Fluid reaction surfaces (i.e. – impellers) – With heating – cooling or thermal insulation means – Changing state mass within or fluid flow through working...
Reexamination Certificate
2001-10-22
2003-06-10
Look, Edward K. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With heating, cooling or thermal insulation means
Changing state mass within or fluid flow through working...
C029S402130
Reexamination Certificate
active
06575702
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to components designed to operate at high temperatures. More particularly, this invention relates to methods for repair and manufacture of airfoils for gas turbine engines, and the articles made and repaired from the use of these methods.
In a gas turbine engine, compressed air is mixed with fuel in a combustor and ignited, generating a flow of hot combustion gases through one or more turbine stages that extract energy from the gas, producing output power. Each turbine stage includes a stator nozzle having vanes which direct the combustion gases against a corresponding row of turbine blades extending radially outwardly from a supporting rotor disk. The vanes and blades include airfoils having a generally concave “pressure” side and a generally convex “suction” side, both sides extending axially between leading and trailing edges over which the combustion gases flow during operation. The vanes and blades are subject to substantial heat load, and, because the efficiency of a gas turbine engine is proportional to gas temperature, the continuous demand for efficiency improvements translates to a demand for airfoils that are capable of withstanding higher temperatures for longer service times.
Gas turbine airfoils on such components as vanes and blades are usually made of superalloys and are often cooled by means of internal cooling chambers and the addition of coatings, including thermal barrier coatings (TBC's) and environmentally resistant coatings, to their external surfaces. The term “superalloy” is usually intended to embrace iron-, cobalt-, or nickel-based alloys, which include one or more other elements including such non-limiting examples as aluminum, tungsten, molybdenum, titanium, and iron. The internal air cooling of turbine airfoils is often accomplished via a complex cooling scheme in which cooling air flows through channels within the airfoil (“internal air cooling channels”) and is then discharged through a configuration of cooling holes at the airfoil surface. Convection cooling occurs within the airfoil from heat transfer to the cooling air as it flows through the cooling channels. In addition, fine internal orifices are often provided to direct cooling air flow directly against inner surfaces of the airfoil to achieve what is referred to as impingement cooling, while film cooling is often accomplished at the airfoil surface by configuring the cooling holes to discharge the cooling air flow across the airfoil surface so that the surface is protected from direct contact with the surrounding hot gases within the engine. TBC's comprise at least a layer of thermally insulating ceramic and often include one or more layers of metal-based, oxidation-resistant materials (“environmentally resistant coatings”) underlying the insulating ceramic for enhanced protection of the airfoil. Environmentally resistant coatings are also frequently used without a TBC topcoat. Technologies such as coatings and internal air cooling have effectively enhanced the performance of turbine airfoils, but material degradation problems persist in turbine airfoils due to locally aggressive conditions in areas such as airfoil leading edges and trailing edges.
A considerable amount of cooling air is often required to sufficiently lower the surface temperature of an airfoil. However, the casting process and the cores required to form the cooling channels limit the complexity of the cooling scheme that can be formed within an airfoil at leading and trailing edges of vanes and blades. The resulting restrictions in cooling airflow often promote higher local temperatures in these areas relative to those existing in other locations on a given airfoil. In typical jet engines, for example, bulk average airfoil temperatures range between about 900° C. to about 1000° C., while airfoil leading and trailing edge surfaces often reach about 1100° C. or more. Maximum surface temperatures are expected in future applications to be over about 1300° C. Of particular concern is the combination of stress with temperature, because metals, including alloys used to make gas turbine airfoils, tend to become weaker, or more easily deformed, as temperatures increase. Thus, while stress of a certain level operating on a cooler section of an airfoil may have little effect on performance, the same stress level may be beyond the performance capability of the material at hotter locations as described above. At such elevated temperatures, materials are more susceptible to damage due to a number of phenomena, including diffusion-controlled deformation (“creep”), cyclic loading and unloading (“fatigue”), chemical attack by the hot gas flow (“oxidation”), wear from the impact of particles entrained in the gas flow (“erosion”), and others.
Damage to airfoils, particularly at edges, leads to degradation of turbine efficiency. As airfoils are deformed, oxidized, or worn away, gaps between components become excessively wide, allowing gas to leak through the turbine stages without the flow of the gas being converted into mechanical energy. When efficiency drops below specified levels, the turbine must be removed from service for overhaul and refurbishment. A significant portion of this refurbishment process is directed at the repair of the specific areas of airfoils. In one repair embodiment, for example, crack-filling processes based on brazing techniques are used to repair localized damage on turbine vanes.
In current practice, the original edge material is made of the same material as the rest of the original blade, often a superalloy based on nickel or cobalt. Because this material was selected to balance the design requirements of the entire blade, it is generally not optimized to meet the special local requirements demanded by conditions at the airfoil leading or trailing edges. The performance of alloys commonly used for repair is comparable or inferior to that of the material of the original component, depending upon the microstructure, defect density, and chemistry of the repair material. For example, many turbine airfoils are made using alloys that have been directionally solidified. The directional solidification process manipulates the orientation of metal crystals, or grains, as the alloy is solidified from the molten state, lining the grains up in one selected primary direction. The resultant alloy has enhanced resistance to creep and fatigue during service when compared to conventionally processed materials. Advanced applications employ alloys made of a single crystal for even further improvements in high-temperature creep and fatigue behavior. However, when these components are repaired by conventional processes, using build-up of weld filler material, the resulting microstructure of the repair is typical of welded material, not directionally solidified or single-crystalline. Other repair methods, such as applying powder mixtures wherein one powder melts and densifies the repaired area during heat treatment, results in microstructures that differ from that of the parent alloy. Such microstructures, present in a conventional airfoil material such as a superalloy, may cause the airfoil to require excessively frequent repairs in advanced designs that rely on the benefits of directional solidification or single crystal processing to maintain performance.
Materials are characterized by several properties to aid in determining their suitability for use in demanding applications such as gas turbine airfoils. The term “creep life” is used in the art to refer to the length of time until a standard specimen of material extends to a preset length or fractures when subjected to a given stress level at a given temperature. Similarly, the term “fatigue life” is used in the art to describe the length of time until a standard specimen fractures when subjected to a given set of fatigue parameters, including minimum and maximum stress levels, frequency of loading/unloading cycle, and others, at a given temperature. The term “oxidation resistance” is used in the art to refer to
Dupree Paul Leonard
Frost Aaron Todd
Hardwicke Canan Uslu
Huang Shyh-Chin
Jackson Melvin Robert
DiConza Paul J.
General Electric Company
Kershteyn Igor
Look Edward K.
Patnode Patrick K.
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