The Role of Side Arcing in the Global Energy Partition during Vacuum Arc Remelting of INCONEL 718
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VACUUM arc remelting (VAR) is a main secondary remelting process for the production of aerospace materials; the aim of this procedure is to produce a controlled, segregation-free ingot structure. There are two types of vacuum arc present during the normal remelting process: (1) liquid cathode–liquid anode arc (the arc between the electrode tip and the surface of the liquid pool); and (2) solid cathode–solid anode arc (the arc between the electrode flanks and the crucible inner surface, usually called side-arcing). The power distributions between cathode and anode in these arcs are different.[1] Recent studies[2,3] of the electric current distributions during VAR of INCONEL* *INCONEL is a trademark of the Special Metals family of companies, Huntington, WV.
D.M. SHEVCHENKO and R.M. WARD, Research Fellows, are with the Metallurgy and Materials Department, University of Birmingham, Birmingham B15 2TT, England. Contact e-mail: [email protected] This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France. Article published online April 2, 2009. 248—VOLUME 40B, JUNE 2009
718 found that the side-arcing accounted for around 45 pct of the total furnace current. If so, the effect of this side-arcing on energy partition would benefit from further study.[4,5] So far, also, the calculations of the heat flux distribution within a VAR furnace[6,7] have been made with the assumption that the heat transfer from the crucible to the cooling water is purely radial, and determined by the difference between the local temperature of the crucible and that of the bulk fluid. However, in practice, vertical (and also azimuthal) flow is possible and a thermal boundary layer is likely to form in the cooling water near the wall with higher temperature than that of the bulk fluid. It will be shown in the 3d simulation results presented later that the thickness of the heated layer is predicted to increase toward the top of the ingot due to the high power input there. Once this layer of heated water is created, it moves upward with the bulk flow for some distance above the ingot top (meniscus). Therefore, the wall just above the meniscus, as it is in contact with this layer of heated water, will have a higher temperature compared to an area below the meniscus for the same power input. II.
EXPERIMENTAL PROCEDURE
A. Outer Crucible Surface Temperature Measurement Historical measurements were used of the outside crucible wall temperature during a melt of a 440-mmdiameter INCONEL 718 electrode into a 508-mmdiameter ingot at 6 kA.[8] The melting parameters such METALLURGICAL AND MATERIALS TRANSACTIONS B
as the melt rate, voltage, electric current, and the bulk cooling water temperature were recorded by the furnace control system. The flow rate of the cooling water used in this experiment was measured with a Doppler flowmeter. The conditions and water cooling layout used were not representative of typical operating procedures. As it was not
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