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Influence of oxygen and carbon activities on surface tension of liquid iron studied using oscillating droplet method


K. Morohoshi1, M. Uchikoshi1, M. Isshiki1 and H. Fukuyama1

1Tohoku University, Japan

Keywords: oscillating droplet method
property: surface tension,
material: liquid iron, oxygen, carbon

Numerical simulations have been developed for process optimization of refining, casting and welding of steel. Accurate thermophysical properties are, therefore, necessary for the simulations as an input data. Surface tension is one of the important properties, which causes Marangoni flow and affects temperature profile and flow pattern in the melt. The Fe-O-C system is the most fundamental system for steel processes, and its surface tension data is essential to simulate Marangoni flow in molten steel. Thus, we aim to clarify the dependences of oxygen and carbon activities on the surface tension using a gas-liquid equilibrium method.

An oscillating droplet method using an electromagnetic levitator (EML), which provides contamination-free measurements, was applied to measure surface tension of liquid iron. High-purity (99.9972 mass %) iron prepared using an ion exchange method [1] was used as a sample. Oxygen and carbon activities were controlled using a gas-liquid equilibrium method with Ar-He-CO-CO2 gas mixtures. The temperature of the sample was measured using a mono-color pyrometer. The images of the oscillating droplet were recorded by a high-speed camera from the top. The frequencies of the surface oscillation of liquid iron were analyzed through fast Fourier transformation (FFT) from the time sequential data of the observed images. The surface tension of the liquid iron was calculated from these frequencies by the modified Rayleigh equation proposed by Cummings and Blackburn [2, 3].

At low carbon activity aC = 10-3, the oxygen activity dependence of the surface tension of the liquid iron was measured in a range of oxygen activity aO2 = 10-12 - 10-8.4. Here, the standard state of the oxygen and carbon activities are 1 bar and a pure graphite at 1 bar, respectively. The surface tension decreases with increasing oxygen activity. From the experimental data, the surface tension of liquid iron was described as functions of oxygen activity and temperature using the Szyszkowski model [4]. Furthermore, at low oxygen activity aO2 = 10-15, the carbon activity dependence of the surface tension was measured in a range of aC = 0.1 - 0.5. Under this condition, the influence of the carbon activity was not clearly observed. 

References
  1. M. Uchikoshi, J. Imaizumi, H. Shibuya, T. Kékesi, K. Mimura, M. Isshiki, Thin Solid Films 461, 94 (2004)

  2. Lord Rayleigh, The Royal Society 29, 71 (1879)

  3. D. L. Cummings and D. A. Blackburn, J. Fluid Mech. 224, 395 (1991)

  4. B. von Szyszkowski, Z. Phys. Chem. 64, 385 (1908)

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