Alloys or metallic compositions – Nickel base – Chromium containing
Reexamination Certificate
1998-05-08
2001-01-23
Sheehan, John (Department: 1742)
Alloys or metallic compositions
Nickel base
Chromium containing
C148S427000, C148S442000
Reexamination Certificate
active
06177046
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of superalloys containing palladium. The invention is particularly drawn to nickel-based superalloys useful in aerospace and power generation turbine applications. The superalloy's weldability, strength and excellent oxidation resistance properties make it useful in turbine blade tip manufacturing or refurbishment as well as in other high temperature components such as combusters, nozzles, flame holders and seals where these properties are desirable or critical.
BACKGROUND OF THE INVENTION
The term “superalloy” is used to represent complex nickel, iron, and cobalt based alloys containing additional metals such as chromium, aluminum, titanium, tungsten, and molybdenum. The term “based” as used herein means that that element is the largest weight fraction of the alloy. The additives are used for their high values of mechanical strength and creep resistance at elevated temperatures and improved oxidation and hot corrosion resistance. For nickel based superalloys, high hot strength is obtained partly by solid solution hardening using such elements as tungsten or molybdenum and partly by precipitation hardening. The precipitates are produced by adding aluminum and titanium to form the intermetallic compound &ggr;′ (“gamma prime”), based on Ni
3
(Ti,Al), within the host material.
The properties of superalloys make them desirable for use in corrosive and/or oxidizing environments where high strength is required at elevated temperatures. Superalloys are especially suitable for use as material for fabricating components such as blades, vanes, etc., for use in gas turbine engines. These engines usually operate in an environment of high temperature and/or high corrosiveness. Therefore a need exists for alloys with high temperature oxidation resistance and/or good hot corrosion resistance.
Nickel based superalloys are well known in this field. For instance, U.S. Pat. No. 4,261,742 to Coupland et al. discloses a superalloy consisting essentially of 5 to 25 wt % chromium, 2 to 7 wt % aluminum, 0.5 to 5 wt % titanium, at least one of the metals yttrium and scandium present in a total amount of 0.01 to 3 wt %, 3 to 15 wt % in total of one or more of the platinum group metals, and the balance nickel. The Coupland et al. superalloy has increased oxidation and hot-corrosion resistance and may be used as a material for fabricating blades or vanes of gas turbine engines or components used in coal gasification systems. Also, U.S. Pat. No. 4,018,569 to Chang discloses an alloy consisting essentially of 8 to 30 wt % aluminum, 0.1 to 10 wt % hafnium, 0.5 to 20 wt % of an element selected from the group consisting of platinum, rhodium and palladium, 0 to 3 wt % yttrium, 10 to 40 wt % chromium, and the balance comprising an element selected from the group consisting of iron, cobalt and nickel. The Chang superalloy has improved environmental resistance which may be used to improve the temperature capability of components in gas turbine engines. However, neither Coupland et al. nor Chang disclose superalloy compositions containing palladium in amounts sufficient to improve the weldability of the superalloy in accordance with the requirements of the present application. These patents are hereby incorporated by reference.
Other patents are known that disclose high temperature nickel containing alloys. Some examples include: U.S. Pat. No. 4,149,881 to D'Silva, U.S. Pat. No. 4,414,178 to Smith, Jr. et al., U.S. Pat. No. 4,719,081 to Mizuhara, and U.S. Pat. No. 4,746,379 to Rabinkin, all hereby incorporated by reference. These patents disclose alloys with various amounts of palladium, chromium and nickel but do not contain aluminum which is a required element of the present invention.
Current and next generation turbofan turbine engines use nickel based superalloys for many of the components in the high temperature sections of an engine. These sections include the later stages of the high pressure compressor, the combuster, the high and low pressure turbine, and the exhaust modules. These components are subjected to a wide variety of service related degradation including oxidation, fatigue, creep, corrosion, and erosion. In nearly all applications, more than one of these phenomena occurs during turbine engine operation. As a result, alloy design principally has been concerned with improving the thermomechanical properties of the alloys. Produceability of the alloy, i.e., weldability, castability, forgeability, and machineability are often considered a secondary or tertiary criterion during alloy design. However, when weldability is considered during alloy design the resulting material may be widely used. For example, Alloy 625 and its derivatives (including Alloy 718) are the most widely used superalloys in the world [H. L. Eiselstein and D. J. Tillack “The Invention and Definition of Alloy 625”, Superalloys 718, 625 and Various Derivatives, Conference Proceedings, Pittsburgh Pa., June 1991, ed. E. A. Loria].
To improve the oxidation resistance and strength of Ni alloys, successive generations of alloys have incorporated increasingly higher levels of aluminum and to a lesser extent titanium. Both Al and Ti are detrimental to weldability.
There are several modes of cracking that can occur during welding. One of the most troublesome is strain age cracking of the weld metal or in the heat affected zone of the base material. Strain age cracking is the principal reason why nickel based superalloys are considered to be difficult to weld [Welding Handbook Vol. 4, Seventh Edition, ed. by W. H. Kearns, p. 233 and 236, ©1982 American Welding Society]. This type of cracking can occur during cooling from weld temperature, during post weld heat treatment, or during the application of subsequent weld passes. The primary reason these alloys exhibit strain age cracking is that the aging kinetics of the &ggr;′ phase is very fast and the alloy can not accommodate the resulting strain without cracking.
FIG. 1
shows the relationship between an alloy's Al+Ti content and weldability [M. Prager and C. S. Shira,
Weld. Res. Counc. Bul
., 128, 1968]. Note that alloys containing greater than about 3 wt % Al are considered difficult to weld, in addition as Ti levels increase the allowable amount of Al present in the alloy also decreases. Also note that this chart was developed before applicant's discovery of the affect of the addition of palladium to superalloys, which allows higher amounts of Al+Ti to be included in the composition at the same level of weldability. This is discussed more fully below.
For alloys that lie close to the line, such as Rene′41 and Waspaloy, special heat treatments have been used to reduce cracking. For example, over aging Rene′41 has been shown to reduce strain age cracking through the coarsening of the &ggr;′ phase [W. P. Hughes and T. B. Berry, “A Study of the Strain-Age Cracking Characteristics in Welded Rene′41-Phase 1”, Welding Journal, August 1967, p 361-370].
It is common for current generation superalloys to have as much as 12% Al with little or no Ti present. The impossibility of welding these alloys has a significant impact on the repairability of components made from such alloys. For example, a turbine blade may be removed from service due to tip wear while the component still has a significant portion of its design life remaining. It is desirable to weld repair the worn area and return the component to service. Currently these components are repaired using a solid solution strengthened alloy such as Alloy 625, Hastelloy X, L605, or HS188. However, these alloys lack the strength and oxidation resistance of the original material; as a result the repaired components suffer rapid degradation during subsequent service.
Several other types of cracking can occur in superalloy weldments. For castings and large grain wrought materials grain boundary liquation cracking or hot shortness may occur. This type of cracking is minimized by
Simkovich George
Whitney Eric J.
Monahan Thomas J.
Oltmans Andrew L.
Sheehan John
The Penn State Research Foundation
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