Revealing the cryogenic-temperature toughness and deformation mechanisms in high manganese austenitic steels

High-Mn austenitic steels are promising structural metallic materials for cryogenic-temperature industries due to their good properties and economy. In this work, the cryogenic-temperature toughness and deformation mechanisms were revealed by Charpy impact testing and checking the microstructural characteristics in deformed Fe-(13- 30) Mn-C steels. The results show that all studied steels showed good toughness at room temperature (i.e., 293 K), while an appropriate gamma SFE level was essential for their low temperature toughness at 77 K. The plastic deformation mechanisms depended on stacking fault energy and accumulated strain or dislocation density levels simultaneously. Twinning-induced plasticity (TWIP) mechanism was found for the three studied steels at 293 K, which was beneficial for good Charpy impact toughness. Enhanced TWIP mechanism was found at 77 K owing to the decreased gamma SFE values. Meanwhile, both strain-induced epsilon HCP martensite and alpha'BCC martensite were indicated in Fe-13Mn-0.9C steel, and few epsilon HCP martensite was examined in Fe-22Mn-0.9C steel. The alpha'BCC martensite was generally nucleated from prior strain-induced epsilon HCP martensite. The cryogenic-temperature toughness would deteriorate remarkably when strain induced alpha'BCC martensite was introduced or mechanical twins were delayed. The statistical dislocation densities in Fe-13Mn-0.9C steel were found to accumulate faster than Fe-22Mn-0.9C steel and Fe-30Mn-1.0C steel, especially at 77 K. And a full nonadditive strengthening mechanism between dislocations and varies obstacles were observed in the deformed steels.

Automated summary

High-Mn austenitic steels are promising structural metallic materials for cryogenic-temperature industries due to their good properties and economy. In this study, the cryogenic-temperature toughness and deformation mechanisms were investigated. The results showed that an appropriate gamma SFE level was essential for their low temperature toughness. The plastic deformation mechanisms depended on stacking fault energy and accumulated strain or dislocation density levels simultaneously. Twinning-induced plasticity mechanism was found to contribute to good toughness at room temperature, while enhanced TWIP mechanism was observed at low temperature. Various martensite phases and changes in dislocation density were also identified.