An arc welding process that uses an arc between a continuous filler metal electrode and the weld pool, employing approximately vertical welding progression

with backing to confine the molten weld metal. The process is used with or without an externally supplied shielding gas and without the application of pressure.

Historical Background

The first available thick- plate single-pass vertical welding process was electroslag welding. Demand arose immediately for equipment that would apply the

process to thinner sections. Almost all vertical joints were being welded with the manual shielded metal arc welding (SMAW) process or by semiautomatic gas metal arc welding (GMAW). In 1961, laboratory studies with an electroslag welding machine adapted to

feed auxiliary gas shielding around a flux cored electrode demonstrated that plate as thin as 10 mm (3/8 in.) could be welded in the vertical position in a single pass. The technique is called electrogas welding (EGW).

Process

Electrogas welding is a machine welding process. The mechanical aspects of electrogas welding are similar to those of the electroslag process from which it was developed. There are two variations of the process commonly used in the United States. Based on the GMAW process, it can feed a solid electrode into the joint; based on the flux cored arc welding process (FCAW), it can incorporate a flux within a tubular electrode. Both variations use retaining shoes (dams) to confine the molten weld metal, which permit welding in the vertical position. Gas shielding, when needed, is provided through inlet ports in the dams or a gas cup around the electrode, or both. When using a

self-shielded FCAW electrode, no gas is added.

A square-groove or single V-groove joint is positioned so that the axis or length of the weld is vertical. Figure E-5 shows typical electrogas welding joint designs. There is no repositioning of the joint once welding has started; welding continues to completion, so that the weld is made in one pass. The nature of the melting and solidification during welding results in a high quality weld deposit. There is little or no angular distortion of the base metal with single-pass welds. The welding action is quiet, with little spatter.

Principles of Operation

The consumable electrode, either solid or flux cored, is fed downward into a cavity formed by the base metals to be welded and the retaining shoes. A sump (starting tab) is used at the beginning of the weld to allow the process to stabilize before the molten weld metal reaches the work. An arc is initiated between the electrode and the sump.

Heat from the arc melts the continuously fed electrode and the groove faces. This is shown schematically in Figure E-6. Melted filler metal and base metal collect in a pool beneath the arc and solidify to form the weld. The electrode may be oscillated horizontally through the joint for uniform distribution of heat and weld metal. As the cavity fills, one or both shoes may move upward. Although the weld travel is vertical, the

weld metal is actually deposited in the flat position at the bottom of the cavity.

Applications

Base metals most commonly joined by electrogas welding are plain carbon, structural and pressure vessel steels. Applications of electrogas welding include storage tanks, ship hulls, structural members and pressure vessels. EGW should be considered for any joint to be welded in a vertical position in materials ranging in thickness from 10to 100mm (3/8 to 4 in.) thick.

Advantages

Some of the advantages associated with EGW have resulted in considerable cost savings, particularly in joining thicker materials, when compared to the more conventional joining methods such as submerged arc welding and flux cored arc welding. Even in some applications involving thinner base materials, EGW may result in cost savings because of its efficiency and simple joint preparation. The following advantages

can be achieved with EGW

(1)Extremely high metal deposition rates; EGW has a deposition rate of 16 to 20 kg (35to 45Ibs) per hour per electrode.

(2) Preheating is normally not required, even on materials of high hardenability.

(3) High-quality weld deposit; the weld metal stays molten for an appreciable time, allowing gases to escape and slag to float to the top of the weld.

(4) Minimum joint preparation and fit-up requirements; mill edges and flame-cut square edges are normally employed.

(5) High duty cycle; the process is automatic and once started, continues to completion; there is little operator fatigue.

(6) Minimum materials handling; the work needs to be positioned only to place the axis of the weld in the vertical or near vertical position; there is no manipulation of the parts once welding has started.

(7) Elimination of weld spatter, which results in 100%filler metal deposition efficiency.

(8) Minimum distortion; there is no angular distortion in the horizontal plane. Distortion is minimal in the vertical plane, and this is easily compensated for.

Limitations

(1) The EGW process welds only carbon and low alloy steels, and some stainless steels.

(2) The joint must be positioned in the vertical or near-vertical position.

(3) Once welding has started, it must be carried to completion or a defective area is likely to result.

(4) Complex material shapes may be difficult or impossible to weld using EGW.

Electrode Variations

Solid Electrode. In a typical electrogas welding installation, a solid electrode is fed through a welding gun, called a nonconsumable guide. See Figure E-6. The electrode may be oscillated horizontally to weld thicker materials. Gas shielding, normally carbon

dioxide (C02)or an argon-carbon dioxide (Ar-C02) mixture, is provided to the weld cavity through gas ports, boxes, or nozzles. Water-cooled copper retaining shoes retain the weld; they move vertically as the machine moves. Vertical movement of the machine

must be consistent with the deposition rate, and may be automatic or controlled by the welding operator.

Electrogas welding with solid electrodes can be used to weld base metals ranging in thickness from approximately 10mm (3/8 in.) to 100 mm (4 in.). Base metal thicknesses most commonly welded are between 13 mm (1/2 in.) and 76 mm (3 in.). Electrode diameters most commonly used are 1.6,2.0, 2.4, and 3.2 mm (1/16, 5/64, 3/32, and 1/8 in.).

Flux Cored Electrode. The principles of operation and characteristics of the self-shielded flux cored electrode are identical to the solid electrode variation, except that no separate gas shielding is needed. See Figure E-7. The flux cored electrode creates a thin layer of slag between the weld metal and copper shoes to provide a smooth weld surface. The flux cored electrode creates a thin layer of slag between the weld metal and copper shoes to provide a smooth weld surface.

Electrogas welding with a flux cored electrode may be done with an external gas shield or a self-shielding electrode. Self-shielded electrodes operate at higher current levels and deposition rates than shielded types. Diameters of flux cored electrodes commonly vary from 1.6 mm to 3.2 mm (1/16 in. to 1/8 in.). The wire (electrode) feeder must be capable of smooth, continuous feeding of small diameter wires at high speeds and larger diameter wires at slower speeds.

Consumable Guide Process

EGW with a consumable guide is similar to consumable guide electroslag welding. This variation of EGW is primarily used for short weldments in ship building, and in column and beam fabrication. Consumable guide EGW uses relatively simple

equipment; the principle difference is that none of the equipment moves vertically during consumable guide welding. Instead, the electrode is fed through a consumable guide tube which extends to about 25 rnm  (1 in.) from the bottom of the joint. As the weld progresses vertically, the electrode melts back to the guide tube. Initially, the wire electrode penetrates about an inch beyond the end of the guide tube. Then a steady-state relationship develops between melting of the end of guide tube and the electrode wire. This relationship remains until the weld is completed. The consumable guide process is shown schematically in Figure E-7.

The American Welding Society publishes ANSI/ AWS A5.26, Specification for Carbon and Low Alloy Steel Electrodes for Electrogas Welding, which prescribes requirements for solid and flux cored electrodes for electrogas welding.

Equipment

The basic mechanical equipment for electrogas welding consists of a direct current power supply, a device for feeding the electrode, shoes for retaining molten metal, an electrode guide, a mechanism for oscillating the electrode guide, and equipment needed for supplying shielding gas, when used. In a typical electrogas welding system, the essential components (with the exception of the power supply) are incorporated in an assembly that moves vertically as welding progresses.

Power Supply. Direct current electrode positive (reverse polarity) is normally used for EGW, with the power supply either constant voltage or constant current. The power source should be capable of delivering the required current without interruption cluring the welding of a seam that may be of considerable length. Power sources used for electrogas welding usually have ratings of 750 to 1000 amperes at 30 to 55 volts and 100% duty cycle. Direct current is usually supplied

by transformer-rectifier power sources, although motor-driven and engine-driven generators may be used.

Wire feed for the electrode is of the push type, such as used with automatic GMAW or FCAW. The wire feeder is normally mounted as an integral part of the

vertical-moving welding machine. Wire feed speeds may vary up to 230 mm/s (550 in./min). The wirefeed system may include a wire straightener to eliminate the cast and helix in the electrode to minimize electrode wander at the joint.

Electrode Guide. Electrode guides are similar to the welding guns used for semiautomatic GMAW or for FCAW. The guide may have a shielding gas outlet to deliver gas around the protruding electrode.

Electrode Guide Oscillator. The horizontal motion needed when welding base metals 30 mm to 100 mm (1-1/4 in. to 4 in.) thick to move the arc back and forth between the shoes and over the weld pool is accomplished by a system that oscillates the electrode guide

and provides adjustable dwell times at either end of the oscillation.

Retaining Shoes. Retaining shoes (also called dams), are pressed against each side of the gap between the base metals to be welded to retain (dam) the molten weld metal in the groove. Nonfusing ceramic backups are sometimes used. Sliding shoes may or may not contain gas ports to supply shielding gas directly into the cavity formed by the shoes and the weld groove. When gas ports are not used in the shoes, a “gas box” arrangement may be mounted on the shoes to surround the el-ectrode and welding arc with shielding gas; these are not required when using self-shielding flux cored electrodes.

Controls. With the exception of the vertical travel control, EGW controls are primarily adaptations of the devices used with GMAW and FCAW. Vertical travel controls, either electrical, optical, or manual, maintain a given electrode extension, with the top of the movable shoe a specific distance above the molten weld pool.

Safety

Specific instructions for safe operation of electrogas welding equipment are available in the manufacturer’s literature. General safety instructions for all welding

and cutting can be found in ANSYASC 249.1, Safety in Welding and Cutting, published by the American Welding Society. Mandatory Federal safety regulations are established by the U.S. Labor Department’s Occupational Safety and Health Administration, and

are available in the latest edition of OSHA Standards, Code of Federal Regulations, Title 29 Part 191 0, from the Superintendent of Documents, U.S. Printing Office, Washington DC 20402.

Personnel should be protected against exposure to noise generated in welding and cutting operations. See Paragraph 19 10.95, Occupational Noise Exposure, Code of Federal Regulations.

The total radiant energy produced by the EGW pro- cess can be higher than that produced by the SMAW process because EGW has a more exposed arc, especially when using an argon shielding gas and when welding on aluminum.

For general information on metallurgical considerations, mechanical properties, process variables, joint design, fit-up and assembly, training of operators, and  troubleshooting guide, refer to: American Welding Society, Welding Handbook, 8th Edition, Vol. 1;  Miami, Florida 1987; and Welding Handbook, 8th Edition Vol. 2; Miami, Florida: American Welding Society 1991.

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