Quenching and Partitioning (Q&P) Steel: Alloy Design, Phase Transformation and Evolution of Microstructure

A low-carbon low-alloy steel suitable for Q&P processing was designed by considering the influence of alloying elements on various aspects of transformation and microstructure like the optimum quenching temperature (QT(opt)) that maximizes the amount of retained austenite at room temperature, the critical cooling rate for martensite transformation, thermal and mechanical stability of retained austenite and cementite precipitation from austenite. A number of semi-empirical equations and the thermodynamic software ThermoCalc were used for the purpose. The designed steel was prepared and pre-processed in the laboratory by melting, casting and hot forging. The samples from the as forged material were then subjected to quenching, austempering and quenching and partitioning treatments in a dilatometer and salt-bath furnace to understand the preliminary phase transformation behavior, microstructural evolution and mechanical properties. An alternative method, utilizing the experimental dilatation data obtained during quenching and conventional austempering processes, was used to quantify the multiphase Q&P microstructures which provided a reasonably accurate estimate of the actual amount of retained austenite at room temperature. The microstructure obtained after Q&P processing resulted in a better combination of strength and elongation than the martensitic and bainitic microstructures which was attributed to a more effective exploitation of the TRIP effect. The TRIP behavior has been found to depend not only on the amount of retained austenite and its carbon content but also on the morphology of austenite and the characteristics of the surrounding phases.