Abstract
This paper presents the effect of high temperature heat treatment on the evolution of eutectic carbides and on the M7C3 → M23C6 carbides reaction in the rapidly solidified hypoeutectic Fe-Cr-C alloy. The investigated alloy was designed using the CALPHAD approach via ThermoCalc in order to obtain a high volume fraction of eutectic carbides with high thermodynamic stability and matrix, where BCC to FCC phase transformation is shifted to temperatures above 1100 °C. The Fe-Cr-C alloy was synthesized in an arc furnace in a high purity argon atmosphere. The studies involved a wide range of experimental techniques to characterize the alloy in the as-cast state, after heat treatment for 4 h at 650, 800 and 1000 °C and during continuous heating including in-situ high temperature X-ray diffraction, high-resolution dilatometry, light microscopy, scanning electron microscopy, transmission electron microscopy and microhardness tests. The microstructure of the investigated alloy in the as-cast state is composed of primary dendrites with secondary branches of Fe-Cr solid solution with eutectic, very fine, rod-like M7C3 (M = Cr and/or Fe) as the predominant carbides and a small amount of M23C6 carbides. It was found that during heating, the M7C3 → M23C6 transformation proceeds in the temperature range between 500 and 600 °C. The first stage of the carbides transformation is the uphill diffusion of chromium from the matrix into M7C3, which is reflected in the abnormal growth of lattice parameters above 400 °C. When the chromium concentration in M7C3 achieves the required level, the orthorhombic M7C3 carbide transforms in-situ into the equilibrium cubic M23C6 carbide with a higher symmetry of the crystal structure. Additionally, the effect of heat treatment on the eutectic carbides' morphology evolution was determined. The M7C3 → M23C6 transformation results in a significant increase in the final volume fraction of carbides, which enhances the coalescence process of rod-like carbides.
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