Coarsening of carbides during different heat treatment conditions

• Coarsening of M7C3 and V4C3 carbides was quantitatively described in detail.
• Cooling mode is a key factor to the simulation for the coarsening of carbides.
• Coarsening of above spherical carbides can be calculated by Ostwald ripening model.
• The interfacial energy between the γ matrix with M7C3 and V4C3 carbides are 0.7 J/m2.

Coarsening of carbides in 1# Fe–5.96Cr–0.35C (wt.%) alloy and 2# Fe–0.5V–0.53C (wt.%) alloy during different heat treatment conditions was investigated by carbon replica, high-resolution transmission electron microscopy (HRTEM) , X-ray diffraction (XRD) and SEM techniques. The equilibrium phases at 850 °C constitute of austenitic matrix (γ) + M7C3 and austenite matrix (γ) + V4C3 for 1# and 2# alloy respectively. Morphology of M7C3 and V4C3 carbides was mainly determined by cooling mode due to the different nucleation sites and growth mechanisms. Under directly aging condition, most carbides nucleate in the grain boundaries and grow into rod-shaped or flake-shaped particles by discontinuous growth mechanism. These particles turn out to be excluded during coarsening simulation using Oswald ripening model to give a more reasonable result. In addition, interfacial energy between M7C3/γ and V4C3/γ for the coarsening of M7C3 and V4C3 during aging at 850 °C is evaluated by fitting experimental data using thermodynamic and kinetic calculations. The interfacial energy is determined to be 0.7 J/m2 for the coarsening of M7C3 and V4C3 in austenitic matrix.