Patent Application
20080264444
The present invention relates the repair of metal components, such as gas turbine engine components. In particular, the present invention relates to the removal of protective coatings during the repair of metal components.
Turbine engine components are exposed to extreme temperatures and pressures during the course of operation. As such, these engine components typically employ high-strength alloys (e.g., superalloys) to preserve the integrity of the components. However, over time, exposed portions of the components are subject to wear, cracking, and other degradations, which can lead to decreases in operational efficiencies and damage to the components.
Due to economic factors, it is common practice in the aerospace industry to restore turbine engine components rather than replace them. However, many of the engine components include protective coatings that need to be removed before the restoration can begin. For example, carbide-based coatings, such as chromium carbide-based coatings, are typically coated onto engine components to increase wear resistance and sliding mechanics between moving parts.
Current techniques for removing carbide-based coatings typically involve machining, grinding, or grit blasting the coatings. However, these techniques may remove portions of the underlying metal components along with the coatings. Thus, if the coating removal processes are not sufficiently monitored, they may reduce the wall thicknesses of the metal components to levels that are too thin for repair. In these situations, the metal component is no longer repairable, and is discarded or recycled. Accordingly, there is a need for a process for removing carbide-based coatings from metal components that also substantially preserves the underlying metal components.
The present invention relates to a method for processing a metal component having a carbide-based coating. The method includes exposing the carbide-based coating to fluoride ions, thereby extracting a carbide material from the carbide-based coating. This provides a residual coating on the metal component, which is then removed from the metal component.
Pursuant to the present invention, coating 14 may be removed by initially exposing metal component 10 to fluoride ions, which react with coating 14 to extract at least a portion of the carbide material (e.g., the chromium-carbide material) from coating 14. Metal component 10 may be exposed to fluoride ions by placing metal component 10 in a chamber containing hydrogen fluoride (HF) gas. The chamber may also include additional gases (e.g., H2) to accommodate desired pressures and reaction rates. While within the chamber, the hydrogen fluoride gas and metal component 10 are then heated to a temperature sufficient to generate the fluoride ions from the hydrogen fluoride gas. Examples of suitable temperatures for generating the fluoride ions include temperatures of at least about 820° C. (about 1500° F.), with particularly suitable temperatures ranging from about 870° C. (about 1600° F.) to about 1100° C. (about 2000° F.). This causes the fluoride ions of the hydrogen fluoride gas to react with coating 14, thereby extracting at least a portion of the carbide material from coating 14.
The amount of carbide material removed from coating 14 is generally dependent on the concentration of the fluoride ions, the temperature used, the surface area of coating 14, and the duration of the extraction. In one embodiment, the extraction is continued until at least about 50% by weight of the carbide material is removed from coating 14. In a more preferred embodiment, the extraction is continued until at least about 75% by weight of the carbide material is removed from coating 14. In an even more preferred embodiment, the extraction is continued until at least about 90% by weight of the carbide material is removed from coating 14. The weight percents of the removed carbide material are based on the pre-extraction weight of coating 14. Examples of suitable durations for the extraction process range from about 10 minutes to about 3 hours, with particularly suitable durations ranging from about 30 minutes to about 1 hour. When the extraction process is complete, metal component 10 may be removed from the chamber and cooled.
Residual coating 18 may be removed from surface 16 with low-pressure abrasive techniques (e.g., low-pressure grit blasting). The duration of the removal process may vary depending on the pressure used. However, the pressure required to remove residual coating 18 is substantially less than what is otherwise required to remove a carbide-based coating not subjected to the fluoride-ion extraction process (i.e., coating 14). Suitable pressures for removing residual coating 18 from surface 16 include removal pressures that are less than 25% of removal pressures required to remove coating 14 from surface 16 in the same duration, with particularly suitable removal pressures including less than 10% of the removal pressures required to remove coating 14 from surface 16 in the same duration, and with even more particularly suitable removal pressures including less than 5% of the removal pressures required to remove coating 14 from surface 16 in the same duration. As used herein, the term “removal pressure” refer to a pressure that is actually applied to the coating (e.g., coating 14 or residual coating 18). For removal techniques that are distance dependant (e.g., grit blasting), the discharge pressure is typically greater than the pressure actually applied to the coating.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
