Formation of core (M7C3)-shell (M23C6) structured carbides in white cast irons: A thermo-kinetic analysis

Abstract

Core-shell structured carbides in white cast irons help reduce interfacial stress and failure, leading to higher resistance to wear. The present study is conducted to understand the mechanism for the formation of core-shell structured carbides in white cast irons through computational thermodynamics. In particular, efforts are made to determine the compositional ranges in which the core-shell structured carbides can form. Arrays of phase diagrams in stable and metastable equilibria were calculated to determine the stable regions of M23C6, M7C3 and Matrix, which are the basic phase assemblage for the core-shell morphology in high-Cr cast irons (HCCIs). Scheil-Gulliver and Lever-Rule solidifications were simulated for nine alloys with compositions selected from the stable/metastable regions to investigate the phase precipitation sequence, as-cast microstructures and compositions. The contour diagrams for the nucleation driving force were mapped and used to analyze the precipitating ability of each type and the configuration of carbide in its appropriate domains in casting during heat treatment. Thorough discussion is given to the effects of parent microstructure, temperature, and overall composition on the nucleation driving force for the M23C6 shell growth in order to guide fabrication of HCCIs with desired core-shell structured carbides.