1. Field of the Invention
This invention relates to austenitic stainless steels and nickel-base alloys, particularly such alloys, and methods of making the same, wherein the alloys are strengthened by nanometer-size hollow oxides which serve as nucleation sites for chromium-rich carbide precipitates within the alloy grains.
2. Prior Art
Strengthening of metallic alloys primarily is achieved through alloy grain size control, solute additions to a base metal to produce solid solution strengthening, and/or dispersion (precipitation or second phase) strengthening effects. These methods have been applied to a variety of metallic alloy systems and are the basis for strengthening of many of the high-value alloys available in the metal market today. For example, according to U.S. Pat. No. 4,758,405, high strength Al alloys have been produced by gas atomization of an Al alloy melt with an inert gas such as argon, helium or nitrogen containing 0.5-2% by volume of oxygen. U.S. Pat. No. 4,999,052 discloses austenitic stainless steels strengthened with nitrogen in solid solution and containing a dispersant such as a nitride, for example, titanium nitride, and/or an oxide such as yttria. The role of nitrogen in iron-base alloys, particularly austenitic stainless steels, has received considerable attention during the past 80 years. Two fairly recent symposia on this subject have provided state-of-the-art reviews. Proceedings of the International Conference on High-Nitrogen Steels-88, Editors J. Foct and A Hendry, Publ. Institute of Metals, London GB (1988); Proceedings of the International Conference on High-Nitrogen Steels-90, Editors G. Stein and H. Witulski, Publ. Verlag Stalil Eisen, MbH, Dusseldorf (1990).
Alloy 654SMO is a relatively new austenitic stainless steel of high strength and good corrosion resistance. B. Wallen, M. Liljas and P. Stenvall, Avesta 654 SMO--a New High Molybdenum, High Nitrogen Stainless Steel, Avesta Corrosion Management, Avesta AB, S-774 80 Avesta, Sweden.
An overview of mechanisms for strengthening austenitic stainless steels is provided by K. J. Irvine et al., "High-Strength Austenitic Stainless Steels," Journal of The Iron and Steel Institute, October 1961.
Alloys 625 and 718 are representative of high strength nickel-base alloys. H. L. Eiselstein et al. "The Invention and Definition of Alloy 625," Inco Alloys International, Inc, P.O. Box 1958, Huntington, W. Va., Superalloys 718, 625 and Various Derivatives, E. A Loria, Ed., The Minerals, Metals & Materials Society, 1991. Such alloys have been produced by the powder metallurgy process. F. J. Rizzo et al. "Microstructural Characterization of PM 625-Type Materials," Crucible Compaction Metals, McKee and Robb Hill Roads, Oakdale, Pa. 15071 and Purdue University, West Lafayette, Ind. 47906, included in Superalloys 718, 625 and Various Derivatives, E. A Loria, Ed., The Minerals, Metals & Materials Society, 1991. See also R. B. Frank "Custom Age 625 Plus Alloy--A Higher Strength Alternative to Alloy 625, Carpenter Technology Corporation, P.O. Box 14662, Reading, Pa. 19612, also included in Superalloys 718, 625 and Various Derivatives, E. A Loria, Ed., The Minerals, Metals & Materials Society, 1991.
However, the options for improving the properties and performance of metallic alloys are becoming limited in terms of new developments, and new, innovative methods are needed in order to provide a new generation of advanced alloys that can stand up to increasingly severe future demands.