J. DuPont, T. Anderson, Lehigh University, Bethlehem, PA; C. Robino, Sandia National Laboratory, Albuquerque, NM
The influence of Gd and B on the solidification behavior and weldability of Ni-Cr-Mo alloy UNS N06455 has been investigated by Varestraint testing, differential thermal analysis, and microstructural characterization. The Gd containing alloys were observed to solidify in a manner similar to a binary eutectic system. Solidification initiated at 1258 C and terminated with a eutectic type reaction involving austenite and a Ni5Gd intermetallic. The solidification cracking susceptibility of the Gd containing alloys reached a maximum at 1 wt% Gd, and decreased with both higher and lower Gd additions. Low cracking susceptibility at Gd concentrations below ~ 1 wt% was attributed to a relatively small amount of terminal liquid that existed over much of the crack susceptible solid + liquid zone. Low cracking susceptibility at Gd concentrations above ~1 wt% was attributed to a reduced solidification temperature range and back-filling of solidification cracks. Simple heat flow equations were combined with solidification theory to develop a relation between the fraction liquid and distance within the solid + liquid zone, and the calculated fL – x curves provide useful insight into composition-solidification-weldability relations of the Gd containing alloys.The addition of B above the 230 ppm level lead to the formation of an additional eutectic type reaction at 1200 oC, and the secondary phase within the eutectic type constituent was tentatively identified as Mo3B2. Boron additions had a strong, deleterious influence on solidification cracking susceptibility. The high cracking susceptibility was attributed to an extension of the crack susceptible solid + liquid zone induced by the additional eutectic reaction and extensive wetting of the grain boundaries by the solute rich liquid. Based on these results, B should not be considered as an alloy addition to these materials if the material is to be used as a welded structural component.
Summary: The influence of Gd and B on the solidification behavior and weldability of Ni-Cr-Mo alloy UNS N06455 has been investigated by Varestraint testing, differential thermal analysis, and microstructural characterization. These alloys are currently being developed as structural materials for nuclear criticality control in applications requiring transportation and disposition of spent nuclear fuel owned by the US Department of Energy.
