Patent Application
20060042729
1. Field of the Invention
The present invention relates to a method of repairing superalloy components. More specifically, the invention provides a method of local heat treatment of superalloy components prior to welding in a manner that resists recrystallization of the material in locations where repair is not necessary, and also resists cracking in the heat affected zone of the weld and deposited weld metal while preserving the material properties of the remainder of the component.
2. Description of the Related Art
Components of various types of equipment that are subject to high temperature, high stress environments, for example, components within combustion turbines, are typically made from materials known as superalloys, which are defined herein as nickel based alloys containing aluminum and/or titanium, or cobalt based alloys. Components made from these materials typically include equiaxed materials, directionally solidified materials, or single crystal materials. After casting, the components are typically subjected to various heat treatments, for example, homogenization, hot isostatic pressing, solutionizing, and/or aging. The heating rate, hold temperature, hold time, and cooling rate of these heat treatment processes are intended to produce optimally sized and shaped grains of precipitate of Ni3(Al,Ti) and carbides within the material. The volume percentage, size, and distribution of these precipitates, along with the type and distribution of the carbide, determine the mechanical properties of the material. An optimum volume percentage and distribution of precipitates is the source of the material's high temperature strength.
During the operation of a combustion turbine having such components therein, the high temperature and stress to which the components are subjected cause precipitation of carbides in the grain boundaries in equiaxed and directionally solidified materials, and also causes coarsening of the Ni3(Al,ti) precipitates, thereby changing the mechanical properties of the material. Prolonged exposure to such conditions may cause cracking within the material.
Such cracks are typically repaired by welding, however, superalloys are difficult to weld. During welding, hot cracking may occur in the heat affected zone due to liquation of low melting phases such as borides, carbides, sulfides and/or phosphides in the grain boundaries. Present efforts to reduce hot cracking include design of the weldments, controlling trace elements within the base metal, using lower strength weld filler metals, and using welding processes with low heat inputs.
Additionally, post weld heat treatment cracking, also known as “strain age cracking,” may occur during the post weld heat treatment which is performed to restore the properties of the components and to relieve residual stresses within the material. Such cracks may extend beyond the heat affected zone through the weld metal or through the parent material. During heat treatment, as residual stress is relaxed, precipitation of Ni3(Al,Ti) occurs rapidly, resulting in volume contraction and strengthening of the material, thereby resulting in a reduction of the ductility of the material. Cracking occurs when the strain associated with stress relaxation exceeds the strain capacity of the heat affected zone. Hot cracks may act as the initiation points for strain age cracks.
The strain-age cracking tendency of superalloys is related to the total amount of alloying elements such as Al and Ti contained within the alloy.
Presently used methods to minimize strain age cracking include solution and overaging pre weld heat treatments. The former method works well with alloys with low Ni3(Al,Ti) volume percents, while the latter method works best for materials with high Ni3(Al,Ti) volume percent. Such heat treatment typically involves heating the entire component in a vacuum furnace to a predetermined temperature and cooling the component to room temperature, with the cooling done quickly or slowly depending on the desired result. A typical hold temperature is the solution temperature where all the Ni3(Al,Ti) precipitates go into solution.
In the case of directionally solidified or single crystal materials, the heat treatment hold temperature is limited to temperatures that are lower than the solution temperature due to recrystallization (formation of new small grains) within the material. Formation of recrystallized grains results in a reduction of the desired mechanical properties of the material. However, such low temperature heat treatment is insufficient to improve the weldability of the material.
Accordingly, there is a need for a method of heat treating superalloys in a manner that improves the weldability of the portion of the component to be repaired without damaging the microstructure and material properties of the remainder of the superalloy component. Such a method would substantially reduce the cost of maintaining equipment using superalloy components by improving the repairability of these components, and avoiding the expense of replacement of damaged components.
The present invention provides an improved method of heat treating superalloy components. The method includes performing a local pre-weld heat treatment only to the region of the component that requires repair. By using a localized heat treatment, temperatures close to, equal to, or greater than the Ni3(Al,Ti) solution temperature may be used. Such localized heat treatment will resist recrystallization in other critical areas such as, in the example of a turbine blade, the remainder of the airfoil and the root.
During localized heat treatment, the heat treated portion of the component will be taken to a temperature between about 1,850° F. and 2,400° F. This portion of the component may be allowed to cool from this temperature to approximately 1,000° F. and 1,800° F. at a controlled cooling rate. The remainder of the component will generally be kept below 1000° F. to resist alteration of the microstructure. Heat conduction through the superalloys that is being given a localized heat treatment is unlikely to be sufficient to increase the temperature of the remainder of the component above about 1,000° F. However, as an additional precaution, a cooling medium may be directed below the portion of the component being given a heat treatment, for example, directing Argon gas below the heat treated portion to carry away the heat.
The region of the components in which welding will be performed may be heat treated using well known local heat treating methods such as induction heating or resistance heating. Particular superalloys with which the present invention may be used include, but are not limited to, CM247, MarM002, IN738, and RENE 80.
Accordingly, it is an object of the present invention to provide a method of resisting cracking during weld repairs of components made from superalloy materials.
It is another object of the invention to provide a method of localized heat treatment of superalloy components.
It is a further object of the invention to maintain the microstructure and mechanical properties of the portion of a superalloy component outside the heat affected zone of a weld repair.
It is another object of the invention to maximize the lifespan and improve the repairability of components made from superalloys that are used in high-temperature, high-stress environments.
These and other objects of the invention will become more apparent through the following description and drawings.
Like reference characters denote like elements throughout the drawings.
The present invention provides an improved method of heat treating superalloys, which resists the formation of recrystallized grains in portions of the component not being repaired, and also resists cracking during and after the welding process. Although primarily intended for use prior to welding, the heat treating method may also be advantageously used after welding, to rejuvenate components after extended service, and as a pre-brazing or post-brazing heat treatment.
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The region of the components in which welding will be performed may be heat treated using well known local heat treating methods such as induction heating, resistance heating, lamp heating, or other known heating methods. Basically, induction heating utilizes a copper coil with a power supply to induce eddy currents in the component, with the eddy currents generating heat. Resistance heat treatment utilizes resistance elements on or near the component being heat treated. The heat treatment may be performed in air, in an inert gas environment, or in a vacuum.
During the localized heat treatment, heat conduction through the remainder of the superalloy component is unlikely to be sufficient to raise the temperature of the remainder of the component above 1,000° F. However, a cooling medium may be directed immediately adjacent to the heat affected zone of the component being repaired, for example, directing argon gas adjacent to the heat affected zone to carry away the heat.
The present invention therefore provides an improved method of heat treating a superalloy component, wherein only a localized portion of the entire component is heat treated. The portion of the component to be repaired may therefore be given a heat treatment at a sufficiently high hold temperature to necessitate the required averaging heat treatment to prevent strain age cracking, while the remainder of the component does not undergo any heat treatment and therefore retains its original microstructure, devoid of any recrystallization. The present invention therefore improves the repairability of superalloy components used in high-temperature, high-stress environments such as the inside of a combustion turbine, thereby increasing the lifespan of these components and decreasing the cost of maintaining a combustion turbine or other equipment utilizing such superalloy components. The heat treatment may be used pre-welding, post-welding, pre-brazing, post-brazing, or for component rejuvenation. The heat treatment may be used with equiaxed materials, directionally solidified materials, or single crystal materials.
While a specific embodiment of the invention has been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
