Rayleigh Number Criterion for Formation of A-Segregates in Steel Castings and Ingots
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INTRODUCTION
A-SEGREGATES are narrow, pencil-like macrosegregation patterns that are often found in the outer columnar zone of large steel ingots and castings. They contain small equiaxed grains and are highly enriched by various solute elements such as C, S, and P. Sometimes, A-segregates are associated with porosity and inclusions. A-segregates diminish the mechanical properties of a casting and should be avoided. For steel ingots, they can negatively affect subsequent rolling or forging operations. Shaped steel castings are sometimes rejected when A-segregates are exposed during machining. A-segregates are, in principle, the same as the freckles that are often observed in directionally solidified Ni-based superalloy castings. A-segregates are initiated by convective instabilities during solidification in the high liquid fraction region of the semi-solid mush, near the primary columnar dendrite tips.[1] In solidification of steel, most of the light alloying elements (like C, S, and P) have a partition coefficient less than unity, implying less solubility in the solid than in the liquid. Therefore, during solidification, these elements will be rejected into the melt. The rejection of the light elements causes the density of the liquid to decrease. Under certain conditions, the density decrease is large enough that the induced buoyancy forces overcome the frictional retarding forces in the M. TORABI RAD, Graduate Research Assistant, and C. BECKERMANN, Professor, are with the Department of Mechanical and Industrial Engineering, The University of Iowa, 2402 SC, Iowa City, IA 52242. Contact e-mail: [email protected] P. KOTAS, Postdoctoral Associate, is with the Department of Mechanical Engineering, Technical University of Denmark, Kgs, 2800 Lyngby, Denmark. Manuscript submitted December 3, 2012. Article published online May 8, 2013 4266—VOLUME 44A, SEPTEMBER 2013
semi-solid mushy zone and convection cells form. Since the mass diffusivity of the liquid is much lower than its heat diffusivity, the segregated light liquid keeps its composition as it flows to the upper regions of the mushy zone where the temperature is higher. In these regions, the segregated melt causes delayed growth or even localized remelting of the solid, which leads to an increase in the local permeability of the mush. The local increase in the permeability allows the segregated liquid to flow more easily, which in turn delays solidification or enhances remelting further, until open channels form that are completely free of solid. The channels are no larger than a centimeter in diameter and are fed by melt flow from the surrounding mush. The channels emit highly segregated liquid into the central, still fully liquid, portion of a casting. The strong flow inside of the channels can separate solid fragments from the dendrites surrounding the channels. If the fragments remain in the channels, then they will eventually grow into the equiaxed grains that are associated with A-segregates. If the fragments are ejected into the central liquid region and surv
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