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Microstructural Evolution in Fe-12Mn Steel

Microstructural Evolution in Fe-12Mn Steel
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The intercritically tempered 9Ni steel has long been used for cryogenic applications due to its high strength, good ductility and toughness by the fine lath martensitic microstructure and the interlath island of precipitated austenite. However, 9Ni steel is inherently expensive due to high Ni content and several investigations have been carried out to replace 9Ni steel with less expensive steels such as high Mn steels. In particular, 12Mn steel has been known to have good combination of strength and low temperature toughness upon proper tempering and addition of microalloying elements. Unlike 9Ni steel, however, microstructure of 12Mn steel consists of ??-martensite with second phase of ?-martensite instead of retained austenite. Moreover, the morphology of ??-martensite in 12Mn steel looks like a ladder, which is different from that of lath type ??-martensite in 9Ni steel. The present study is aimed at understanding the microstructural evolution of this technologically important 12Mn steel. It shows that the long lath of ??-martensite consists of alternating layer of twin-related ??-martensite, dividing the long lath into short twin-related domains whose boundaries are perpendicular to those of long laths. In addition, thin films of ?-martensite are often present along the lath boundaries. The twin-related ??-martensite can be explained by K-S relations and each domain of twin-related K-S variants is classified as different Bain group from each other which has large misorientation of the {100} cleavage planes. It can suppress the propagating cleavage fracture. Also the domain which scale is under micron is effective to high strength. The crystal orientation and the morphology have a unique relationship: the domain boundary is parallel with twin plane{112} and the lath boundary is parallel with common {110} plane of ??-martensite domains. Three twin-related K-S variant pairs in 1 packet make Three different Bain group combinations and three different domain boundaries. Formation of such structure is considered to reduce the strain due to austenite ? martensite transformation.
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