Metal treatment – Stock – Nickel base
Reexamination Certificate
2001-08-29
2002-12-10
King, Roy (Department: 1742)
Metal treatment
Stock
Nickel base
C420S443000, C420S448000, C420S449000
Reexamination Certificate
active
06491769
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to Ni—Co—Cr base alloys and, more particularly, to a high strength, sulfidation resistant Ni—Co—Cr alloy for long-life service at 538° C. to 816° C. The alloy of the present invention provides a combination of strength, ductility, stability, toughness and oxidation/sulfidation resistance so as to render the alloy range uniquely suitable for engineering applications where sulfur-containing atmospheres are life limiting.
2. Discussion of the Related Art
Over the years, researchers have continually developed alloys meeting requirements for both high strength at intermediate temperatures and corrosion resistance under severe environmental conditions. This quest for increasing performance is far from over as designers and engineers continuously seek to increase productivity, lower operating costs, improve yields and extend service lives. All too often, however, researchers terminated their efforts when the target combination of properties was achieved. Such is the case, for example, in two industrial areas in critical need of advanced alloys to maintain progress. These industrial applications are diesel exhaust valves and alloys for coal-fired boilers. These applications have in common that their developers require ever-increasing strength at increasingly higher temperatures, improved resistance to sulfur-containing atmospheres as atmospheres become more demanding and increases in service lives to assure trouble-free operation over the life of the equipment. Heavy-duty diesel engines in off-road construction equipment, often operating in remote corners of the globe where refined, low sulfur fuels are not available, are suffering exhaust valve failure due to sulfidation attack. Maintenance of these engines, usually requiring original equipment mechanics, can become prohibitively expensive and time-consuming. These same engines are now being designed for higher temperatures to increase power and efficiency. This has only served to exacerbate the alloy challenge.
Ultra supercritical boiler designers are creating a similar problem in coal-fired boilers as utilities seek to improve efficiency by raising steam pressure and temperature. Today's boilers with efficiencies around 45% typically operate at a 290 bar steam pressure and 580° C. steam temperature. Boiler designers are setting their sights on 50% efficiency or better by raising the steam conditions as high as 375 bar/700° C. To meet this requirement in the boiler tubing, the 100,000 hour stress rupture life must exceed 100 MPa at 750° C. (mid radius tube wall temperature needed to maintain a 700° C. steam temperature at the inner wall surface). Raising steam temperature has made coal ash corrosion more troublesome, placing a further requirement on any new alloy. This corrosion requirement is less than 2 mm of metal loss in 200,000 hours for exposures in the temperature range of 700° C. to 800° C. For economy, the boiler tube must be as thin-walled as possible (i.e., <8 mm wall thickness) and be fabricable into long lengths in high yield on conventional tube making equipment. This places a major constraint on the maximum work-hardening rate and yield strength tolerable in manufacture and field installation, physical property characteristics running counter to the need for superior strength in valve and boiler tube service.
To meet the new strength and temperature requirements of an advanced diesel exhaust valve or a future boiler tube alloy, designers must exclude the usual ferritic, solid solution austenitic and age-hardenable alloys heretofore employed for this service. These materials commonly lack one or more of the requirements of adequate strength, temperature capability and stability or sulfidation resistance. For example, the typical age-hardenable alloy, in order to develop high strength at intermediate temperatures, must be alloyed with insufficient chromium for peak sulfidation resistance in order to maximize the age-hardening potential of the alloy. Adding chromium not only degrades the strengthening mechanism but, if added in excess, can result in embrittling sigma, mu or alpha-chromium formation. Since 538° C. to 816° C. is a very active range for carbide precipitation and embrittling grain boundary film formation, alloy stability is compromised in many alloys in the interest of achieving high temperature strength and adequate sulfidation resistance.
The present invention overcomes the problems of the prior art by providing a Ni—Co—Cr-base alloy range possessing exceptional resistance to sulfur-containing atmospheres containing limiting amounts of Al, Ti, Nb, Mo and C for high strength at 538° C. to 816° C. while retaining ductility, stability and toughness.
The present invention contemplates a newly-discovered alloy range that extends service conditions for the above-described critical industrial applications notwithstanding the seemingly incongruous constraints imposed by the alloying elements economically available to the alloy developer. Past alloy developers commonly claimed broad ranges of their alloying elements which, when combined in all purported proportions, would have faced these counter influences on overall properties. The present inventors have discovered that a narrow range of composition does exist that allows one to fabricate a high strength alloy for service at 538° C. to 816° C. with both sulfidation resistance, phase stability and workability. A better appreciation of the alloying difficulties can be presented by defining below the benefits and impediments associated with each element employed in the present invention.
SUMMARY OF THE INVENTION
A high strength, sulfidation resistant Cr—Co—Ni base alloy for long-life service at 538° C. to 816° C. containing, in % by weight, about 23.5-25.5%Cr, 15.0-22.0%Co, 0.2-2.0%Al, 0.5-2.5%Ti, 0.5-2.5%Nb, up to 2.0%Mo, up to 1.0%Mn, 0.3-1.0%Si, up to 3.0%Fe, up to 0.3%Ta, up to 0.3%W, 0.005-0.08%C. 0.01-0.3%Zr, 0.001-0.01%B, up to 0.05% rare earth as misch metal, 0.005-0.025%Mg plus optional Ca, balance Ni, including trace additions, such as up to 0.05%La, up to 0.05%Y, plus impurities. The alloy provides a combination of strength, ductility, stability, toughness and oxidation/sulfidation resistance so as to render the alloy range uniquely suitable for engineering applications where sulfur-containing atmospheres are life limiting.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The combination of elements set forth above unexpectedly and surprisingly possesses all of the critical attributes required of high strength applications in sulfur-containing atmospheres. It has been discovered that sulfidation resistance can be achieved by alloying within a narrow range of Cr (23.5-25.5%Cr) without destroying phase stability resulting from embrittling phases by concurrently limiting certain elements to very narrow ranges, namely, Mo to less than 2%, C to less than 0.08%, Fe to less than 3.0% and the total Ta plus W content to less than 0.6%. Less than 23.5%Cr results in inadequate sulfidation resistance and greater than 25.5%Cr produces embrittling phases even with the alloy restrictions defined above. It should be mentioned that, unless otherwise specified, all percentages of the various alloy constituents set forth herein are percent by weight.
Oftentimes, in striving for maximum corrosion resistance, the resultant alloys lack the required high temperature strength. This has been solved by the instant invention by balancing the weight percent of precipitation hardening elements to a narrow range where the resulting volume percent of hardening phase is between about 10 and 20% within the Ni—Co—Cr matrix. Excessive amounts of the hardener elements not only reduce phase stability and lower ductility and toughness, but also render valve and tubing manufacturability extremely difficult, if not impossible. The selection of each elemental alloying range can be rationalized in terms of the function each element is expected to perform within the compositional range of the present inve
Baker Brian A.
Smith Gaylord D.
Dropkin, Esq. Robert F.
Inco Alloys International, Inc.
King Roy
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
Wilkins, III Harry D
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