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Isothermal Transformation in Fe–N Hypereutectoid Alloy

  • Nakada Nobuo
    International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University Department of Materials Science and Engineering, Graduate School of Engineering, Kyushu University
  • Fukuzawa Norihide
    Graduate Student of Engineering, Kyushu University
  • Tsuchiyama Toshihiro
    International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University 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 International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University
  • Koyano Tamotsu
    Cryogenics Division, Research Facility Center, University of Tsukuba
  • Iwamoto Takashi
    Steel Research Laboratory, JFE Steel Corporation
  • Omori Yasuhiro
    Steel Research Laboratory, JFE Steel Corporation

Bibliographic Information

Other Title
  • Isothermal Transformation in Fe^|^ndash;N Hypereutectoid Alloy

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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 Ae1 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 Fe4N over the entire temperature range, and the time-temperature-transformation (TTT) 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 + Fe4N) 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 Fe4N formed by concentration and ordering of the nitrogen. Since Fe4N 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

    The Iron and Steel Institute of Japan

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