Effect of superheat on the solidification structures of AISI 310S austenitic stainless steel
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where c and d are constants determined by the distance from the edge of the ingot. The relationship between primary arm spacing and superheat could be expressed by the equation At =
p+q
In (1/AT)
where p and q are constants determined by the distance from the edge of the ingot. The parameter "grain width ratio" has been introduced to describe the relationship between the shape and the nucleation and growth kinetics of the columnar grains.
I.
INTRODUCTION
THE solidification parameters that determine the nature of the cast structure are the thermal gradient, growth rate, and alloy variables such as the slope of the liquidus, solute distribution coefficient, and initial solute concentration. The ratio of thermal gradient (G) to growth rate (R) has been shown to be a sig-nificant parameter with respect both to the mode of growth and the final grain structure produced in solid solution alloys.Vm In his extensive work on AISI 310S stainless steel, Fiegenschuh[61 studied the effects of casting temperature and cooling rate on the cast structure of this material. He observed an exponential relationship between casting temperature and average grain size at constant cooling rate (40 Kmin-'). He derived the following relation between the cooling rate and the average grain size: d~, = 13.66v -~
[1]
where d~, is the average grain size and v the cooling rate in Kh-L This equation was based on an earlier model ~j which is of the form d~ ',~ = k'v,."
[2]
where k' and n are constants and vt. is the linear cooling rate at the liquidus point of the alloy.
S. OZBAYRAKTAR, Senior Research Scientist, is with De Beers Diamond Research Laboratories, Johannesburg, 2000 South Africa. A. KOURSARIS, Associate Professor, is with the School of Process and Materials Engineering, University of the Witwatersrand, Johannesburg, 2050 South Africa. Manuscript submitted October 8. 1994. METALLURGICAL AND MATERIALS TRANSACT{ONS B
The cooling rates employed by Fiegenschuh .'~i in his casting experiments were relatively slow compared to cooling rates prevailing during continuous casting. The cooling rate could be as high as 1440 K s - ' at 0.5 mm from the surface of continuous cast slabs. It drops sharply to about 360 Ks '~ at I mm and to about 75 Ks -~ at 3 mm from the surface57~ At high cooling rates, AISI 310S was foundry] to solidify through a peritectic reaction rather than directly to austenire. Although the effect of casting temperature on the evolution of macrostructure is well documented, its effect on the microstructure of cast steels has not been explored in detail. It was expected that as the macrostructure was refined, the microstructure should change likewiseYJ However, in his work on the effects of nucleation and constitutional supercooling on the properties of cast steel, ChurchV0J showed that higher casting temperatures resulted in smaller primary and secondary arm spacings. He found that cooling rates for the castings poured from higher temperatures were greater, resulting in a shorter mushy zone due to steeper thermal gradie
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