A welding process that produces coalescence of metals with molten slag that melts the filler metal and the surfaces of the workpieces. The weld pool is shielded by this slag, which moves along the full cross section of the joint as welding progresses. The process is initiated by an arc that heats the slag. The arc is then extinguished by the conductive slag, which is kept molten by its resistance to electric current passingbetween the electrode and the workpieces. See ELECTROSLAG WELDING ELECTRODE and CONSUMABLE GUIDE ELECTROSLAG WELDING.
The electroslag welding process is most often used to join metals in the vertical or near vertical position, usually in a single pass. However, it has been shown that ESW can be used at angles of 45″ or greater from vertical. Some of the advantages associated with ESW have resulted in considerable cost savings, particularly in joining thicker materials. Savings have been achieved where components are joined to make larger units instead of initially producing massive castings or forgings. ESW is often less expensive than more conventional joining methods such as submerged arc welding in thicker section weldments. Even in some applications involving thinner base materials, ESW has resulted in cost savings because of its efficiency and simple joint preparation. The ESW process offers many opportunities for reducing welding costs on specific types of joints.
Applications
Many types of carbon steels can be electroslag welded in production, such as AISI 1020, AISI 1045, ASTM A36, ASTM A441, and ASTM A515. They can generally be welded without post-weld heat treatment.
In addition to carbon steels, other steels are successfully electroslag welded. They include AISI 4 130, AISI 8620, ASTM A302, HY80, austenitic stainless steels, ASTM A514, ingot iron, and ASTM A387. Most of these steels require special electrodes and a
grain refining post-weld heat treatment to develop required weld or weld heat-affected zone properties.
Advantages
(1) Extremely high metal deposition rates; ESW has a deposition rate of 16 to 20 kg (35 to 45 Ibs) per hour per electrode.
(2) Capability to weld very thick materials in one pass; there is one equipment setup and no interpass cleaning, since there is only one pass.
(3) Preheating is normally not required, even on materials of high hardenability.
(4) 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.
(5)Minimum joint preparation and fit-up requirements; mill edges and flame-cut square edges can usually be used.
(6) High duty cycle; the process is automatic and once started, continues to completion; there is little operator fatigue.
(7) Minimum materials handling; the work needs to be positioned only to place the axis of the weld in vertical or near vertical position; there is no manipulation of the parts once welding has started.
(8) Elimination of weld spatter, which results in 100% filler metal deposition efficiency.
(9) Low flux consumption; approximately 1 pound of flux is used for each 20 pounds of weld metal.
(10)Minimum distortion; there is no angular distortion in the horizontal plane. Distortion is minimum in the vertical plane, but this is easily compensated for.
(11) Minimum welding time; ESW is the fastest welding process for thick metal.
Limitations
(1) The ESW process welds only carbon and low alloy steels, and some stainless steels.
(2)Joining must be positioned in the vertical or near vertical position.
(3) Once welding has started, it must be carried to completion or a defective re-start area is likely to result.
(4)ESW cannot be used on materials thinner than about 19 mm (3/4 in).
(5)Complex material shapes may be difficult or impossible to weld using ESW.
Principles of Operation
To set up for an electroslag weld, a square groove joint is positioned so that the axis or length of the weld is vertical or nearly vertical. The process is initiated by starting an electric arc between the electrode and the joint bottom. Granulated welding flux is then added and melted by the heat of the arc. As soon as a sufficiently thick layer of molten slag (flux) is formed, all arc action stops, and the welding current passes from
the electrode through the slag by electrical conduction. Welding is started in a sump or on a starting tab to allow the process to stabilize before the welding action reaches the work. Figure E-9 is a schematic representation of an electroslag welding operation.
Heat generated by the resistance of the molten slag to passage of the welding current is sufficient to fuse the welding electrode and the edges of the workpiece.
The interior temperature of the bath is in the vicinity of 1925°C (3500°F). The surface temperature is approximately 1650°C (3000°F). The melted electrode and base metals collect in a pool beneath the molten slag bath and slowly solidify to form the weld. There
is progressive solidification from the bottom upward, and there is always molten metal above the solidifying weld metal.
Run-off tabs are required to allow the molten slag and some weld metal to extend beyond the top of the joint. Both starting and run-off tabs are usually removed flush with the ends of the joint.
Equipment
The equipment used for electroslag (and electrogas) welding is very similar to that required for submerged arc welding (SAW) or flux cored arc welding (FCAW). The same power sources can be used for either process, with one exception: both a-c and d-c power supplies are used with the electroslag process, while in the electrogas process, a-c power supplies are not used. Standard power sources used for either process should have a minimum open circuit voltage of 60 V and be capable of delivering 600 A continuously
(100% duty cycle). The power supplies should be equipped with remote controls. The number of power supplies required depends on the number of welding electrodes being used to fill the joint. One power supply is required for each welding electrode. Special
constant-voltage d-c power supplies designed for electroslag and electrogas welding are available. Typical power supplies are transformer-rectifiers having 74 V open circuit and a current rating of 750 A at 50 V output, 100% duty cycle. The primary input is 60 Hz,
three phase, 2301460 V.
Safety
As in any type of welding, reasonable care must be exercised in the set-up, welding, and post-welding procedures for ESW. Various potential hazards exist, some minor and others serious, but all can be eliminated. Failure to use safety protection equipment or follow safe practices can result in physical danger to personnel, as well as damages to production parts, equipment, and facilities.
For detailed safety information, refer to the manufacturer’s instructions and the latest editions of the following publications: ANSI 249.1, Safety in Welding and Cutting, and ANSI 287.1 Practice for Occupational and Educational Eye and Face Protection. Reference: American Welding Society, Welding Handbook, Vol. 1,8th Edition. Miami, Florida: American Welding Society, 1987.
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, DC 20402.
Other considerations involve equipment, safety, consumables, applications, quality control, qualifications, training, troubleshooting, and definitions associated with the process. Reference: American Welding Society, Welding Handbook, Vol. 2, 8th Edition. Miami, Florida: American Welding Society, 199 1.