Quenching and partitioning (Q&P) is a novel processing route that can create microstructures that combine high strength, ductility and easy formability without the excess use of alloying elements by also keeping the production cost low. Therefore, this method is ideal to be applied to steels that are used in the automotive industry. The material is initially annealed at a high temperature to form austenite. Then it is quenched to a temperature below the martensite start temperature to form a controlled fraction of martensite. Second annealing follows, at a lower temperature than the first one, that stabilises the austenitic phase at room temperature. It has been found that the austenite grain size at the end of the first annealing (prior austenite grain size) has a significant impact on the processing method, the final microstructure and consequently on the final properties of the material. In this thesis, phase-field modelling was employed to investigate the role of prior austenite grain size on the quenching and partitioning process. Simulations of the processing were performed for three different prior austenite grain sizes (6, 25 and 67 μm). Microstructures consisting of about 10.5% austenite and 89.5% martensite were created by simulating the first quenching of the Q&P process. Then, the evolution of the microstructure and carbon distribution were investigated for partitioning at 400 ◦C for times up to 4000 s, and quenching to room temperature. The results gave insights into the effect of the prior austenite grain size on the morphology of the microstructure and the kinetics of carbon during partitioning. It was found that a finer microstructure was derived with decreasing PAGS. Moreover, partitioning becomes more efficient because of the shorter distance that carbon has to diffuse, and the even spatial distribution of austenite. Finally, wide grain size distributions of the austenite that is present during partitioning can lead to significant variations in the carbon content among the austenite grains of the final microstructure, especially for short partitioning times. This affects the efficiency of the partitioning process and has an impact on the mechanical stability of the austenite in the final microstructure and consequently on the TRIP phenomenon.

Bainitic steels have a right combination of strength and toughness when compared to that of martensitic and pearlitic steels. For the same reason, it is becoming a popular kind of steel among the automobile industry. It is important to study the bainite transformations to have an explicit knowledge about the same and to design bainitic steels accordingly based on the application which has to be satisfied. This requirement has led to an immense research on the bainitic steels in the recent past. One of the aspects which is studied recently is the behaviour of the transformation kinetics of bainite during the isothermal treatments. Factors like, austenite grain size, chemical composition (Si, Mn and C) and the presence of prior athermal martensite have been found to affect the transformation kinetics of bainite. The presence of prior athermal martensite (PM) has been reported to have an accelerating effect on the transformation kinetics of bainite by few of the authors. The two reasons given for the acceleration by two different school of thoughts are: 1.) due to the creation of additional potential nucleation sites from the martensite-austenite (’-) interfaces with the presence of prior athermal martensite (PM) and 2.) due to the strain induced (through dislocations) in the austenite matrix with the presence of PM. Therefore, further investigations are required to concrete the mechanism responsible for the acceleration. On the other hand, carbon content has been reported to have a decelerating effect on the kinetics of bainite formation due to the reduction in the driving force (based on the thermal stability of austenite). Thus, the effect due to the presence of PM and the effect due to the carbon content have opposite influence on the kinetics. It is important to study the effect of PM on the bainite transformation kinetics for steels with different compositions in order to understand if the alloying elements also have an effect on the kinetics. Having said that the carbon content has an effect on the kinetics, it will be interesting to study the effect of carbon content on the kinetics along with the presence of PM.

The primary goal of the current research is to examine the effect of prior athermal martensite (PM) on the kinetics of bainite transformation in a medium carbon steel (0.57 wt% C). In order to investigate the effect of carbon content, the current study will be compared with the research by A. Navarro-López et al. 1 which was on a low carbon steel (0.2 wt% C). There is clear evidence from the results of the current study that there is a competition between the decelerating effect of carbon content and the accelerating effect of PM at the beginning of the isothermal transformation. The results show that the carbon content has a significant role on the intensity of the acceleration effect due to the presence of prior athermal martensite.