Оxygen solubility in melts of Ni – Co system at complex deoxidation by aluminium and silicon

Abstract

Alloys of Ni – Co system are widely used in industry. Oxygen is one of the harmful impurities in these alloys; in metal it is present in dissolved form or in the form of nonmetallic inclusions. Getting the finished metal with a minimum oxygen concentration is one of the main tasks of production process of these alloys. With the complex deoxidation of the metal melt, the activity of oxides resulting from the deoxidation process is less than one, due to this, with the same content of deoxidizing elements, it is possible to obtain a metal with a lower oxygen concentration, therefore, more deeply deoxidized. At joint deoxidation with two deoxidizers, a stronger deoxidizer takes a predominant part in the reaction, however, if oxides of the deoxidizing elements form chemical compounds, it contributes to participation of a weaker deoxidizer in deoxidation process. A thermodynamic analysis of the joint influence of aluminum and silicon on the solubility of oxygen in the melts of Ni – Co system has been performed. Deoxidation reaction products may be formed as mullite (3Al2O3·2SiO2 ), and kyanite (Al2O3·SiO2 ). Presence of silicon in the melt enhances the deoxidizing ability of aluminum: insignificantly in the case of formation of compound 3Al2O3·2SiO2 and significantly in the case of formation of compound Al2O3·SiO2 . Oxygen solubility curves in the case of formation of compound Al2O3·SiO2 pass through a minimum, the position of which depends on the content of aluminum in the melt and doesn’t dependent on silicon content. Aluminum content in the minimum points is insignificantly reduced from nickel to cobalt as in the case of melts of Ni – Co–Al. Further additives of aluminum lead to an increase in oxygen concentration. Areas of compounds Al2O3 , 3Al2O3·2SiO2 , Al2O3·SiO2 and SiO2 depending on aluminum and silicon contents in the melt are determined. In melts of the Ni – Co system, the deoxidizing ability of aluminum and silicon increases with an increase in cobalt content in the melt. However, silicon enhances the deoxidizing ability of aluminum the weaker the higher cobalt content.