Stress-Induced Change of the Microstructure and Strength of Modified 9Cr-1Mo Steel under Fatigue and Creep Loading at Elevated Temperature.

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  • SHINOZAKI Taichi
    Department of Finemechanics, Graduate School of Engineering, Tohoku University
  • SUZUKI Ken
    Fracture and Reliability Research Institute, Tohoku University
  • MIURA Hideo
    Fracture and Reliability Research Institute, Tohoku University

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Other Title
  • 高温負荷環境における応力誘起ラスマルテンサイト組織消失メカニズムの解明

Abstract

<p>The change of the lath martensitic structure in Modified 9Cr-1Mo steel was observed in the specimens after the fatigue and creep tests by using EBSD (Electron Back-Scatter Diffraction) in order to elucidate the mechanism of the disappearance of the strengthening micro texture. The Kernel Average Misorientation (KAM) value obtained from the EBSD analysis was used for the quantitative evaluation of the change in the lath martensitic strengthening structure. It was found that the average KAM values of the fractured specimens decreased clearly after 107-108 cycles of the fatigue loading at temperatures higher than 500oC when the amplitude of the applied stress exceeded a critical value. Moreover, KAM value decreased as time passed in interrupted creep tests. This change corresponded to the disappearance of the lath martensitic structure. The critical value decreased monotonically with the increase of the test temperature. This microstructure change decreased the strength of the alloy drastically. It was found that the change of the microstructure started at a certain time at each test temperature as a function of the amplitude of the applied stress. There was the critical stress at which the microstructure started to change at each test temperature higher than 500oC, and the activation energy of the change was determined as a function of temperature and the amplitude of the applied stress. The dominant factor of the microstructure change was the stress-induced acceleration of the atomic diffusion of the component elements in the alloy.</p>

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