GOALI: Crack Repair of Irradiated Stainless Steel by Friction Stir Processing

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This Grant Opportunity for Academic Liaison with Industry (GOALI) award supports fundamental research to uncover the mechanisms of sigma phase and chromium carbide formation in friction stir processed austenitic stainless steel. An Eulerian finite element approach will be used to predict temperatures, strains, and strains rates in the stir zone so that fraction recrystallization can be predicted for different tool speeds and feed rates. The modeling work will be combined with experiments to test the hypothesis that sigma phase and chromium carbide formation can be minimized or eliminated when friction stir processing is done under conditions that avoid dynamic recrystallization. Phase identification will be performed on processed specimens by electron backscatter diffraction and by transmission electron microscopy for model validation. Hot cell experiments on irradiated stainless steel will test a second hypothesis that friction stir processing can be done without causing helium cracking in the heat affected zone of the stirred region. This research will provide fundamental understanding of the role of recrystallization on sigma phase and chromium carbide formation in friction stir processed austenitic stainless steel. A new approach for repairing stress corrosion cracks in nuclear components will emerge from the fundamental research results. As current fusion welding processes cause helium cracking during repair of components that have been irradiated for twenty or more years, this new approach has the potential of overcoming a major technical challenge in maintaining our fleet of aging reactors, which is critical to the national energy infrastructure and the economy. Additional societal benefits include the education of students who will be prepared to contribute to the nuclear industry, and an outreach program working with high school teachers on new curriculum development, aimed at encouraging female students to pursue careers in science and engineering.