FIELD OF THE INVENTION
This invention relates generally to the field of materials technology, and more particularly to the repair of superalloy components such as gas turbine disks.
BACKGROUND OF THE INVENTION
Nickel-based superalloy materials are known for use in high temperature, high stress environments such as in the hot combustion gas path of a gas turbine engine. In one application, the nickel-based superalloy known as Alloy 706 (AMS Specification 5701) is used to form the turbine rotor discs of a gas turbine engine. The discs have a generally annular shaped hub portion and an outermost rim portion shaped into a plurality of steeples or dovetails for engaging a respective plurality of turbine blades. Several discs are joined together along an axis of rotation to form a gas turbine rotor.
Turbine discs formed of Alloy 706 have experienced failures during operation. These disks were formed with a two-step heat treatment; i.e. 970° C. solution anneal followed by a 730° C. +620° C. aging treatment (heat treatment B in AMS Specification 5701). This material exhibits a degree of notch sensitivity, i.e., its Larson-Miller Parameter values for a notched bar are lower than those for a smooth specimen at equivalent stress levels, and this is a suspected damage mode for the failed turbine disks. This type of behavior is also known as stress-assisted grain boundary oxidation (SAGBO). To avoid future failures, the failed disks may be replaced with disks formed of a material exhibiting improved notch sensitivity. One example of such a material is Alloy 706 material subjected to a three step heat treatment; i.e. 970° C. anneal followed by a 845° C. stabilizing treatment followed by a 730° C. +620° C. aging treatment (heat treatment A in AMS Specification 5701). Another material that may be used for the replacement disks is Alloy 718 (AMS Specification 5663). However, regardless of the material selected, there is a significant cost associated with the replacement of failed turbine disks.
It is known in the art to repair turbine disks made of low alloy Ni—Cr—Mo—V or Cr—Mo—V steels, such as are used in steam turbine applications. However, repairs have not previously been performed on the stronger nickel-based superalloys that are used in modern gas turbine engines, since fusion welding of such materials in typical disk thicknesses is generally not possible without cracking.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in following description in view of the drawings that show:
FIG. 1 is a partial perspective view of a gas turbine disk being repaired with a single steeple repair technique.
FIG. 2 is a partial perspective view of a gas turbine disk being repaired with a 360° repair technique.
FIG. 3 is a cross-sectional view of a gas turbine disk being repaired with the installation of a ring of replacement steeple material.
FIG. 4 is a partial plan view of a gas turbine disk being repaired with a linear friction welding technique.
FIG. 5 is a cross-sectional view of a gas turbine disk being repaired with a rotary friction welding technique.
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have discovered a method for repairing a damaged nickel-based superalloy turbine disks. The method includes removing a damaged rim portion of the disk and installing a replacement rim portion onto the disk with a process that avoids the weld cracking problems of the prior art and that protects the properties of the underlying original disk material.
FIG. 1 illustrates a nickel-based gas turbine disk 20 including a plurality of steeples 22 shaped to engage the root portions of a plurality of blades (not shown) there between. The disk 20 may be formed of Alloy 706, for example. FIG. 1 illustrates the disk 20 at a stage of repair wherein a damaged one of the steeples (not shown) has been removed from repair region 24, such as by grinding, machining, electric arc gouging or other known method. The surface (also not shown) created by the removal of the damaged portion of the disk 20 may be conditioned to bright metal, such as with denatured alcohol, acetone or other known cleaning process. The surface may further be inspected to confirm that all damaged material has been removed, such as by dye penetrant testing, for example.
In place of the removed damaged material, a replacement steeple 26 is formed by a weld build-up process that does not adversely affect the properties of the underlying material of the original disk 20 and that is not subject to an unacceptable level of reheat cracking. In one embodiment, the welding filler metal is selected to be in accordance with AMS Specification 5832 for Alloy 718 welding wire in order to provide a desired degree of strength and resistance to service related damage. Welding is accomplished with a set of low heat input parameters utilizing a laser, electron beam, or gas tungsten arc welding process. The preheating temperature is controlled to be no more than 100° C. below the aging temperature for the deposited alloy so as to continuously age the weld deposit and to develop desired mechanical properties without the need for additional heat treatment, which could otherwise have an adverse effect on the properties of the underlying original disk material. For the embodiment of Alloy 718 welding wire, the minimum preheat temperature would be 620° C. In one embodiment, the preheat temperature is maintained to be at least the aging temperature of the alloy. In addition, the interpass temperature is controlled to be below the solution annealing temperature of the alloy (925° C. in this embodiment), also to ensure a desired aging response. Multiple layers of material are used to achieve a gross steeple shape, as illustrated in FIG. 1. Welding tabs 28 may be used where appropriate. The gross steeple shape is then final machined or ground to achieve the desired final steeple shape consistent with the original steeples 22.
FIG. 2 illustrates a further embodiment wherein all of the steeples have been removed from a damaged turbine rotor disk 40. Multiple layers 41 of nickel-based superalloy weld metal are then deposited to create a ring 42. New steeples (not shown) are then formed from the ring 42 by any known material removal process. The preheat temperature and the interpass temperature are controlled during the welding process in the manner described above with respect to the process of FIG. 1 so as to provide a desired degree of aging to underlying layers of weld metal and to protect the underlying material of the original disk 40 from harmful heat treatment effects.
FIG. 3 illustrates an alternative process for replacing all of the steeples of a damaged turbine disk 50. In order to minimize the effect of welding on the underlying original disc material and in order to reduce the time required for the repair, a ring 52 of replacement nickel-based superalloy material is welded onto the hub portion of original disk 50 using a welding process that preserves the underlying original disk material and that avoids reheat cracking in the weld metal. In one embodiment, a narrow groove configuration utilizing a gas tungsten arc process may be employed to form attachment weld 54. Following the attachment of the ring 52, the geometry of the steeples is restored into the ring 52 with a material removal process such as machining or grinding. As described above, the filler metal is selected to meet required properties and the preheat and interpass temperatures are controlled to provide a desired degree of aging of the alloy material without additional post-weld heat treatment. FIG. 4 illustrates a further embodiment of a gas turbine disk 60 wherein a damaged steeple (not shown) has been removed from between two undamaged steeples 62 and a replacement steeple 64 is installed in its place. In this embodiment, the replacement steeple 64 is joined to the original disk 60 by a linear friction welding process. Linear friction welding is a solid phase joining technique that uses a linear reciprocating motion to generate friction heat, as opposed to the more common rotary motion used in conventional friction welding. The weld is accomplished by oscillating a surface of the steeple against a surface of a nickel-based superalloy turbine disk while applying a force there between to cause inter-diffusion between the adjoined material. Once the oscillations are ceased, the melted material will solidify and join the steeple to the disk. Linear friction welding allows the replacement steeple 64 to be welded to the underlying original disk material 60 between two existing original steeples 62 if desired. In other embodiments, groups of adjoined adjacent replacement steeples may be simultaneously joined along an arc length of an original disk 60 using a linear friction welding technique. This solid phase joining technique provide high integrity, low distortion joints in these difficult to weld nickel-based superalloy materials. This method allows the replacement steeple 64 to be fabricated from the same material/heat treatment as the original disk 60 (such as Alloy 706, heat treatment B) or from a different material and/or different heat treatment (such as such as Alloy 706, heat treatment A or Alloy 718). In other embodiments, the replacement steeple may be formed of directionally solidified or single crystal material and joined to the polycrystalline original disk 60.
In the method of FIG. 4, the original damaged steeple(s) is/are removed such as by machining and new replacement steeple(s) 64 is/are formed. Appropriate heat treatment and/or non-destructive examination techniques may be performed on the original disk 60 and/or the replacement steeple 64. The replacement steeple 64 is then joined to the disk 60 by linear friction welding. The relative motion may be achieved by holding the disk 60 stationary and subjecting the steeple 64 to reciprocating motion while a force is applied there between. Typical linear friction welding parameters for such applications may be:
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Friction force per unit area50-300Mpa
Forge force75-450Mpa
Burn-off0.5-5mm
Oscillation amplitude1-7.5mm
Oscillation frequency20-120Hz
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The welding process will produce a weld flash of waste material around the perimeter of the joint, and this weld flash is removed and the weld inspected. Post weld heat treatment may be performed, if desired, any final machining done and a final nondestructive examination conducted, as appropriate for the application.
A further embodiment is illustrated in FIG. 5, where a damaged superalloy turbine disk 70 is repaired by removing all of the original steeples (not shown) and by welding on a replacement ring of superalloy material 72 using a rotary friction welding technique. The mating surfaces 74, 76 of the original disk 70 and ring 72 are angled relative to the rotating axis 78 of the disk 70. One of the disk 70 and ring 72 is then rotated about the axis 78 while the surfaces 74, 76 are forced together to create the friction weld there between. The replacement steeples (not shown) are then formed in the ring 72 by a material removal 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.
Patent Application
20050050705
