Thermodynamics of nucleation and supersaturation for the aluminum-deoxidation reaction in liquid iron
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I. INTRODUCTION
DEOXIDATION of liquid iron to a level such that the oxygen contained in the liquid iron is in equilibrium with the solid a-alumina is generally difficult. Thus, the oxygen content in the iron often exceeds the equilibrium level. The excess oxygen which exceeds the equilibrium value with solid a-alumina is hereafter referred to as excess oxygen. Although reports concerning excess oxygen have been published,[1–6] a persuasive explanation of the presence of excess oxygen beyond the equilibrium value has not yet been reported. Suito et al.[1] recently reported that the low oxygen content (0.002 to 0.004 mass pct) in Fe-0.0001-33 mass pct Al before melting was found to persist long after melting, although the oxygen content in the liquid iron could be lowered to the equilibrium value after repeated stirring when the oxygen content was high (' 0.1 mass pct) before the addition of an Fe-Al deoxidizer. Suito et al. considered that this excess oxygen should be in the supersaturated state. Li and Suito[2] reported that the logarithm of the critical degree of supersaturation (log S8) is 3.5. The parameter S8 shows a maximum degree of supersaturation (S), over which nucleation occurs. Repetylo et al.[3] observed the change in aluminum and oxygen contents with time after the addition of aluminum to liquid iron, which was maintained at 1873 K in an induction furnace under a deoxidized argon atmosphere. The aluminum and oxygen contents in liquid iron, which has an initial oxygen content between 0.01 and 0.055 mass pct and a concentration product (5 (mass pct Al)2 (mass pct O)3) of approximately 3 3 1029, decreased. The concentration product approached the equilibrium value (10214) with time; however, the equilibrium value was never reached, and the final value of approximately 5 3 10213 and an oxygen content of between 0.002 and 0.006 mass KYOKO WASAI, Research Associate, and KUSUHIRO MUKAI, Professor, are with the Department of Materials Science and Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan. Manuscript submitted August 19, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
pct was obtained. Repetylo et al. concluded that the residual oxygen and aluminum should be present in liquid iron as a fine alumina suspension, the upward movement of which might be very slow. Similarly, Rode et al.[4] reported that the excess oxygen is attributable to the suspension of an alumina deoxidation product and showed that covering liquid iron with CaO-Al2O3 slag and the bubbling of argon gas in liquid iron were effective methods of eliminating the suspension. Previously, liquid iron in equilibrium with solid a-alumina was analyzed[7] based on an associated solution model. The results of this analysis showed that not only the thermodynamic properties, but also inconsistencies between measurements performed in various studies, could all be explained reasonably using this model. Considering the applicability of this model to the liquid iron and solid a-alumina system, the excess oxygen might also be explai
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