STP792

    Research Toward New Alloys for Generator Retaining Rings

    Published: Jan 1983


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    Abstract

    The research reported here was undertaken to develop an alloy suitable for use in the retaining rings of two-pole electrical generators which would have three key properties: (1) a yield strength of 1380 MPa (200 ksi) or greater with good residual toughness, (2) resistance to hydrogen embrittlement and stress-corrosion cracking, and (3) processability through heat treatment after hot forming, to avoid the necessity of cold forming of the ring. The principal alloy developed during the course of this work was an iron-based superalloy, designated EPRI-T, which has the nominal composition Fe-34.5Ni-5Cr-3Ti-3Ta-0.5Al-1.0Mo-0.3V-0.01B. The alloy is strengthened through the formation of cubic γ′ precipitates of the composition Ni3(Ti,Ta,Al). When given appropriate aging treatment from the as-forged condition, the alloy achieves yield strnegth in excess of 1380 MPa (200 ksi) with good residual toughness and promising resistance to cracking in gaseous hydrogen and salt water. The composition and processing of the alloy are the result of sequential metallurgical development, the steps of which are described in the body of the paper. The alloy was chosen from a class of iron-based superalloys to achieve high strength in thick sections while maintaining reasonable costs, melting practice, and hot formability. The nickel content of the alloy was adjusted to insure that the alloy would be paramagnetic austenite after the aging reaction to form the Ni3X strengthening precipitates. Tantalum was included among the γ′-forming elements to increase the lattice mismatch of the precipitate and improve alloy strength. Chromium was added to the composition to stabilize the austenite phase and eliminate stress-corrosion cracking susceptibility due to martensitic transformation of the austenite. The Mo-V-B group was included to inhibit intergranular precipitation of the equilibrium η phase and hence suppress a tendency toward intergranular fracture in the alloy.

    Keywords:

    steels, microstructure, generator retaining rings, austenitic steel, iron-based superalloys, ultrahigh-strength steel, fracture toughness, precipitation hardening, hydrogen embrittlement, stress-corrosion cracking, environmental fatigue, strain-induced martensitic transformation


    Author Information:

    Morris, JW
    Professor of metallurgy and research scientist, University of CaliforniaGeneral Electric Co., BerkeleySchenectady, Calif.N.Y.

    Chang, KM
    Professor of metallurgy and research scientist, University of CaliforniaGeneral Electric Co., BerkeleySchenectady, Calif.N.Y.


    Paper ID: STP31766S

    Committee/Subcommittee: E04.05

    DOI: 10.1520/STP31766S


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