Influence of the Mold Current on the Electroslag Remelting Process
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CRITICAL applications such as aeronautics, energy, and tooling require high-quality and defect-free metal, which cannot be fully achieved after primary melting. One way to achieve this quality is the use of remelting processes such as electroslag remelting (ESR) which allows a good control of the alloy chemistry and solidification structure of the ingot.[1] The materials concerned encompass high value-added metallic alloys such as special steels or nickel-based superalloys. Remelting is caused by means of an electric current, usually an alternative one, which flows from the primary ingot—acting as a consumable electrode—whose lower part, as shown in Figure 1(a), is immersed in a highly resistive calcium fluoride-based slag, to a water-cooled crucible, thus generating Joule heating in the slag. The electrode melts by contact, forming metal droplets which fall down through the slag to the liquid pool of the secondary ingot. Because of the water cooling, the MATHILDE HUGO, formerly Graduate Student with the Institut Jean Lamour - UMR 7198 CNRS/Universite´ de Lorraine (Laboratory of Excellence DAMAS), Parc de Saurupt, CS 50840, 54011, Nancy Cedex, France, is now Engineer with Sopra Banking Software, Tour Manhattan, 5 place de l’Iris, 92095, La De´fense Cedex, France. BERNARD DUSSOUBS, Research Engineer, and ALAIN JARDY, Senior Research Fellow, are with the Institut Jean Lamour - UMR 7198 CNRS/Universite´ de Lorraine, France (Laboratory of Excellence DAMAS). Contact e-mail: [email protected] JESSICA ESCAFFRE and HENRI POISSON, Research Engineers, are with Aubert & Duval, BP1, 63770, Les Ancizes, France. Manuscript submitted January 20, 2015. Article published online June 1, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
latter solidifies continuously while growing, and its contraction leads to the building of an air gap between the solidified ingot and crucible. It must be noticed that the slag is also in contact with the water-cooled mold, which results in the formation of a layer of solidified slag at the slag/mold interface, as shown in Figure 1(b). As the secondary ingot rises, this layer is partially remelted and/or crushed between the metal and crucible, resulting in a slag skin which acts as a thermal insulator and provides ESR ingots with a smooth lateral surface. After the end of melting, complete solidification, and removal of the mold, this slag skin may remain in the form of a few-millimeter-thick slag crust around the ingot. The thickness of this crust, at different heights, can be measured to compare model predictions with the actual observation. Besides, chemical reactions occur at the electrode/ slag, droplets/slag, and pool/slag interfaces, thus influencing the final ingot chemistry. Non-metallic inclusions are dissolved in the liquid slag.[1] The strategic importance of the remelted products makes it essential to acquire a detailed understanding of the process. Actual remelting experiments are very costly; so their repeated use cannot be considered as a suitable solution to get a better insight in
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