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Electrogas-weldingwins the productivity race.SOLUTIONS with Effective, Practical Advice
Electrogas-welding (EGW) is a high productivity process derived and adapted from the older Electroslag Welding, (ESW). The original purpose of developing EGW was of adapting the ESW process for welding thinner plates. The benefits sought were high deposition rates and efficient welding in one pass in the vertical position. A short note on EGW was published (2) in Issue 31 of Practical Welding Letter for March 2006. Click on PWL#031 to see it.
Electrogas-welding is similar to the Gas Metal Arc Welding (GMAW) process, except that the progression of the weld is up, towards the top. The weld position however is flat, with the molten metal continually being deposited at the bottom of the moving cavity. In Electrogas-welding, fixed or moving metal or ceramic shoes (or dams), water cooled if needed, are set up before starting, to bridge the gap between abutting plates and to contain the molten metal until solidification is completed.
Bare solid filler metal is used with shielding gas. Otherwise, tubular filler metal, filled with flux (flux cored) is generally used without auxiliary shielding gas. However these flux cored wires are of a special type, containing fewer slag building ingredients (than regular FCAW filler wires) because excessive slag buildup on the molten metal surface can affect arc stability and cause slag entrapment. Although there is a range of thicknesses where both ESW and EGW could be used, the advantages for Electrogas-welding are lower heat input, more refined microstructure and improved Heat Affected Zone (HAZ) properties. Moreover the EGW process can be more easily (than ESW) restarted if interrupted for any cause. In case such an occurrence stops the process before completion, one must immediately force a wedge into the open joint to avoid that the cooling, shrinking weld, grip the welding head in the joint opening. Over large areas the Electrogas-welding shielding gas could be less protective (than molten slag in ESW), causing the appearance of unacceptable defects. Other thing being equal ESW is preferred for thick plates in that it should cause less inclusions and other internal defects. For Electrogas-welding, the reported thickness range of plates is between 13 and 38 mm (0.5 and 1.5 in.). Thicker plates, up to about 75 mm (3 in.) can be welded usually with two or more electrodes dipping into the weld pool, possibly with an oscillation movement being imparted to the welding head, to distribute more uniformly heat and filler material. Besides filler wires, static consumable guides (metal tubes) are sometimes used, for relatively short weldments, up to 1.5 m (3 to 4 ft), to provide between 5 to 30% of the needed weld metal. Their chemical composition must be suitable to that of the weld sought. Direct current with electrode positive (reverse polarity) is used for Electrogas-welding with power source of the constant voltage type, suitable for 100% duty cycle (capable of working for hours without overheating). The shielding gas used for steel is carbon dioxide (CO2) or various mixtures of Argon and carbon dioxide. The upward travel speed of the weld being performed is a complex function of all primary parameters. The range of welding speeds is therefore quite large, between 38 to 200 mm/min (1.5 to 8 in./min). The slowest travel speeds and low cooling rates tend to form large grains with coarse columnar growth. Hardness results low, and also low toughness values are obtained, unless suitably alloyed electrodes are used and controlled welding conditions are implemented. As welded condition may meet requirements for certain applications. Postweld heat treatment may improve the toughness. Typical applications of Electrogas-welding represent a whole collection of different manufactured structures, including storage tanks, pressure vessels, nuclear components, ship subassemblies, bridges, oil drilling platforms, power generating equipment, heavy presses, rolling mills, and other constructions where the advantages of the process determine its economy. The following documents should be consulted: ANSI/AWS A5.26/A5.26M:1997 (R2009)
ANSI/AWS C5.7:2000 (R2006)
ANSI/AWS A5.32M/A5.32:2011
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