The effect of temperature on the evolution of eutectic carbides and M7C3 → M23C6 carbides reaction in the rapidly solidified Fe-Cr-C alloy

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 degrees 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 degrees 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 degrees 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 degrees 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. (C) 2016 Elsevier B.V. All rights reserved.