An experimental and theoretical analysis of the phase equilibria in the Fe-Cr-V-C system
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I. INTRODUCTION
ACCURATE phase diagram and thermodynamic information are essential for the understanding and modeling of microstructure evolution. The Fe-Cr-V-C system is a critical alloy system for tool steels and high-speed steels due to the very strong carbide forming elements Cr and V. These kinds of steel normally contain 1 to 3 wt pct C and up to 25 pct phase fraction of carbides. The development of high-speed steels and tool steels has in the last decade led to a significant increase of the amount of Cr and V in the alloys. The elements, Cr and V, are important carbide forming elements, and these carbides yield good mechanical properties for the special fields of applications where these materials are used. Knowing the phase relations in the Fe-Cr-V-C system, that is, the relative stability between Cr and V carbides or the solubility of Cr or V in the V- or Cr-rich carbides, is essential for understanding the behavior of these steels in heat treatments and is the basis for improving the properties or designing new alloys by controlling the amount of alloying elements. Several thermodynamic descriptions of the phase equilibria in this quaternary system have previously been presented.[1,2] They are all based on old experimental information, from commercial alloy systems. Thermodynamic calculations with the old descriptions all show the same tendency: the distribution of Cr and V between the matrix and the carbides is unsatisfactory. To make a significant improvement of this description, we felt that new experimental information was critical. The previously available thermodynamic descriptions are a valuable aid to calculate compositions of model alloys to be used for the experimental determinations. The idea was to produce alloys with as few elements as possible that include the same combination of matrix and carbides that are found in commercial tool steel and high-speed steel. For the general validation, commercial alloy systems were included in the experimental program. Within the Swedish Centre for Computational Thermodynamics (CCT), a thermodynamic database specialized for tool steels and high-speed steels is under construction. Lee and J. BRATBERG, M.Sc., and K. FRISK, Doctor, are with the Computational Thermodynamics Research Group, Swedish Institute of Metals Research, SE-114 28 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted January 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
Lee’s[1] thermodynamic description of the Fe-Cr-V-C system was integrated in a commercial available database (cal. 1), and the present work is a part of a revised thermodynamic database (cal. 2). A well-established method, the CALPHAD method,[3,4] was adopted in the present work to evaluate thermodynamic parameters for the quaternary Fe-Cr-V-C system. The most distinctive character of the methodology is its aim to couple the phase diagrams and thermochemical properties in an attempt to explicitly characterize all of the possible phases in a system. This includes phases that are stable, me
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