A B phases as a function of temperature in IN740H for the base metal nominal composition (Fig. 21A, B) and for a value of 0.99 fraction solid that is representative of the interdendritic composition — Fig. 21C, D. (Figure 21B, D shows the same results as Figs. 21A and 22C, but over a narrower temperature range for clarification.) Note that, if the heat treatment was based on the nominal composition of the base metal, the results suggest that the initial PWHT step could be conducted slightly beyond 1300°C before any problems from localized melting (also note that the MC carbide will still be stable at these high temperatures). However, when dendritic segregation is properly considered (Fig. 21C, D), the results demonstrate that the initial PWHT temperature should not exceed about 1150°C to avoid localized melting in the interdendritic regions. These results emphasize the importance of designing PWHT schedules based on the actual phase stability in the fusion zone as affected by compositional gradients. Figure 22 shows the results of DICTRA calculations for IN740H that demonstrate the homogenization kinetics. Results are shown for the concentration gradient of Nb, since this is the slowest diffusing element in the system and therefore the rate-limiting step. The calculations show that a PWHT at 1100°C for four h should eliminate the microsegregation, and this has been confirmed by experimental measurements (Fig. 23). Once the homogenization is complete, a higher temperature solution temperature can be utilized if needed for more complete dissolution of secondary phases. Although it is recognized that the 1100°C/4-h treatment is not practical for a PWHT conducted in the field, the results can be applied to shop fabrication conditions. As mentioned previously, the potential for a more practical solution through design of tailored filler metals is needed and is currently being pursued. Fusion Welding of Gd-Enriched Ni Alloys for Spent Nuclear Fuel Applications Nuclear fuel plays an important role in energy production in the United States and throughout the world, and the use of nuclear power is expected to rise over the coming years. One of the major challenges associated with nuclear fuel is the handling of spent nuclear fuel (e.g., nuclear waste). Safe disposition of spent nuclear fuel requires the development of thermal neutron absorbing structural materials for WELDING JOURNAL 39-s WELDING RESEARCH Fig. 18 — EDS traces acquired across the dendritic substructure of a weld in IN740H. The data points reflect the average and standard deviation from multiple measurements, while the lines represent results from Scheil solidification simulations. Fig. 19 — SEM photomicrograph showing how increased localized concentration of γ ′-forming elements produces enhanced grain boundary precipitation in fusion welds on Alloy IN740H. Fig. 20 — ΔCo values at 800°C associated with the γ ′-forming elements Nb, Ti, and Al that demonstrate the significantly enhanced driving force for precipitation in the interdendritic regions of welds in Alloy IN740H.
Welding Journal | February 2014
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