Standard gibbs energies of formation of the carbides of chromium by emf measurements
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CrF2, 'Cr-C' (+)
The measurements have been carried out in the temperature range 1002 to 1176 K. The AG ~ values obtained in the present work are generally in agreement with some of the earlier emf measurements but differ significantly from those obtained by Cr203-CO equilibrium studies.
I.
INTRODUCTION
C H R O M I U M plays an important role as an alloying element in tool and heat-resisting steels. A knowledge of the stabilities of the carbides of chromium will be useful in the understanding of heat treatment and thermomechanical working of chromium steels. Furthermore, the study of the hydrogen attack on steel requires an understanding of the stabilities of the carbides in steel, which, in turn, would require a clear idea of the thermodynamics of the Cr-C binary system. Chromium carbides are also of great importance in the hard metal industry. One recent application is the use of chromium in stabilizing the nonstoichiometric uranium monocarbide, which would hinder the formation of uranium dicarbide. A knowledge of the stabilities of chromium carbides would be useful in the temperature effect of such a stabilization. The study of the thermodynamics of the carbides of chromium is interesting even from a theoretical viewpoint. It is generally known that the transition metals having lesser atomic numbers than chromium would form separate carbides in steel in view of their affinities for carbon. On the other hand, the carbides of the transition metals with higher atomic numbers than Cr in the periodic table get dissolved in cementite. In the case of chromium itself, in addition to the occurrence of complex carbides in alloy steels, the precipitation of simple chromium carbides is also found in certain high-chromium steels and superalloys. A knowledge of the thermodynamic properties of the carbides of chromium would be of great value in the understanding of the chemical behavior of chromium in ferroalloys. II.
PREVIOUS WORK
The phase diagram for the Cr-C binary system is shown in Figure 1 .tq It is seen that chromium forms three stable carbides, viz., Cr23C6, Cr7C3, and Cr3C2. Four different experimental methods have been employed in the deter-
DU SICHEN, Research Associate, S. SEETHARAMAN, Docent and Senior Lecturer, and L.-I. STAFFANSSON, Professor, are with the Department of Theoretical Metallurgy, The Royal Institute of Technology, S-100 44 Stockholm, Sweden. Manuscript submitted January 26, 1989. METALLURGICAL TRANSACTIONS B
mination of the Gibbs energies of formation of these carbides. These are (a) a solid-gas equilibration technique involving Cr203 and C O , [2'3'4] (b) a solid-gas equilibration technique involving CH42 and H2, [51 (c) solid-state galvanic cell measurements involving C ~ 2 , [6'7] o x i d e Is] o r carbide electrolyte, tgl and (d) the Knudsen effusion method, t~~ In addition, a number of thermodynamic evaluations of the Cr-C binary system t"-lSJ have also been carded out. Thus, there is an abundance of thermodynamic data in the literature, but the discrepancy between different studies i
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