Patent Application
20060075624
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.
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.
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 D1 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 D2, 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.
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 D2. 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 D2 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
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.
