Evolution of microstructure during the "quenching and partitioning (Q&P)" treatment

Most studies on the Q&P process focus on enhancing the resultant amount of retained austenite. The quenching temperature (QT), which maximizes the amount of retained austenite is known as optimum quenching temperature (QT(opt)) and is the primary design parameter of the Q&P process. The results show that the existing models do not give a close estimate of the experimental QT(opt) and the corresponding amount of retained austenite. The detailed microstructural and dilatometry analysis showed bainite transformation, incomplete carbon partitioning from martensite and carbide precipitation during the partitioning treatment as well as segregation of the carbon atoms around dislocations in martensite, which were ignored in these models. In addition, existing empirical equations do not predict the progress of martensite transformation accurately. These factors were considered in this work to improve the existing models. The martensite transformation kinetics was deduced from the experimental dilatation curve and was fitted to existing empirical equations to get the model constants. The amount of bainite was calculated using lever rule after considering the carbon enrichment of austenite due to martensite decarburization during the partitioning treatment. The carbon concentration of bainitic-ferrite was taken based on the carbon content of this parent austenite from which bainite forms and the corresponding Zener ordering temperature with respect to the partitioning temperature. The total amount of carbon trapped at dislocations and defects was estimated using various empirical relations. The revised model, after considering these factors, gives a better prediction of QT(opt) and the corresponding amount of retained austenite.