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 (E-T) composed of fracture initiation energy (E-I) and propagation energy (E-P) 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 El and E-P decreased slightly with decreasing temperature, but the EC/E-T ratio was kept to be about 0.5. In the 0.4C-18Mn steel, the E-I remained almost constant or slightly decreased with decreasing temperature, while the E-P/E-T ratio steadily decreased, thereby leading to the lower (about 30%) cryogenic-temperature E-T than that of the 1.0C-18Mn steel. Under the dynamic compressive loading, a considerable number of epsilon-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 gamma ->epsilon-martensite transformation was attributed to the decrease in stacking fault energy, and resulted in the very low E-P and resultant E-T. (C) 2015 Elsevier B.V. All rights reserved.