X-ray-based measurement of composition during electron beam melting of AISI 316 stainless steel: Part II. Evaporative pr
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I. INTRODUCTION
IN Part I of this study, the application of an energy dispersive X-ray (EDX) spectrometer for in-situ measurements of composition was described.[1] In the electron beam (EB) melting of specialty alloys, accurate on-line composition measurement is needed to help reduce and potentially eliminate composition related rejections due to alloy losses associated with evaporation. Furthermore, the capacity to have continuous on-line reading of alloy composition eliminates uncertainty as to the uniformity of the final ingot composition. In Part II, the viability of using this characterization technique to measure the bulk composition during the EB melting of specialty alloys is further investigated using a combined experimental and computational approach. Specifically, a series of melting experiments of AISI 316 steel were performed using a laboratory scale EB melting furnace. The evaporation rate, and hence rate of compositional change, was measured using the EDX in-situ detector. A computational model of the heat, mass, and momentum transfer was implemented to allow the influence of surface evaporation vs bulk mixing to be determined. In addition, direct measurements of the composition of the vapor phase were made by analysis of short duration deposits on an acetate film. M. RITCHIE, Engineer and Technical Sales Specialist, is with North American Pipe and Steel, Delta, BC, Canada V4H 1B9. P.D. LEE, Senior Lecturer, is with the Department of Materials, Imperial College, London, United Kingdom SW7 2BP. Contact e-mail: [email protected] A. MITCHELL, Emeritus Professor, and S.L. COCKCROFT, Associate Professor and Head, are with the Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. T. WANG, Development Engineering, is with Alstom Ltd., 5401 Baden, Switzerland. Manuscript submitted November 29, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
A general background on the evaporation mechanisms is given first, followed by a short overview of prior studies of fluid flow and evaporation in EB furnaces. The experimental methods and computational model theory are then described and the results of the laboratory experiments and simulations are presented and discussed, illustrating that even with the very high evaporative rates in industrial EB melting furnaces, the EDX in-situ detector can be used to monitor bulk compositions. II. BACKGROUND A. Evaporation Kinetics Numerous authors have studied the role of evaporation in vacuum metallurgical systems, with the general conclusion that the evaporating solute atoms can be traced from the bulk of the liquid metal to the location where they finally condense in six steps:[2] (1) transport from the bulk liquid to a diffusion boundary layer, (2) diffusion across the boundary layer to the surface of the melt, (3) desorption from the surface of the melt, (4) diffusion across a gas boundary layer, (5) transport through the bulk gas, and (6) condensation. The EB furnaces typically operate at pressures of 10⫺1 to 10⫺3 Pa w
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