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Electroslag-weldingBest for Thick Vertical Welds.SOLUTIONS with Effective, Practical Advice
Electroslag-welding is a process providing one of the highest weld deposition rates and is therefore most cost-effective for suitable applications. Its unique capability permits to weld in one pass groove joint of two plates, thick 1 to 12 inches (25 to 300 mm) Electroslag-welding was derived from a metal manufacturing process called Electroslag Remelting (or Refining) (ESR). This consists in the progressive remelting of an ingot (used as a consumable electrode) in a molten slag bath. The conductive slag is heated and melted by an electric current passing through it from the electrode to the water cooled copper crucible. The consumable ingot lower end in contact with the molten slag, is fused at the high temperature and drips down progressively. The metal drops undergo cleaning and refining in their passage through the molten slag, and sink at the bottom where they collect and solidify. The purpose is obtaining a purer and refined remelted ingot free from objectionable inclusions and other defects. Materials treated this way display improved mechanical properties and are specified for demanding applications like aerospace, nuclear etc. Electroslag-welding, however, is set to proceed in the vertical-up direction from the bottom to the top. The vertical joint to be filled consists generally in the gap between the edges of two thick plates, while the lateral walls confining the joint space are cooled copper shoes or dams either stationary (along the whole length of the joint) or capable to move up along the joint as soon as the weld metal is solidified. For starting, a sump is prepared at the bottom of the joint and filled with solid flux. An arc is generated between the consumable electrode (wire or strip) continuously supplied and the base metal. The power supply is usually of direct current constant voltage type. The electrode wire is guided in the weld pool through an insulated consumable guide tube. If also the consumable guide is melted during welding, (providing up to 15% additional filler material) its composition must be compatible with the prescribed chemistry. Once the molten flux bath is stabilized, the arc is extinguished and heating proceeds by resistance.
The flux, that provides a suitable molten cover, must be replenished from time to time to make up for the slight slag consumption. The molten slag melts the consumable filler and the abutting base metal surfaces, and shields the metal from oxidation. The process is being employed essentially for welding low carbon steels. With appropriate precautions, it could be applied also to structural steels capable of higher mechanical properties. Electroslag-welding has been used for heavy high rise constructions, for building bridges, for shipbuilding, for vessels and containers, for rails and for other industrial applications. Electroslag-welding has been known for several decades, and enjoyed intensive use around the seventies and eighties of the past century. Electroslag-welding is similar to processes that cast in place a large volume of molten metal with high heat content. See Thermite Welding. Originally, in the large gap that was common at the time, a substantial amount of molten steel was collected. The large mass took a long time to solidify and cool down. Therefore the weld metal showed large grains and poor impact properties. Electroslag-welding, as an approved process for building bridges, suffered a set back for a certain time, between 1977 and 2000, when a moratorium in its usage was imposed by the Federal Highway Administration of the United States Department of Transportation, as it was suspected that large weld grain size and relatively low impact properties could endanger the stability of bridges. As a consequence of research, the narrow gap technology was investigated. It was demonstrated that grain size could be reduced and impact properties improved by limiting heat input. A better Electroslag-welding process was developed, called Narrow Gap Improved - Electroslag-welding (NGI-ESW) that demonstrated that higher weld properties could be achieved and guaranteed. It was published that Electroslag-welding was going to be approved in the new edition of AWS D1.5, Bridge Welding Code, that was expected in 2009, but it took longer to appear. ANSI/AWS D1.5M/D1.5:2010 Electroslag-welding is currently approved and applied for specific bridges building projects in the United States. The Advantages of NGI-ESW are:
The Limitations are:
A short note on this process was published in section 2 of Issue 7 of Practical Welding Letter for March 2004. Click on PWL#007 to see it. This publication was honored to be quoted also by Wikipedia in their page on A note on Applications of ESW was published (11) in Issue 49 of Practical Welding Letter for September 2007.
The U.S. Department of Transportation, Federal Highway Administration The three major improvements quoted include: (1) - Consistent defect-free welds. The NGI-ESW process has virtually eliminated the occurrence of internal weld defects, including slag inclusions, ferrite vein cracking, hot cracking, and lack of fusion (2) - Fatigue Performance. In full size fatigue testing of long girders containing 2 in. thick flange, under no circumstances did any of the NGI-ESW welds develop fatigue cracks. (3) - Impact Toughness in the weld and heat-affected zones. By substantially reducing heat input and by designing the weld metal alloy chemistry to produce a uniformly tough microstructure, the NGI-ESW weld metal CVN (Charpy V-Notch impact test) properties taken at the weld centerline, the region of lowest toughness, exceed the proposed CVN values of 20 ft-lb. (27 joules) at 0° F (-20° C).
In conclusion, the NGI-ESW process, approved by the major structural building Codes, is likely to enjoy a renewed productive period of increased applications.
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