Metal treatment – Stock – Chromium – molybdenum – or tungsten base
Reexamination Certificate
2002-07-19
2003-11-25
Sheehan, John (Department: 1742)
Metal treatment
Stock
Chromium, molybdenum, or tungsten base
C420S429000
Reexamination Certificate
active
06652674
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to Mo—Si—B alloys and, particularly, Mo—Si—B alloys with improved oxidation resistance due to additions of transition elements selected from the group consisting of Fe, Ni, Co, Cu and mixtures thereof.
Molybdenum has excellent high temperature strength which makes it attractive for structural applications at elevated temperatures. The utility of molybdenum and molybdenum-based alloys however are often limited by their poor elevated temperature oxidation resistance. In an oxidizing environment, the first oxidation product that molybdenum forms is molybdenum trioxide. Molybdenum trioxide has a high vapor pressure and sublimes at substantial rates above 1100° F., resulting in accelerated metal loss from the alloy. Molybdenum and molybdenum-based alloys are therefore largely limited to use in non-oxidizing environments at elevated temperatures without some form of externally applied oxidation protective coating.
U.S. Pat. Nos. 5,595,616 and 5,693,156 disclose a new class of high temperature oxidation resistant molybdenum alloys, Mo—Si—B alloys. In these alloys, the silicon and boron which remain after the initial molybdenum trioxide surface layer volatizes, oxidize to form a protective borosilicate-based oxide scale. If properly processed, these alloys can exhibit mechanical properties similar to other molybdenum-based alloys while also maintaining good oxidation resistance at elevated temperatures (1500° F.-2500° F.). This combination of mechanical properties and oxidation resistance makes these materials very attractive for high temperature structural applications.
The oxidation resistance of these Mo—Si—B alloys is largely a function of the silicon and boron content in the alloy. Increasing the silicon content in the presence of boron, improves the oxidation resistance of the alloy but also results in increased silicide volume fraction. High silicide volume fraction not only makes the alloy difficult to process, it makes it more difficult to achieve mechanical properties equivalent to other molybdenum-based alloys. The '595 patent discloses that quaternary additions of a variety of elements, specifically C, Hf, Ti, Zr, W, Re, Al, Cr, V, Nb and Ta, could improve the oxidation resistance of the Mo—Si—B alloy without increasing the silicide volume fraction. Alloys with the specified quaternary additions exhibited enhanced oxidation resistance at 2200° F. and 2500° F. relative to the ternary Mo—Si—B alloys of equivalent silicide content.
Naturally, it would be highly desirable to further improve the oxidation resistance of Mo—Si—B alloys over a wide range of temperature.
Accordingly, it is a principle object of the present invention to provide an improved Mo—Si—B alloy that exhibits excellent oxidation resistance at elevated temperatures, that is, temperatures in excess of 2200° F.
SUMMARY OF THE INVENTION
The foregoing object is achieved by way of the present invention wherein the oxidation resistance of the ternary Mo—Si—B alloys are improved at elevated temperatures by minor additions of certain transition elements, such as Fe, Ni, Co, Cu. While earlier alloying additions resulted in the formation of an oxide scale which was protective for tens of hours at 2500° F., the described additions result in the formation of an oxide scale which is protective for hundreds of hours (700hrs+) at 2500° F. Minor additions of these elements improve the high temperature oxidation resistance of the alloy without any significant effect on the lower and intermediate temperature oxidation resistance of the alloys.
REFERENCES:
patent: 3690686 (1972-09-01), Prasse et al.
patent: 5505793 (1996-04-01), Subramanian et al.
patent: 5595616 (1997-01-01), Berczik
patent: 5693156 (1997-12-01), Berczik
patent: 60-33335 (1985-02-01), None
patent: Wo 96/22402 (1996-07-01), None
Berczik Douglas Michael
Myers James F.
Raban Rafael
Woodard Shiela Rhea
Bachman & LaPointe P.C.
Sheehan John
United Technologies Corporation
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