Observations of the columnar-to-equiaxed transition in stainless steels
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INTRODUCTION
AS part of the solidification process, the columnar-toequiaxed transition (CET) has been examined for many years. Experimental observations have shown[1] that the position of the transition and the equiaxed grain size in chill castings of aluminum copper alloys is dependent on the melt superheat, the melt composition, and the freezing rate. In addition, it is evident that nucleating sites or dendrite fragments are required, as well as constitutional supercooling, for equiaxed grains to grow ahead of an advancing dendritic interface. More recently, Mahapatra and Weinberg[2] showed that the position of the CET in vertically solidified cylinders of tin/lead alloys is related to the temperature gradient in the melt immediately ahead of the liquidus isotherm. The transition occurs when the decreasing temperature gradient reaches 0.11 7C/mm for Sn 1.0 wt pct Pb and 0.13 7C/mm for Sn 15 wt pct Pb. Similar observations by Weinberg and Ziv[3] for Al 3 wt pct Cu alloys give a gradient of 0.06 7C/mm at the transition. Relating the occurrence of the CET to one particular growth parameter, in this case the temperature gradient in the liquid, is thought by Spittle and Tadayon[4] to be simplistic. Villafuerte and Kerr[5] have examined the CET in ferritic stainless steel gas-tungsten arc welds. They ascribe the observed CET to heterogeneous nucleation of ferrite on Ti-rich inclusions present in the melt. Increasing the welding speed, in some cases, increased the equiaxed fraction of the weld. The factors contributing to the CET have been considered by Hunt,[6] and expressions have been derived for the WARREN J. POOLE, Assistant Professor, and FRED WEINBERG, Professor Emeritus, are with the Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted September 2, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleation process, growth of columnar and equiaxed grains, and constitution supercooling as a function of the growth conditions. Using these expressions, they define when columnar or equiaxed growth takes place and when a transition occurs. The nucleating process requires assumptions for the number of effective nuclei in the melt and the constitutional supercooling associated with the onset of grain growth. The constitutional supercooling is related to solute segregation during solidification, which is difficult to quantify.[1] In the model calculations, the constitutional supercooling is determined from the solute segregation which occurs at the leading dendrite tips, assuming solute movement at the tips is diffusion controlled. Burden and Hunt[7] measured supercooling at the tips of columnar dendrites, growing upward in small tubes, for Al Cu alloys. They established the dependence of supercooling on the alloy composition, the growth velocity, and the temperature gradient during solidification. These expressions were then used to characterize the dendrite tip supercooling in the model. Whether the same expressions can be applied to o
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