Disappearance of strengthened micro texture of modified 9Cr-1MO steel caused by stress-induced acceleration of atomic diffusion at elevated temperatures

Yifan Luo, Hideo Miura

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

The change of the lath martensitic structure in the modified 9Cr-1Mo steel was observed in the specimens after the intermittent fatigue and creep tests using EBSD (Electron Back-Scatter Diffraction) analysis. The Kernel Average Misorientation (KAM) value and the image quality (IQ) value obtained from the EBSD analysis were used for the quantitative evaluation of the change in the lath martensitic texture. It was found that the lath martensitic texture started to disappear clearly after 10 7 -10 8 cycles under the fatigue loading at temperatures higher than 500 o C when the amplitude of the applied stress exceeded a critical value. Similar change also appeared in the creep test. The critical value decreased monotonically with the increase of the test temperature. This microstructure change decreased the strength of the alloy drastically. In order to explicate the dominant factors of the change quantitatively, the changes of the microstructure and the strength of the alloy were continuously measured by applying an intermittent creep test at elevated temperatures. It was found that the effective activation energy of atomic diffusion decreased drastically under the application of mechanical stress at elevated temperatures. The effective diffusion length for the disappearance was about 9 µm, and this value was much larger than the initial pitch of the lath martensitic texture of about 0.5 µm, and smaller than the average size of the initial austenite grains of about 20 µm. Therefore, the stress-induced acceleration of atomic diffusion was attributed to the disappearance of the initially strengthened micro texture. The change of the micro texture caused the drastic decrease in the yielding strength of this alloy. Finally, the prediction equation of the lifetime of the alloy was proposed by considering the stress-induced acceleration of atomic diffusion under the application of mechanical stress at elevated temperatures.

Original languageEnglish
Title of host publicationMechanics of Solids, Structures, and Fluids
PublisherAmerican Society of Mechanical Engineers (ASME)
ISBN (Electronic)9780791852149
DOIs
Publication statusPublished - 2018 Jan 1
EventASME 2018 International Mechanical Engineering Congress and Exposition, IMECE 2018 - Pittsburgh, United States
Duration: 2018 Nov 92018 Nov 15

Publication series

NameASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)
Volume9

Other

OtherASME 2018 International Mechanical Engineering Congress and Exposition, IMECE 2018
CountryUnited States
CityPittsburgh
Period18/11/918/11/15

ASJC Scopus subject areas

  • Mechanical Engineering

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