Effect of bismuth on phase equilibria in iron-base binary systems

The paper presents the results of the thermodynamic modelling of ternary systems Fe-C-Bi, Fe-Ni-Bi and Fe-Mn-Bi in the temperature range from 0 to 1800 °C at pressure equal to 1 bar. The modelling was made using FactSage software package (version 7.0). The vertical sections of phase diagrams of the listed ternary systems are constructed for the constant bismuth concentration equal to 0.1 wt. %, which is typical for commercial bismuth-doped free-machining steels. Calculation results show that when 0.1 wt. % of Bi is added to iron-base binary alloys, a liquid or solid bismuth-base phase is formed at temperatures below the melting temperature of the bulk of metal in all the studied ranges of concentrations. This amount of bismuth is completely absorbed by the iron-base melt. At the same time, high partial pressure of bismuth at the temperatures when the iron base melt is in a liquid form indicates that the loss of bismuth is possible when adding bismuth to steel. These results of calculation do not contradict to the known facts about the behaviour of bismuth steels in melting and further treatment. Thus, even despite the absence of optimized descriptions of the Bi-C, Bi-Ni and Bi-Mn binary systems, the information contained in FSstel and FactPS databases of the FactSage package can be correctly and effectively used for the modeling of phase equilibria in bismuth-doped free-machining steels.