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Investigations Near the Fusion Boundary of Grade 91 Steel Dissimilar Metal Welds with Nickel Based Filler Metals

Kuper, Michael W
http://rave.ohiolink.edu/etdc/view?acc_num=osu1543505600533312

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
In this study, the formation, evolution, and failure of dissimilar metal welds (DMWs) involving Grade 91 steel using nickel based filler metals were evaluated. First, the curiosity of stable d ferrite found in the heat affected zone (HAZ) of Grade 91 DMWs was investigated since this phase was not present in the HAZ of matching filler metal welds. This difference could have signified a change in the thermal histories of the weld, a change in the chemical potential gradients present, or a combination of both. In the first investigation, it was found that the thermo-mechanical properties of the nickel based filler metal contributed to longer dwell times within the temperature range of stable d ferrite within the HAZ as compared to the autogenous and matching filler metal welds. This occurred because solidification temperature range of nickel based filler metals overlaps the stable d ferrite temperature range and because of the lower thermal conductivity of the nickel based filler metal. These factors enabled carbide dissolution and carbon diffusion, if the presence of a chemical potential gradient existed. Since these welds involved steel base metal and nickel based filler metals, the chemical potential gradients were relevant and were also investigated. The effect of the chemical potential gradient across the dissimilar fusion boundary was also investigated. It was found that there was a strong carbon chemical potential gradient between the Grade 91 base material and the nickel based filler metal caused by a difference in carbon concentration and carbide forming elements. A diffusion simulation was used to predict the magnitude of carbon migration during welding, which resulted in a carbon depleted HAZ. Carbon depletion in the HAZ stabilized the d ferrite phase, shown through statistical analysis of the hardness distribution and a strong correlation between the carbon concentration and amount of d ferrite found in the HAZ. A mechanism was proposed for the retention of d ferrite, which included the driving force from the carbon chemical potential gradient and the long dwell times at high temperatures. After that, the solidification mechanism was investigated, since the multiple phase transformations of the base metal on cooling hide the evidence of solidification. However, the phase boundary was identified and the details of the initial fusion boundary were uncovered using a variety of characterization techniques and the reconstruction of prior austenite grain orientations. As a result, two different mechanisms were proposed for DMW solidification, and each of them identified the fusion boundary to be within, or at the edge of, the narrow band of martensite grains found between the band of d ferrite and the fusion boundary. Fresh martensite was also identified within the highly diluted region of the partially mixed zone (PMxZ) after the PWHT. In order to explain the presence of fresh martensite in the PMxZ after the PWHT, phase transformations within the PMxZ were studied. In particular, the composition gradient of the PMxZ was reproduced at intervals, which determined that the A1 and A3 temperatures were locally reduced, resulting the region transforming to and from austenite during the PWHT. Because of this, the martensitic PMxZ would have reduced toughness. Additionally, a narrow region of the PMxZ operated as austenite during service, but martensite at room temperature. This region experienced cycling phase transformations and moving phase boundary that had unknown consequences on mechanical properties. The premature damage region was also found within the PMxZ and was characterized using transition electron microscopy (TEM), energy dispersive spectroscopy (EDS), and transmission kikuchi diffraction (TKD). This region contained coarse nickel-rich carbides, which may have contributed to the failure mechanism. Finally, the high temperature mechanical behavior of cross-weld specimens was investigated. It was found that under high levels of restraint (as in a weld), that the thermal expansion coefficient of the materials was sufficient to cause yielding in a cross weld specimen. This was problematic, since these regions of the power plant cycle on and off, potentially on a daily basis. Additionally, mechanical testing at 650 °C indicated strain concentration at grain boundaries near the fusion boundary. Finally, during the high temperature tensile test, strain localization was found to be higher in the PMxZ than the HAZ.
Boian Alexandrov (Advisor)
Michael Mills (Advisor)
Stephen Niezgoda (Committee Member)
243 p.
Engineering; Materials Science
dissimilar metal welds; Grade 91 steel; premature failure; characterization; carbon diffusion; delta ferrite; chemical analysis; phase transformations; mechanical testing; heat treatments; computational modeling

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Citations

  • Kuper, M. W. (2018). Investigations Near the Fusion Boundary of Grade 91 Steel Dissimilar Metal Welds with Nickel Based Filler Metals [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1543505600533312

    APA Style (7th edition)

  • Kuper, Michael. Investigations Near the Fusion Boundary of Grade 91 Steel Dissimilar Metal Welds with Nickel Based Filler Metals. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1543505600533312.

    MLA Style (8th edition)

  • Kuper, Michael. "Investigations Near the Fusion Boundary of Grade 91 Steel Dissimilar Metal Welds with Nickel Based Filler Metals." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1543505600533312

    Chicago Manual of Style (17th edition)

osu1543505600533312
190
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This open access ETD is published by The Ohio State University and OhioLINK.