Investigation of microstructural evolution and carbon redistribution in ausformed nanostructured bainitic steel via 3D atom probe tomography and its structure-property relationship

Thermomechanical treatment is an important method to refine the microstructure and accelerate the kinetics of phase transformations in steels. In this study, different ausforming strains as 7 %, 15 %, 30 %, and 60 % were used to ausform steel samples at 570 °C temperature followed by isothermal holding at 350 °C for 10 days. X-ray diffraction was used to analyse the different phases present after thermomechanical treatment. Long isothermal holding for 10 days was used to develop a fully nanostructured bainitic microstructure consisting of bainitic ferrite and retained austenite films. Refinement of bainitic laths was observed with an increase in deformation strain. Atom probe tomography was further employed to study the carbon redistribution in the form of solid solution, clusters, segregation, and precipitation in a sample ausformed at 570 °C with 7 % strain. Atomic clustering was observed and quantified for the first time inside bainitic ferrite laths in nanostructured bainite, due to the dislocation substructure formed inside austenite as a result of ausforming. The maximum carbon content found in nano-scale particles was found to be 14.13 atomic % establishing the presence of clusters. The effect of ausforming on microstructural evolution, austenite stabilization, and carbon redistribution has been presented systematically in this work.