Alloy 22, a highly corrosion-resistant Ni-Cr-Mo alloy, is the material the U.S. Department of Energy currently plans to use as the outer container of the waste package for the disposal of high-level nuclear waste. Changes in material microstructure and microchemistry resulting from fabrication processes such as welding and subsequent heat treatment may impair the corrosion resistance and mechanical properties of the outer container material. Solution annealing has been proposed to eliminate any detrimental phases and residual stresses created by the longitudinal and
circumferential welding of hot rolled plates during the fabrication of the cylindrical containers. Precipitation of topologically close-packed (TCP) phases in Alloy 22 and the upper stability temperature of these phases (also known as the solvus temperature), as influenced by alloy compositional variation and element segregation in the welds, were evaluated using both experiments and thermodynamic calculations. The equilibrium calculations with Thermo-Calc and Ni-DATA predicted that P-phase would be the only equilibrium TCP phase at higher temperatures. All aging and solution annealing treatments of the welded material conducted in this investigation promoted precipitation of the secondary phases. These observations are supported by the prediction of a substantial increase in solvus temperature for the P-phase in the interdendritic regions as a result of the segregation of molybdenum. Results from this work suggest that the standard solution annealing treatment for Alloy 22 may be inadequate to produce a homogeneous single-phase structure. Further evaluation of the effects of fabrication processes on phase stability of Alloy 22 weldments and their corrosion resistance is necessary to assess the lifetimes of the waste packages.
Acknowledgment: This paper is an independent product of the CNWRA and does not necessarily
reflect the views or regulatory position of the NRC.