An ice-water model study on formation of blowhole-free skin with reference to solidification of steel

Abstract

A cold model investigation on the formation of blowhole-free initial skin was undertaken in the laboratory by freezing water containing dissolved carbon dioxide. Plane front unidirectional solidification was simulated. Experiments were conducted in a stagnant bath to study the effect of CO2 concentration in water and the rate of solidification on blowhole-free skin thickness. Some experiments were also conducted with artificial gas bubbling. Initially, the variation of thickness of ice with time deviated from the parabolic rate law of solidification. The thickness of blowhole-free skin decreased with increasing concentration of dissolved CO2 and solidification rate. Mild stirring increased the thickness of blowhole-free skin. Differential equations, along with requisite initial and boundary conditions, were solved by a numerical analysis technique to compute the concentration of CO2 in water at the moving ice-water interface. This revealed that very small supersaturations were required to form blowholes although the experiments were conducted under very clean and controlled conditions. Implications of some of the findings in connection with solidification of steel are indicated.