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Microstructure and Damage Evolution During Short Term Creep of Modified 9Cr-1Mo Steel used in Generation IV Nuclear Energy Systems

Tammana, Deepthi
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1397476705

Abstract Details

2014, MS, University of Cincinnati, Engineering and Applied Science: Materials Science.
Major applications for high Cr steels include high temperature applications, advanced nuclear core material applications for Generation IV technology. Grade 91, Modified 9Cr-1Mo steel is being considered for the pressure vessels, turbine and core applications (ducts) of very high temperature reactors, super-critical water cooled reactor systems. All of these components undergo high temperature creep while in service, hence detailed study during the creep of these materials is essential. There are two creep regions with different creep characteristics: short-term creep region, where precipitates and sub-grains are thermally stable, and long-term creep region, where thermal coarsening of precipitates and sub-grains appear. In this study, creep behavior, damage evolution and microstructural changes during the short creep at different stresses varying from 80 to 140MPa were studied. Creep tests to different creep time intervals ranging from 25% to 75% of total creep life to failure at a temperature of 650°C at three of the stress levels in the range mentioned above were conducted and various microstructural parameters including the cavity nucleation, carbide evolution and sub-grain development were characterized in detail, by Scanning electron microscopy (SEM), Electron backscatter diffraction (EBSD) and Transmission electron microscopy (TEM). Micro hardness measurements were also carried out using Vickers and Knoop indenters and related to microstructural evolution. Calculation of stress exponent indicated dislocation creep as the major mechanism of creep deformation at stresses above 100MPa. Minor change in creep mechanism can be noticed at 100MPa, mechanism being slightly different below and above 100MPa. Stress exponent calculated along with microstructure characterization results confirm dislocation creep is the deformation mechanism. It was observed that though cavity density close to the fracture area remains almost same at stress levels below 100MPa, but is comparatively lower at higher stress levels. The carbide particle diameters at every stage of creep are greater as the stress level decreases. At higher stress levels the damage initiation and failure occurs in the final quarter of life, whereas at lower stress levels damage initiation and evolution can be seen over a large time interval. The contribution of static recovery of sub-grains due to creep deformation causes the breakdown in creep strength in Grade 91 steels. Creep deformation initially proceeds with sub-grain and carbide coarsening until necking is observed. Increasing strain during the creep test after the necking stage, results in formation of more sub-grains and refinement in sub-grain size. Formation and growth of sub-grain boundaries and coarsening of carbides is the main microstructure evolution mechanism. Higher fractions of sub-grains are formed at high stress levels leading to the recrystallization of microstructure. The near fracture areas show highly recrystallized microstructures. The fracture mechanism is formation of voids at the vicinity of the deformed carbides, which are elongated and located at the grain boundaries and are subjected to high strain. These voids grow quickly and coalesce resulting in micro grain boundary cracks and dimples or larger voids. However, the cavity formation, coalescence and propagation occur after 90% of the creep life even at lower levels of stress. Comparison of the results from the present work with the earlier work and literature in this field gave further support for the above damage mechanisms.
Vijay Vasudevan, Ph.D. (Committee Chair)
Vesselin Shanov, Ph.D. (Committee Member)
Donglu Shi, Ph.D. (Committee Member)
103 p.
Materials Science
Modified 9Cr-1Mo steel; Creep; microstructure; damage evolution

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Citations

  • Tammana, D. (2014). Microstructure and Damage Evolution During Short Term Creep of Modified 9Cr-1Mo Steel used in Generation IV Nuclear Energy Systems [Master's thesis, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1397476705

    APA Style (7th edition)

  • Tammana, Deepthi. Microstructure and Damage Evolution During Short Term Creep of Modified 9Cr-1Mo Steel used in Generation IV Nuclear Energy Systems. 2014. University of Cincinnati, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1397476705.

    MLA Style (8th edition)

  • Tammana, Deepthi. "Microstructure and Damage Evolution During Short Term Creep of Modified 9Cr-1Mo Steel used in Generation IV Nuclear Energy Systems." Master's thesis, University of Cincinnati, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1397476705

    Chicago Manual of Style (17th edition)

ucin1397476705
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This open access ETD is published by University of Cincinnati and OhioLINK.