Ally Fraser - Research
Dissimilar Metal Welds (DMWs) are critical for design, development, and manufacturing of Very High Temperature Reactors (VHTR). Premature failures of DMWs in service are associated with the sharp change in composition, microstructure, and properties that exist across the fusion line. A proposed solution, sponsored by the U.S. Department of Energy is a joint whose composition changes gradually from ferrous to austenitic steel to mitigate carbon diffusion and property mismatch. 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. The DMWs exhibited large changes in hardness over short distances due to carbon diffusion shown in Figure 1, while the GTJ did not exhibit drastic changes in hardness values after aging shown in Figure 2. The GTJs exhibited no local softening near the fusion line. Future work includes hardness tests after longer aging times with finer spaced traces to continue investigation of carbon diffusion. Tensile tests with digital image correlation will also be conducted to measure the localized strain rate within the welds.
Figure 1. Hardness trace on DMW samples with a 5 g load spaced 25 mm apart at a steep angle across the fusion line. After aging the DMW for 1000 hrs, there is a steep increase in hardness across the fusion line due to carbon diffusion from the ferritic to austenitic steel. The compositional gradient occurs across a length on the order of microns, allowing easier carbon diffusion.
Figure 2. Hardness traces on GTJ samples taken at a 100 g load spaced 125 mm apart across the graded region for as welded, 500 hrs, 1000 hrs, and 2000 hrs aging conditions. The hardness initially rises due to solid solution strengthening with additions of Ni and Cr, then decreases as martensite tempers with aging. Aging past 500 hrs has little influence on the hardness values. The lack of the softened zone suggests resistance to premature failure due to large concentration gradients and carbon diffusion at elevated temperatures.