrode is suitable for a weld consisting of a single pass. Such an electrode is not suitable for a multiple-pass weld.

An M designator in this position indicates that the electrode is classified using 7540% argon/balance COB

X X T - X M

shielding gas. When the M designator does not appear, it signifies that either the shielding gas used for classification is CO, or that the product is a self-shielded product.

Optional Supplemental Designators 7 I Designates that the electrode meets the requirements of the diffusible hydrogen test (an optional supplemen-

tal test of the weld metal with an average value not exceeding 2 mL of H2 per 1 OOg of deposited metal where Z is 4, 8, or 16).

- Designates that the electrode meets the requirements for improved toughness by meeting a requirement of 20 ft.Ibf at -4OOF (27J at -4OOC). Absence of the J indicates normal impact requirements.

*The combination of these constitutes the electrode classification.

Source: Adapted from American Welding Society (AWS) Committee on Filler Metals and Allied Materials, Specification for Carbon Steel Electrodes for Flux Cored Arc Welding, ANSVAWS A5.20-95, Miami: American Welding Society, Figure A1 .

Figure 5.1 &Classification System for Mild Steel FCAW Electrodes

However, some classifications may be suitable for vertical or overhead positions, or both. In those cases, a 1 would be used instead of the 0 to indicate all- position capability.

The letter T indicates that the electrode is of a tubular construction (a flux cored electrode). The suffix number 1 (in this example) indicates a general group- ing of electrodes that contain similar flux or core components and have similar usability characteristics. The group designators described in Figure 5.14 are expanded in Table 5.2, which lists usability characteristics of mild steel FCAW electrodes. Other possible designators can be included with this suffix, such as an M, indicating that the electrode was tested in mixed

gas, J to designate enhanced impact properties, or an H followed by a number signifying diffusible hydrogen testing results.

As shown in Figure 5.14, mild steel FCAW electrodes are classified on the basis of whether they are self- shielded or whether they are intended to be used with C02 or mixed gas, which is usually considered to be 75% to 80% argon and the balance C02. The classification also specifies the type of current, usability out of position, the chemical composition, and the as-welded mechanical properties of deposited weld metal. Some classifications are listed as being suitable for multiple- and single-pass welding, while others are stated to be suitable for single-pass welding only. Electrodes for


Table 5.2 Shielding and Polarity Requirements for Mild Steel FCAW Electrodes

AW S Class if i ca t i o n Recommended Weld Passes External Shielding Medium Current and Polarity

EXXT-1 Multiple co2 DCEPC EXXT-1 M Multiple Mixed gasa DCEP EXXT-2 Single co2 DCEP EXXT-2M Single Mixed gasa DCEP EXXT-3 Single None DCEP EXXT-4 Multiple None DCEP EXXT-5 Multiple co2 DCEP EXXT-5M Multiple Mixed gasa DCEP EXXT-6 Multiple None DCEP EXXT-7 Multiple None DCENd EXXT-8 Multiple None DCEN EXXT-9 Multiple co2 DCEP EXXT-9M Multiple Mixed gasa DCEP EXXT-10 Single None DCEN EXXT-11 Multiple None DCEN EXXT-I 2 Multiple GO2 DCEP EXXT-12M Multiple Mixed gasa DCEP EXXT-13 Single None DCEN EXXT-14 Single None DCEN EXXT-G Multiple Note b Note b EXXT-GS Single Note b Note b

Notes: a. Mixed gas normally refers to 75% to 80% argon/balance C02 b. As agreed upon by supplier and user c. Direct current electrode positive. d. Direct current electrode negative.

single-pass welding have more deoxidizing elements such as manganese and silicon and can be used to weld over mill scale or rust without resulting in porosity. When these electrodes are used for more than a single pass, these deoxidizers will increase the effective alloy content of the weld metal, excessively increasing hardness and reducing ductility. These same effects will also be observed when an electrode classified with COZ shielding gas is used with a less reactive gas (argon or combinations containing argon, for example). Elec- trodes are designed to produce weld metal having specified chemical composition and mechanical properties when the welding and testing are performed according to the specification requirements.

Electrodes are produced in standard diameter sizes ranging from 0.8 to 4.0 mm (0.030 to 5/32 in.). Special sizes may also be available. Weld properties may vary appreciably, depending on a number of conditions,

including electrode size, welding amperage, plate thickness, joint geometry, preheat and interpass temperatures, surface conditions, base metal composition and admixture with the deposited metal, and shielding gas (if required). Many electrodes are designed primarily for welding in the flat and horizontal positions. They may also be suitable for use in other positions, depending on electrode diameter, choice of heat input, and the level of operator skill. Selected electrodes with diameters below 2.4 mm (3/32 in.) may be used for out-of- position welding at welding currents on the low side of the manufacturer's recommended range.

The classifications, descriptions, and intended uses of mild steel electrodes as designated in ANSUAWS A5.20 are described below.

EXXT-1 and EXXT-1M. Electrodes of the EXXT-1 group are classified with COZ shielding gas. However,

228 CHAPTER5

other gas mixtures, such as argon and C02, may be used to improve the arc characteristics, especially for out-of-position work, when recommended by the manufacturer. Increasing the amount of argon in the argon/ C02 mixture will increase the manganese and silicon contents in the weld metal. The increase in manganese and silicon will increase the yield strength and tensile strength and may affect impact properties.

Electrodes of the EXXT-1M group are classified with 75% to 80% argodbalance C 0 2 shielding gas. The use of these electrodes with argon/COz shielding gas mixtures with reduced amounts of argon, or with C 0 2 shielding gas alone, may result in some deterioration of arc characteristics and out-of-position welding characteristics. In addition, a reduction of the manganese and silicon contents in the weld will reduce yield and tensile strengths and may affect impact properties.

Both the EXXT-1 and EXXT-1M electrodes are designed for single- and multiple-pass welding using direct current electrode positive (DCEP) polarity. The larger diameters (usually 2.0 mm [5/64 in.] and larger) are used for welding in the flat position and for welding fillet welds in the horizontal position (EXOT-1 and EXOT-1M). The smaller diameters (usually 1.6 mm [ 1/16 in.] and smaller) are generally used for welding in all positions (EXlT-1 and EXlT-1M). The EXXT-1 and EXXT-1M electrodes are characterized by a spray transfer, low spatter loss, flat-to-slightly convex bead contour, and a moderate volume of slag that completely covers the weld bead. Most electrodes of this classification have a rutile-base slag and produce high deposition rates.


rates of these electrodes are similar to those of the EXXT-1 and EXXT-1M classifications.

EXXT-2 and EXXT-2M. Electrodes of these classifications are essentially EXXT-1 and EXXT-1M with higher percentages of manganese or silicon, or both, and are designed primarily for single-pass welding in the flat position and for welding fillet welds in the horizontal position. The higher levels of deoxidizers in these classifications allow the single-pass welding of heavily oxidized or rimmed steel.

Weld metal composition requirements are not specified for single-pass electrodes, since checking the composition of the undiluted weld metal will not provide an indication of the composition of a single-pass weld. These electrodes provide good mechanical properties in single-pass welds.

Should the user choose to make multiple-pass welds using EXXT-2 and EXXT-2M electrodes, it should be noted that both the manganese content and the tensile strength of the weld metal made with this filler metal will be high. These electrodes can be used for welding base metals that have heavy mill scale, rust, or other foreign matter that cannot be tolerated by some electrodes of the EXXT-1 and EXXT-1M classifications. The arc transfer, welding characteristics and deposition

EXXT-3. Electrodes of this classification are self- shielded, used with DCEP, and produce a spray transfer. The slag system is designed to make very high welding speeds possible. The electrodes are used for single-pass welds in the flat, horizontal, and vertical positions (up to a 20" incline, downward progression) on sheet metal. Since these electrodes are sensitive to the effects of base metal quenching, they are not generally recommended for T-joints or lap joints in material thicker than 4.8 mm (3/16 in.) and butt, edge, or corner joints in materials thicker than 6.4 mm (1/4 in.). The electrode manufacturer should be consulted for specific recommendations.

EXXT-4. Electrodes of this classification are self- shielded, operate on DCEP, and have a globular transfer. The slag system is designed to make very high deposition rates possible and to produce a weld that is very low in sulfur, which makes the weld highly resistant to hot cracking. These electrodes are designed for low penetration beyond the root of the weld, making them suitable for use on joints that are poorly fitted and for single- and multiple-pass welding.

EXXT-5 and EXXT-5M. Electrodes of the EXXT-5 classification are designed to be used with C02 shielding gas; however, as with the EXXT-1 classification, argon-C02 mixtures may be used to reduce spatter in accordance with the manufacturer's recommendations. Electrodes of the EXXT-5M classification are designed for use with 75% to 80% argon-balance C 0 2 shielding gas. Electrodes of the EXOT-5 and EXOT-5M classifications are used primarily for single- and multiple-pass welds in the flat position and for welding fillet welds in the horizontal position. These electrodes are characterized by a globular transfer, slightly convex bead contour, and a thin slag that may not completely cover the weld bead. These electrodes have a lime-fluoride base slag. Weld deposits produced by these electrodes typi- cally have impact properties and resistance to hot and cold cracking that are superior to those obtained with rutile-base slags. The EXlT-5 and EXlT-5M electrodes, using direct current electrode negative (DCEN), can be used for welding in all positions. However, these electrodes have less operator appeal than electrodes with rutile-base slags.

EXXT.6. Electrodes of this classification are self- shielded, operate on DCEP, and have a spray transfer. The slag system is designed to give good low- temperature impact properties, good penetration into the root of the weld, and excellent slag removal, even in a deep groove. These electrodes are used for single-

FLUX C O R E D ARC WELDING

and multiple-pass welding in the flat and horizontal positions.

C H A P T E R 5 229

EXXT-7. Electrodes of this classification are self- shielded, operate on DCEN, and have a transfer range from small droplet transfer to a spray transfer. The slag system is designed to allow the large droplets to be used for high deposition rates in the horizontal and flat positions, and to allow the smaller spray particles to be used for all welding positions. The electrodes are used for single- and multiple-pass welding and produce very low-sulfur weld metal, which is highly resistant to cracking.

EXXT-8. Electrodes of this classification are self- shielding, operate on DCEN, and have a small droplet or spray-type transfer. The electrodes are suitable for all welding positions, and the weld metal has very good low-temperature notch toughness and crack resistance. The electrodes are used for single- and multiple-pass welds.

EXXT-9 and EXXT-9M. Electrodes of the EXXT-9 group are classified with C02 shielding gas. However, gas mixtures of argon and C02 are sometimes used to improve usability, especially for out-of-position applications when recommended by the manufacturer. Increas- ing the amount of argon in the argodC02 mixture will affect the weld metal analysis and mechanical properties of weld metal deposited by these electrodes, just as it will for weld metal deposited by EXXT-1 and EXXT- 1M electrodes.

Electrodes of the EXXT-9M group are classified with a 75% to 80% argodbalance C02 shielding gas. The use of these electrodes with argon/C02 shielding gas mixtures with reduced amounts of argon, or with 100% C02 shielding gas, may result in some deterioration of arc characteristics and out-of-position welding characteristics. In addition, a reduction of the manganese and silicon contents in the weld will have some effect on the properties of weld metal from these electrodes, just as it will on properties of weld metal deposited by EXXT-1M electrodes.

Both the EXXT-9 and EXXT-9M electrodes are designed for single- and multiple-pass welding. The larger diameters (usually 2.0 mm [5/64 in.] and larger) are used for welding in the flat position and for welding fillet welds in the horizontal position. The smaller diameters (usually 1.6 mm [1/16 in.] and smaller) are often used for welding in all positions.

The arc transfer, welding characteristics, and deposition rates of the EXXT-9 and EXXT-9M electrodes are similar to those of the EXXT-1 and EXXT-1M classifications. EXXT-9 and EXXT-9M electrodes are essentially EXXT-1 and EXXT-1M electrodes that deposit weld metal with improved impact properties.

Some electrodes in this classification require that joints be relatively clean and free of oil, excessive oxide, and scale in order to obtain welds of radiographic quality.

EXXT-I 0. Electrodes of this classification are self- shielded, operate on direct current electrode negative (DCEN), and have a small droplet transfer. The electrodes are used for single-pass welds at high travel speeds on material of any thickness in the flat, horizontal, and vertical (up to 20" incline) positions.

EXXT-11. Electrodes of this classification are self- shielded, operate on DCEN, and have a smooth, spray- type transfer. They are general-purpose electrodes for single- and multiple-pass welding in all positions. These electrodes are generally not recommended for welding on thicknesses greater than 19 mm (3/4 in.) unless preheat and interpass temperature control is maintained. The electrode manufacturer should be consulted for specific recommendations.

EXXT.12 and EXXT-I 2M. Electrodes of these classifications are essentially EXXT-1 and EXXT-1M electrodes that have been modified to improve impact toughness and to meet the lower manganese requirements of the A-1 Analysis Group in the ASME Boiler and Pressure Vessel Code, Section IX.ll Therefore, they have an accompanying decrease in tensile strength and hardness. Since welding procedures influence all weld metal properties, users should check hardness on any application in which a specific hardness level is a requirement.

The arc transfer, welding characteristics, and deposition rates of the EXXT-12 and EXXT-12M electrodes are similar to those of the EXXT-1 and EXXT-1M classifications.

EXXT-13. Electrodes of this classification are self- shielded, operate on DCEN, and are usually welded with a short-arc transfer. The slag system is designed so that these electrodes can be used in all positions for the root pass on circumferential welds on pipe. The electrodes can be used on all pipe wall thicknesses, but are recommended for the first pass only. They generally are not recommended for multiple-pass welding.

EXXT-I 4. Electrodes of this classification are self- shielded, operate on DCEN, and have a smooth spray- type transfer. The slag system is designed with characteristics so that these electrodes can be used to weld in all positions and also to make welds at high speed. They are used to make welds on sheet metal up to

11. American Society for Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Committee, 1998, Welding and Brazing Qualifi- cations, Section IX of Boiler and Pressure Vessel Code, New York: American Society of Mechanical Engineers.

Next Page

Front MatterTable of Contents5. Flux Cored Arc Welding5.1 Introduction5.2 Fundamentals5.2.1 Process Variations

5.3 Applications5.3.1 Advantages5.3.2 Limltations

5.4 Equipment5.4.1 Semiautomatic Equipment5.4.1.1 Power Source5.4.1.2 Electrode Feed Control5.4.1.3 Welding Guns

5.4.2 Automatic Equipment5.4.3 Materials5.4.4 Fume Extraction

5.5 Materials5.5.1 Base Metals5.5.1.1 Mild Steels5.5.1.2 High-Strength Steels5.5.1.3 Chromium-Molybdenum Steels5.5.1.4 Stainless Steels5.5.1.5 Abrasion-Resistant Steels5.5.1.6 Nickel Alloys

5.5.2 Shielding Gases5.5.2.1 Carbon Dioxide5.5.2.2 Gas Mixtures

5.5.3 Electrodes5.5.3.1 Electrode Classifications5.5.3.2 Mild Steel Electrodes5.5.3.3 Low-Alloy Steel Electrodes5.5.3.4 Chemical Composition5.5.3.5 Electrodes for Surfacing5.5.3.6 Stainless Steel Electrodes5.5.3.7 Protection from Moisture

5.6 Process Control5.6.1 Welding Current5.6.2 Arc Voltage5.6.3 Polarity5.6.4 Electrode Extension5.6.5 Travel Speed5.6.6 Electrode Angle and Welding Position5.6.7 Shielding Gas Flow5.6.8 Deposition Rate and Efficiency

5.7 Joint Designs and Welding Procedures5.7.1 Designs for Gas-Shielded Mild Steel and Low-Alloy Electrodes5.7.1.1 Self-Shielded Electrodes Mild and Low-Alloy Steel5.7.1.2 Stainless Steel Electrodes

5.7.2 Edge Preparation and Fitup Tolerances

5.8 Weld Quality5.9 Troubleshooting5.10 Economics5.11 Safe Practices5.11.1 Safe Handling of Gas Cylinders and Regulators5.11.2 Gases5.11.2.1 Ozone5.11.2.2 Nitrogen Dioxide5.11.2.3 Carbon Monoxide

5.11.3 Metal Fumes5.11.4 Radiant Energy5.11.5 Noise5.11.6 Electric Shock

5.12 Conclusion5.13 Bibliography

AppendicesIndex of Major SubjectsIndex of Ninth Edition

AWS WHB 9 FCAW

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