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Dissolution Equilibrium of Magnesium Vapor in Liquid Iron X I A O D O N G Z H A N G , Q I Y O N G HAN, and DONG CHEN The dissolution equilibrium of magnesium vapor in liquid iron was investigated at 1873 K in a two-temperature zone furnace using the vapor pressure method. A sealed molybdenum reaction chamber and TiN crucible were used in the experiment. The equilibrium constant for the reaction Mg(g~ = [Mg] and the free energy of solution obtained are

steel, t51 It was found that magnesium is a good deoxidizer in steelmaking processes if it can actually be added to a steel bath. t6] Furthermore, after treating steel with magnesium, nozzle blockage is reduced, and toughness and ductility are greatly improved, t7,8] Because of the low boiling point and high chemical activity of magnesium, the thermodynamic data for magnesium in iron at steelmaking temperature are very scarce. It is the purpose of this work to investigate the dissolution of magnesium in liquid iron at different magnesium vapor pressures by equilibrating liquid iron with a known magnesium vapor pressure. The equilibrium constant and the free energy of solution of magnesium in liquid iron are also evaluated. Pure iron was melted and deoxidized using metallic calcium (99 pct purity). The composition of the iron before deoxidation was (weight percent): 0.002C, 0.005Si, 0.008S, 0.005P, 0.001Mn, 0.02Ni, and 0 . 1 8 0 . After deoxidation, the oxygen content was less than 0.007 wt pct. The TiN crucible was made from TiN powder (99.9 pct purity) by a hot pressing method. Magnesium with 99.9 pct purity was used. Experiments were carried out in a two-temperature zone furnace which was heated by two groups of molybdenum heating elements controlled by two individual temperature controllers and thermocouples. The higher temperature zone was kept at 1873 K, while the lower temperature zone was varied between 1343 and 1373 K in order to change pressure of magnesium vapor in the reaction chamber. The equipment is shown schematically in Figure 1. The TiN Crucible containing the iron charge was located in the higher temperature zone of the furnace, and the magnesium, contained in a pure iron crucible, was located in the lower temperature zone. Both the iron and magnesium charges were sealed in the molybdenum reaction chamber with an iron covet. The temperature of the low-temperature zone was measured by means of a P t / R h l 0 - P t thermocouple fitted inside the

K = aM___.~g~ 0.023 eMg

AG~

pct = 5 8 , 7 0 0

(J/mol)

Magnesium and its compounds have been used successfully in ironmaking processes to desulfurize steel and to produce spheroidal graphite cast iron. t~-41 In the mid-1970s, magnesium was introduced to clean liquid

XIAODONGZHANG, formerlywith the University of Science and Technology, Beijing, is Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. QIYONG HAN, Professor, is with the Department of Physical Chemistry of Metallurgy, Universityof Science and Technology,Beijing, Peo