Evaluation of a New High Strength Austenitic Alloy for Generator Retaining Rings

A new high strength, nonmagnetic, iron-base alloy has been developed for large retaining ring application. This alloy is based on the development efforts recently completed in a previous collaboration between EPRI and the University of California, but takes advantage of the fact that tantalum, whose price is subject to fluctuations, can be successfully replaced partially or fully by niobium. Among the niobium modified versions evaluated, the alloy containing 1Nb + 1Ta provided the optimum combination of desired properties. This alloy, which has the approximate composition Fe-34.5Ni-5Cr-3Ti-1Nb-1Ta-0.5A1-1.0Mo-0.3V-0.01B, has demonstrated the capability to reach yield strength levels of around 200 ksi (1379 MPa), following controlled hot working, modest cold expansion and a double aging heat treatment. Compared to the currently used 18Mn-5Cr alloy, the new alloy exhibited superior stress corrosion resistance (in humid air and 3.5 percent NaCl) and comparable toughness in air and dry hydrogen. Physical properties such as density, magnetic permeability, thermal conductivity and coefficient of thermal expansion were found to be compatible with the requirements for retaining ring application. It is anticipated that successful achievement of similar properties in large-scale heats and subsequent commercialization can result in improved generator reliability, size capability and efficiency. This paper is a progress report on this development.