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Thesis (Ph. D.)--University of Texas at Austin, 2005. / Supervisor: Joseph J. Beaman. Vita. Includes bibliographical references.

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. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The electroslag remelting process combines both a chemical refining ability and a better control on solidification which produces materials of more uniform properties. In spite of the advantages of this process, segregation-prone alloys are still difficult to produce via ESR, particularly when large ingot diameters are involved. In this context the study of externally applied electromagnetic fields is of great interest. An AC operated stirring device was designed and laboratory-scale experiments were conducted. The solidification structures resulting from the stirring as well as the mixing conditions prevailing in the liquid pool were investigated. Also, modelling studies on the electromagnetic body force produced by the stirrer and the resulting thermal field were conducted. The start of stirring results in a clearly defined band in both steady- and unsteady-state regimes. The microstructure was not significantly modified in the center of the ingot but a loss in directionality was found at the mould wall. It seems apparent that the stirred liquid does not penetrate significantly inside the interdendritic region. Mixing studies revealed that the degree of mixing in the pool is enhanced by the stirring. The theoretical calculations show that most of the electromagnetic body force is confined to the magnetic skin depth at the mould wall and the metal-slag interface. Using a previously developed two-dimensional heat transfer model it was not possible to reproduce the experimental pool profiles obtained when stirring was applied. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Previous attempts at a quantitative description of alloy losses in electroslag remelting (ESR) operations have not adequately reflected the true nature of this process. In the current work a mass transfer model was developed which takes into account four reaction sites and mass transfer of all species at each site. An experimental program was subsequently devised to test extensively the provisions of the model. The overall system was simplified by avoiding complex alloys and multicomponent slags and also by excluding air. Care was taken to deal only with species whose thermochemistry was known in,both slag and metal phases. The electrode material used was mild steel in which manganese was the onlyoxidizable species. Slags used throughout the melt program were CaV - 20% CaO, for which system the activity data for FeO, MnO were available. The melt program provided data on Mn losses in a variety of ESR operating conditions including steady and unsteady state, live and insulated mold configurations. The effects of melting in air and of direct current operation were also investigated. Several melts undertaken with Armco iron were designed to permit direct calculation of certain mass transfer coefficients. The mass transfer model was applied to each melt and the predictions compared to the experimental data. There was generally good agreement between the two, confirming that the manganese losses were controlled entirely by mass transfer effects. Further study of the model showed mass transfer of species in the slag phase to be the predominant rate controlling step. The relative roles of the various reaction sites in contributing to the overall mass transfer were also elaborated by this analysis. In order to assess the full potential of this mass transfer approach, the model was extended to the manganese, sulphur reaction system and to larger, commercial-sized ESR furnaces. Projections of alloy losses based on this scale-up study appeared to be consistent with the sparse information that is currently available. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The aim of this work was to determine whether (ASTM E-45) D-type inclusions are a necessary component of ESR ingots. Therefore, experiments were undertaken to ascertain the origin of the inclusions. Pure iron (FVE) and electrolytic nickel were melted through CaF₂+CaO+Al₂0₃ slags, and the changes in the metal compositions (with respect to calcium, aluminum and oxygen) were measured. The composition changes could not be accounted for by simple chemical slag/metal reactions. Hence, electrochemical slag/metal reactions were inferred. These reactions were detected, and the accompanying rectification of the 60 Hz A.C. melting current was measured. It was thus concluded that both chemical and electrochemical slag metal reactions contribute to the final composition of the metal. The cooling and solidification paths of the various metal compositions were then studied. With the use of the classical nucleation theory it was found that the metal compositions after reacting with the slag, provide sufficient supersaturation for inclusion nucleation at the latter stages of solidification. As in the latter stages of solidification little time and material is available for inclusion growth it was concluded that small inclusions are to be expected in the ESR solidification mode. Since the chemical and electrochemical reactions are an inherent part of the electroslag process, we are able to conclude that small globular oxide inclusions are a necessary result of the normal method of electroslag melting. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The objective of this research was to investigate how non-metallic inclusions (inclusions) are physically and chemically transformed, removed and controlled from electrodes to the final ESR-product. Several 1020, 4340 and rotor (Ni-Mo-V) steel electrodes were refined by two ESR-units (7.5 mm and 200 mm in mould diameter) under different slag systems. Refining of these electrodes was done under different deoxidation practices, namely pure Al, CaSi, CaSiAlBa and AISi alloys. Through this research it was found that inclusions in the electrode are physically and chemically transformed in the electrode tip by the thermal gradients. Inclusions are chemically altered by the presence of liquid slag at the liquid film and they are entirely dissolved in the matrix when the droplet is completely formed. No ingot inclusions were identifiable as of electrode origin and it is concluded that all electrode inclusions are either dissolved or removed by the slag. The effects of the slag with and without deoxidizers on the chemical composition of the liquid pool and ingot were traced during refining and hence the chemistry of inclusions was determined by extracting slag and liquid metal samples during refining. The total oxygen content was measured by the vacuum fusion technique, chemical analyses of slag by spectrophotometric techniques, electron microanalysis by SEM and EPMA and x-ray (crystallographic) analysis. The assays were used to formulate and corroborate the deoxidation and precipitation mechanisms. The chemical composition of inclusions in refined ingots are more strongly influenced by the deoxidation practice than by the electrode or the slag composition in low Si0₂ content slags. The precipitation of complex Al-Ca-Si inclusions is predictable in high silica slags (>10.0 wt%) and the most appropriate slag system to perform an efficient deoxidation is the 50 wt% CaF2, 30 wt% Al₂0₃ and 20 wt% CaO. [Figure 1] The deoxidation in ESR ingots takes place by the process of cooperative reactions between slag and deoxidizers in the following sequences: [Figure 2] An excessive deoxidation with Ca raises the Al content in the ingot according to: [Figure 3] Radial inclusion size distribution as well as dendrite arm spacings in samples extracted from liquid pool and ingots were determined. It was found that the inclusion size obeys the normal distribution and there is a normal variation of the inclusion size along radial distances. Hence the inclusion composition and size is a function of local solidification conditions and also of the local thermochemical conditions. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Khatiboleslam Sadrnezhaad. / Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / M.S.

Thesis: M.S., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 1977 / Includes bibliographical references. / by Robert Joseph Furlong, Jr. / M.S. / M.S. Massachusetts Institute of Technology, Department of Materials Science and Engineering