Patent Publication Number: US-7896986-B2

Title: Heat treatment of superalloy components

Description:
BACKGROUND OF THE INVENTION 
     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 Ni 3 (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&#39;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 Ni 3 (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 Ni 3 (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 Ni 3 (Al,Ti) volume percents, while the latter method works best for materials with high Ni 3 (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 Ni 3 (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. 
     SUMMARY OF THE INVENTION 
     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 Ni 3 (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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph illustrating the weldability of various alloys based on susceptibility to strain age cracking. 
         FIG. 2  is a graph indicating the typical thermal cycle involved during welding and post weld heat treatment. 
         FIG. 3  is a scanning electron microscope image magnified 10,000 times illustrating the Ni 3 (Al,Ti) precipitate size resulting from pre-weld heat treatment with a high hold temperature. 
         FIG. 4  is a scanning electron microscope image magnified 10,000 times showing the Ni 3 (Al,Ti) precipitate size resulting from pre-weld heat treatment with a low hold temperature. 
         FIG. 5  is a graph illustrating the difference in temperature and time between prior whole component heat treatment and the localized heat treatment of the present invention. 
         FIG. 6  is an isometric view of a blade for a combustion turbine. 
         FIG. 7  is a metallograph magnified 100 times illustrating recrystallization in a directionally solidified nickel base alloy. 
     
    
    
     Like reference characters denote like elements throughout the drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     Referring to  FIG. 1 , the difficulty in welding various superalloys based on their aluminum and titanium concentrations is illustrated. As shown in  FIG. 1 , increasing concentrations of both aluminum and/or titanium in nickel based superalloys increases the difficulty of welding these materials. The graph shows that the alloys CM247, MarM002, IN738, and RENE 80 are particularly difficult to weld. All of these alloys are examples of alloys with which the present invention may be used. 
     Referring to  FIG. 2 , the thermal cycle during welding and post heat treatment is illustrated. Welding subjects the material to a very high temperature for a relatively short period of time, resulting in residual stress within the material. During the post-weld heat treatment, two competing processes occur simultaneously. The desired relief of residual stresses and the undesired Ni 3 (Al,Ti) precipitation occur at the same time. Carbides may also precipitate out of the material.  FIG. 3  illustrates the enlarged grain structure that results from high temperature heat treatment of superalloys. This grain structure is contrasted with  FIG. 4 , illustrating the small grain size that is desired for these superalloys. This precipitation of Ni 3 (Al,Ti) particles increases the strength of the material, with a concurrent reduction in ductility. The combination of the relief of residual stresses and precipitation of Ni 3 (Al,Ti) particles results in strain age cracks. The present invention therefore seeks to avoid the formation of recrystallized grains as shown in  FIG. 6  in other areas of the component for which repair is not needed, while simultaneously preventing strain age cracking in the heat affected zone and filler metal of the weld. 
       FIG. 5  illustrates the pre-weld heat treating process of the present invention as compared with the previous heat treating method. Both the previous method and the present invention utilize similar heating rates, as indicated by the portion  10  of the graph. However, a previous method of heat treating the entire component used a hold temperature below the Ni 3 (Al,Ti) solution temperature, as indicated by graph segment  12 , while the present heat treating method uses a heat treating temperature close to, at, or above the Ni 3 (Al,Ti) solution temperature, with the hold temperature preferably in the range of about 1,850° F. to about 2,400° F., as indicated by graph segment  14 . The hold time at the desired temperature for the present invention may be approximately equal to or, if desired, longer than the hold time of the previous heat treating method, also illustrated by the line segments  12 ,  14 . After the appropriate hold time, the previous heat treating method cools the entire component to below 1,000° F. over a short time period that may be about two hours, represented by the line segment  16 . The present invention cools the component over a time period totaling about three to ten hours, represented by the combination of the line segments  18  and  20 . During the initial phase of cooling, cooling is allowed to proceed slowly from the hold temperature, as indicated by the line segment  18 . At the point  22 , where the temperature of the heat treated material is about 1,200° F. to about 1,700° F., the component is cooled more rapidly, as indicated by the line segment  20 . As is well known in the art of heat treating, the slow cooling rate represented by the line segment  18  permits for continued diffusion of the molecules within the material, while the faster cooling rate illustrated by the line segment  20  limits further diffusion of the molecules. 
     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. 
       FIG. 6  illustrates a blade  24  of a combustion turbine, which is representative of a component that may be repaired using the present invention. The blade  24  includes a root depending downward from a platform  28 . An airfoil  30  extends upward from the platform  28 . When the blade  24  is installed within a turbine, the root  26  will be retained by the turbine discs using a fir-tree configuration and a locking mechanism. The tip  32  of the airfoil  30  will undergo the greatest stress during use, and is therefore the most likely location for crack formation. In such a case, after welding, only the heat affected zone  34  requires a heat treatment of the present invention, thereby preserving the metallurgy of the remainder of the blade  24 . If desired, a coolant such as argon gas may be applied to the region  36  of the airfoil  30 . 
     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.