Inclusion precipitation diagram for the Fe-O-Ca-AI system

Abstract

The inclusion precipitation diagram for the Fe-O-Ca-Al system at 1823 K was developed in terms of Henrian activities from thermodynamic data. In the diagram, stability surfaces separate the O-Ca-Al saturated and unsaturated volumes and the location of a point represents the chemistry of the steel. By allowing the chemistry of the steel to change by the formation of reaction products, the sequence of precipitation of the inclusion phases can be predicted. From the inclusion precipitation diagram, projections of interphase boundaries on the Ca-Al, Ca-O and Al-O planes and the stability lines for constant aluminum contents, it was concluded that 1) the Ca:Al ratio in liquid iron determines the identity of the inclusion phases, 2) the calcium and aluminum contents can vary over a narrow range for each calcium aluminate phase, 3) in steels containing aluminum with an activity >0.01, calcium has little effect as a deoxidizer, and 4) close control of the calcium and aluminum contents is required to obtain a specific calcium aluminate inclusion phase. Activity coefficients are hypothesized that relate the ratios of the calcium and aluminum contents of liquid steel, the identity of the precipitating inclusion phases and the calculated activity ratioshCa/hAl.