Doctor of Philosophy, The Ohio State University, 2024, Welding Engineering

This research focuses on the non-destructive characterization of microstructures in Grade 91 (9Cr-1Mo-V) and Grade 92 (Fe-9Cr-2W-0.5Mo) steel welds. These alloys are creep strength-enhanced ferritic (CSEF) steels commonly used in fossil-fuel-fired and nuclear power plants. The weld integrity of these two steels is crucial for power plants' safe and reliable operations. Two welding processes, cold metal transfer (CMT) and flux-cored arc welding (FCAW), were investigated for the Grade 91 steel weld test samples. For the Grade 92 weld samples, three different heat inputs (low, medium, and high) of gas tungsten arc welding (GTAW) were utilized to replicate traditional field welding processes. The non-destructive evaluation (NDE) method used for this research was immersion ultrasonic testing (UT). Using a newly patented micro-resolution ultrasonic imaging methodology specifically designed to operate in the through-transmission configuration. A 20 MHz focused probe with an ultrasonic beam focal diameter between 250-300 μm was used as the transmitter. As the receiver, a laser vibrometer with a 6-10 μm beam diameter was used to produce highly resolved ultrasonic images with longitudinal and mode-converted shear waves. Based on the micro-resolution ultrasonic C-scan images obtained during this investigation, three microstructural regions, such as weld metal (WM), heat-affected zone (HAZ), and base metal (BM), were clearly identifiable. Various levels of ultrasonic amplitudes distributed over the three regions were correlated with electron beam backscattered diffraction (EBSD) images using grain size, grain boundaries, and dislocation densities. The results showed that areas with relatively higher ultrasonic amplitude levels were associated with smaller grains and higher dislocation densities, while areas with lower amplitude levels were associated with larger grains and lower dislocation densities. In addition, ultrasonic velocity data obtained across the three differ (open full item for complete abstract)

Committee: Desmond Bourgeois (Advisor); Dave Farson (Committee Member); Xun Liu (Committee Member) Subjects: Engineering

Doctor of Philosophy, The Ohio State University, 2018, 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 (open full item for complete abstract)

Committee: Boian Alexandrov (Advisor); Michael Mills (Advisor); Stephen Niezgoda (Committee Member) Subjects: Engineering; Materials Science

Master of Science, The Ohio State University, 2016, Welding Engineering

Grade 91 steel has been heavily utilized in the construction of coal fired power plants in an effort to achieve superior efficiency. This steel has good creep strength and high temperature oxidation resistance allowing power plants to push the boundaries on the maximum operating temperature. However, Grade 91 steel is known to be susceptible to Type IV cracking in the base metal heat affected zone (HAZ). In an effort to better understand this type of failure, a study on the metallurgical reactions occurring within the HAZ during welding was conducted, particularly within the fine grained (FG) and intercritical (IC) regions where Type IV cracking is most commonly found to occur. The course grained (CG), FG, IC and sub-critical (SC) regions of the HAZ in Grade 91 steel were simulated using a GleebleTM 3800 Thermo-Mechanical Simulator. Dilatometry, single sensor differential thermal analysis (SS-DTA) and in-situ XRD measurements were used to determine the phase transformations at occur during simulation of weld thermal histories. From these measurements, ferrite was shown to form on cooling from peak temperatures within the ICHAZ temperature range. Using XRD measurements to calculate the volume fraction of a particular phase, it was found that more ferrite formed when the sample was cooled at 3oC/s than at 26oC/s. Dilatometry measurements agree well with this finding. From the dilation curves, the magnitude of the volume change associated with the ferrite transformation is greater for slower cooling rates and decreases with faster cooling rates. In addition to ferrite, evidence of an austenite transformation on cooling was found in samples heated to peak temperatures below the Ac1 temperature. This austenite that forms on cooling decomposes to ferrite and martensite later in the thermal cycle. The transformations to austenite and ferrite on cooling during a simulated weld thermal cycle have not been previously reported in Grade 91 steel. This phase tran (open full item for complete abstract)

Committee: Boian Alexandrov Dr. (Advisor); Antonio Ramirez Dr. (Committee Member) Subjects: Materials Science; Metallurgy