Prediction of Precipitation Sequences within Grains in 18Cr–8Ni Austenitic Steel by Using System Free Energy Method

Abstract

The applicability of theoretical energy analysis to the evolution of microstructures in heat-resistant steels was explored by using the system free energy method to predict the precipitation of M₂₃C₆ and σ phase within grains in 18Cr–8Ni austenitic steels. The chemical free energy of Fe–C–Cr–Ni quaternary steel and the interfacial and elastic strain energies between austenitic (γ) matrix and the M₂₃C₆ and σ phase were estimated for the system free energy of microstructures wherein coherent or incoherent M₂₃C₆ and the incoherent σ phase precipitated within γ grains. By identifying the minimum-energy path through a determination of system free energy hierarchies, the precipitation initiation curves of precipitates in Fe–0.07C–18.95Cr–9.57Ni steel for temperatures between 823–973 K were theoretically predicted. The calculated curves agreed well with experimental results for Type 304H austenitic steels; this suggests that the system free energy method is suitable for predicting the evolution of microstructures in heat-resistant steels.