BACKGROUND
[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.
SUMMARY
[0005] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006]
FIG. 1 is a sectional view of a metal component containing a carbide-based coating.
FIG. 2 is a sectional view of the metal component containing a residual coating after
the carbide-based coating is exposed to fluoride ions.
FIG. 3 is a sectional view of the metal component after the residual coating is removed.
DETAILED DESCRIPTION
[0007] FIG. 1 is a sectional view of metal component 10, which includes substrate 12 and
coating 14. Metal component 10 may be any type of component capable of containing
coating 14, such as turbine engine components. Substrate 12 is a metal substrate (e.g.,
nickel-based alloys and superalloys, cobalt-based alloys and superalloys, and combinations
thereof) of metal component 10, and includes surface 16. Coating 14 is a carbide-based
coating formed on surface 16 of substrate 12 (e.g., via plasma spray deposition) to
provide wear resistance and sliding properties during use. As used herein, the term
"carbide-based coating" refers to a coating that includes at least one carbide material.
Examples of suitable carbide materials for use in the carbide-based coating include
chromium carbide materials (e.g., Cr
3C
2, Cr
7C
3, and Cr
23C
6), tungsten carbide materials (e.g., WC), and combinations thereof. Coating 14 may
also include other materials, such as nickel chromium (NiCr) alloys, cobalt (Co) alloys,
and combinations thereof. An example of a suitable chromium carbide-based coating
for coating 14 includes about 75% by weight of a chromium carbide material and about
25% by weight of a nickel chromium alloy. Suitable coating thicknesses for coating
14 range from about 25 micrometers (about 1 mil) to about 500 micrometers (about 20
mils).
[0008] 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., H
2) 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.
[0009] 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.
[0010] FIG. 2 is a sectional view of metal component 10 after the extraction process, which
includes residual coating 18 disposed on surface 16 of substrate 12. Residual coating
18 is the remaining coating of coating 14 (shown in FIG. 1) after the extraction process.
Because of the carbide material removal, residual coating 18 primarily includes the
non-carbide portion of coating 14 (e.g., the nickel chromium alloy) and any residual
amount of the carbide material that was not extracted. However, because a substantial
portion of the carbide material was removed, residual coating 18 is structurally weaker
than coating 14. Thus, residual coating 18 can be removed from surface 16 of substrate
12 without requiring the high-intensity machining, grinding, or grit blasting that
are typically used to remove carbide-based coatings.
[0011] 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.
[0012] FIG. 3 is a sectional view of metal component 10 after the residual coating 18 is
removed. After residual coating 18 is removed, the resulting metal component 10 may
undergo the necessary repair processes to restore metal component 10 to operable condition.
Because residual coating 18 (shown in FIG. 2) can be removed with a low-pressure technique,
the risk of damaging surface 16 during the removal process is reduced. Accordingly,
pursuant to the present invention, coating 14 (shown in FIG. 1) may be removed from
substrate 12 while substantially preserving the dimensions of surface 16.
[0013] 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 scope of the invention.
1. A method for processing a metal component (10) having a carbide-based coating (14),
the method comprising:
exposing the carbide-based coating (14) to fluoride ions, thereby extracting a carbide
material from the carbide-based coating (14) to provide a residual coating (18) on
the metal component (10); and
removing the residual coating (18) from the metal component (10).
2. The method of claim 1, further comprising heating the metal component (10) to a temperature
of at least about 820°C.
3. The method of claim 2, wherein the metal component (10) comprises a material selected
from the group consisting of a nickel-based alloy, a nickel-based superalloy, a cobalt-based
alloy, a cobalt-based superalloy, and combinations thereof.
4. The method of any preceding claim, wherein at least about 50% by weight of the carbide
material is extracted from the chromium carbide-based coating (14), based on a pre-extraction
weight of the carbide-based coating.
5. The method of claim 4, wherein at least about 75% by weight of the carbide material
is extracted from the carbide-based coating (14).
6. The method of claim 5, wherein at least about 90% by weight of the carbide material
is extracted from the carbide-based coating (14).
7. The method of any preceding claim, wherein the residual coating (18) is removed from
the metal component (10) with a first removal pressure that is less than 25% of a
second removal pressure required to remove the carbide-based coating (14) from the
metal component in a same duration.
8. The method of claim 7, wherein the first removal pressure is less than 10% of the
second removal pressure.
9. The method of claim 8, wherein the first removal pressure is less than 5% of the second
removal pressure.
10. The method of any preceding claim, wherein the carbide material is selected from the
group consisting of chromium carbide materials, tungsten carbide materials, and combinations
thereof.
11. The method of any preceding claim, wherein the carbide-based coating is exposed to
hydrogen fluoride.
12. The method of claim 11, further comprising:
heating the metal component (10) to react the hydrogen fluoride with the carbide-based
coating (14), thereby providing the residual coating (18) on the metal component.
13. The method of claim 12, wherein the metal component (10) and hydrogen fluoride and
heated to a temperature of at least about 820°C.
14. The method of claim 12 or 13, wherein the hydrogen fluoride reacts with the carbide-based
coating (14) until at least about 50% by weight of a carbide material is extracted
from the carbide-based coating (14), based on a pre-extraction weight of the carbide-based
coating.