Surface tension of liquid Fe-Cr-O alloys at 1823 K
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I. INTRODUCTION
KNOWLEDGE of surface tension of liquid iron in the presence of surface-active elements such as oxygen and sulfur is important to the understanding of the processing of liquid steel, because the surface-active elements would preferentially occupy the surface sites and considerably decrease the surface tension. For example, the nitrogen absorption (or desorption)[1–12] kinetics is related to the surface tension of liquid steels. When the interfacial chemical reaction in nitrogen absorption is the rate-determining step, surface-active elements occupy the reaction sites, so that the reaction only (or preferentially) occurs on the vacant sites. If we consider a thermodynamic equilibrium [1] among the surface-active element i in the bulk, the sites occupied by i (i), and the vacant sites (_), the equilibrium constant is given by Eq. [2].[13] i () (i)
[1]
ui Ki (1 ui)ai
[2]
where i is the fraction of the surface sites occupied by the surface-active element i, ai is the activity of i normalized to 1 wt pct as the standard state, and Ki is the adsorption coefficient of i. The adsorption coefficient Ki can be obtained from the information of the change of the surface tension with respect to ai. The gas bubble behavior in front of an advancing solidliquid interface[14,15] is another example. In front of an advancing solid-liquid interface in the continuous casting process, the behavior of gas bubbles is determined by the surface tension gradient, which, in turn, is determined by the concentration gradient of the surface-active element in the boundary layer. Therefore, the surface tension of liquid steels needs to be expressed as a function of the concentration (or activity) of the surface-active element. JOONHO LEE, formerly Postdoctoral Researcher, Department of Materials Engineering, University of Tokyo, is Assistant Professor, Osaka University, Osaka, Japan 565-0817. KOJI YAMAMOTO, Graduate Student, and KAZUKI MORITA, Associate Professor, are with Department of Materials Engineering, University of Tokyo, Bunkyo, Tokyo, Japan 113-8656. Contact e-mail: [email protected] Manuscript submitted May 23, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
Belton expressed the surface tension, , of a liquid metal containing a surface-active element as a function of the activity of the surface-active element using Eq. [3].[13] s sP RTsat i ln (1 Kiai)
[3]
where P is the surface tension of pure metal, R is the gas constant, T is the temperature, and sat i is the surface adsorption of i at saturation. Hence, if we determine the values of P, sat i , and Ki for a given metal (or alloy) at a given temperature, we may anticipate the interfacial behavior of liquid steel (nitrogen adsorption, bubble behavior, etc.). It is probable that knowledge of the surface tension of liquid Fe-Cr-O alloy is necessary in order to better understand the processing of stainless steels containing oxygen. Numerous researchers have reported on the surface tension of liquid Fe-Cr alloys,[16–23] but these re
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