Abstract:
During a solution heat treatment grain recrystallization may occur of a textured article, especially at the surface rim. The present invention provides a method for restoring the microstructure of a textured article, such as a gas turbine engine blade, which comprises coating the surface of the article with a high temperature stable surface coating or by a controlled development of an oxide scale and subsequently performing a solution heat treatment, thereby maintaining said thermally stable surface coating.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application claims priority to the provisional application Ser. No. 60/286,662, filed on Apr. 27, 2001, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to textured articles and in particular to the restoration of the microstructure of textured articles. The invention is further related to the field of refurbishment of gas turbine airfoils. 
     BACKGROUND OF THE INVENTION 
     An example of the background technology of the present invention is U.S. Pat. No. 5,611,670 which shows a gas turbine blade made of a nickel based superalloy. Superalloys are nickel- or cobalt-based alloys, typically comprising chromium, titanium, tantalum, aluminum, tungsten and other elements, with excellent high temperature resistance, thereby maintaining high strength properties. Accordingly, superalloys are widely used in high temperature applications where additionally high mechanical strength is required. A typical application is the casting of airfoils for gas turbines, jet engines as well as stationary gas turbines, e.g. for industrial applications like power generation. Further improvements in mechanical strength is achieved by casting the superalloy as a columnar or as a single crystal. A textured article has no or very few grain boundaries. 
     As additional background, European Patent Application EP 1 038 982 A1 describes a process for manufacturing single crystal superalloy articles. After casting the article is subjected to a heat treatment in order to further improve the mechanical strength. The heat treatment is a high temperature solution heat treatment which homogenizes the microstructure of the alloy itself formed by different crystal phases. However, this heat treatment may lead to a grain re-crystallization occurrence, initiated by dislocations in the crystal structure. This grain re-crystallization destroys locally the single crystal structure which may lead to a dramatic decrease in the mechanical strength of the article. Accordingly, grain re-crystallization is a cause for rejection of single crystal castings if present beyond a preset maximum for re-crystallized grains and can result in low yields of acceptable heat-treated single crystal castings. By heat treating in a carburizing atmosphere, carbon is introduced into the casting and forms carbides therein that reduce or localize the occurrence of grain re-crystallization. 
     European Patent EP 0 525 545 B1 describes the refurbishment of corroded superalloy articles. In particular gas turbine airfoils are subjected to corrosion by hot gases. Typically, a corrosion protective coating is provided on the body made from the superalloy. Widely used coatings are of the MCrAlY type, where M is iron, cobalt and/or nickel, and Y, for yttrium or another rare earth element or another element such as lanthanum. This type of coating is usually applied by a plasma spray process. However, despite a corrosion protective coating, the airfoils are still under corrosion and erosion attack which leads to the need for servicing after a certain period of time. Corrosion results from contaminants in the fuel and/or air, and oxidation may also occur at high temperatures. Depending on the conditions of operation, an oxide layer of varying thickness may form on the surface of the airfoil. Also, and very significantly, sulfur can penetrate into the base material to form sulfides. Furthermore, internal oxides and nitrides may form within the metal near the surface. 
     Instead of completely exchanging airfoils, it is often a cost saving option to refurbish the airfoils, i.e. providing a new protective coating. This requires complete removal of the old coating, which is realized by applying mechanical stripping as well as chemical treatment, e.g. with acid. After removal of a substantial part of the old coating, the surface is aluminized. Subsequently, the aluminide layer is removed, thereby also removing oxidized and corroded regions at the surface. 
     U.S. Pat. No. 5,413,648 discloses a directionally solidified article with a plastic deformation damage at the surface, which is prone to recrystallization. This problem is overcome by removing a part of the deformed surface region. 
     European patent No. EP 1 036 850 A1 discloses a single crystal having a surface coating for preventing recrystallization fracture by reinforcing the grain boundaries. After a heat treatment the texture of the article at the surface shows no single crystal structure anymore, because the surface has grain boundaries, which are reinforced by grain boundary strengthening elements like Zr, Hf, B or C. 
     U.S. Pat. No. 6,271,668 shows the need of a coating during one step of refurbishment of gas turbine components, which is applied on the surface of the component. The coating is heat treated by which a surface region of the article is aluminized. This heat treatment is performed at low temperatures in order to avoid detrimental diffusion of atoms from corrosion products. The coating is removed together with the corroded layers before further heat treatments are performed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for restoring the microstructure of a textured article, e.g. a single crystal or a directionally solidified article, which comprises creating on the surface of the article a high temperature stable surface coating and subsequently performing a solution heat treatment, thereby maintaining said thermally stable surface coating. 
     As stated previously, grain recrystallization may occur during a solution heat treatment of the article. The present invention underlies the discovery that grain recrystallization occurs at lower temperatures at the surface of an article compared to bulk regions. The energy needed for forming new grains with grain boundaries is lower at the surface. By applying a coating on the surface and maintaining this surface coating during the solution heat treatment, grain recrystallization is suppressed due to the now provided bulk conditions. Accordingly, an effective solution heat treatment can be processed, thereby restoring the microstructure of the textured article without introducing grain recrystallization. A full description of the effect of suppressing grain recrystallization by surface coating was given by one of the applicants in the publication “Recrystallization In Single Crystals Of Nickel Base Superalloys”, R. Bürgel, P. D. Portella, J. Preuhs, Superalloys 2000, edited by T. M. Pollock, pages 229-238, the teaching of which is incorporated herein by reference, in particular with respect to the alloy compositions disclosed in table I (balance Ni) and heat treatment parameters in table II. 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Cr 
                 Co 
                 Mo 
                 W 
                 Ta 
                 Nb 
                 Al 
                 Ti 
                 C 
                 Hf 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 CMSX-11B 
                 12.5 
                 7 
                 .5 
                 5 
                 5 
                 .1 
                 3.6 
                 4.2 
                 — 
                 .04 
               
               
                 PWA 1483 
                 12.2 
                 9 
                 1.9 
                 3.8 
                 5 
                 — 
                 3.6 
                 4.1 
                 .07 
                 — 
               
               
                 SRR 99 
                 8.5 
                 5 
                 — 
                 9.5 
                 2.8 
                 — 
                 5.5 
                 2.2 
                 0.2 
                 — 
               
               
                 CMSX-6 
                 10 
                 5 
                 3 
                 — 
                 2 
                 — 
                 4.8 
                 4.7 
                 — 
                 .08 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
             
             
               
                 CMSX- 
                 solutioning, 
                 1204° C./2 h + 1227° C.2/h + 1249° C./3 h + 
               
               
                 11B 
                 SHT* 
                 1260° C./6 h; heating with 1 K/min 
               
               
                   
                 age hdn. 
                 1120° C./5 h + 870° C./24 h + 760° C./30 h 
               
               
                 PWA 
                 solutioning* 
                 1260° C./1 h 
               
               
                 1483 
                 age hdn. 
                 1090° C./4 h 
               
               
                 SRR 99 
                 solutioning* 
                 1270° C./0, 5 h + 1280° C./1 h + 1290° C./ 
               
               
                   
                   
                 2 h + 1300° C./0, 5 h + 1305° C.0, 5 h; 
               
               
                   
                   
                 heating with 1 K/min 
               
               
                   
                 age hdn. 
                 1080° C./4 h + 870° C./16 h 
               
               
                 3 CMSX- 
                 solutioning* 
                 1227° C./2 h + 1238° C./2 h + 1271° C./2 h + 
               
               
                 6 
                   
                 1277° C./3 h + 1280° C./2 h; heating 
               
               
                   
                   
                 with 1 K/min 
               
               
                   
                 age hdn. 
                 1080° C./4 h + 870° C./16 h 
               
               
                   
               
               
                 *SHT: Solution heat treatment. Additional steps at lower temperatures may precede the solutioning to equilibrate the furnace.  
               
             
          
         
       
     
     The article is preferably made from a superalloy, which may be nickel-based or cobalt-based. The microstructure in such a nickel-based superalloy is formed by a γ-phase and a γ′-phase. The temperature required for solution heat treatment is considerably high and is at least the solution temperature of the γ′-phase. By maintaining such a temperature during the solution heat treatment, an effective restoring of the microstructure is achieved. This high temperature normally increases the risk of recrystallization. This risk is substantially reduced by applying the surface coating before performing the solution heat treatment. The temperature of the solution heat treatment is preferably above 1100° C., more preferred above 1150° C. and even more preferred above 1200° C. 
     Preferably, the article is a gas turbine blade or vane. The rejection rate of textured castings because of grain recrystallization is in particular high for gas turbine blades because very high mechanical stresses resulting from centrifugal forces require high mechanical strength which is strongly influenced and decreased by grain recrystallization. Moreover thermal fatigue cracking of blades or vanes may start from non-textured areas. 
     Preferably, the surface coating is an aluminide coating, which is preferably placed on the surface by a Chemical Vapor Deposition (CVD) process, which is well known in the art, e.g. for aluminizing articles for establishing corrosion resistance. 
     Some preferred composition of the surface coating with its main components are listed here (weight prozent): 
     Ni: 24%, Cr: 17%, Al: 45%, 
     Ni: 49%, Cr: 4%, Al: 37%, 
     Ni: 49%, Cr: 7%, Al: 35%. 
     The Al-content should be at least 2 wt % for an aluminide coating. 
     The surface coating could also be for instance an oxide film, e.g. developed by oxidation of the surface of the article, e.g.: alumina, NiO, chromoxide or mixtures of it. The oxidation of the article can be performed in air or in certain atmospheres with a given oxygen partial pressure in a pre-treatment leading to an oxide coating and then performing 3a solution heat treatment. The oxidation of the article can also be performed in one step during heat-up of the solution heat treatment. First an atmosphere suitable for oxidation is present and after oxidation has occurred a vacuum is applied in order not to oxidise the blade material too much. 
     Nevertheless the whole solution heat treatment can also be performed in a controlled oxidizing atmosphere. The oxidation of the surface is especially useful, if the article shows a plastic deformation of the surface, e.g. by strong grinding or other mechanical surface work such as shot peening. Nevertheless a thermally stable oxide layer can be applied to the surface by plasma spraying without oxidation of the surface of the article. 
     The invention is in particular useful for the refurbishment of a gas turbine blade. The invention provides a method for refurbishing a gas turbine blade made from a textured superalloy body coated with a protective coating and comprising the following subsequent steps: Coating the protective coating with a high temperature stable surface coating; performing a solution heat treatment of the superalloy, thereby maintaining the thermally stable surface coating; removing jointly the surface coating and the protective coating and finally providing a new protective coating on the body for the next engine operation period. 
     This refurbishment not only includes the re-coating of a protective coating system but also includes a solution heat treatment for re-establishing the full mechanical strength properties of the textured structure. Since the gas turbine blade was already subjected to erosion and corrosion attack, plastic deformations in the surface rim are likely to occur. As mentioned above, those plastic deformations are a source of grain recrystallization. Accordingly, in the past, the refurbishment did not include full solution heat treatment of the gas turbine blade because grain recrystallization would have occurred. The heat treatment had to be restricted to a lower temperature heat treatment for bonding the new protective coating. Restoring of the microstructure was not or not completely possible with such a treatment. 
     Now, a solution heat treatment can be performed without recrystallization because the surface coating or the oxide film also covers areas where the old protective coating is eroded and establishes bulk conditions for the gas turbine blade body which leads to a higher temperature threshold for grain recrystallization. 
     Depending on the actual condition of the old protective coating, the following method can be applied instead of the above-mentioned method. The following subsequent steps are performed, achieving the same advantages mentioned above: removing the protective coating; coating the surface of the gas turbine blade or vane with a high temperature stable surface coating; subsequently performing a solution heat treatment, thereby maintaining the thermally stable surface coating; removing the surface coating and providing a new protective coating on the body. 
     The invention is explained in greater detail below and by reference to exemplary embodiments shown in the drawings wherein like numerals refer to equivalent elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1,  2  and  3  show schematically the microstructure of a superalloy. 
     FIG. 4 is a gas turbine blade. 
     FIGS. 5 to  11  shows several steps of the method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention involves heat treating single crystal or directionally solidified articles made from a nickel-based superalloy. 
     A schematic view of a single crystal article  1  made from a nickel-based alloy is shown in FIG.  1 . The article has a body  3 . According to the single crystal structure, no grain boundaries are present. In the case of directionally solidified articles (FIG. 2) there are grain boundaries  35  only along the z-axis  33 . The z-axis is e.g. a radial direction of a turbine blade when in use in a gas turbine. The grain boundaries extend mainly along dotted lines along the z-axis  33 . 
     However, the alloy has a texture formed by mainly two crystal phases  5 , 7 . The first phase  5  is a γ-phase, the second phase is a γ′-phase, both crystal structures are well known to those skilled in the art. 
     The body  3  has a surface  10  on which a surface coating  11  can be deposited e.g. by a Chemical Vapor Deposition (CVD) process with a CVD coating apparatus. The surface coating  11  can be an aluminide coating. Further shown is a lattice defect  9 , i.e. a dislocation. Such lattice defects may be caused from mechanical impact on the surface. The mechanical strength of the alloy is influenced by the homogeneity of the microstructure. Therefore, a solution heat treatment is applied at a temperature or a range of temperatures to the article which means holding a high temperature for a certain time, e.g. 1 to 5 hours. For an efficient solution heat treatment, the temperature has to be at least the solution temperature of the γ′-phase  7 . By applying this solution heat treatment, a restoration of the microstructure is established. 
     However, prior art solution heat treatment had as a consequence a significant risk of introducing grain recrystallization. This means that areas, i.e. grains, with different lattice orientations develop. Accordingly, grain boundaries are formed which lower the mechanical strength. When a certain depth or amount of recrystallization is exceeded, the article has to be rejected. 
     FIG. 3 illustrates this recrystallization. A new grain  15  is developed by growth into the original grain  13 . A high dislocation concentration  9  as shown in FIG. 1 is the source for the recrystallization. 
     As a discovery underlying the invention, the occurrence of recrystallization can be suppressed by the surface coating  11 . The energy for developing new grains and thereby storing energy in the grain boundaries is higher in the bulk as compared to the surface. The coating  11  implements bulk conditions at the surface  10 , thereby increasing the threshold temperature for recrystallization. Accordingly, a solution heat treatment without recrystallization can be performed. 
     The surface coating  11  can also be generated by oxidation during a heat treatment in air or in a atmosphere with an oxygen partial pressure. The surface coating  11  is generated from the material of a surface rim of the article  1 . 
     A gas turbine blade  21  made from a single crystal nickel-based superalloy is shown in FIG.  4 . The body  23  of the blade  21  is coated with a corrosion protective coating  29 . On the protective coating  29 , a thermal barrier coating  31  is placed. After a certain time period of operation, the blade  23  needs to be refurbished because the coating system is at least partially worn off by oxidation, corrosion and erosion. 
     The thermal barrier coating  31  and/or the protective coating  29  is removed by chemical and/or mechanical stripping, e.g. by grinding. During this procedure of mechanical stripping often a surface damaged surface rim is generated, usually with a depth of 20 μm from the surface. 
     The surface coating  11  is deposited on the remainder of the protective coating  29 , thereby closing the complete surface of the body  23 . Subsequently the solution heat treatment is applied, as described above. After the heat treatment, the surface coating  11 , the protective coating  29  and the thermal barrier coating  31  are removed by mechanical stripping and chemical treatment and a new protective coating as well as a new thermal barrier coating are provided. 
     Another embodiment, the protective coating  29  and the thermal barrier coating  31  could be removed first, then the surface coating deposited and the solution heat treatment performed. Subsequently, a new protective coating as well as a new thermal barrier coating are provided. 
     FIGS. 5 to  11  show schematically the steps of restoring the microstructure of a textured article and/or the refurbishing of a textured article. FIG. 5 shows the article  1  with the body  3  having a dislocation accumulation  9 . On the body  3  there is e.g. a layer  38  with corrosion products on which the protective layer  29  and thermal barrier coating  30  is laying. 
     In a first step of the method of the invention, the protective layer  29 , the thermal barrier coating  30  and the corrosion products in the layer  38  are removed (FIG.  6 ). Secondly, on the surface  10  of the body  3  the surface coating  11  is applied by a coating apparatus  42 .(FIG. 7) Different kinds of coating apparatus or processes can be used. The surface coating  11  can also be applied by oxidising the body  3  at higher temperatures HT (FIG. 8) in an atmosphere with oxygen O 2 . The article with the surface coating  11  according to FIGS. 7,  8  is now applied to a solution heat treatment as indicated in FIG.  9 . The thermal stable surface coating  11  is maintained during the hole solution heat treatment. 
     FIG. 10 shows the article  1  with his body  3  after the solution head treatment and removal of the surface coating  11 . No grain boundary inside the single crystal or no additional grain boundary inside the directionally solidified article  1 , especially in the surface rim is present. A new protective layer  29  and a new thermal barrier coating  30  can now be applied again on the body  3  of the article  1  (FIG.  11 ). 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications would be obvious to one skilled in the art intended to be included within the scope of the following claims.