Patent Publication Number: US-7722330-B2

Title: Rotating apparatus disk

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of U.S. patent application Ser. No. 10/961,626, filed Oct. 8, 2004 now U.S. Pat. No. 7,316,057, which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of materials technology, and more particularly, to a method of fabricating a large component such as a gas turbine or compressor disk. 
     BACKGROUND OF THE INVENTION 
     The use of nickel-iron based superalloys to form disks for large rotating apparatus such as industrial gas turbines and compressors is becoming commonplace as the size and firing temperatures of such engines continue to increase in response to power, efficiency and emissions requirements. The requirement for integrity of such components demands that the materials of construction be free from metallurgical defects. 
     Turbine and compressor disks are commonly forged from a large diameter metal alloy preform or ingot. The ingot must be substantially free from segregation and melt-related defects such as white spots and freckles. Alloys used in such applications are typically refined by using a triple melt-technique that combines vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR), usually in the stated order or in the order of VIM, VAR and then ESR. However, alloys prone to segregation, such as Alloy 706 (AMS Specification 5701) and Alloy 718 (AMS Specification 5663), are difficult to produce in large diameters by VAR melting because it is difficult to achieve a cooling rate that is sufficient to minimize segregation. In addition, VAR will often introduce defects into the ingot that cannot be removed prior to forging, such as white spots, freckles, and center segregation. Several techniques have been developed to address these limitations: see, for example, U.S. Pat. Nos. 6,496,529 and 6,719,858, incorporated by reference herein in their entireties. 
     Alternative methods such as powder metallurgy and metal spray forming are available for producing large diameter segregation free ingots, however, these methods have not been demonstrated as being commercially useful either for yielding acceptable properties or for their cost effectiveness. Accordingly, enhanced methods of producing large diameter preforms from segregation prone metallic materials are needed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an ingot having an inner core portion and an outer portion. 
         FIG. 2  is a flow diagram illustrating steps in a method of forming a rotating apparatus disk including forming the ingot of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A large ingot  10  including nickel-iron based superalloy material is formed by a process that will minimize the possibility of segregation and other melt related defects, and is thus well suited for subsequent forging operations. Ingot  10  includes an inner core portion or inner ingot  12  that may be formed using a traditional triple melt technique including vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). Advantageously, the inner ingot  12  is formed to have a size wherein the triple melt technique or other technique used provides a sound ingot; that is, one uniform and free of a detrimental degree of microsegregation, macrosegregation and other solidification defects, even using segregation-prone materials such as Alloy 706 or Alloy 718. Depending upon the material and the particular process parameters selected, an inner ingot  12  having a dimension such as diameter D 1  as large as 30 inches or more may be produced using known triple melt techniques. Refining/casting techniques other than triple melt processes may be used to form the inner ingot  12  provided that the resulting ingot is substantially defect free in accordance with the design requirements of the particular application. 
     The ingot  10  further includes an outer portion  14  that is formed by adding material to the inner ingot  12  after the inner ingot  12  has been formed to form the final ingot  10  having a desired dimension. The outer portion  14  is added to build up the ingot  10  to the required dimension, such as diameter D 2 , without the necessity of relying upon the triple melt process to produce an ingot of that dimension. In this manner, segregation-free ingots  10  may be produced that are larger than those that can be produced with a single prior art process that is prone to such defects, such as the prior art triple melt process alone, resulting in less scrap and therefore potentially lower overall cost for producing a large component. 
       FIG. 2  illustrates steps in one method  20  that may be used to produce a large component such as a gas turbine or compressor disk utilizing the ingot  10  of  FIG. 1 . An inner ingot  12  is first produced at step  22  using a known triple melt process or other fabrication technique that provides a high level of assurance of acceptable metallurgical properties. The material, process and resulting ingot size are specifically selected in step  22  to provide a low risk of segregation or other defects when producing an ingot  12  having a dimension such as diameter D 1  that is less than a desired final ingot dimension. 
     The outer surface  16  of inner ingot  12  may then be cleaned, if desired, such as by machining or grit blasting at step  24  in preparation for a material addition step  26 . Any appropriate material addition process is used at step  26  to increase the dimensions of the ingot from that achieved in step  22  to the required final dimension, such as a desired diameter D 2 . The inner ingot  12  is used as a core to which material is joined to form larger ingot  10 . Materials addition processes used in step  26  may include powder metallurgy or metal spray deposition, for example. A welding process may be used in step  26  in selected applications. If powder metallurgy is used, a hot isostatic pressing step may be included within materials addition step  26 . 
     The final ingot  10  having the required dimension D 2  is then subjected to a forging process at step  28  to achieve a desired final shape. Heat-treating of the partially and/or fully formed component during or following the forging step  28  may be accomplished at step  30  as desired. The resulting component shape such as disk  32  is thus fabricated to have sound metallurgical properties in sizes that are larger than available with prior art techniques at comparable scrap rates. 
     There will be a degree of bonding that occurs between the inner core material  12  and the added material  14  along the surface  16 , with the strength and type of bond depending upon the type of material addition process that is used in step  26 . Advantageously, forging of the ingot  10  at an elevated temperature during step  28  may serve to improve the bond between the two layers  12 ,  14 , creating a sound metallurgical bond. 
     It is known that the hub area of a turbine disk should have maximized resistance to low cycle fatigue cracking and crack propagation in order to ensure long turbine disk life. The hub area should also have good notch ductility to minimize the harmful effects of stress concentrations in critical regions. In contrast to the hub, tensile stress levels are lower in the rim area of a turbine disk, but operating temperatures are higher and creep resistance becomes an important consideration. The process of  FIG. 2  permits the core ingot material  12  to be the same material or a different material than the added material  14 , with the respective materials migrating to the hub and rim areas of the finished disk  32  during the forging step  28 . For example, Alloy 718 material may be added to a core  12  of Alloy 706 material to achieve a disk having an Alloy 718 rim around an Alloy 706 hub. Furthermore, the added material  14  may be graded across its depth by varying the material or deposition process during material addition step  26 . In a rotating apparatus disk embodiment, the graded added material  14  will migrate to form a rim region of the disk  32  having a graded material property across a radius of the disk. In one embodiment a graded layer  14  may be useful when applying a nickel-iron based superalloy material over a core ingot of a steel material such as 9Cr-1Mo steel or a NiCrMoV low alloy steel. For such an embodiment, the final ingot  10  and the resulting disk  32  would include a layer of added rim material  14  that is graded in composition from primarily the steel hub material in a region closest to the core ingot  12  to primarily a nickel-iron based superalloy material at its outmost region. The layer of material  14  would be graded in composition across its depth from a first percentage of the steel material and a first percentage of a nickel-iron based superalloy material closest to the core ingot  12  to a second percentage of the steel material and a second percentage of a nickel-iron based superalloy material remote from the core ingot to form a final ingot. Thus, the improved properties of the nickel-iron based superalloy material are obtained in the region where they are most needed without risking segregations or other defects that may occur when forming the entire disk out of the superalloy material using a triple melt process. 
     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.