Thermodynamic analysis of two-stage heat treatment in TRIP steels

Abstract

In this work, we present a detailed thermodynamic analysis of the two-stage heat treatment (intercritical annealing (IA) and banite isothermal transformation (BIT)) necessary to stabilize retained austenite in transformation-induced plasticity (TRIP) assisted steels. Through a set of experiments on alloys with nominal composition Fe–0.32C–1.42Mn–1.56Si (wt.%), we monitored the evolution of the volume fraction of retained austenite at room temperature as a function of the IA and BIT temperatures. We also investigated the thermodynamic limit for the bainitic transformation during BIT under the displacive (partitionless) transformation assumption. The fraction of retained austenite at the end of the two-stage heat treatment was calculated by taking into account the corresponding start of the martensitic transformation (TMs). Comparisons with experiments suggest good qualitative agreement in the fraction of retained austenite when considering the effect of the IA temperature. On the other hand, the analysis of the effect of BIT on the amount of retained austenite showed qualitative disagreement with the observations. To further analyze this discrepancy, we utilize a modified thermodynamic analysis with empirical observations as input, and conclude that the assumption of thermodynamic equilibrium at IA is not valid at lower IA temperatures. Moreover, the unexpected high carbon enrichment in retained austenite indicates the importance of the kinetic effects. We conclude that the thermodynamic limit for the bainitic transformation can be used at least to provide a lower bound to the expected fraction of retained austenite under specific IA+BIT treatment schedules.