Experimental investigation of the thermodynamics of the Fe-Ti-C austenite and the solubility of titanium carbide
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I.
INTRODUCTION
T H E solubility of carbides, nitrides, and carbonitrides of Ti, Nb, V, etc., in alloyed austenite has been the focus of many investigations in the past two decades. The mutual stability is generally understood in terms of solubility limits of the binary stoichiometric compounds of these elements in austenite devoid of solute interactions. The carbides and nitrides of Ti and Nb are nonstoichiometric, and hence, their composition can vary when precipitated in steels. As the Group IV and V transition metals are very strong carbide and nitride formers, their interaction with carbon and nitrogen in austenite is very pronounced. The effects of nonstoichiometry and the solute interactions on the solubility of these precipitates have not been addressed in the past investigations, rE2"31All of these investigations were performed at low carbon levels (below 0.2 wt pct) and, hence, are not capable of predicting the solubility at higher carbon levels, where the effects of solute interactions become significant. In this article, the nature and the magnitude of these interactions in Fe-Ti-C austenite and solubility of titanium carbide will be investigated. II.
EXPERIMENTAL PROCEDURE
A. Experimental Technique A modern Cahn 1000 microbalance with capabilities of lifting large sample weights (up to 100 grams) and K. BALASUBRAMANIAN, formerly with McMaster University, is Research Scientist with the Defence Metallurgical Research Laboratories, Hyderabad 50025B, India. A. KROUPA, Research Associate, on leave from Institute of Physical Metallurgy, Brno, Czechoslovakia, and J.S. KIRKALDY, Emeritus Professor, Department of Materials Science and Engineering and Institute for Materials Research, are with McMaster University, Hamilton, ON L8S 4L7, Canada. Manuscript submitted April 22, 1988. METALLURGICAL TRANSACTIONS A
very high sensitivity [1 microgram (mcg) weight change], together with a dynamic equilibrium system, has been used in this investigation of the thermodynamics of FeTi-C austenite and the solubility of binary titanium carbide. In this dynamic equilibrium method, the Fe-Ti samples are equilibrated at high temperatures (1273 to 1473 K) in a flowing carburizing gas, the carbon potential of which is controlled by metering the component gases (methane and hydrogen) in the mixture. Accurate chemical analysis of equilibrated samples is essential for obtaining the necessary thermodynamic quantities. Dynamic weight changes measured using a Cahn 1000 microbalance have been used to obtain the carbon content. A sensitive microbalance is ideally suited for continuously monitoring small weight changes, and hence, the attainment of equilibrium is firmly established once the weight change with time becomes negligible. Dunwald and Wagner t41 used a microbalance in their classic study on the thermodynamics of ferrite in the Fe-C system.
B. Experimental Apparatus The experimental apparatus consists of five parts, namely, gas metering and blending system, gas cleaning furnaaes and chambers, high-temperature furnace assembl
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