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Electroslag welding is a highly productive welding process for thick materials. It involves the melting of the surfaces of the metal workpieces and the filler metal with a molten slag to cause coalescence. An electric arc is passed through the slag to heat it, but the arc itself is extinguished by the slag.

The process was patented by Robert K Hopkins in the United States in February 1940 (patent 2191481) and developed and refined at the Paton Institute, Kiev, USSR during the 1940s. The Paton method was released to the west at the Bruxelles Trade Fair of 1950. The first widespread use in the U.S. was in 1959, by General Motors Electromotive Division, Chicago, for the fabrication of engine blocks.

Electroslag welding easily lends itself to mechanization, thus reducing the requirement for skilled manual welders. It is commonly used to weld in a vertical orientation, and is particularly popular with steels. In the 1970s, it was used extensively in bridges, ships, and other large metal structures. However, in 1977 the Federal Highway Administration banned its use in welds for some structural members of bridges, due to concerns of weld imperfections and poor properties.

Benefits of the process include its high metal deposition rates—it can lay metal at a rate between 15 and 20 kg per hour (35 and 45 lb/h) per electrode—and its ability to weld thick materials. Many welding processes require more than one pass for welding thick workpieces, but often a single pass is sufficient for electroslag welding. The process is also very efficient, since joint preparation and materials handling are minimized while filler metal utilization is high. The process is also safe and clean, with no arc flash and low weld splatter or distortion.

One electrode is commonly used to make welds on materials with a thickness of 25 to 75 mm (1 to 3 in), and thicker pieces generally require more electrodes. The maximum workpiece thickness that has ever been successfully welded was a 0.91 m (36 in) piece that required the simultaneous use of six electrodes to complete.

• Cary, Howard B. and Scott C. Helzer (2005). Modern Welding Technology. Upper Saddle River, New Jersey: Pearson Education. ISBN 0-13-113029-3.
• Pires, J Roberto; Loureiro, Altino; Bolmsjö, Gunnar (2005). Welding Robots: Technology, System Issues and Application. New York: Springer, p11. ISBN 1852339535.