Combining in-situ technology to study the influence of bainite morphology on the strength and toughness properties of medium-carbon bainitic steel

The microstructure of bainitic steel was controlled through continuous cooling in the medium-temperature region. The influence of cooling rate on the microstructure and its effects on the strength and toughness of bainitic steel were investigated using transmission electron microscope (TEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). Experimental results indicate that as the cooling rate increases from 0.1 to 4°C/s, the microstructure evolves from granular bainite to lath bainite, characterized by a high-density interface. At higher cooling rates, the samples exhibit enhanced strength and impact energy, attributed to dislocation strengthening and fine-grained strengthening mechanisms. Additionally, the presence of high-angle grain boundaries and filmy retained austenite within the microstructure contributes to improved toughness. Conversely, at a cooling rate of 0.1°C/s, martensite/austenite(M/A) constituents and low-stability massive retained austenite do not favorably influence toughness improvement; however, increased retained austenite delays necking onset and promotes uniform plastic deformation. Combining in-situ phase transformation and tensile analysis, it is evident that phase transformations significantly affect tensile strain distribution. Specifically, temperature selection and the driving force for phase transformation influence the variation in size and orientation, thereby affecting strain distribution during tensile testing.