Tellurium Thermodynamics in Ferrous Alloys

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0096619SchlesingerA thermodynamic model for predicting the activity of tellurium in molten steels is needed to develop safer and more effective alloying practice. This would improve the viability of tellurium as a replacement for lead in free-machining steels. However, little is known about the high-temperature Fe-Te system in general, and the thermodynamics of dilute Fe-Te melts are entirely unknown. The same is true for ternary and higher-order systems. Fe-C-Te and Fe-Mn-Te are of particular importance, given the presence of carbon in most steels and the strong interaction between dissolved manganese and tellurium often noticed in tellurium steels. The research program investigates the thermodynamics of dilute Fe-(C, Mn)-Te molten alloys by measuring the solubility of tellurium vapor in these melts as a function of temperature and composition. The experimental program centers on the use of a self-sealing two-zone furnace tube with molten tellurium at the bottom and the ferrous alloy held in a crucible at the top. Varying the temperature of the tellurium changes its vapor pressure (and thus its solubility); this sets the activity of tellurium with respect to the liquid reference state. Analyzing the ferrous alloy after equilibration determines the mole fraction of tellurium, and thus the activity coefficient. Measurements are made in both pure iron and in iron-carbon (up to saturation level) and iron-manganese (10 wt.%) melts, at temperatures ranging between the liquidus and 1600 degrees C. The experimental results will be fitted against the thermodynamic model developed by Bale and Pelton for non-dilute metallic solutions. This model allows the prediction of tellurium activity (and thus partial pressure) in ferrous melts as a function of composition and temperature. %%%As environmental concerns over the use of lead in free-machining steels grow, increasing attention is being paid to potential alternative additions. One of these alternatives is tellurium, which, added in small quantities (generally less than 0.5 wt.-%), is even more effective than lead at promoting machinability. However, tellurium is expensive and difficult to alloy, given its low boiling point (989 degrees C); its high vapor pressure at steelmaking temperatures presents workplace hygiene concerns of its own in addition to potential losses to the dust.***