Evolution of Tantalum Content During Vacuum Induction Melting and Electroslag Remelting of a Novel Martensitic Steel
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INTRODUCTION
ELECTROSLAG remelting (ESR) is a widely used process to produce materials in which cleanliness is of upmost importance.[1] A consumable electrode is cast using vacuum induction melting (VIM) with a mold geometry optimized for ESR. The electrode is then placed in a water-cooled copper crucible that contains a ceramic slag (or flux) consisting of combinations of CaF2, CaO, Al2O3, SiO2, and MgO.[2] Although there are slags for general purpose melting such as 40 to 30 to 30 CaF2-CaO-Al2O3, the slag composition can, and should, be optimized for electrical efficiency, prevention of Al pickup, and/or lubrication, among other factors. During ESR, electrical current passes from the electrode through the slag layer and forming ingot to the bottom of the crucible. The current superheats the electrically resistive slag which, in turn, causes the electrode to melt. Liquid metal droplets travel from the bottom of the
MARTIN DETROIS is with the National Energy Technology Laboratory, 1450 Queen Ave. SW, Albany, OR 97321, and also with Leidos Research Support Team, 626 Cochrans Mill Road, P.O. Box 10940, Pittsburgh, PA. Contact e-mail: [email protected] PAUL D. JABLONSKI and JEFFREY A. HAWK are with the National Energy Technology Laboratory. Manuscript submitted December 19, 2018. Article published online May 31, 2019. 1686—VOLUME 50B, AUGUST 2019
electrode to the crucible where the ESR ingot forms. During their travel within the slag, the droplets are superheated which causes favorable reactions that (under usual circumstances) remove tramp elements. Casting and remelting of metals have important features that lead to the production of final ingots that possess greater cleanliness with reduced segregation.[3] During VIM, the induction heating contributes to the motion of the liquid metal in the furnace crucible, resulting in a better mixing of the constituting elements of the alloy. Furthermore, removal of tramp elements such as oxygen is enabled during VIM by CO boil or in Cr-containing alloys where Cr-oxides form and float on top of the liquid metal or attach to the VIM skull.[4] During ESR, desulfurization of the liquid droplets occurs from their reaction with the CaO from the slag.[5] Furthermore, ESR is particularly effective at removing inclusions that either dissolve or remain in the slag depending on their composition, size, and density.[6] Several factors influence the overall cleanliness of the final ingot and/or the efficiency of the removal of tramp elements and other undesirable inclusions during VIM and ESR. First, the selection of the melt stock is critical to the application intended for the alloy. Most alloys are made from industry-grade melt stock that contains impurities, such as oxygen, nitrogen, and sulfur, usually in relatively high concentrations. For instance, Cr has an affinity for oxygen and selecting 500 ppm oxygen-containing Cr vs 5000 ppm influences the cleanliness of the METALLURGICAL AND MATERIALS TRANSACTIONS B
final product after melting. In some instances, industries use scrap metal to
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