Effect of Mn on the chemical driving force and bainite transformation kinetics in medium-manganese alloys

This work presents insights into the manganese influence on the driving force and bainite transformation kinetics. Three different medium-Mn steels were subjected to theoretical calculations and dilatometric study in order to determine the Mn impact on bainite formation. The theoretical approach shows that the increase of manganese leads to a lower bainite fraction formed during the isothermal stage. This implicates the carbon enrichment of the austenite during thermal treatment. The less bainite is formed, the higher is the fraction of residual austenite which enrichment of carbon is globally low. Meanwhile, the manganese influences the incubation and transformation time. As the manganese content increases, the incubation period and formation time of bainite are longer because the chemical driving force essential to start and complete austenite into bainite transformation decreases. This was proved by theoretical calculations and dilatometric analysis, which show that even a small increase in manganese content leads to a longer time necessary to occur the bainitic transformation. For the steel containing 5% manganese, the driving force was too small that the transformation could occur even after 3 h. Additionally, the XRD analysis was conducted to determine the retained austenite fraction and its carbon enrichment. These results were compared with the theoretical values to determine the accuracy of the applied model.