Simple model of microsegregation during solidification of steels
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NTRODUCTION
SOLIDIFICATION phenomena play a major role in such diverse operations as casting, crystal growth, and welding. Solidification proceeds at various rates, which are sometimes far from equilibrium. Thus, the microstructure obtained is generally not homogeneous and gives rise to variations in composition with position at both small and large scales, which is known as segregation. Solute segregation is important because it leads to nonequilibrium phases, cracks, and other problems, which lower the mechanical properties of the final product. Over the last three decades, attention has focused on segregation of aluminum and steel alloys, owing to their great commercial importance and susceptibility to this solidification problem. Segregation affects all processes, including foundry, ingot, and continuous casting. Segregation is classified, according to its scale, as macrosegregation or microsegregation. Macrosegregation occurs on the scale of the grains or the entire casting and can be observed with the naked eye. It arises from large-scale fluid flow, caused by forced, natural, and solutal convection. It requires the transport of solute-rich or -poor liquid and solid phases during solidification over distances much larger than the dendrite arm spacing. One unavoidable cause is the interdendritic flow of liquid due to solidification shrinkage and changes in the liquid density. These density changes can be caused by temperature changes or by changes in the liquid composition.[1,2,3] Macrosegregation is also affected by nozzles, which direct the liquid; electromagnetic forces, which enhance mixing;[4,5,6] and by thermal or mechanical bulging or deformation of the casting during solidification.[7] Microsegregation refers to a composition variation within YOUNG-MOK WON, formerly a Postdoctoral Research Associate, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, is Researcher, Pohang Iron & Steel Co., Ltd., Gyungbuk, 790-785 Korea. BRIAN G. THOMAS, Professor, is with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted October 10, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
the columnar or equiaxed dendritic solidification structure, which has a length scale on the order of only a few micrometers. Clearly, macrosegregation prediction is very complex. Among many other things, it depends on an accurate prediction of microsegregation. Microsegregation is caused by the redistribution of solute during solidification, as solute is generally rejected into the liquid.[8] Its fundamental cause is the difference between the thermodynamic equilibrium solubility of alloy elements in the different phases that coexist in the mushy region during solidification. This is combined with the inability of solidstate diffusion to fully return the composition to its equilibrium constant level after solidification is complete, owing to the short times and small diffusion coefficients involved. Quantita
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