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ABSTRACT
High-Mn austenitic steel is expected to be a next-generation structural material for cryogenic use by maintaining a stable fcc crystal structure even at cryogenic temperature by adding a large amount of C and Mn which are γ-stabilizing elements. In some cases, Cr is added together with C as a solid solution strengthening element to the high-Mn austenitic steel. Depending on the conditions of ageing heat treatment, the formation of Cr carbide may affect the mechanical properties. In this study, the formation behaviour of Cr carbide and the effect on cryogenic Charpy impact toughness in the ageing heat treatment at from 773 to 1273 K for 30 s to 18 ks of a high-Mn austenitic steel with 0.5%C-25%Mn-5%Cr.
Regardless of the ageing heat treatment conditions, no cleaved but 100% ductile fracture surface was observed in all samples after the Charpy impact test at 77 K. On the other hand, M23C6 containing Cr was formed on the austenite grain boundaries in the sample aged for a long time at 1073 K, and the Charpy absorbed energy showed lower than those of the other ageing conditions. As the results of both detailed microstructure observation of M23C6 and adjacent austenite matrix and DICTRA simulation, it was clarified that M23C6 mainly act as a fracture initiation point but propagation to the matrix becomes ductile because no sharp alloy deficiency layer is formed at the M23C6/austenite matrix interface. As a result of calculation of stacking fault energy using a thermochemical model, it was confirmed that γ stability at cryogenic temperature could be sufficiently secured even in the alloy-deficient region near M23C6 as well as in the stable matrix.