Flux Entrapment and Titanium Nitride Defects in Electroslag Remelting of INCOLOY Alloys 800 and 825
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INTRODUCTION
ELECTROSLAG remelting (ESR) is a secondary metal processing route used for microstructure control and chemical refining of nickel-based superalloys and specialty steels. In ESR, the heat required for melting is provided by the current flowing through a circuit formed by the consumable electrode, the slag, and the solidifying ingot, as shown in Figure 1. The slag serves as a heat source, a barrier to the atmosphere, and a medium for liquid metal refining. As the electrode is heated, droplets of molten metal pass through the slag and into the liquid metal pool below. The slag is the resistive element in the circuit, and thus the energy efficiency of the process depends on the electrical conductivity of the molten slag. The slag barrier between the atmosphere and the liquid metal prevents direct contact between the liquid metal and atmospheric O2 and N2, helping to retain reactive alloying elements. As metal passes through the slag in droplets 1 to 10 mm in diameter and interacts with the slag at the slag-liquid metal interfaces, it is refined by both physical and chemical means.[1,2] For example, the density difference between the alloy and non-metallic inclusions is the driving force for their separation, and sulfur is removed
JONATHAN D. BUSCH, Graduate Student, and MATTHEW J. M. KRANE, Associate Professor, are with the Purdue Center for Metal Casting Research, School of Materials Engineering, Neil Armstrong Hall of Engineering, Purdue University, 701 West Stadium Avenue, West Lafayette, IN 47907-2045. Contact e-mail: jbbusch@ purdue.edu JOHN J. DeBARBADILLO, Manager, is with the Process and Product Development, Special Metals Corporation, 3200 Riverside Drive, Huntington, WV 25705. Manuscript submitted July 6, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
from the liquid metal due to a chemical reaction with slag.[2] The consumable electrode for ESR is generally produced by vacuum induction melting (VIM) or in an electric arc furnace (EAF) followed by argon oxygen decarburization (AOD). Alloys that more readily react with O2 and N2 or those for more demanding applications tend to be processed with VIM rather than EAF-AOD. The AOD process is designed to significantly decrease the C and S content of the melt, while minimizing the loss of expensive alloying elements such as Cr. After transferring the molten metal from the EAF, the AOD process can be divided into three main steps: decarburization, reduction, and tapping and teeming. During the decarburization stage, an Ar-O gas mixture is bubbled through the molten metal. The C is removed as it reacts with O to form CO, while the Ar minimizes the partial pressure of CO, thus sustaining the reaction. Maintaining a low partial pressure of CO also minimizes the rate of Cr oxidation to the slag. Generally, the AOD process starts with a high O mixture to heat the system via exothermic oxide formations and ends with a high Ar mixture to attain the desired C content. Si and CaO are often added during the reduction stage, maximizing Cr recovery from the slag and re
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