Download Full Text (961 KB)
Material Science and Engineering
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.
Fraser, Allison, "Development of Graded Transition Joints for Avoiding Dissimilar Metal Weld Failures" (2016). David and Lorraine Freed Undergraduate Research Symposium Winning Posters. 2.