Revealing carbide precipitation effects and their mechanisms during quenching-partitioning-tempering of a high carbon steel: Experiments and Modeling

Quenching-Partitioning-Tempering (Q-P-T), as a modified process of quenching and partitioning (Q&P), is a promising process to treat ultra-high strength steels with a good balance of strength and ductility. The essences of the Q-P-T process are to stabilize metastable austenite via carbon partitioning from martensite into austenite during partitioning and to strengthen the martensite matrix via nano-precipitation of micro-alloyed carbides during tempering. The competitive reactions, e.g. carbon segregation to dislocations, transition carbide precipitation and austenite decomposition, could occur during partitioning/tempering, which are expected to play a substantial role in the microstructures of the Q-P-T steels. In this contribution, the complex microstructure evolution during the Q-P-T processing of an Fe-0.67C-1.48Mn-1.53Si-0.038Nb steel was systematically characterized by various techniques. A concise QPT-LE (Local Equilibrium) thermo-kinetic model with dual interfaces (martensite/carbide and martensite/austenite) migration was established to predict the evolution of austenite fraction and its carbon content based on the consideration of competitive reactions mentioned above. In the QPT-LE model the effect of carbide precipitation is introduced, which is different from CCE (constrained carbon equilibrium) thermodynamic model and QP-LE thermo-kinetic model. Therefore, the QPT-LE model can be used to reveal the effects of carbide precipitation on the retained austenite fraction and its carbon content, while the prediction accuracy of carbide fraction can be further improved by considering the effect of carbon segregation to dislocation. In general, the QPT-LE model can better predict the experimental results compared with popular CCE model and QP-LE model.