Tuning Bainitic Microstructures by Complex Thermo-mechanical Treatments under Constant Stress

Ausforming processes are those thermomechanical treatments in which austenite is plastically deformed before either a martensitic or a bainitic transformation takes place. Although the deformation of an austenitized steel at intermediate temperatures has many benefits, it can also induce displacive transformations, sometimes unavoidably at the industrial level. Although the addition of a mechanical driving force, associated to the applied stress, has been shown to accelerate the bainitic transformations and promote transformation plasticity and variant selection, little information is found on the effect of stress on parameters such as bainitic ferrite plate thickness or volume percentage of retained austenite. Some works suggest that constant stresses can coarsen bainitic ferrite plates and increase the amount of transformed bainitic ferrite, although no systematic work has been conducted in this regard. This work aims to better understand the characteristics of bainitic microstructures formed during the application of stress or under constant stress and discuss the mechanisms affecting the bainitic transformations. Among the obtained results, it can be highlighted that, at temperatures below the bainite start temperature, bainitic ferrite plates formed during straining are more refined that isothermally formed plates at the same temperature, whereas constant stresses leads to an increased fraction of coarser bainitic ferrite plates, as compared to those microstructures obtained at the same temperature.