Interpretation of cryogenic-temperature Charpy fracture initiation and propagation energies by microstructural evolution occurring during dynamic compressive test of austenitic Fe-(0.4,1.0)C-18Mn steels

In the present study, Charpy impact energy (ET) composed of fracture initiation energy (EI) and propagation energy (EP) of austenitic Fe-(0.4,1.0)C-18Mn steels was evaluated in the temperature range from room to cryogenic temperatures by an instrumented Charpy impact tester, and was interpreted by microstructural evolution of dynamically compressed specimens. In the 1.0C-18Mn steel, the EI and EP decreased slightly with decreasing temperature, but the EP/ET ratio was kept to be about 0.5. In the 0.4C-18Mn steel, the EI remained almost constant or slightly decreased with decreasing temperature, while the EP/ET ratio steadily decreased, thereby leading to the lower (about 30%) cryogenic-temperature ET than that of the 1.0C-18Mn steel. Under the dynamic compressive loading, a considerable number of ε-martensites were formed in the 0.4C-18Mn steel, whereas they were not found in the 1.0C-18Mn steel, and their volume fractions increased steadily with decreasing temperature. This γ→ε-martensite transformation was attributed to the decrease in stacking fault energy, and resulted in the very low EP and resultant ET.