The effect of gas flow rate on the evolution of the surface oxide on a molten low carbon Al killed steel


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Abstract

The oxide phase formation on a molten Al killed low carbon steel surface under a flowing Ar atmosphere with an oxygen partial pressure of Po2 = 1–5 × 10−5 atm has been visualized with a Confocal Scanning Laser Microscope (CSLM) equipped with a gold image furnace. In this study, the effect of gas flow rate variation (170–300 cm3/min) on the oxide evolution under isothermal conditions of 1600°C was investigated. Al2O3, rather than the thermodynamically stable phase FeAl2O4, was found to precipitate under all the experimental conditions studied and the apparent rate of evolution was found to increase with increasing gas flow rate. The oxide evolved as a network that started from the container wall and grew towards the crucible center. At low flow rates the growth was a result of primarily crystal growth resulting in distinctly dendritic crystals. As the flow rate was increased, growth due to the attachment of discrete inclusions to the advancing front was observed which resulted in a final oxide network that constituted of smaller facetted particles. In the latter case, the transport of the individual inclusions to the advancing front could be caused by surface Marangoni flow due to gradients in both temperature and dissolved oxygen concentration.

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