Alloys or metallic compositions – Ferrous – Nine percent or more chromium containing
Patent
1999-05-25
2000-09-26
Yee, Deborah
Alloys or metallic compositions
Ferrous
Nine percent or more chromium containing
420 67, 420 68, 420583, C22C 3822, C22C 3000
Patent
active
061238988
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to an improvement of the low thermal expansion alloys composed of the basic Cr--W--Fe system alloy by which the coefficients of thermal expansion of said alloy are approximated to the coefficient of thermal expansion of stabilized zirconia. The present invention is furthermore aimed on several objectives. These objectives may include (1) an improvement of oxidation resistance by forming a surface oxide layer consisting of Cr.sub.2 O.sub.3 containing Al.sub.2 O.sub.3 on the extremely outer surface and an inner layer with dispersed Al.sub.2 O.sub.3, (2) an enhancing mechanical properties by adding Co element, (3) a reducing specific resistance by adding at least one element properly selected from the element group consisted of Ti, Zr, and Hf, (4) an adding B element in order to prevent the grain boundary segregation of W element, and (5) an adding M element in order to improve the oxidation resistance; where said M element is appropriately selected one or more than two elements from the element group consisting of Y, Hf, Ce, La, Nd, and Dy. Then, by accomplishing the aforementioned objectives, the present invention directly relates to provide an alloy with low coefficient of thermal expansion, which is furthermore characterized by (a) a formation of an intermediate layer which serves as a thermal barrier coating and is formed between zirconia system ceramic and substrate material. The low thermal expansion alloy is, moreover, characterized by (b) a coefficient of thermal expansion which is approximated to the coefficient of thermal expansion of stabilized zirconia--which is a promising third generation ceramic material applicable as solid oxide fuel cell, and (c) some other excellent physical and mechanical properties including high-temperature strength, oxidation resistance and low specific resistance.
2. Background Art
Recently, there is a significant trend of more severe operational conditions of higher temperature and higher pressure in a lot of plant machinery being operated at high temperature regimes including a generator gas turbine, reflecting to saving energy and global environmental issues. As a result, metallic materials which have been used under the critical conditions are subjected to a severe extent of damage and deterioration processes.
Hence an anti-corrosion coating is normally applied to surfaces of static and dynamic blades made of high strength superalloys in gas turbines for both airplanes and on-ground applications. However, the anti-corrosion coating is not a perfect solution to solve the current damage and deterioration problem due to a high temperature corrosion.
By the thermal barrier coating (TBC) as an example, the ceramic material having a lower thermal conductivity is coated onto the metallic material with temperature gradient in order to prevent unwanted temperature rise on the metal surface layer. The thermal barrier coating has been applied to the fuel chamber in the gas turbine system for more than 10 years, and recently is applied to cooling blades, too. The test conducted on a real blade indicated that the thermal barrier effect was observed with a temperature ranging from 50.degree. C. to 100.degree. C.
The thermal barrier coating is consisted of mainly two parts; the first layer is a ceramic sprayed layer made of mainly ZrO.sub.2 which is a solid-solution with a stabilizer such as MgO, Y.sub.2 O.sub.3, CaO or the like, and has a heat conductivity of 0.005.about.0.006 (cal/cm.s..degree.C.), being much less than those values for Al.sub.2 O.sub.3 (0.04.about.0.08) or TiO.sub.2 (0.01.about.0.02). The second layer is an intermediate sprayed layer composed of Ni--Al system alloy, Ni--Cr system alloy or M--Cr--Al--Y system alloy (whereas M represents Fe, Ni, Co element or the like). The second intermediate layer is formed for a purpose of reducing the thermal expansion difference between said ceramic sprayed layer and alloy substrate, and of improving corrosion resistance. This intermediate layer can be fabricated
Taimatsu Hitoshi
Ueda Masami
Gossett PLLC Dykema
Sumitomo Special Metals Co. Ltd.
Yee Deborah
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