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Effects of Martensitic Transformation on Cryogenic-Temperature Tensile and Charpy Impact Properties in Austenitic High-Mn Steels

Effects of Martensitic Transformation on Cryogenic-Temperature Tensile and Charpy Impact Properties in Austenitic High-Mn Steels
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Many researches on how to effectively control the global warming and to reduce greenhouse gases have been conducted all over the world. Recently, austenitic high-Mn steels have been used for cryogenic-temperature storage or transportation applications because the storage or transportation of liquefied natural gas is globally demanded. In the cryogenic-temperature applications, fcc metals are widely used because of their sufficient low-temperature toughness, whereas bcc metals show a ductile-brittle transition phenomenon. Austenitic stainless steels, 9% Ni alloys, or Ni-based invar alloys are generally used in large-scale plants or industries, but many efforts have been directed towards replacing expensive austenitic stainless steels and Ni-based alloys to high-Mn steels whose strength, ductility, and toughness are comparable. Efforts have also been made to come up with a nice combination of high strength and toughness in austenitic high-Mn steels composed of single phase of austenite by utilizing deformation mechanisms which are varied with stacking fault energy (SFE). The SFE increases with increasing Mn content, and the TWinning Induced Plasticity (TWIP) mechanism is well activated in the SFE range of 20~40 mJ/m2. Mechanical twins formed during the deformation prevent the movement of dislocations as they play a role in refining grains, which is known as dynamic Hall-Petch effect, and the necking is suppressed during the tensile deformation due to the high strain hardening rate. Thus, austenitic high-Mn steels show high tensile strength and ductility simultaneously. The reduction in Mn content is not easy because of the existence of stabilized austenite at room temperature and the sufficient formation of twins inside austenite grains. TWIP steels containing Al with reduced Mn content have also been developed to improve formability and to prevent delayed fracture. Here, Al plays an important role in decreasing twin formation because it works for increasing SFE. These Al-added TWIP steels have excellent formability due to the decreased twin formation and the increased slip activation. When the temperature decreases to cryogenic temperature (-196 °C), however, the deformation mechanisms can be changed as the SFE decreases by about 20~60%. The TRansformation Induced Plasticity (TRIP) mechanism can take place, depending on Mn content and test temperature. This variation in deformation mechanism greatly affects the strength and toughness by forming complex microstructures mixed with twins and martensite. Austenitic high-Mn steels containing about 20 wt.% of Mn and 2 wt.% of Al are newly developed for cryogenic-temperature applications, but very few studies on microstructural evolution and deformation mechanisms have been conducted. Furthermore, detailed deformation and fracture mechanisms related with alloying elements, test temperature, and SFE are not studied. In this study, austenitic microstructures were fabricated by varying Mn and Al contents, and their tensile and Charpy impact properties were examined at room and cryogenic temperatures. Cryogenic-temperature deformation mechanisms were investigated by analyzing deformed microstructures occasionally containing twins and martensites, and the correlation between microstructural evolution processes and tensile and Charpy impact properties was verified.
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