Coarsening Kinetics of M23C6 Carbide in a Tantalum Containing 12% Cr Steel: Modeling and Experimental Validation

Abstract

Coarsening kinetics of M₂₃C₆ (M = Cr, W, Fe) precipitate in a Tantalum containing 12% Cr tempered martensitic steel is studied at the aging temperature of 650 °C, employing DICTRA simulations. Presence of M₂₃C₆, Laves phase, and MX phase at 650 °C is confirmed through thermodynamic simulations and transmission electron microscopy (TEM), with the mean radius of ≈11, 41, and 37 nm, respectively. Predicted size of M₂₃C₆ precipitate is substantiated with TEM investigation, and the effect of interfacial energy, alloying elements, temperature, and formation of Laves phase on the coarsening kinetics of M₂₃C₆ is discussed. A combination of microscopy and precipitation kinetic simulations suggests that the interfacial energy of carbide and tempered martensitic matrix lies in the range of 0.5–0.7 Jm⁻². Simulation results suggest that the addition of Mn increases the coarsening rate coefficient of carbide, while Co decreases it, due to their effect on the diffusion coefficients of Cr in the matrix. Elevated temperature results in an increase in the diffusion coefficient of Cr in the matrix, which thereby leads to an increase in the coarsening rate of the carbide. Further, it is observed that, formation of Laves phase (Fe₂W) leads to slight increase in the coarsening rate coefficient of M₂₃C₆ carbide.