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A Study of the Effects of Mechanical Surface Treatments on Residual Stresses, Microstructure and Stress Corrosion Cracking Behavior of Alloy 600

Telang, Abhishek
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1447070304

Abstract Details

2015, PhD, University of Cincinnati, Engineering and Applied Science: Materials Science.
Stress corrosion cracking (SCC) of Alloy 600 has been a major problem in commercial light water reactor (LWR) nuclear power plants. Localized corrosion and intergranular SCC (IGSCC) have been observed in Alloy 600 in the high temperature (288-340 °C) pure water environment of LWRs. Additionally, IGSCC of Alloy 600 has been reported even at room temperature under certain conditions in thiosulfate and tetrathionate solutions. In general, SCC can be attributed to the presence of tensile stress, an aggressive environment and a susceptible microstructure. Therefore, SCC mitigation techniques address these factors by modifying the environment, metallurgical processing treatments and alleviating the tensile stresses by mechanical surface treatments/stress relief. This study investigated the application of laser shock peening (LSP) as a technique to mitigate SCC in Alloy 600. LSP induced large compressive residual stresses (-550 MPa) that decreases gradually through depth. The pressure pulse generated during the LSP treatment causes plastic deformation, resulting in high dislocation density, twins and formation of misoriented sub-grains/crystallites that have sizes in the range of 50-300 nm in the near-surface region. Slow strain rate tests (SSRTs) and constant load tests performed in tetrathionate solution at room temperature were used to evaluate the effect of LSP on the SCC behavior. LSP treated samples had a significantly longer time to failure and reduced susceptibility to SCC as compared with untreated sensitized Alloy 600. These improvements were attributed to LSP induced compressive residual stresses, increased yield strength (YS) and hardening caused by near-surface microstructural changes. SSRTs in simulated PWR environment also show similar results with higher YS, tensile strength and strain to failure. Additionally, the gage section shows fewer cracks and smaller crack lengths in the LSP treated samples as compared with the untreated samples. The other approach involved using mechanical surface treatments/cold work followed by annealing to engineer the Alloy 600 microstructure for increased resistance to corrosion and IGSCC. We demonstrated a novel method of surface grain boundary engineering (SGBE) in Alloy 600 using iterative cycles of ultrasonic nanocrystalline surface modification (UNSM) treatment and strain annealing. Three cycles of UNSM and strain annealing at 900-1000 °C were used to modify the microstructure to a depth of 250 µm from the surface. This surface treatment based method increased the fraction of low coincident site lattice (CSL) grain boundaries whilst decreasing the fraction and connectivity of random high angle boundaries (HABs) in the near surface region. Similar results were achieved using thermo-mechanical processing (TMP) with iterative cycles of 10% cold work and strain annealing in Alloy 600. A disrupted random HAB network and large fraction (70%) of CSL boundaries (Σ3-Σ27) reduced the propensity to sensitization. SSRTs in tetrathionate solutions at room temperature show that SGBE and TMP lowered the susceptibility to intergranular SCC. Detailed analysis using EBSD showed cracks arrested at J1 (1-CSL) and J2 (2-CSL) type of triple junctions. The probability of crack arrest, calculated using percolative models, was higher after SGBE and TMP in Alloy 600 and explains the improved IGSCC resistance.
Vijay Vasudevan, Ph.D. (Committee Chair)
Seetha Ramaiah Mannava, Ph.D. (Committee Member)
Dong Qian, Ph.D. (Committee Member)
Rodney Roseman, Ph.D. (Committee Member)
Vesselin Shanov, Ph.D. (Committee Member)
204 p.
Materials Science
Stress Corrosion Cracking; Microstructure; Laser Shock Peening; Microstructure; Grain boundary; Residual Stress

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Citations

  • Telang, A. (2015). A Study of the Effects of Mechanical Surface Treatments on Residual Stresses, Microstructure and Stress Corrosion Cracking Behavior of Alloy 600 [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1447070304

    APA Style (7th edition)

  • Telang, Abhishek. A Study of the Effects of Mechanical Surface Treatments on Residual Stresses, Microstructure and Stress Corrosion Cracking Behavior of Alloy 600. 2015. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1447070304.

    MLA Style (8th edition)

  • Telang, Abhishek. "A Study of the Effects of Mechanical Surface Treatments on Residual Stresses, Microstructure and Stress Corrosion Cracking Behavior of Alloy 600." Doctoral dissertation, University of Cincinnati, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1447070304

    Chicago Manual of Style (17th edition)

ucin1447070304
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This open access ETD is published by University of Cincinnati and OhioLINK.