In situ CLSM observation of Austenite microstructural evolution during hot deformation

In-situ observations of microstructure evolution during the thermomechanical processing of low-carbon steel have been carried out using a high-temperature tensile testing system (HiTTS) integrated with a confocal laser scanning microscope (CLSM). Experiments were conducted within the temperature range between 800 to 1200 degrees C, and employing a strain rate of 0.001 s-1 to analyze the evolution of austenite microstructure at different temperatures and to identify the deformation and restoration mechanisms. The findings suggest that at temperatures below 900 degrees C, planar slip is the dominant deformation mechanism, and slip transfer is more favorable at twin boundaries than at grain boundaries. On the other hand, dynamic recrystallization (DRX) is identified as the primary restoration mechanism above 900 degrees C. The study identifies various nucleation sites for DRX grains, with triple junctions and grain boundaries serving as the nucleation sites at 900 degrees C. As the temperature increases to 1000 degrees C and above, new nucleation sites, such as inside annealing twin boundaries and free twin ends, are observed. The microstructure results suggest that the morphology of the twin boundary changes and loses its character during deformation. The role of annealing twin boundaries on DRX and bulging mechanisms associated with the various nucleation sites are discussed in great detail. Finally, the operational details, including temperature variations along the gauge length and thermal profile adjustments, including overshooting and undershooting and high-temperature surface reactions such as oxidation, decarburization, and evaporation, are meticulously examined.