Microstructure characterization of nanocrystalline bainitic steel during tempering

Abstract The microstructure characterization and mechanical properties of tempered nanocrystalline bainitic steel are investigated in the present study. It is found that the plastic relaxation in the thin film retained austenite usually relies on the dislocation debris. On the other hand, an accommodation nanotwin structure can be found in the blocky retained austenite. Furthermore, the different accommodation mechanisms in the bainitic steel result from the non-uniform carbon distribution in the retained austenite. Microhardness data mainly revealed that, in the specimens tempered at 425 °C, the average Vickers hardnesses were 600 HV 0.1 for tempering of 5 min, 601 HV 0.1 for 10 min, and 595 HV 0.1 for 60 min. For the specimens tempered at 475 °C, the average Vickers hardnesses were 598 HV 0.1 for tempering of 5 min, 596 HV 0.1 for 10 min, and 593 HV 0.1 for 60 min. Moreover, due to measurement error, the hardness of the specimens that underwent the tempering process could be regarded as having almost maintained the initial hardness because the carbide precipitates in the interface further prevented the coalescence of bainitic ferrite plates during tempering treatment. High resolution transmission electron microscopy provided new experimental evidence on the carbide precipitation sequence, a subject critical to the understanding of the phase transformation mechanism during the tempering process in bainitic steel. The presence of high carbon transition carbides (η-carbides) is the result of the low amount of dislocation debris, which acts as effective carbon traps, near the blocky retained austenite.