Metal working – Method of mechanical manufacture – Impeller making
Reexamination Certificate
1998-12-22
2001-05-22
Cuda, I (Department: 3726)
Metal working
Method of mechanical manufacture
Impeller making
C029S889700, C029S402160, C029S402210
Reexamination Certificate
active
06233822
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to repair of high pressure turbine shrouds. More particularly, it relates to the method of repairing high pressure turbine shrouds utilizing a high velocity oxyfuel (HVOF) and materials used for such repairs.
2. Discussion of Prior Art
In gas turbine engines, a shroud typically surrounds the tips of the rotor blades in the turbine section of the engine. Pressurized air and fuel are burned in a combustion chamber to add thermal energy to the medium gases flowing therethrough. The effluent from the chamber comprises high temperature gases, which are flowed downstream in an annular flow path through the turbine section of the engine. Nozzle guide veins at the inlet to the turbine directed the medium gases onto a multiplicity of blades which extend radially outward from the engine rotor. An annular shroud that is supported by the turbine case surrounds the tips of the turbine blades to contain the medium gases flowing thereacross to the flow path. The clearance between the blade tips and the shroud is minimized to prevent the leakage of medium gases around the tips of the blades. Shrouds provide a rubbing surface for the tip of the blade. The design intent is for the blade tip to rub into the shrouds, thus reducing the amount of air that can bypass the turbine airfoils. Minimizing the air that can bypass the turbine airfoils increases the efficiency of the engine. A secondary function of the shroud is to thermally shield the case from the hot flow path gas.
The shroud thus is exposed to abrasion from the rotating turbine blade tips. Simultaneously, the shroud also is exposed to the hot flow path gases that are burned in a combustion chamber. These gases over a period of time not only result in corrosion and high temperature oxidation of the shroud, but also function to cause erosion of the shroud surfaces. Thus, the shroud must be designed to be at once resistant to corrosive and oxidation effects of the hot gases, erosion resistant to the constant flow of the hot gases over the shroud surfaces and abrasion resistant, or rub compliant, as a result of the contact with the turbine blade seal teeth.
Over a period of time, as the engine it utilized, the surfaces of the shrouds tend to be worn from the rubbing surfaces of the blades' tips. In addition, some erosion takes place as the hot gases mechanically erode the shroud flow path surfaces. Additionally, some corrosion and oxidation of the shroud surfaces also occurs due to the corrosive action of the gases on the shroud surfaces.
Because of the high cost of the shroud materials, rather than dispose of the shrouds that are made from expensive superalloy material and machined to exacting and tight tolerances, it is desirable to repair the shrouds by restoring the shrouds to their original dimensions in accordance with preselected tolerances as determined by the engine's size as well as to restore the corrosion resistant properties to the flow path surfaces. In the past, this restoration has been accomplished by low pressure plasma spray (LPPS) or by use of thermally densified coatings (TDC). While both of these methods provide repairs and restorations that are effective, both suffer from some limitations. For example, the VPS and LPPS processes spray MCrAlY in a vacuum chamber on a heated substrate, making the process very sensitive to leaks, as the partial vacuum must be maintained in order to successfully accomplish the repair. Only a limited number of parts can be processed at any one time with the LPPS process. Additionally, LPPS requires a preheat, and coupled with the welding process, can result in considerable part distortion. While this method has the advantage of being able to provide a repaired shroud that can be used at higher temperatures than other methods, the deposition of the material also is accomplished at a much slower rate. The result is than the shroud is either restored to minimum or below minimum dimensions, or significant cost is incurred in adding additional material to the shroud during repair. The result is that this method is slow, time consuming and considerably expensive. The TDC process utilizes brazed preforms, which may be in the form of powders, to build up the sides and the flow paths on all the surfaces. The preforms typically include epoxy as a bonding agent. The result is that the parts typically include some undesirable, and sometimes unacceptable porosity. Of course, the quality of parts repaired by the TDC process is dependent on the quality of the preforms. The materials that are utilized in a TDC process typically contain melting point depressants such as silicon and boron or combinations of these elements. Because these materials are designed to melt at temperatures of about 2300° F. or less, they must be applied at temperatures below the incipient melting temperature of the base material. Shrouds that are repaired using these materials cannot be utilized in applications above about 2250° F.
What is desired is a method of repairing high pressure turbine shrouds after engine running to extend the life of the shrouds and provide cost effective operation of the engine while applying oxidation-resistant, corrosion-resistant and rub-compliant materials that can withstand temperatures higher than about 2250° F.
BRIEF SUMMARY OF THE INVENTION
The present invention is a method for repairing turbine shrouds removed from turbine service. The repair restores the corrosion and oxidation resistance characteristics to the shroud while at the same time restoring the dimensional characteristics to the shroud flow path surfaces, the shroud forward and aft rails and the left and right sides of the shroud. The method comprises a series of steps. Because of the extremely high temperatures, in excess of 2300° F., from the hot gases of combustion that a shroud is exposed to, loose surface contaminants, including products of combustion and oxidation by-products, form on the exposed surfaces of the shroud at these temperatures. After the turbine shroud is removed from service, these loose surface contaminants must first be removed. This cleaning exposes any coating materials that may have been applied to the shroud prior to placing it in service. The next step involves removing the remaining coating materials applied to the shroud prior to being placed in service that still remain on the shroud. These coatings may have been applied to provide corrosion resistance characteristics, oxidation resistance characteristics, abradability characteristics, or all of these characteristics, to the shroud. After removal of the coatings, activated diffusion healed (ADH) or partitioned alloy component healing (PACH) material is applied to the exposed surfaces of the shroud to fill any existing voids, such as cracks or holes, that may have occurred over the operational life of the shroud or than may have been formed in the shroud during its original manufacture. After the voids have been filled, the applied compliant material is machined to provide a smooth surface for the remainder of the restoration steps. Next, the ends of the shrouds are repaired by weld depositing a superalloy material compatible with the base material of the shroud, if needed. This build-up restores the shroud base material that had been worn away during operation of the engine. Next, a corrosion and oxidation resistant, rub-compliant material is sprayed onto the flow path surfaces of the shroud as well as to the side surfaces (forward and aft rails) of the shroud using a high velocity oxyfuel process (HVOF). In this process, the filler material originates as a powder that is sprayed onto the substrate in the HVOF process. While this filer material may be any corrosion resistant, oxidation resistant and rub tolerant powder, MCrAlY and superalloys, typically Ni-base superalloys have been found to be suitable. Sufficient material is sprayed onto the repaired substrate to at least restore the substrate to the minimum dimensions required for a new shroud. The shroud is then m
Charles Patricia A.
Grossklaus, Jr. Warren D.
Cuda I
General Electric Company
Gressel Gerry S.
Hess Andrew C.
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