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Study of thermal instabilities in electroslag melting. Jackson, Robert Orrin

Abstract

Structural and compositional changes resulting from interruptions in the steady-state heat balance of an electroslag remelted ingot have been investigated on a laboratory scale electroslag furnace. An attempt was also made to solve some of the fundamental problems introduced by the proposed production of large commercial ingots in a tandem electrode change furnace. The investigation was carried out on three commercially available alloy steels: 1) EN-25, 2) AISI 4340, and 3) AISI 630 (17-4 P.H.). Power interruption experiments on EN-25 and AISI 4340 steels revealed only minor structural changes but did show carbon concentration banding. Carbon rich bands were also produced by periodic variations in the slag skin thickness. Power interruption experiments on AISI 630 produced some structural changes but no change in the concentrations of the major alloying elements was detected. The mixing action in the liquid metal pool appears to be due to a slow convective motion which causes the liquid to approach a state of complete mixing at the solidification rates found in the ESR process. A general heat balance was calculated for a 10 cm dia. ESR ingot. The various volume fractions solidified were calculated for different durations of the "power-off" mode. The results of the heat balance were extended to a large (61 cm dia.) commercial ingot and the volume solidified during a 60 second power interruption was estimated. A heat transfer program was written to determine the unsteady state temperature profiles in an electrode as a function of temperature of the slag bath and time after immersion. The resulting profiles indicated that in order to avoid any major structural and/or compositional changes during an electrode change operation, electrode preheating is mandatory.

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