U.S. patent application number 10/702987 was filed with the patent office on 2005-05-12 for method for hvof or lpps restoration coating repair of a nickel-base superalloy article.
This patent application is currently assigned to General Electric Company. Invention is credited to Budinger, David Edwin, Grossklaus, Warren Davis JR., Jackson, Melvin Robert, Miller, Joshua Leigh, Miller, Matthew Nicklus, Tholke, Brent Ross.
Application Number | 20050098243 10/702987 |
Document ID | / |
Family ID | 33541627 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050098243 |
Kind Code |
A1 |
Budinger, David Edwin ; et
al. |
May 12, 2005 |
Method for HVOF or LPPS restoration coating repair of a nickel-base
superalloy article
Abstract
A method for repairing a nickel-base superalloy article, such as
a gas turbine stationary flowpath shroud having flowpath cooling
holes therein that has previously been in service, includes the
steps of providing the nickel-base superalloy article that has
previously been in service; and applying a restoration to a surface
of the article. The restoration is applied by the steps of
providing a restoration nickel-base alloy, wherein the restoration
nickel-base alloy preferably has no more than about 15 weight
percent chromium and no more than about 0.01 percent yttrium,
thereafter applying a restoration coating of the restoration
nickel-base alloy to the surface of the article by a hyper-velocity
oxyfuel metal spray process or a low-pressure plasma spray process,
and thereafter heating the article with the restoration coating
applied to the surface thereof to a sufficiently high temperature
to diffusion bond the restoration coating to the surface of the
article. The article is then returned to service.
Inventors: |
Budinger, David Edwin;
(Loveland, OH) ; Tholke, Brent Ross; (Cincinnati,
OH) ; Miller, Matthew Nicklus; (Maineville, OH)
; Grossklaus, Warren Davis JR.; (West Chester, OH)
; Miller, Joshua Leigh; (West Chester, OH) ;
Jackson, Melvin Robert; (Niskayuna, NY) |
Correspondence
Address: |
MCNEES WALLACE & NURICK LLC
100 PINE STREET
P.O. BOX 1166
HARRISBURG
PA
17108-1166
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
33541627 |
Appl. No.: |
10/702987 |
Filed: |
November 6, 2003 |
Current U.S.
Class: |
148/527 |
Current CPC
Class: |
B23K 2101/001 20180801;
B23K 10/027 20130101; B23K 2103/26 20180801; C23C 4/129 20160101;
C23C 4/18 20130101; B23P 6/007 20130101 |
Class at
Publication: |
148/527 |
International
Class: |
C22F 001/10 |
Claims
What is claimed is:
1. A method for repairing a nickel-base superalloy article
comprising the steps of providing the nickel-base superalloy
article that has previously been in service; and applying a
restoration to a surface of the article by the steps of providing a
restoration nickel-base alloy, thereafter applying a restoration
coating of the restoration nickel-base alloy to the surface of the
article by a hyper-velocity oxyfuel metal spray process or a
low-pressure plasma spray process, and thereafter heating the
article with the restoration coating applied to the surface thereof
to a sufficiently high temperature to diffusion bond the
restoration coating to the surface of the article.
2. The method of claim 1, including an additional step, after the
step of applying is fully completed, of returning the article to
service.
3. The method of claim 1, including an additional step, after the
step of applying is fully completed, of returning the article to
service, wherein the article is heated to a temperature of not less
than about 1800.degree. F. during service.
4. The method of claim 1, wherein the step of providing the
nickel-base superalloy article includes the step of providing a gas
turbine stationary flowpath shroud as the article.
5. The method of claim 1, wherein the step of providing the
nickel-base superalloy article includes the step of providing a gas
turbine stationary flowpath shroud having flowpath cooling holes
therein as the article.
6. The method of claim 1, wherein the step of providing the
nickel-base superalloy article includes the step of providing a gas
turbine stationary flowpath shroud having flowpath cooling holes
therein as the article, and wherein the method includes an
additional step, after the step of heating, of redrilling the
cooling holes.
7. The method of claim 1, wherein the step of providing the
nickel-base superalloy article includes the step of providing the
article having a nominal composition in weight percent of about 7.5
percent cobalt, about 7.0 percent chromium, about 1.5 percent
molybdenum, about 5 percent tungsten, about 3 percent rhenium,
about 6.5 percent tantalum, about 6.2 percent aluminum, about 0.15
percent hafnium, about 0.05 percent carbon, about 0.004 percent
boron, about 0.01 percent yttrium, balance nickel and
impurities.
8. The method of claim 1, wherein the step of providing the
restoration nickel-base alloy includes the step of providing the
restoration nickel-base alloy having no more than about 15 weight
percent chromium and no more than about 0.01 percent yttrium
9. The method of claim 1, wherein the step of providing the
restoration nickel-base alloy includes the step of providing the
restoration nickel-base alloy having no more than about 12 weight
percent chromium and no more than about 0.01 percent yttrium
10. The method of claim 1, wherein the step of providing the
restoration nickel-base alloy includes the step of providing the
restoration nickel-base alloy having a nominal composition in
weight percent of about 3.1 percent cobalt, about 7.6 percent
chromium, about 0.1 percent maximum molybdenum, about 3.85 percent
tungsten, about 0.02 percent maximum titanium, about 1.65 percent
rhenium, about 0.55 percent silicon, about 5.45 percent tantalum,
about 7.8 percent aluminum, about 0.15 percent hafnium, about 0.02
percent carbon, balance nickel and impurities.
11. The method of claim 1, wherein the step of providing the
restoration nickel-base alloy includes the step of providing the
restoration nickel-base alloy having a nominal composition in
weight percent of 0.01-0.03 percent carbon, 0.1 percent maximum
manganese, 0.5-0.6 percent silicon, 0.01 percent maximum
phosphorus, 0.004 percent maximum sulfur, 7.4-7.8 percent chromium,
2.9-3.3 percent cobalt, 0.01 percent maximum molybdenum, 3.7-4.0
percent tungsten, 5.3-5.6 percent tantalum, 0.02 percent maximum
titanium, 7.6-8.0 percent aluminum, 1.5-1.8 percent rhenium, 0.005
percent maximum selenium, 0.3 percent maximum platinum, 0.01-0.02
percent boron, 0.03 percent maximum zirconium, 0.12-0.18 percent
hafnium, 0.1 percent maximum niobium, 0.1 percent maximum vanadium,
0.1 percent maximum copper, 0.2 percent maximum iron, 0.0035
percent maximum magnesium, 0.01 percent maximum oxygen, 0.01
percent maximum nitrogen, balance nickel with other elements 0.5
percent maximum.
12. The method of claim 1, wherein the step of applying the
restoration coating includes the step of applying the restoration
coating to a thickness of from about 0.030 to about 0.150
inches.
13. The method of claim 1, wherein the step of heating includes the
step of heating the article to a temperature of from about
2025.degree. F. to about 2075.degree. F.
14. The method of claim 1, wherein the step of heating includes the
step of heating the article to a temperature of from about
2025.degree. F. to about 2075.degree. F. for a time of at least
about 3-1/2 hours.
15. A method for repairing a nickel-base superalloy article
comprising the steps of providing the nickel-base superalloy
article comprising a gas turbine stationary flowpath shroud having
flowpath cooling holes therein that has previously been in service,
wherein the article has a nominal composition in weight percent of
about 7.5 percent cobalt, about 7.0 percent chromium, about 1.5
percent molybdenum, about 5 percent tungsten, about 3 percent
rhenium, about 6.5 percent tantalum, about 6.2 percent aluminum,
about 0.15 percent hafnium, about 0.05 percent carbon, about 0.004
percent boron, about 0.01 percent yttrium, balance nickel and
impurities; and applying a restoration to a surface of the article
by the steps of providing a restoration nickel-base alloy, wherein
the restoration nickel-base alloy has a nominal composition in
weight percent of about 3.1 percent cobalt, about 7.6 percent
chromium, about 0.1 percent maximum molybdenum, about 3.85 percent
tungsten, about 0.02 percent maximum titanium, about 1.65 percent
rhenium, about 0.55 percent silicon, about 5.45 percent tantalum,
about 7.8 percent aluminum, about 0.15 percent hafnium, about 0.02
percent carbon, balance nickel and impurities, thereafter applying
a restoration coating of the restoration nickel-base alloy to the
surface of the article by a hyper-velocity oxyfuel metal spray
process or a low-pressure plasma spray process, and thereafter
heating the article with the restoration coating applied to the
surface thereof to a temperature of about 2050.degree. F. to
diffusion bond the restoration coating to the surface of the
article, prior to returning the article to service.
16. The method of claim 15, wherein the method includes an
additional step, after the step of heating, of redrilling the
cooling holes.
17. The method of claim 15, including an additional step, after the
step of applying is fully completed, of returning the article to
service.
18. The method of claim 15, including an additional step, after the
step of applying is fully completed, of returning the article to
service, wherein the article is heated to a temperature of not less
than about 1800.degree. F. during service.
19. The method of claim 15, wherein the step of applying the
restoration coating includes the step of applying the restoration
coating to a thickness of from about 0.030 to about 0.150
inches.
20. An article comprising: a nickel-base superalloy article that
has previously been in service; and a restoration applied to a
surface of the article, wherein the restoration is made of a
nickel-base superalloy different from that of the nickel-base
superalloy article, and wherein the restoration nickel-base
superalloy has no more than about 15 weight percent chromium and no
more than about 0.01 weight percent yttrium.
Description
[0001] This invention relates to the repair of a nickel-base
superalloy article and, more particularly, to a repair for
restoring a dimension of the article.
BACKGROUND OF THE INVENTION
[0002] In an aircraft gas turbine (jet) engine, air is drawn into
the front of the engine, compressed by a shaft-mounted compressor,
and mixed with fuel. The mixture is combusted, and the resulting
hot combustion gas is passed through a turbine mounted on the same
shaft. The turbine includes a rotating turbine disk with turbine
blades supported on its periphery, and a stationary (that is, not
rotating) gas turbine flowpath shroud that confines the combustion
gas to flow through the annulus between the turbine disk and the
shroud, and thence against the turbine blades. The constrained flow
of hot combustion gas turns the turbine by contacting an airfoil
portion of the turbine blade, which turns the shaft and provides
power to the compressor. The rotating turbine blades and the gas
turbine stationary flowpath shroud are heated to high temperatures
by the hot combustion gas. To aid them in withstanding the high
external temperatures, they are typically cooled by flows of
compressed cool air that are conducted through their interiors and
exit at cooling holes in their surfaces. The hot exhaust gases flow
from the back of the engine, driving it and the aircraft
forward.
[0003] During service, the turbine disk, the turbine blades, and
the gas turbine stationary flowpath shroud are all corroded,
eroded, and oxidized by the hot combustion gas, and material is
also lost by rubbing. Some of the metal of the turbine blades and
the gas turbine stationary flowpath shroud is burned away, reducing
the dimensions of the components below that which is acceptable for
economic operation of the gas turbine engine. Rotor excursions, due
to causes such as power bursts or hard landings, produce rubs
between the turbine blades and the shroud that dig into the shroud.
Consequently, with increasing periods of service, the clearance gap
between the turbine blades and the gas turbine stationary flowpath
shroud is increased. Eventually, the efficiency of the gas turbine
suffers because hot combustion gas leaks through the clearance gap
between the tips of the turbine blades and the gas turbine
stationary flowpath shroud and does not perform work to turn the
turbine blades.
[0004] When the gas turbine engine is overhauled, it is
conventional practice to restore the dimensions of the components
to within their original manufactured tolerances, thereby regaining
the efficiency of the gas turbine. In the case of the gas turbine
stationary flowpath shroud, techniques are known to conduct this
repair with thermally densified coatings, see for example U.S. Pat.
No. 5,561,827, whose disclosure is incorporated by reference. In
this approach, a preform is prepared and bonded to the flowpath
surface of the gas turbine stationary flowpath shroud, and the
cooling holes are redrilled. This approach has been successful for
restoring the dimensions of the gas turbine stationary flowpath
shroud, and, in conjunction with techniques for restoring the
turbine blades, for returning the gas turbine to its specification
dimensions and thence to its original efficiencies.
[0005] However, in some instances there has been observed a
preferential oxidation at the openings of the cooling holes of the
gas turbine flowpath shroud, after it is returned to service. This
preferential oxidation may cause the bore of the cooling holes to
be significantly reduced, so that the flow of cooling air is
reduced. The damage to the repaired gas turbine stationary flowpath
shroud caused by the hot combustion gas during the subsequent
service is consequently accelerated. Various alternative repair
techniques for the gas turbine stationary flowpath shroud have been
tried, but these alternatives either result in the same
preferential oxidation at the cooling holes or in premature crack
initiation in the restoration that is propagated into the
substrate.
[0006] There is a need for a repair procedure for the gas turbine
stationary flowpath shrouds that is satisfactory in restoring its
dimensions, but at the same time does result in the preferential
oxidation around the cooling holes and premature cracking of the
restoration and substrate during service. The present invention
fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
[0007] The present invention provides a method for repairing a
nickel-base superalloy article that has previously been in service
and has its dimensions reduced. The present approach is used to
restore the dimensions of the article, specifically the thickness
dimension in the case of the gas turbine stationary flowpath
shroud, to within acceptable tolerances. The present approach is
effective to build up the dimensions, but does not result in a
restoration that preferentially oxidizes at the cooling holes. The
restoration is also resistant to premature cracking in service. The
method uses an application technique that is well established for
other applications. The present approach is particularly useful
where the repaired article is heated to quite high temperatures by
the hot combustion gas, such as temperatures of not less than about
1800.degree. F, and preferably not less than about 1900.degree.
F.
[0008] A method for repairing a nickel-base superalloy article
comprises the steps of providing the nickel-base superalloy article
that has previously been in service, and applying a restoration to
a surface of the article. In the case of most interest, the article
is a gas turbine stationary flowpath shroud with cooling holes in
the flowpath surface. The restoration is applied by the steps of
providing a restoration nickel-base alloy. The restoration
nickel-base alloy preferably has no more than about 15 weight
percent chromium, more preferably no more than about 12 weigh
percent chromium, even more preferably no more than about 10 weight
percent chromium, and most preferably from about 7 to about 8
weight percent chromium. The restoration nickel-base alloy
preferably has no more than about 0.01 percent yttrium, and more
preferably substantially no yttrium (i.e., no more than about 0.001
percent), thereafter applying a restoration coating of the
restoration nickel-base alloy to the surface of the article by a
hyper-velocity oxyfuel (HVOF) or a low-pressure plasma spray (LPPS)
metal spray process, and thereafter heating the article with the
restoration coating applied to the surface thereof to a
sufficiently high temperature to diffusion bond the restoration
coating to the surface of the article. The cooling holes are
redrilled as necessary, either before or, preferably, after the
heating. After this application of the restoration coating is fully
completed, the article is returned to service, preferably in an
application wherein the article is heated to a temperature of not
less than about 1800.degree. F., more preferably not less than
about 1900.degree. F., during service.
[0009] In a preferred embodiment, the article is Rene.TM. N5 alloy
having a nominal composition in weight percent of about 7.5 percent
cobalt, about 7.0 percent chromium, about 1.5 percent molybdenum,
about 5 percent tungsten, about 3 percent rhenium, about 6.5
percent tantalum, about 6.2 percent aluminum, about 0.15 percent
hafnium, about 0.05 percent carbon, about 0.004 percent boron,
about 0.01 percent yttrium, balance nickel and impurities. The
preferred restoration nickel-base alloy preferably has a nominal
composition in weight percent of about 3.1 percent cobalt, about
7.6 percent chromium, about 0.1 percent maximum molybdenum, about
3.85 percent tungsten, about 0.02 percent maximum titanium, about
1.65 percent rhenium, about 0.55 percent silicon, about 5.45
percent tantalum, about 7.8 percent aluminum, about 0.15 percent
hafnium, about 0.02 percent carbon, balance nickel and impurities.
In another embodiment, the restoration nickel-base alloy has a
nominal composition in weight percent of 0.01-0.03 percent carbon,
0.1 percent maximum manganese, 0.5-0.6 percent silicon, 0.01
percent maximum phosphorus, 0.004 percent maximum sulfur, 7.4-7.8
percent chromium, 2.9-3.3 percent cobalt, 0.01 percent maximum
molybdenum, 3.7-4.0 percent tungsten, 5.3-5.6 percent tantalum,
0.02 percent maximum titanium, 7.6-8.0 percent aluminum, 1.5-1.8
percent rhenium, 0.005 percent maximum selenium, 0.3 percent
maximum platinum, 0.01-0.02 percent boron, 0.03 percent maximum
zirconium, 0.12-0.18 percent hafnium, 0.1 percent maximum niobium,
0.1 percent maximum vanadium, 0.1 percent maximum copper, 0.2
percent maximum iron, 0.0035 percent maximum magnesium, 0.01
percent maximum oxygen, 0.01 percent maximum nitrogen, balance
nickel with other elements 0.5 percent maximum.
[0010] The restoration alloy that forms the restoration coating may
be a nickel-base superalloy different from that of the nickel-base
superalloy article. However, the restoration alloy is not
necessarily a nickel-base superalloy and, if it is, the restoration
alloy need not be heat treated to produce gamma prime precipitates.
The purpose of the restoration coating is to restore dimensions,
not primarily to provide strength although it may do so.
[0011] The restoration coating is preferably applied to a thickness
of from about 0.030 to about 0.150 inches, more preferably to a
thickness of from about 0.060 to about 0.100 inches.
[0012] In the step of heating to effect diffusion bonding, the
article is preferably heated to a temperature of at least
1950.degree. F., more preferably to a temperature of from about
2025.degree. F. to about 2075.degree. F., and most preferably to a
temperature of about 2050.degree. F. The heat treatment is
preferably for a time of at least about 1 hour, more preferably for
about 3-1/2 to 4-1/2 hours, and most preferably for about four
hours.
[0013] The restoration coating is resistant to oxidation, and in
particular is resistant to preferential oxidation around the
cooling holes. The restoration coating is itself preferably a
nickel-base superalloy that may be strengthened by the
precipitation of gamma-prime phase or a related phase. This
moderate strength aids in resisting any cracking of the restoration
during subsequent service. The hyper-velocity oxyfuel (HVOF, also
sometimes called high-velocity oxyfuel) metal spray process and the
low-pressure plasma spray (LPPS) process are proven application
techniques that are used in other circumstances.
[0014] Other features and advantages of the present invention will
be apparent from the following more detailed description of the
preferred embodiment, taken in conjunction with the accompanying
drawings, which illustrate, by way of example, the principles of
the invention. The scope of the invention is not, however, limited
to this preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a block flow diagram of a preferred approach for
practicing an embodiment of the method of the invention;
[0016] FIG. 2 is a fragmentary schematic front elevational view of
a portion of a gas turbine; and
[0017] FIG. 3 is a cross-sectional view on line 3-3 of FIG. 2 of a
gas turbine stationary flowpath shroud assembly and its relation to
a turbine blade;
[0018] FIG. 4 is a perspective view of a segment of the gas turbine
stationary flowpath shroud;
[0019] FIG. 5 is a bottom view of the gas turbine stationary
flowpath shroud;
[0020] FIG. 6 is a front view of the gas turbine stationary
flowpath shroud; and
[0021] FIG. 7 is an enlarged side schematic elevational view of the
gas turbine stationary flowpath shroud during the restoration
process.
DETAILED DESCRIPTION OF THE INVENTION
[0022] FIG. 1 depicts in block diagram form a preferred embodiment
of a method for repairing a nickel-base superalloy article. In
practicing the method, the nickel-base superalloy article that has
previously been in service is provided, step 20. The present
approach is applicable to any operable article. FIGS. 2-7 depict
the use of the method in relation to a presently preferred article,
a gas turbine stationary flowpath shroud.
[0023] FIG. 2 presents a simplified depiction of the relevant
portions of a gas turbine 40 illustrating only the components of
interest. The gas turbine 40 includes a turbine disk 42 that is
fixed to and rotates with a center shaft 44. A plurality of turbine
blades 46 extend radially outwardly from a periphery 48 of the
turbine disk 42. A gas turbine stationary flowpath shroud 50 forms
a tunnel-like structure in which the turbine disk 42, the shaft 44,
and the turbine blades 46 turn. (The gas turbine stationary
flowpath shroud 50 is termed "stationary" and does not rotate as
the turbine disk 42, the shaft 44, and the turbine blades 46
rotate. The stationary gas turbine stationary flowpath shroud 50 is
to be distinguished from the rotating shrouds that are found near
the tips of some types of gas turbine blades.) The gas turbine
stationary flowpath shroud 50 is formed by a series of stationary
shroud segments 52 that together define the cylindrical gas turbine
stationary flowpath shroud 50. A combustion gas flow 54 flowing
from the combustors (not shown) of the gas turbine engine is
perpendicular to the plane of the drawing of FIG. 2.
[0024] FIG. 3 illustrates the gas turbine stationary flowpath
shroud 50 and one of the stationary shroud segments 52 in greater
detail. Each stationary shroud segment 52 is supported on a shroud
hanger structure 56. The gas turbine stationary flowpath shroud 50
and the stationary shroud segment 52 have a flowpath surface 58
that faces but is spaced apart from a tip 60 of the turbine blade
46. In the operation of the gas turbine 40, it is important that
the separation of the flowpath surface 58 and the tip 60, termed
the clearance gap CG, be within specified tolerance limits. During
service, both the flowpath surface 58 and the tip 60 are eroded,
corroded, and oxidized by the hot combustion gas 54, and
occasionally rubbed together, with a consequent loss of material.
The value of CG therefore increases over time, until it becomes so
large that an unacceptable amount of the combustion gas flow 54
leaks between the flowpath surface 58 of the stationary flowpath
shroud 50 and the tip 60, so that it does not contact the turbine
blade 46 and impart energy to it. The result is the loss of
efficiency of the gas turbine 40.
[0025] One technique for increasing the temperature at which the
turbine blade 46 and the gas turbine stationary flowpath shroud 50
may operate is to pass a flow of a cooling gas, typically air,
through their interiors. The stationary shroud segment 52 therefore
has interior cooling channels 62 through which the cooling gas
passes to emerge at cooling holes 64 in the flowpath surface 58.
The number and pattern of the cooling holes 64 in the stationary
shroud segment 52 is selected to provide the desired cooling. FIGS.
4-6 illustrate one approach to the positioning of the cooling holes
64. FIG. 4 depicts from the side opposite the flowpath surface 58
the entrance end 70 of the cooling channels 62, while FIGS. 5-6
illustrate a pattern of the cooling holes 64 from bottom (FIG. 5)
and front elevational (FIG. 6) views. (The turbine blade 46 also
has internal cooling channels 66 with cooling holes 68, but the
present embodiment is concerned only with the stationary shroud
segment 52 and not the turbine blades 42.)
[0026] The article such as the gas turbine stationary flowpath
shroud 50 is most preferably made of a nickel-base superalloy. As
used herein, "nickel-base" means that the composition has more
nickel present than any other element. The nickel-base superalloys
are of a composition that is strengthened by the precipitation of
gamma-prime phase or a related phase. In a preferred case, the
article is a nickel-base alloy that is also a superalloy and has a
nominal composition in weight percent of about 7.5 percent cobalt,
about 7.0 percent chromium, about 1.5 percent molybdenum, about 5
percent tungsten, about 3 percent rhenium, about 6.5 percent
tantalum, about 6.2 percent aluminum, about 0.15 percent hafnium,
about 0.05 percent carbon, about 0.004 percent boron, about 0.01
percent yttrium, balance nickel and impurities.
[0027] The article that has previously been in service is cleaned
to remove dirt, oxidation and corrosion products, and other foreign
matter resulting from the prior service, step 21. The cleaning is
preferably accomplished by fluoride ion cleaning.
[0028] A restoration is applied to a surface, here the flowpath
surface 58, of the article, here the stationary shroud segment 52,
step 22. The application step 22 first includes providing a
restoration nickel-base alloy, step 24. The restoration nickel-base
alloy is preferably a nickel-base superalloy that is strengthened
by the precipitation of gamma prime or a related phase. The
restoration nickel-base alloy preferably has no more than 15 weight
percent chromium, more preferably no more than 12 weigh percent
chromium, even more preferably no more than 10 weight percent
chromium, and most preferably from 7 to 8 weight percent chromium.
The restoration coating has no more than about 0.01 percent
yttrium, and preferably substantially no yttrium (i.e., no more
than about 0.001 percent). If larger amounts of chromium and
yttrium are present in the restoration coating, such as 20 percent
chromium and 0.07 percent yttrium, the restoration coating is
operable, but there is a tendency for oxidation within the shroud
cooling holes 64 that gradually plugs the cooling holes 64 and
reduces the cooling air flow through them.
[0029] In a preferred embodiment, the restoration nickel-base alloy
has a nominal composition in weight percent of about 3.1 percent
cobalt, about 7.6 percent chromium, about 0.1 percent maximum
molybdenum, about 3.85 percent tungsten, about 0.02 percent maximum
titanium, about 1.65 percent rhenium, about 0.55 percent silicon,
about 5.45 percent tantalum, about 7.8 percent aluminum, about 0.15
percent hafnium, about 0.02 percent carbon, balance nickel and
impurities. In another embodiment, the restoration nickel-base
alloy has a nominal composition in weight percent of 0.01-0.03
percent carbon, 0.1 percent maximum manganese, 0.5-0.6 percent
silicon, 0.01 percent maximum phosphorus, 0.004 percent maximum
sulfur, 7.4-7.8 percent chromium, 2.9-3.3 percent cobalt, 0.01
percent maximum molybdenum, 3.7-4.0 percent tungsten, 5.3-5.6
percent tantalum, 0.02 percent maximum titanium, 7.6-8.0 percent
aluminum, 1.5-1.8 percent rhenium, 0.005 percent maximum selenium,
0.3 percent maximum platinum, 0.01-0.02 percent boron, 0.03 percent
maximum zirconium, 0.12-0.18 percent hafnium, 0.1 percent maximum
niobium, 0.1 percent maximum vanadium, 0.1 percent maximum copper,
0.2 percent maximum iron, 0.0035 percent maximum magnesium, 0.01
percent maximum oxygen, 0.01 percent maximum nitrogen, balance
nickel with other elements 0.5 percent maximum.
[0030] As seen in FIG. 7, a restoration coating 72 of the
restoration nickel-base alloy is thereafter applied, step 26, to a
pre-repair flowpath surface 74 by a hyper-velocity oxyfuel (HVOF)
metal spray process or by a low-pressure plasma spray (LPPS)
process. The preferred HVOF metal spray process is a known
technique for applying metallic deposits in other contexts, see for
example U.S. Pat. No. 6,233,822, whose disclosure is incorporated
by reference. The LPPS process is also known in the art for other
applications. The restoration coating 72 may be applied in any
operable thickness t.sub.C that returns the dimensions of the
article to the desired values, but it is preferably applied in a
thickness of from about 0.030 to about 0.150 inches, more
preferably to a thickness of from about 0.060 to about 0.100
inches.
[0031] The stationary shroud segment 52 with the restoration
coating 72 applied to the pre-repair flowpath surface 74 is
thereafter heated to a sufficiently high temperature to diffusion
bond the restoration coating to the flowpath surface, step 28.
Preferably, the stationary shroud segment and the applied
restoration coating 72 are heated to a temperature of from about
2025.degree. F. to about 2075.degree. F. for a time of at least
about 3-1/2 hours, most preferably to about 2050.degree. F. for
about 4 hours. The exposed surface of the restoration coating 72 is
the new, repaired flowpath surface 58.
[0032] The application process 22 typically overcoats the visible
cooling holes with the restoration coating 72. It is therefore
necessary to redrill the cooling holes, step 30. The redrilling
step 30, like the originally drilling of the cooling channels 62,
is preferably performed by laser drilling.
[0033] Optionally, an environmental coating may be applied to
protect portions of the article. The environmental coating is
typically a diffusion aluminide applied by vapor phase aluminiding
(VPA), a process known in the art. Any portion of the environmental
coating that deposits on the flowpath surface 58 is machined away
prior to the initial operation of the engine.
[0034] After the application step 22 and redrilling step 30 are
complete, the repaired article with restored dimensions is returned
to service, step 32. Although the present approach may be used in
relation to any service temperature at which the article is
operable, it is most preferably used where the service temperature
is very high, as in high-temperature gas turbines 40. Such high
services temperatures are typically not less than about
1800.degree. F., and preferably not less than about 1900.degree.
F.
[0035] Thus, an article comprises a nickel-base superalloy article
that has previously been in service, and a restoration applied to a
surface of the article. The restoration is made of a nickel-base
superalloy different from that of the nickel-base superalloy
article. The restoration nickel-base superalloy has no more than
about 15 weight percent chromium and no more than about 0.01 weight
percent yttrium.
[0036] The present approach has been reduced to practice using the
preferred approach discussed above. High-pressure-turbine
stationary shroud segments of the preferred substrate material were
coated with the preferred restoration coating material. The
restoration coating 72 adheres well to the substrate material after
the heating step 28. The restoration coating 72 machines well in
the redrilling of the cooling holes, step 30.
[0037] The restoration coating material must be resistant to
oxidation and corrosion damage, and these two properties were
evaluated using coated specimens prepared by the preferred approach
discussed above. For the oxidation-resistance determination,
individual pin samples of the preferred Rene.TM. N5 substrate
material were (1) left uncoated as a control, (2) coated with a
prior coating material that had a chromium content of 20 weight
percent and a yttrium content of 0.1 weight percent and was applied
by the HVOF process, and (3) coated with the preferred restoration
coating material by the HVOF process. The samples were subjected to
cyclic oxidation at 2075.degree. F. maximum temperature with 0.5
mach combustion gas velocity, and a heating cycle of a rapid heat,
5 minutes at temperature, and a rapid cool for 2000 cycles. The
weight after oxidation testing was compared with the weight prior
to oxidation testing, to determine a weight loss. The uncoated pin
had a weight loss of 0.0083 grams, the pin coated with the prior
coating material had a weight loss of 0.1729 grams, and the pin
coated with the present restoration coating material had a weight
loss of 0.0067 grams. The present restoration coating material had
better oxidation resistance than the other samples, under the
relatively high-temperature testing conditions.
[0038] In the accelerated corrosion testing, individual samples of
the same types as used in the oxidation testing were prepared, and
uncoated pin samples were also prepared as a baseline. The pin
samples were exposed at 1700.degree. F. for 10 minutes to Mach 0.5
flowing combustion gas, then exposed to 5.4 cubic centimeters per
minute of salt solution added to the Mach 0.5 flowing combustion
gas, then heated to 2075.degree. F. for 5 minutes, and then rapidly
cooled. Each sample was given 200 cycles of this type. The pins
were sectioned and inspected metallographically. The effective
diameter loss was 0.0149 inches for the specimens prepared by the
present approach, 0.0286 inches for the specimens coated with the
prior coating material applied by the HVOF process, and 0.0428
inches for the uncoated specimens. The present approach yielded
significantly better hot-salt corrosion performance.
[0039] Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various
modifications and enhancements may be made without departing from
the spirit and scope of the invention. Accordingly, the invention
is not to be limited except as by the appended claims.
* * * * *