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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.

In order to apply safely a turbine-bucket alloy which has been successful in short-life high-temperature aircraft gas turbines to long-life heavy-duty gas turbines, it was necessary to make many laboratory tests. These included tensile, rupture, creep, fatigue, damping, oxidation, and corrosion tests. Many of these tests were made on specimens which had been aged for 5000 and 10,000 hr. Rupture data were obtained to 15,000 hr. Operating experience has confirmed laboratory tests and now M-252 is destined to become a standard bucket material for heavy-duty gas turbines.

This paper presents a discussion of important engineering properties of some martensitic 12 per cent Cr alloys for use at temperatures up to 1200 F. Detailed high-temperature creep-relaxation and stress-rupture data are given for 12 per cent Cr (Type 403) and for six alloy modifications including 12 per cent Cr-Co-W-V, 12 per cent Cr-W-V, 12 per cent Cr-Mo-V, 12 per cent Cr-Mo-W-V, 12 per cent Cr-Cb, and 12 per cent Cr-Ni-W. Other properties such as thermal expansion, modulus of elasticity, resistance to stress corrosion, and so forth, are discussed. Some of the modified 12 per cent Cr alloys offer a combination of properties which makes them attractive for a number of high-temperature applications.