Abstract:
A heat treatment assembly and heat treatment methods are disclosed for producing different microstructures in the bore and rim portions of nickel-based superalloy disks, particularly suited for gas turbine applications. The heat treatment assembly is capable of being removed from the furnace and disassembled to allow rapid fan or oil quenching of the disk. For solutioning heat treatments of the disk, temperatures higher than that of this solvus temperature of the disk are used to produce coarse grains in the rim of each disk so as to give maximum creep and dwell crack resistance at the rim service temperature. At the same time, solution temperature lower than the solvus temperature of the disk are provided to produce fine grain in the bore of the disk so as to give maximum strength and low cycle fatigue resistance.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435;42 U.S.C.2457). 
     FIELD OF THE INVENTION 
     The invention relates to an apparatus and method of operation thereof for heat treating a disk so as to produce a dual microstructure superalloy disk particularly suited for gas turbine applications. 
     BACKGROUND OF THE INVENTION 
     There are numerous incidents where operating conditions experienced by an article, or a component of a machine, place different material property requirements on different portions of the article or component. Examples include a crank shaft in an internal combustion engine, a piston rod in a hydraulic cylinder, planatary gears for an automotive transmission, and a turbine disk for a gas turbine engine. Gas turbine disks are often made from nickel-base superalloys, because these disks need to withstand the temperature and stresses involved in the gas turbine cycle. In the bore portion of the disk where the operating temperature is somewhat lower, the limiting material properties are often tensile strength and low-cycle fatigue resistance. In the rim portion of the disk, where the operating temperatures are higher than those of the bore, because of the proximity to the combustion gases, resistance to creep and cracking are the limiting properties. 
     Advanced nickel-base, gamma prime strengthened superalloys have been introduced to the field that allow improved engine performance through higher disk temperatures as compared to current engines. This is achieved by using high levels of gamma prime and refractory elements. However, there is a long term need for disks with higher rim temperature capabilities of 1400° F. or more. This increased temperature capability would allow higher compressor exit temperatures of a gas turbine and allow the full utilization of advanced combustion and airfoil concepts for aerodynamic applications. These disks require high creep resistance and dwell crack growth resistance of coarse grain microstructures in the rim region near 1400° F., while still maintaining the high strength and low cycle fatigue resistance of fine grain microstructures in the bore region near 800-1200° F. 
     The chief determinant of achieving grain size in powder metallurgy superalloy disks is the temperature at which the alloy is solution heat treated. As is known in the art, solution heat treatment is concerned with the solvus temperature; i.e., the temperature at which all of the gamma prime strengthening precipitate of the superalloy goes into solution. To perform the desired solution heat treatment in this invention, it is necessary to solution heat treat the disk in a way whereby the rim is heated to a higher solution heat treatment temperature than the bore. Furthermore, it would be necessary at the same time, as known in the art, to be able to directly quench the disk after the solution heat treatment to achieve high tensile strength and low cycle fatigue resistance in the bore and high creep resistance in the rim. 
     For most gas turbine applications, disks are currently heat treated at uniform solution temperature either below the gamma prime solvus temperature (subsolvus heat treatments), or above the solvus temperature (supersolvus heat treatments). Several recent approaches have been established which differ from the traditional subsolvus or supersolvus heat treatment. One approach, more fully described in U.S. Pat. No. 5,312,497, uses induction heating to preferentially heat the rim of a disk, while a pressurized gas is run through the bore of the disk to keep the bore and web cooler. Another approach, more fully described in U.S. Pat. Nos. 5,527,020 and 5,527,402, uses simpler top and bottom thermal caps placed over the bore of the disk to blow pressurized air through the center of a single disk, while the disk is being held at a constant temperature in a gas fired furnace. In this way, the bore of the disk is maintained at a sufficiently cooler temperature than the rim of the disk, thus, achieving desired subsolvus solution of the bore and desired supersolvus solution of the rim. 
     Uniform disk temperature heat treatments produce either fine or coarse grain microstructures throughout the disk. The fine grain microstructure has inferior creep and dwell crack growth resistance for rim service temperatures. Similarly, the coarse grain microstructure has inferior tensile and low cycle fatigue resistance for bore service temperatures. The approach described in U.S. Pat. No. 5,312,497, using induction heating of the rim with pressurized gas cooling of the bore can only be applied to one disk at a time, and is thereby very expensive. The practice of U.S. Pat. No. 5,312,497 is also very sensitive to induction coil-disk geometry tuning, disadvantageously yielding difficult process control. The approach described in U.S. Pat. Nos. 5,527,020 and 5,527,402, also is limited to heat treating one disk at a time. The practice of U.S. Pat. Nos. 5,527,020 and 5,527,402, while having reduced complexity compared to the practice of U.S. Pat. No. 5,312,497, still requires specialized air pressure lines going into a furnace that must remain operable for process viability. Accordingly, there still remains a need to provide heat treatment devices, and methods of use thereof, that provide different microstructures in the bore and rim portions of nickel-base superalloy disks without suffering the drawbacks of the prior art techniques. 
     OBJECTS OF THE INVENTION 
     It is a primary object of the present invention to provide a heat treatment apparatus and method of use thereof. The heat treatment yields rim portions of superalloy disks as having higher temperature capabilities associated with coarse grain microstructures, while at the same time maintaining high strength and low cycle fatigue resistance of fine grain microstructures in the bore portions of superalloy disks near 800-1200° F. 
     It is another object of the present invention to provide for different microstructures in the bore and rim portions of nickel-base superalloy disks and accomplish such by the use of standard production furnaces without auxiliary cooling. 
     It is further desired to provide differential microstructures in the rim and bore portions of nickel-base superalloy disks while still maintaining the option for rapid cooling upon completion of the solution heat treatment using conventional fan or oil quenching operations. 
     A further object of the present invention is to provide for design of the heat treatment device using a finite element computer code and solvus data of the disk alloy. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a heat treatment apparatus and methods of use thereof which produce different microstructures in the bore and rim portions of nickel-base superalloy disks particularly suited for gas turbine engines. 
     In one embodiment, an apparatus is provided that is insertable and removable from a heat treatment furnace for differentially heat treating a superalloy disk to obtain a dual microstructure disk. The disk comprises an inner section termed the bore with a bore hole, an intermediate section termed the web portion, an outer section termed the rim portion, and first and second faces on opposite sides of the disk. The disk has predetermined diameter and thickness dimensions. The apparatus comprises first and second thermal blocks, respectively, arranged on the first and second faces of the disk. Each of the first and second thermal blocks has predetermined diameter and thickness dimensions related to the predetermined diameter and thickness dimensions of the disk by a predetermined relationship. The diameters of the first and second thermal blocks are less than the diameter of the disk. The first and second thermal blocks each have have upper and lower faces with the lower face of the first thermal block having an alignment pin positionable in correspondence with the bore hole of the disk and the upper face of the second thermal block having an alignment pin positionable in correspondence with the bore hole of the disk so that the first and second thermal blocks along with the disk are brought together and expose at least the rim portion of the disk. The apparatus further comprises first and second insulation jackets that surround the first and second thermal blocks. Each insulating jacket consists of an alignment plate, outer shell, and insulating media. The first and second alignment plates are respectively fastened to the upper face of the first thermal block and to the lower face of the second thermal block. The alignment plates have diameters greater than the thermal blocks. The apparatus still further comprises first and second outer shells respectively located outside of the first and second alignment plates with high temperature insulating media filling the cavity between the outer shells and thermal blocks. 
     The invention provides a method for differentially heat treating a superalloy disk having a gamma prime solvus temperature so as to obtain a dual microstructure disk. The method includes providing first and second thermal blocks respectively arranged on first and second faces of a disk. Each of the first and second thermal blocks has predetermined diameter and thickness dimensions related to the predetermined diameter and thickness dimensions of the disk by a predetermined relationship. The first and second thermal blocks each has upper and lower faces, with the lower face of the first thermal block having an alignment pin positionable in correspondence with the bore hole of the disk, and the upper face of the second thermal block having an alignment pin positionable in correspondence with the bore hole of the disk. The diameters of the first and second thermal blocks are less than the diameter of the disk. The method further includes providing first and second alignment plates each with a diameter greater than the diameter of the first and second thermal blocks and having means for being respectively fastened to the upper face of the first thermal block and to the lower face of the second thermal block. The method further comprises providing first and second outer shells respectively located outside of the first and second alignment plates with high temperature insulating media filling the cavity between the thermal blocks and outer shells. The method further includes the following steps: (1) positioning each of the alignment pins of the first and second thermal blocks in correspondence with the bore hole of the disk; (2) bringing together the first and second thermal block, the first and second shells with the associated high temperature insulating media and the disk thereby exposing the rim portion of the disk; (3) selectively attaching a thermocouple to either the first or second thermal block; (4) placing the brought together disk, the first and second thermal blocks, the first and second shells with the associated high temperature insulating media, and the thermocouple in a furnace; (5) heat treating the disk with heat sink assembly in a standard production furnace maintained at a temperature which is above the gamma prime solvus temperature of the disk for a first predetermined duration; (6) removing the disk and heat sink assembly from the furnace when the thermocouple reaches the subsolvus temperature of the disk alloy; (7) freeing the disk from the heat sink assembly; and (8) quenching the disk. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a disk shape for a gas turbine engine; 
     FIG. 2 is a cross-sectional view of a heat treatment apparatus of the present invention used for differentially heating a disk so as to provide a dual microstructure thereof; 
     FIG. 3 is composed of FIGS.  3 (A) and  3 (B) which show the predicted thermal gradients in a disk and the thermal block of the heat treatment apparatus at a specific time at an elevated temperature based on calculations obtained using a finite element computer code; and 
     FIG. 4 illustrates a macro etched section of a turbine disk created by the practice of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, wherein the same reference number indicates the same element throughout, there is shown in FIG. 1 an article which is differentially heat treated in accordance with the practice of the present invention. More particularly, FIG. 1 shows a typical disk  10  for a gas turbine and is generally illustrated by reference number  10 . Each of the various disks  10  contemplated by the practice of the present invention has predetermined diameters and thickness dimensions covering a wide range of sizes all handled by a heat treatment device to be described hereinafter with reference to FIG.  2 . 
     The disk  10  has a typical diameter of thirteen (13) inches, a typical height of two (2) inches at its central region and a typical height of one (1) inch at its outer region. The disk  10  is comprised of an outer section rim portion, to be further defined hereinafter with reference to FIG. 2, occupying a predetermined region at the outer region of the disk  10  and generally shown by reference number  12 , an inner section bore portion  14  generally shown by reference number  14 , and a connecting or intermediate section web portion generally shown by reference number  16 . A central bore hole  18 , through the bore portion  14 , is illustrated and is an essential feature of the turbine disk  10 . The disk  10  additionally comprises a first face  20 , and a second face  22 , each of which extends over the rim, web, and bore portions of the disk  10  and are on opposite sides of the disk  10 . The disk  10  is advantageously solution heat treated by the use of a heat treatment device  24  of the present invention, which may be further described with reference to FIG.  2 . 
     FIG. 2 illustrates the heat treatment assembly  24 , which rests on a production heat treatment grate  32  of a standard gas-fired furnace to be described hereinafter and comprises top and bottom heat sinks  34  and  36 . The heat sinks,  34  and  36 , except for their insulative members to be further described, can be fabricated from any metal or alloy which can withstand the heat treatment temperatures. Carbon steel serves well for this purpose and can also be used to minimize cost of the heat treatment assembly  24 . The heat treatment assembly  24  also contains a thermocouple  26  that is preferably placed in a thermal block  28  near the bore portion  14  of the disk  10 . The thermocouple  26  is connected to a temperature indicator  30  by way of signal path  30 A. The thermocouple  26  derives an electrical signal representative of the temperature of the thermal block  28  and, more importantly, the temperature of the bore portion  14  of the disk  10 . 
     Each heat sink  34  or  36  has four components; a thermal block, an alignment pin, an alignment plate, and an outer shell, which for heat sink  34  are respectively shown with reference numbers  28 ,  44 ,  46 , and  48  and, similarly, these four components are respectively shown for heat sink  36  with reference numbers  50 ,  52 ,  54 , and  56 . For the disk  10  shown in FIG. 1, the thermal block  28  has typical dimensions of a diameter of six (6) inches and a height of two (2) inches, whereas thermal block  50  has typical dimensions of a diameter of six (6) inches and a height of three (3) inches. The alignment plates  46  and  54  have typical dimensions of a diameter of eight (8) inches and a thickness of 0.25 inches. The outer shells  48  and  56  are essentially pipe sections preferably comprised of carbon steel and have a typical diameter of eight (8) inches. 
     The rim portion  12  is defined herein as that portion of the disk  10  extending outside of the shells  48  and  56 . The thermal blocks  28  and  50  are defined herein as having diameters which are less than the diameters of the shells  48  and  56  and also less than the diameters of the disk  10  receiving the heat treatment of the present invention. 
     The thermal blocks  28  and  50  are solid metal cylinders and are used to chill the central portion of the disk  10 . The alignment pins  44  and  52  and alignment plates  46  and  54  are respectively connected by appropriate means, such as bolts, to the thermal blocks  28  and  50  to assure maintaining the concentricity of the disk  10 , thermal blocks  28  and  50 , and outer shells  48  and  56  during the heat treatment of the disk  10  to be described hereinafter. The alignment pins  44  and  52  and the alignment plates  46  and  54  provide concentric alignment of the thermal blocks  28  and  50  and the outer shells  48  and  56  relative to the geometric center of the disk  10  so as to ensure that the coarse and fine grain macrostructures resulting from the practice of the present invention, to be further described hereinafter with respect to FIG. 3, are also concentric after disk  10  is heat treated. 
     Insulating jackets, which are comprised of the outer shells  48  and  54  and high temperature insulating media, generally identified by reference number  58 , minimize the temperature rise of the thermal blocks  28  and  50  and the central portion of the disk  10 . Any high temperature insulating media, such as Kaowool™, can be used to fill the gaps between the outer shells  48  and  56  and the thermal blocks  28  and  50  as shown in FIG.  2 . 
     The thermal block  28  has an upper face  28 A and a lower face  28 B, similarly, the thermal block  50  has an upper face  50 A and a lower face  50 B. The lower face  28 B of the thermal block  28  is mated with face  20  of the disk  10 , whereas the upper face  50 A of the thermal block  50  is mated with the face  22  of the disk  10 . The thermal block  50  has the alignment pin  52  protruding from its upper face  50 A, whereas the thermal block  28  has an alignment pin  44  protruding from its lower face  28 B. The alignment pins  44  and  52  are positionable in correspondence with the bore hole  18  of the disk  10 . The diameters of the thermal blocks  28  and  50  are less than the diameter of the disk  10  by a predetermined amount so as to expose the outer periphery of the disk  10 . 
     The alignment plates  46  and  54  have respective peripheries  46 A and  54 A. Further, the alignment plates  46  and  54 , each has a diameter greater than the diameter of the thermal blocks  28  and  50  and each has appropriate means, such as bolts (not shown) for being respectively fastened to the upper face  28 A of the thermal block  28  and to the lower face  50 B of the thermal block  50 . 
     The outer shells  48  and  56  are respectively located outside of, but near the periphery  46 A and  54 A of the alignment plates  46  and  54 . The outer shells  48  and  56  are dimensioned so as to slide over the respective alignment plates  46  and  54 . The outer shells  48  and  56  are preferably spaced apart from each other by an amount, which is somewhat greater than the predetermined thickness of the rim portion  12  of the disk  10 . For one embodiment, the outer shell  48  rests on the disk  10 , whereas the outer shell  56  is free of contact with the disk  10 . This results in maximum thermal contact of disk  10  and thermal block  50 . 
     The heat treatment assembly  24  still further preferably comprises a special purpose rack  62  comprised of a heat resistant material and having a frame  64  for holding the disk  10 . The frame  64  also has a supporting legs  66 . The frame  64  has a clearance hole  68  with a typical diameter of nine (9) inches so that the frame  64  may slide over the outer shell  56 . 
     The heat sinks  34  and  36  are designed to enhance and maximize the natural thermal gradient between the interior bore  14  and periphery rim  12  of the disk  10 . These heat sinks  34  and  36  and the accompanying thermal cycle, to be further described hereinafter with reference to FIG. 3, operatively cooperate to produce a fine grain bore  14  and a coarse grain rim  12  in the disk  10  in a standard furnace without the aid of auxiliary cooling. The dimensions of the thermal blocks  28  and  50  and the outer shells  48  and  56 , having the typical values previously described with reference to FIG. 2, are related to the dimensions of the disk  10  by a predetermined relationship that may be determined using commercially available finite element heat transfer computer code. An example of one embodiment of the present invention and the thermal gradients thereof are depicted in FIG. 3, which is composed of FIGS.  3 (A) and  3 (B). FIG. 3 shows the thermal gradient in a typical disk  10  used in a turbine application and schematically illustrated in FIG. 3, as having mated thereto a thermal block, such as thermal block  28 , and an insulating jacket, defined by outer shell  48 . The thermal gradients are illustrated for a specified time at an elevated temperature. 
     FIGS.  3 (A) and  3 (B) are interrelated, wherein FIG.  3 (A) shows a Finite Element Analysis (FEA) prediction and FIG.  3 (B) illustrates associated temperatures. The (FEA) prediction is the condition occurring after subjecting the disk  10  and thermal block  28  to an elevated temperature of 2150° F. for a duration of about 1.8 hours. FIG.  3 (B) illustrates a temperature range  70  segmented into three temperatures ranges, which define regions  72 ,  74 , and  76  having the clear and two different shaded portions shown in FIG.  3 (B). These regions  72 ,  74  and  76  are shown in FIG.  3 (A) as being associated with thermal block  28  and disk  10 . As can be seen in FIG. 3, the temperature of the thermal block  28  and more importantly the central portion of the disk  10  corresponds to the lowest  72  (subsolvus) region, whereas the temperature at the rim  12  of the disk  10  corresponds at the highest  76  (supersolvus) region. Region  72  represents the fine grain region of the disk  10  and region  76  represents the coarse grain region of the disk  10  after completion of the heat treatment of the invention. 
     In operation, and in reference to FIG. 2, the first and second heat sinks  34  and  36  are selected in a manner as previously described and assembled with the disk  10  and heat treatment rack  62  on a standard production heat treatment grate  32 . Thermocouple  26  is then preferably attached to thermal block  28 , but it may alternatively be attached to thermal block  50 . 
     The solution heat treatment of the present invention may be provided by standard gas-fired furnaces which may be of the type used by Ladish Company Inc., Wyman-Gordon Forging, or other heat treatment companies. 
     The heat treatment cycle is dependent on the alloy making up the disk  10 , that is, its gamma prime solvus temperature and its incipient melting point. The method of the present invention first handles the heat treatment assembly  24 , which is at room temperature, so as to be inserted into a furnace maintained at a temperature above the gamma prime solvus temperature of the alloy. It is desired that the furnace temperature be as high as possible without producing incipient melting of the alloy. For the class of alloys used for turbine applications, the upper limit of the furnace temperature is generally less than 2200° F. The method of the invention monitors the temperature of the bore portion  14  of the disk  10  by means of the thermocouple  26  and temperature indicator  30 . 
     The heat treatment assembly  24  is removed from the furnace when the thermocouple  26  in the thermal block  28  reaches the subsolvus solution temperature of the disk alloy which is generally less than about 2100 F. At this point, the rim  12  of the disk  10  will have exceeded the solvus temperature of the alloy and, therefore, have a coarse grain microstructure, while the bore portion  14  of the disk  10  will have been maintained below the solvus temperature and, therefore, have a fine grain microstructure. 
     The heat sinks  34  and  36  are removed prior to the quenching to facilitate faster cooling of the disk  10 . As is known in the art, this rapid quenching achieves high strength and creep resistance in the disk  10 . Rapid removal of the heat sinks are facilitated by rack  62 . Upon lifting rack  62 , disk  10  and heat sink  34  can be removed from the furnace without heat sink  36 . Once rack  62 , disk  10 , and heat sink  34  are out of the furnace, heat sink  34  can be rapidly removed from disk  10  and rack  62 . This can be accomplished by any number of techniques readily available at heat treat shops which routinely handle metal parts at high temperature as heat sink  34  is not clamped to disk  10 . Once heat sink  34  is removed, disk  10  now resting on rack  62  can be moved to existing cooling facilities for fan cooling or oil quenching. 
     It should now be appreciated that the practice of the present invention provides for a heat treatment assembly  24  in which the disk  10  is brought into contact with the heat sinks  34  and  36 , and the solution treatment is performed in a desired manner. The heat sinks  34  and  36  are brought together with the disk  10  in a non-clamped manner so as to allow a relatively easy disassembly thereof. The removal of the heat sinks  34  and  36  allow the disk  10  to be easily moved and placed into an appropriate quenching station by moving the special rack  62  carrying the disk  10 . 
     The heat treatment assembly  24  provides a compact arrangement for performing the desired heat treatment of the disk  10 . Because of this compact arrangement multiple disks can be heat treated simultaneously, in accordance with the practice of the present invention, in a standard production furnace so as to decrease cost with minimal modification to the present invention. 
     In the practice of the invention, the method hereinbefore described was performed on a disk  10  mated with the heat treatment assembly  24  of FIG. 2, and some of the results thereof may be further described with reference to FIG.  4 . 
     FIG. 4 shows macroetched section of an actual superalloy disk  10  after receiving the heat treatment of the present invention utilizing the heat treatment assembly  24 . From FIG. 4 it should be noted that a fine grain region exists in the center of disk  10  (clear texture) at the bore portion  14  and a coarse grain region exists at the rim portion  12  of the disk  10  (speckled texture). The transition between the fine and coarse grain regions is generally identified in FIG. 4 by dimensional line  78 . The grain size of the circle portion A located in the center of the disk  10  at the bore portion  14  is about 13 ASTM (American Society for Testing and Materials) and the grain size of the circle portion B located at the rim portion  12  of the disk  10  is about 7 ASTM. 
     It should now be appreciated that the practice of the present invention provides for a method to handle various articles, some of which are particularly suited as gas turbine disk, and all of which have a dual microstructure in which the rim portion of the article being treated has creep and dwell crack growth resistance of coarse grain microstructure operable at a temperature near 1400° F., while still maintaining the high strength and low cycle fatigue resistance of fine grain microstructure in the bore portion of the article being treated operable at a temperature of 800-1200° F. 
     It should now be appreciated that the practice of the present invention provides for a heat treatment assembly that accommodates superalloy disks. The practice of the present invention yields these disks having high creep resistance and dwell crack growth resistance of coarse grain microstructure in the rim portion  12  with an operating temperature near 1400° F., while at the same time maintaining high strength and low cycle fatigue resistance of fine grain microstructure in the bore region  14  having an operating temperature of between 800-1200° F. 
     The invention has been described with reference to the preferred embodiments and some alternates thereof. It is believed that many modifications and alterations to the embodiments as discussed herein will readily suggest themselves to those skilled in the art upon reading and understanding the detailed description of the invention. It is intended to include all such modifications and alterations insofar as they come within the scope of the present invention.