Isothermal Transformation in Fe-N Hypereutectoid Alloy

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Author(s)

    • NAKADA Nobuo
    • Department of Materials Science and Engineering, Graduate School of Engineering, Kyushu University
    • TSUCHIYAMA Toshihiro
    • Department of Materials Science and Engineering, Graduate School of Engineering, Kyushu University
    • TAKAKI Setsuo
    • Department of Materials Science and Engineering, Graduate School of Engineering, Kyushu University
    • KOYANO Tamotsu
    • Cryogenics Division, Research Facility Center, University of Tsukuba

Abstract

In order to understand the mechanism of isothermal transformation of Fe–N alloy, the isothermal transformation microstructure that forms in a wide temperature range below <i>Ae</i><sub><i>1</i></sub> was investigated in Fe–2.6 mass%N hypereutectoid alloy by means of the electron back scatter diffraction method in addition to the conventional microstructural observation methods. High-nitrogen austenite fully decomposed to ferrite and Fe<sub>4</sub>N over the entire temperature range, and the time-temperature-transformation (<i>TTT</i>) diagram had a C shape with a nose temperature around 700 K. The hardness linearly increased with decreasing transformation temperature because the microstructure became finer, but the morphology of the (ferrite + Fe<sub>4</sub>N) structure changed discontinuously at around 800 K. From the microstructural and crystallographic analyses, it was concluded that the microstructure formed at higher temperature is a lamellar eutectoid structure, braunite, while the other is an upper bainitic structure containing bainitic ferrite formed through a displacive mechanism and Fe<sub>4</sub>N formed by concentration and ordering of the nitrogen. Since Fe<sub>4</sub>N is a counterpart of the cementite in Fe–C alloy, the respective structures are similar to pearlite and upper bainite in carbon steel.

Journal

  • ISIJ International

    ISIJ International 53(1), 139-144, 2013-01-15

    The Iron and Steel Institute of Japan

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