Abstract:
A method of repairing a turbine blade and the blade repaired thereby. The blade comprises a platform that has become bowed as a result of high temperature creep, with the result that the platform has a concave surface and an oppositely-disposed convex surface. The method generally comprises welding the concave surface to build up a weldment on the concave surface, cooling the weldment during which the weldment shrinks, the convex surface becomes flatter, and the concave surface beneath the weldment becomes flatter, and then removing a surface portion of the weldment so as to create a substantially flat weldment surface overlying the substantially straightened concave surface.

Description:
BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to methods for repairing components subjected deformation as a result of high temperature creep, such as turbine blades of gas turbine engines. More particularly, this invention relates to a method of repairing a gas turbine engine blade whose platform has become bowed as a result of high temperature creep.  
           [0003]    2. Description of the Related Art  
           [0004]    High temperature nickel-base superalloys are widely used in the manufacture of components for the high temperature sections of gas turbine engines. Such components, which include nozzles, combustors, and turbine vanes and blades, are under strenuous high temperature conditions during engine operation that can lead to various types of damage or deterioration. Because the material and processing costs of superalloys are relatively high, restoration and repair of damaged or worn superalloy components are typically preferred to replacement. For this purpose, various weld repair methods have been developed, including those using tungsten inert gas (TIG) and plasma transferred arc (PTA) welding processes performed at room and elevated temperatures to improve welding yields and ensure that the mechanical properties of the superalloy are maintained. Particularly suitable welding processes, referred to as superalloy welding at elevated temperatures (SWET), are disclosed in U.S. Pat. Nos. 6,020,511, 6,124,568 and 6,297,474. SWET welding processes are performed within an enclosure in which a controlled atmosphere and temperature are maintained to inhibit cracking and oxidation of a superalloy component being repaired.  
           [0005]    A particular example of damage that can occur is the result of high temperature creep to which rotating gas turbine components are susceptible. Platforms of gas turbine engine blades can become bowed as a result of high temperature creep, necessitating scrappage of the blade if the bow is excessive. In view of the high material and processing costs of turbine blades, it would be desirable if blades that have sustained excessive platform bow could be repaired and restored to extend their service life.  
         SUMMARY OF INVENTION  
         [0006]    The present invention provides a method of repairing a turbine blade having a platform that has become bowed as a result of high temperature creep during engine operation, with the result that the platform has a concave surface and an oppositely-disposed convex surface. The method generally comprises welding the concave surface to build up a weldment thereon, and then cooling the weldment during which the weldment shrinks. During cooling, the convex surface becomes flatter and the concave surface beneath the weldment becomes flatter. A surface portion of the weldment is then removed so as to create a substantially flat surface that is defined by the weldment. In addition to the method, the present invention also encompasses the resulting repaired turbine blade.  
           [0007]    According to the above, a particular aspect of the method of this invention is that the weldment is built up on the concave surface of the platform, instead of the convex surface. While shrinkage of the weldment during cooling would suggest that the weldment would actually cause the concave surface to become more concave, thereby exacerbating the bow in the platform, the opposite has been shown to occur. While not wishing to be held to any particular theory, the cause for the straightening (flattening) of the platform is believed to be related to the mixing of the thermally-deformed metal (of the platform) with new metal (of the weldment) and the generation of in-plane stresses that cause the convex surface to contract and/or cause the concave surface to expand. These stresses are then believed to be maintained by the weldment remaining on the previously concave surface of the platform. To promote the ability of the weldment to maintain the restored shape of the platform, the weldment is preferably formed of a material of similar strength and properties to the platform. For example, both the platform and the weldment are formed of gamma-prime strengthened nickel-base superalloys.  
           [0008]    In view of the above, it can be seen that a significant advantage of this invention is that turbine blades whose platforms have been damaged as a result of high temperature creep can be repaired to extend their service life beyond that otherwise possible. The process is relatively uncomplicated, and provides a reliable and economical method to reducing maintenance costs.  
           [0009]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]    [0010]FIGS. 1 and 2 are an end view and cross-sectional view, respectively, of a turbine blade with a bowed platform.  
         [0011]    [0011]FIGS. 3 and 4 are end and partial cross-sectional views similar to FIGS. 1 and 2, respectively, and depict the blade as it appears with a weldment built up on the platform in accordance with the present invention.  
         [0012]    [0012]FIG. 5 is a partial cross-sectional view similar to FIG. 2, and depicts the result of post-weld cooling and machining of the platform in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    An example of a high pressure turbine blade  10  is represented in FIG. 1. The blade  10  has a platform  12  and an airfoil  14  that extends roughly perpendicular from an outer surface  16  of the platform  12 . The blade  10  also has a dovetail  20  (FIGS. 2 through 4) that extends roughly perpendicular from an oppositely-disposed inner surface  18  of the platform  12 , and serves to anchor the blade  10  to a turbine disk (not shown). Hot combustion gases are directed at the airfoil  14  and the outer surface  16  of the platform  12  during operation of the gas turbine engine in which the blade  10  is installed. Because the blade  10  is also subjected to high stresses during engine operation, the blade  10  is also subjected to high temperature creep. As a result of its severe operating conditions, the blade  10  is preferably formed from a nickel-base superalloy, more preferably a gamma-prime strengthened nickel base superalloy, though it is foreseeable that other materials could be used. Particularly suitable superalloys for the blade are gamma prime-strengthened alloys such as Ren é  80  and Ren é  142 , both of which are known alloys having high gamma prime content. Ren é  80  has the following nominal composition by weight: about 14% chromium, 9.5% cobalt, 4.8% titanium, 3% aluminum, 4% molybdenum, 4% tungsten, 0.17% carbon, 0.75% hafnium, 0.01% zirconium, and 0.015% boron, the balance nickel and incidental impurities. Ren e  142  has the following nominal composition by weight: about 12% cobalt, 6.8% chromium, 6.15% aluminum, 1.5% molybdenum, 4.9% tungsten, 6.35% tantalum, 2.8% rhenium, 1.5% hafnium, 0.12% carbon, and 0.015% boron, the balance nickel and incidental impurities. Both alloys are formulated as directionally-solidified (DS) alloys.  
         [0014]    In its original as-manufactured condition, the platform  12  and its surfaces  16  and  18  are substantially planar. In contrast, the platform  12  shown in FIG. 2 is represented as being deformed (bowed) as a result of high temperature creep, with the outer surface  16  of the platform  12  being convex and the inner surface  18  of the platform  12  being concave. As depicted in FIG. 2, the curvature of the bowed platform  12  is generally in a direction toward the airfoil  14  and away from the dovetail  20 . In accordance with the invention, the platform  12  and its surfaces  16  and  18  can be returned to their substantially planar as-manufactured condition by building up a weldment on the inner surface  18  as represented in FIGS. 3 and 4.  
         [0015]    In preparation for welding, the inner surface  18  of the platform  12  preferably undergoes a surface treatment (blending) to remove oxides, evidence of environmental attack, and any other surface contaminants that could interfere with the welding operation. The surface treatment may be performed using an abrasive hand tool or other suitable equipment. In FIGS. 3 and 4, the inner (concave) surface  18  of the platform  12  is shown as having been surface welded to build up a weldment  24  that extends out to the perimeter of the platform  12 , generally covering a crescent-shaped portion of the inner surface  18  as depicted in FIG. 3. As seen in FIGS. 3 and 4, the weldment  24  preferably avoids the fillet  28  at the intersection between the dovetail  20  and the inner surface  18 , since the fillet  28  is thicker and therefore more susceptible to cracking during welding. The material for the weldment  24  preferably has similar properties to the material for the blade  10 . For example, for a blade  10  formed of one of the previously-noted gamma-prime strengthened nickel base superalloy, the material for the weldment  24  is also preferably a gamma-prime strengthened nickel base superalloy, more preferably Ren é  80  or Ren é  142 . Notably, attempts to use solution-strengthened nickel-base superalloys to repair blades formed of gamma-prime strengthened nickel base superalloys have not produced the desired results.  
         [0016]    In order to be effective, the weldment  24  should not penetrate the entire thickness of the platform  12  (the distance between the outer and inner surfaces  16  and  18 ), but instead preferably penetrates roughly half the thickness of the platform  12 , e.g., to a depth of about 0.030 inch (about 0.8 mm) for a platform  12  having a typical thickness of about 0.065 inch (about 1.65 mm). In addition, the weldment  24  is preferably deposited in multiple adjacent weld passes, each along a substantially crescent-shaped path corresponding to the crescent-shaped edge of the weldment  24  shown in FIG. 3. The weldment  24  is also preferably deposited to a single weld-pass (bead) depth, and is thinner toward the ends of the weldment  24  (near the fillet  28  and the platform perimeter) and thicker at the middle where the surface  18  of the platform  12  is displaced farthest from its desired position. As a result, the weldment  24  has a generally crescent-shaped cross-section and the surface of the weldment  24  is roughly planar, as depicted in FIG. 4. The method of this invention does not require any weldment deposited on the outer (convex) surface  16  of the platform  12 .  
         [0017]    While various welding techniques may be capable of achieving the objects of this invention, the weldment  24  is preferably built up using the TIG welding techniques and apparatuses of the type disclosed in U.S. Pat. Nos. 6,020,511, 6,124,568 and 6,297,474, all of which are incorporated herein by reference. Conventional TIG welding techniques that do not provide the controlled atmosphere and temperature environment provided by the preferred TIG welding techniques have been found to create the desired movement of the platform  12 , but have caused cracking in the platform  12  during cooldown. A standard TIG power supply may be used, or a polarity-reversing plasma transferred arc (PTA) supply. Suitable approximate TIG welding parameters are summarized in Table I below.  
                   TABLE I                           Electrode   0.062 inch (1.6 mm) diameter pointed tungsten, 2%           thoriated       Torch Gas   20 to 25 CFH argon       Backup Gas   45 to 55 CFH argon (TIG welding enclosure)       Weld   Ren é 80 (0.030 inch (0.8 mm) diameter weld wire)       material       Weld current   20 to 35 amps                  
 
         [0018]    Following the welding operation, the blade  10  is allowed to cooldown in accordance with known practices to avoid weld-induced cracking. As a result of being welded in the manner described above, the platform  12  straightens during cooling to a degree that, aside from the weldment  24 , substantially reestablishes the original as-manufactured planar form of the platform  12 . FIG. 5 depicts the appearance of the blade  10  following cooldown from the welding operation and following blending of the surface of the weldment  24  with the adjacent exposed region of the surface  18 , to yield a substantially flat weldment surface  26  that is generally parallel to the underlying and now straightened inner surface  18  of the platform  12 . As such, the weldment  24  is significantly reduced in thickness, though substantially remaining on the surface region on which the weldment  24  was originally deposited. In practice, a remaining built-up weldment thickness of up to about 0.005 inch (about 0.1 mm) above the adjacent surface region is believed to be suitable, resulting in the platform  12  very nearly having its as-manufactured dimensions.  
         [0019]    The remaining weldment  24  is believed to not only straighten the platform  12 , but also strengthen the platform  12  to inhibit re-bowing. The strength of a gamma-prime strengthened nickel base superalloy, such as one of the two alloys noted above, is particularly effective for this reason. The mechanism by which the platform  12  becomes straightened is not well understood. However, several events are believed to contribute to the desired results. First, the welding operation results in the mixing within the weld penetration zone of the metal damaged by high temperature creep and the new metal (weldment  24 ) deposited by welding. The deformation in the platform  12  is observed to be corrected as the weldment  24  shrinks during cooling, and this shrinkage is believed to induce tensile stresses in the original metal within the platform  12  that pull the platform  12  toward its original contour. These tensile stresses are believed to be induced by the weldment  24  in the outer surface  16 , while compressive stresses are induced in the inner surface  18 . Once the original shape of the platform  12  is reestablished, blending of the weldment  24  enables the original dimensions of the platform  12  to be nearly reestablished, yet allows sufficient weldment  24  to remain to maintain the desired shape. Engine testing of blades formed of and repaired with Ren e  80  in accordance with the parameters disclosed in Table I above have shown that the restored platform shape is not lost during a period in which platforms of identical new blades are bowed beyond acceptable service limits.  
         [0020]    While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.