nuclear criticality control. These materials will be used for the internal baskets that separate spent fuel assemblies and are required for structural support, spent nuclear fuel geometry control, and nuclear criticality safety. Given the large quantity of material required for this application, the material should be producible with conventional fabrication methods such as ingot casting and hot working. Ultimately, the material will be formed and welded into an internal structure that will cradle the fuel and maintain a specified geometry, so the material must also exhibit good weldability. Recent research (Refs. 36–38) has focused on development of gadolinium- (Gd) enriched Ni-based alloys for this application. Gd serves as an effective alloy addition for this application because it has a very high neutron absorption cross section (Refs. 23, 39). The results of recent research has also shown that Gd additions at the nominal 2 wt-% level can successfully be added to the commercial Ni-based C-4 alloy while maintaining adequate hot workability, weldability, and mechanical properties. The current plan for fusion welding of this alloy involves the use of a Gd-free commercial filler metal such as Alloy C-4 or 59. With this approach, the Gd content B in the fusion zone will vary with weld metal dilution, and the dilution level is strongly affected by the welding parameters (Refs. 40, 41). Considering the case in which a base metal containing 2 wt-% Gd is welded with a Gd-free filler metal, the concentration of Gd in the fusion zone will vary linearly with dilution and can be close to 0 wt-% Gd (at low dilution values) to 2 wt-% Gd (for an autogenous weld in which the dilution is 100%). The welding parameters are typically selected based on the required weld size and joint design requirements, and a wide range of welding parameters can be expected in practice. This, in turn, can produce a wide range of fusion zone Gd concentration values. The Gd concentration can also vary throughout the fusion zone of a given weld in multiplepass welding applications. Recent research has shown that Gd controls the solidification behavior of this alloy. In particular, the solidification temperature range and amount of terminal eutectictype constituents that form at the end of solidification are essentially dominated by the Gd concentration (Ref. 38). In addition, it has also been well established that the solidification cracking susceptibility is also strongly affected by the solidification temperature range and amount of terminal eutectic-type constituents that form during solidification of the fusion zone (Refs. 42, 43). Thus, it is important to determine the influence of Gd concentration on the solidification cracking susceptibility of Alloy C-4 so that these structural materials can be welded while maintaining their structural integrity. Figure 24 shows the influence of Gd on the solidification cracking susceptibility of Alloy C-4. The cracking susceptibility is low for the Gd-free base alloy. The cracking susceptibility then increases with increasing Gd concentration up to ~1 wt-% Gd and then decreases to a level that is similar to the base alloy at Gd concentrations of 1.5 to 2.5 wt-%. Typical microstructures of the Varestraint samples are shown in Fig. 25. No significant crack healing due to backfilling of solute-rich liquid occurred in the alloy with 1.01 wt-% Gd. Similar results were obtained with the alloy that had 0 and 0.46 wt-% Gd. However, a significant amount of crack healing was observed in the alloys that had additions of Gd at the 1.49 wt-% level and above. The Varestraint weldability results show that the cracking susceptibility reaches a maximum at ~ 1 wt-% Gd, and decreases with both higher and lower Gd additions. Previous research (Refs. 42, 43) has shown that the solidification cracking susceptibility of engineering alloys is strongly affected by the solidification temperature range and amount of terminal eutectic-type constituent that forms at the end of solidification. Solidification of these alloys initiates at the liquidus temperature by the formation of primary γ− austenite. Essentially, no Gd is dissolved in the austenite matrix. Thus, as solidification proceeds, the liquid becomes increasingly enriched in Gd until the Liquid → γ + Ni5Gd eutectic-type reaction is reached, at which point solidification is terminated. FEBRUARY 2014, VOL. 93 40-s WELDING RESEARCH Fig. 21 — Stability of various 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) in fusion welds. Fig. 22 — 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. A C D
Welding Journal | February 2014
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