Development of Graded Transition Joints for Avoiding Dissimilar Metal Weld Failures
Department: Materials Science and Engineering
Advisor: John DuPont
View: UGRS Research Poster (PDF)
Dissimilar Metal Welds (DMWs) are critical for design, development, and manufacturing of Very High Temperature Reactors (VHTR). When the DMW is in service, experience has demonstrated that premature failures of DMWs can be caused by carbon diffusion across the weld interface from the ferritic to austenitic material, driven by the large concentration gradient. This creates creep voids near the weld interface that are the leading cause to premature failure in service. A proposed solution is a joint whose composition changes gradually from ferrous to austenitic steel to mitigate carbon diffusion during service due to the smaller concentration gradient. This research proposes that a graded transition joint (GTJ) will exhibit minimal changes in hardness when compared to the DMW, suggesting carbon diffusion will be limited.
DICTRA simulations were performed to determine a suitable grade length for the GTJ. A 20 mm grade length was calculated to have the least amount of carbon diffusion after aging at 465˚C for 20 years. Samples of DMWs and GTJs were fabricated, prepared, and hardness tested in the as welded and aged conditions. The DMWs exhibited large changes in hardness over short distances due to carbon diffusion, while the GTJ does not exhibit drastic changes in hardness values after aging. Longer aging times affect the hardness in DMWs but not GTJs, with the GTJs exhibiting no local softening near the fusion line. Future work will include hardness tests after longer aging times with finer spaced traces to continue investigation of carbon diffusion.
About Allison Fraser:
Allison is currently a junior at Lehigh University pursuing a B.S. in Materials Science and Engineering with a minor in Mechanics of Materials. Allison has been conducting research with Professor John DuPont as a recipient of the Clare Boothe Luce Research Scholars Award, studying the microstructural stability of graded transition joints at high temperature. She is currently conducting mechanical tests on graded transition joints at elevated temperatures to understand how strain is partitioned along the joint. Outside of research, Allison is involved in the Society of Women Engineers, Women in Science and Engineering, volunteering as a tour guide for the Materials Science and Engineering department. She has also been riding horses for 15 years. Allison is considering pursuing a PhD in Materials Science and Engineering to further develop her interest in metallurgy.