Abstract:
A method of producing a component wherein a directionally solidified columnar grained cast superalloy material is heat treated such that a secondary phase of the alloy is only partly solved, thereby providing improved transverse stress rupture strength compared to fully solved alloys.

Description:
[0001]    This application is a continuation of U.S. application Ser. No. 10/641,995, filed 15 Aug. 2003 and incorporated by reference herein, which is the U.S. National Stage of International Application No. PCT/EP02/11856, file 23 Oct. 2002 and is also a continuation-in-part of U.S. application Ser. No. 10,429,950, filed 5 May 2003 (now abandoned), which in turn is a continuation of U.S. application Ser. No. 09/103,097, filed 23 Jun. 1998 (now abandoned). 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a heat treatment of alloys, especially nickel base superalloy and, more particularly, to castings having a columnar grain microstructure. 
       BACKGROUND OF THE INVENTION 
       [0003]    U.S. Pat. No. 4,597,809 describes single crystal castings made from a nickel base superalloy having a matrix with a composition consisting essentially of, in weight %, of 9.5% to 14% Cr, 7% to 11% Co, 1% to 2.5% Mo, 3% to 6% W, 1% to 4% Ta, 3% to 4% Al, 3% to 5% Ti, 6.5% to 8% Al+Ti, 0% to 1% Nb, and balance essentially nickel with the matrix containing about 0.4 to about 1.5 volume of a phase based an tantalum carbide as a result of the inclusion in the alloy of about 0.05% to about 0.15% C and extra Ta in an amount equal to 1 to 17 times the C content. 
         [0004]    Single crystal castings produced from the aforementioned nickel base superalloy exhibit inadequate transverse grain boundary strength. The present inventors attempted to produce directionally solidified (DS) columnar grain castings of the nickel base superalloy. However, the directionally solidified (DS) columnar grain castings produced were unacceptable as DS castings as a result of the castings exhibiting essentially no transverse grain boundary strength and no ductility when tested at a temperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi). The transverse grain boundary strength and ductility were so deficient as to render DS columnar grain castings produced from the aforementioned nickel base superalloy unsuitable for use as turbine blades of gas turbine engines. 
         [0005]    WO 99/67435 discloses nickel base superalloy castings having boron added to improve transverse stress rupture strength and ductility of DS castings. The castings are heat treated at 1250° C. for 4 h so that a full solution of the secondary phase (γ′-phase) is performed. Due to the occurrence of grain boundary cracks after the full solution heat treatment, the producibility is so deficient as to render DS columnar grain castings produced from the aforementioned nickel base superalloy unsuitable for use as turbine blades of gas turbine engines. 
         [0006]    An object of the present invention is to provide a heat treatment of alloys, especially of as-cast alloys, e.g. DS columnar grain castings based on the aforementioned single crystal nickel base superalloy, having substantially improved transverse stress rupture strength and ductility as well as producibility to an extent that the DS castings are acceptable for use in high temperature applications such as turbine blades of a gas turbine engine. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention involves a novel heat treatment of cast alloys, such as superalloys, having at least one addition, such as boron, which improves grain boundary strength in the nickel base superalloy. In order to significantly improve transverse stress rupture strength and ductility of directionally solidified (DS) columnar grain castings of such alloys, the present invention innovatively utilizes a heat treatment which solves a secondary phase only partly, e.g. no full solution heat treatment is performed. Boron is often added to superalloy compositions in an effective amount to substantially improve transverse stress rupture strength and ductility of directionally solidified columnar grain castings produced from the boron-modified superalloy. The boron concentration preferably is controlled in the range of about 0.003% to about 0.0175% by weight of the superalloy composition to this end. In conjunction with addition of boron to the superalloy composition, the carbon concentration preferably is controlled in the range of about 0.05% to about 0.11% by weight of the superalloy composition. When using such alloys to produce cast components, the present invention innovatively processes the casting through only a partial solution heat treatment of a secondary phase, thereby unexpectantly providing improved producibility when compared to the prior art process of full solution heat treating of such alloy castings. 
         [0008]    A preferred nickel base superalloy in accordance with an embodiment of the present invention consists essentially of, in weight %, of about 11.6% to 12.70% Cr, about 8.50% to 9.5% Co, about 1.65% to 2.15% Mo, about 3.5% to 4.10% W, about 4.80% to 5.20% Ta, about 3.40% to 3.80% Al, about 3.9% to 4.25% Ti, about 0.05% to 0.11% C, about 0.003% to 0.0175% B, and balance essentially Ni. The boron modified nickel base superalloy can be cast as DS columnar grain castings pursuant to conventional DS casting techniques such as the well known Bridgman mould withdrawal technique. 
         [0009]    DS castings produced in this manner typically have a plurality of columnar grains extending in the direction of the principal stress axis of the casting with the &lt;001&gt; crystal axis generally parallel to the principal stress axis. DS columnar grain castings pursuant to the present invention preferably exhibit a stress rupture life of at least about 100 hours and elongation of at least about 2.5% when tested at a temperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi) and will find use as turbine blades, vanes, outer air seals and other components of a industrial and aero gas turbine engines. 
         [0010]    The above objects and advantages of the present invention will become more readily apparent form the following detailed description. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Exemplarily, a nickel base superalloy is chosen which consists essentially of, in weight %, of about 9.5% to 14% Cr, about 7% to 11% Co, about 1% to 2.5% Mo, about 3% to 6% W, about 1% to 6% Ta, about 3$ to 4% Al, about 3% to 5% Ti, about 0% to 1% Nb, and balance essentially Ni and B present in an amount effective to substantially improve transverse stress rupture strength of a DS casting as compared to a similar casting without boron present. 
         [0012]    The inclusion of boron, as an addition, which improves the grain boundary strength in the alloy, is chosen in an amount effective to provide substantial transverse stress rupture strength and ductility of a DS columnar grain casting produced from the alloy as compared to a similar casting without boron present. 
         [0013]    Preferably, the nickel base superalloy is modified by the inclusion of boron B in the range of about 0.003% to about 0.0175%, preferably 0.010% to 0.015%, by weight of the superalloy composition to this end. 
         [0014]    In conjunction with addition of boron to the superalloy composition, the carbon C concentration is controlled in a preferred range of about 0.05% to about 0.11% by weight of the superalloy composition. Also Silizium Si, Zirkonium Zr and Hafnium Hf can be used as addition. 
         [0015]    Furthermore all combinations of B, C, Si, Zr, Hf are possible. 
         [0016]    The transverse stress rupture strength and ductility as well as the producibility of DS castings produced from nickel base superalloy with the modified heat treatment first described herein are provided to an extent that the castings are rendered commercially acceptable for use as turbine blades and other components of gas turbine engines. 
         [0017]    A particularly preferred boron-modified nickel base superalloy casting composition consists essentially of, in weight %, of about 11.6% to 12.70% Cr, about 8.5% to 9.5% Co, about 1.65% to 2.15% Mo, about 3.5% to 4.10% W, about 4.80% to 5.20% Ta, about 3.40 to 3.80% Al, about 3.9% to 4.25% Ti, about 0.05% to 0.11% C, about 0.003% to 0.0175% B, and balance essentially Ni and castable to provide a DS columnar grain microstructure. 
         [0018]    The DS microstructure of the columnar grain casting typically includes about 0.4 to about 1.5 volume % of a phase based an tantalum carbide. 
         [0019]    Although not wishing to be bound by any theory, it is thought that boron and carbon tend to migrate to the grain boundaries in the DS microstructure to add strength and ductility to the grain boundaries at high service temperatures, for example 816 degrees C. (1500 degrees F.) typical of gas turbine engine blades. DS columnar grain castings produced from the above boron modified nickel base superalloy typically have the &lt;001&gt; crystal axis parallel to the principal stress axis of the casting and exhibit a stress rupture life of at least about 100 hours and elongation of at least about 2.5% when tested at a temperature of 750 degrees C. (1382 degrees F.). and stress of 660 MPa (95.7 Ksi) applied perpendicular to the &lt;001&gt; crystal axis of the casting. 
         [0020]    For example, the following DS casting tests were conducted and are offered to further illustrate, but not limit, the present invention. 
         [0021]    An alloy #1 having a nickel base superalloy composition in accordance with the aforementioned U.S. Pat. No. 4,597,809 and alloys #1A and #2 and #3 of boron modified nickel base superalloy were prepared with the following compositions, in weight percentages, set forth in Table I: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Alloy 
                 Cr 
                 Co 
                 Mo 
                 W 
                 Ta 
                 Al 
                 Ti 
                 C 
                 B 
                 Ni 
               
               
                   
               
             
             
               
                 #1 
                 12.1 
                 9.0 
                 1.8 
                 3.7 
                 5.2 
                 3.6 
                 4.0 
                 0.07 
                 0.001 
                 balance 
               
               
                   #1A 
                 12.1 
                 9.0 
                 1.8 
                 3.7 
                 5.2 
                 3.6 
                 4.0 
                 0.08 
                 0.010 
                 balance 
               
               
                 #2 
                 12.1 
                 9.0 
                 1.8 
                 3.7 
                 5.2 
                 3.6 
                 4.0 
                 0.09 
                 0.011 
                 balance 
               
               
                 #3 
                 12.1 
                 9.0 
                 1.8 
                 3.7 
                 5.2 
                 3.6 
                 4.0 
                 0.08 
                 0.014 
                 balance 
               
               
                   
               
             
          
         
       
     
         [0022]    Each alloy was cast to form DS columnar grain non-cored castings having a rectangular shape for transverse stress rupture testing pursuant to ASTM E-139 testing procedure. 
         [0023]    The DS castings were produced e.g. using the conventional Bridgman mould withdrawal directional solidification technique. 
         [0024]    For example, each alloy was melted in a crucible of a conventional casting furnace under a vacuum of 1 micron and superheated to 1427 degrees C. (2600 degrees F.). The superheated alloy was poured into an investment casting mould having a face coat comprising zircon backed by additional slurry/stucco layers comprising zircon/alumina. The mould was preheated to 1482 degrees C. (2700 degrees F.) and mounted on a chill plate to effect unidirectional heat removal from the molten alloy in the mould. The melt-filled mould on the chill plate was withdrawn from the furnace into a solidification chamber of the casting furnace at a vacuum of 1 micron at a withdrawal rate of 6-16 inches per hour. 
         [0025]    The DS columnar grain castings were cooled to room temperature under vacuum in the chamber, removed from the mould in conventional manner using a mechanical knock-out procedure. 
         [0026]    It is noted that the data for Alloys #1, #1A and #2 contained herein are identical to that described in prior art International Application WO 99/67435 by inventors Winfried Esser, et al. After being cast, Alloys #1, #1A and #2 were heat treated at 1250 degrees C. (2282 degrees F.) for 4 hours (full solution heat treatment of the secondary γ′ phase) as was described in WO 99/67435. The present inventors, including said Winfried Esser, have now recognized the benefit of using only a partial heat treatment regiment, resulting in the data for Alloy #3 first presented herein. The alloy for Heat #3 was heat treated at a temperature and for a duration in such way that the solution of a secondary phase in the matrix is only partly performed. 
         [0027]    The nickel based superalloy has as a secondary phase the γ′-phase. 
         [0028]    The inventive heat treatment is performed at 1213° C. for at least 1 h, which is not the solution temperature of a secondary phase (e.g. y′ phase) for this alloy. 
         [0029]    Also the temperature of 1250° C. (called the full solution temperature), which is normally used for a full solution treatment, can be used, but only for a shorter time than in the prior art so that the secondary phase is not completely solved in the matrix. 
         [0030]    The not solubilized amount of the secondary phase in the matrix is smaller than 90, 70, 50 or 30 vol % according to the geometry and producibility after the heat treatment, because grain boundary cracks are avoided, in order to increase the yield rate of specimens and desired mechanical properties of the specimen. 
         [0031]    The alloy can have a single crystal structure or only having grains along one direction. 
         [0032]    Optionally an ageing heat treatment can be performed for this composition at 1080° C. for at least 2 h after this solution heat treatment. Optionally followed by a second ageing heat treatment at 870° C. for at least 12 h. 
         [0033]    Especially the inventive heat treatment is used for hollow specimen, especially blades, vanes, or liners because cracks do appear more often in walls, especially in thin walls, than in massive specimens after the normally used heat treatment after casting. 
         [0034]    The inventive heat treatment leads to an increased grain boundary strength during this heat treatment, so that the yield rate (components without cracks) after the heat treatment is increased. 
         [0035]    Also the transverse stress rupture of the component as final product is increased during use of the component at working conditions, because grain boundary strength is increased. 
         [0036]    The inventive method yields also good results for massive components, e.g. of a gas turbine. 
         [0037]    The castings were also analysed for chemistry, and machined to specimen configuration. 
         [0038]    Stress rupture testing was conducted in air at a temperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi) applied perpendicular to the &lt;001&gt; crystal axis of the specimens. 
         [0039]    The results of stress rupture testing are set forth in TABLE II below where LIFE in hours (HRS) indicates the time to fracture of the specimen, ELONGATION is the specimen elongation to fracture, and RED OF AREA is the reduction of area of the specimens to fracture. The test data of Table II for Alloys #1, #1A, #2 and #3 correspond to Alloys #1, #1A, #2 and #3, respectively of TABLE I. The Alloy #1 data represent an average of two stress rupture test specimens, while the #1A, #2 and #3 data represent a single stress rupture test specimen. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
                   
                   
                 Stress 
                   
                   
                   
               
               
                   
                 #of 
                 Temperature 
                 MPa 
                 Life 
                 Elonga- 
                 Red of 
               
               
                 Alloy 
                 Tests 
                 ° C. (° F.) 
                 (KSI) 
                 (HRS) 
                 tion (%) 
                 Area (%) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 #1 
                 2 
                 750 (1382) 
                 660 (95.7) 
                 0 
                 0 
                 0 
               
               
                 prior art 
               
               
                   #1A 
                 1 
                 750 (1382) 
                 660 (95.7) 
                 275 
                 3.1 
                 4.7 
               
               
                 prior art 
               
               
                 #2 
                 1 
                 750 (1382) 
                 660 (95.7) 
                 182 
                 2.6 
                 6.3 
               
               
                 prior art 
               
               
                 #3 
                 1 
                 750 (1382) 
                 660 (95.7) 
                 173 
                 3.7 
                 10.7 
               
               
                 inven- 
               
               
                 tion 
               
               
                   
               
             
          
         
       
     
         [0040]    It is apparent from TABLE II that the DS columnar grain specimens produced from heat #1 exhibited in effect essentially no (e.g. zero hours stress rupture life) transverse grain boundary strength when tested at a temperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi). That is, the specimens failed immediately to provide an essentially zero stress rupture life. Moreover, the elongation and reduction of area data were essentially zero. These stress rupture properties are so deficient as to render the DS columnar grain castings produced from heat #1 unacceptable for use as turbine blades of gas turbine engines. 
         [0041]    In contrast, TABLE II reveals that DS columnar grain specimens produced from heat #1A exhibited a stress rupture life of 275 hours, an elongation of 3.1%, and a reduction of area of 4.7 and specimens from heat #2 exhibited a stress rupture life of 182 hours, an elongation of 2.6%, and a reduction of area of 6.3% when tested at a temperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi). 
         [0042]    The present invention, revealed in Alloy #3 which underwent only a partial solutioning of the secondary phase, is effective to provide DS columnar grain castings with substantially improved transverse stress rupture strength and ductility; as evidenced by the dramatically improved Elongation and Reduction of Area test results in Table II. These properties are achieved without adversely affecting other mechanical properties, such as tensile strength, creep strength, fatigue strength, and corrosion resistance of the DS castings. The present invention is especially useful to provide large DS columnar grain industrial gas turbine (IGT) blade castings which have the alloy composition described above to impart substantial transverse stress rupture strength and ductility to the castings and which have a length of about 20 centimeters to about 60 centimeters and above, such as about 90 centimeters length, used throughout the stages of the turbine of stationary industrial gas turbine engines. The above described boron-modified nickel base superalloy casting composition can be cast as DS columnar grain or single crystal components. 
         [0043]    While the invention has been described in terms of specific embodiments thereof, it is not intended to be limited thereto but rather only to the extent set forth in the following claims.