The formation of a mixed martensitic/bainitic microstructure and the retainment of austenite in a medium-carbon steel during ultra-fast heating

Abstract

The current work focuses on the microstructure evolution of a medium-carbon steel during an ultra-fast heating cycle. Electron microscopy methods such as Scanning Electron Microscopy, Electron BackScatter Diffraction and Transmission Electron Microscopy are utilized for the characterization of microstructural constituents of the studied steel samples. The applied heating rate during this treatment was 100 °C/s and the heating temperature was 930 °C, slightly above the Ac3 temperature, and subsequent quenching in water. It is found that the microstructure consists of martensite (60 %), lower bainite (37 %) and retained austenite (3%). The formation of bainite during this treatment is attributed to the heterogeneity of the chemical composition in the microstructure due to the lack of time for carbon to diffuse from cementite in long distances. Regions that are located away from the former pearlitic colonies are not enriched in carbon and manganese along the austenitization process and, according to the Controlled Cooling Transformation diagram, will form bainite upon cooling. The retainment of austenite in films is observed between the bainitic plates and on the martensitic lath boundaries, where carbon and manganese are segregated. Such microstructures are utilized by the current 3rd Generation Advanced High Strength Steels for the optimization of the mechanical properties.