U.S. patent application number 10/606436 was filed with the patent office on 2004-12-30 for clean atmosphere heat treat for coated turbine components.
Invention is credited to Burns, Steven M., Hahn, Steven P..
Application Number | 20040261923 10/606436 |
Document ID | / |
Family ID | 33418688 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040261923 |
Kind Code |
A1 |
Burns, Steven M. ; et
al. |
December 30, 2004 |
Clean atmosphere heat treat for coated turbine components
Abstract
A method for heat treating at least one workpiece, such as a
coated turbine engine component, is provided. The method comprises
the steps of cleaning a furnace to be used during the heat treating
method, which cleaning method comprising injecting a gas at a
workpiece center location and applying heat, and diffusion heat
treating the at least one workpiece in a gas atmosphere with the
gas being injected at the workpiece center location. After the
diffusion heat treatment step, the coated workpiece(s) may be
subjected to a surface finishing operation such as a peening
operation.
Inventors: |
Burns, Steven M.; (West
Hartford, CT) ; Hahn, Steven P.; (Avon, CT) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510
US
|
Family ID: |
33418688 |
Appl. No.: |
10/606436 |
Filed: |
June 25, 2003 |
Current U.S.
Class: |
148/712 |
Current CPC
Class: |
C23C 8/80 20130101; C21D
1/76 20130101; C21D 2241/01 20130101; C23C 8/06 20130101; C21D 1/74
20130101 |
Class at
Publication: |
148/712 |
International
Class: |
C22F 001/02 |
Claims
What is claimed is:
1. A method for heat treating at least one workpiece comprising the
steps of: cleaning a furnace to be used during said heat treating
method; said cleaning method comprising injecting a gas at a
workpiece center location and applying heat; and diffusion heat
treating said at least one workpiece in a gas atmosphere with said
gas being injected at said workpiece center location.
2. A method according to claim 1, wherein said cleaning method
comprises injecting said gas into said furnace at said workpiece
center location at a flow rate sufficient to create a pressure
differential which carries contaminants away from said workpiece
center location toward an exit.
3. A method according to claim 2, wherein said gas injecting step
comprises injecting said gas at a partial pressure of at least 0.8
Torr.
4. A method according to claim 2, wherein said gas injecting step
comprises injecting said gas into said furnace at a rate of 30
liters per minute to 70 liters per minute.
5. A method according to claim 2, wherein said gas injecting step
comprises injecting an inert gas.
6. A method according to claim 2, wherein said gas injecting step
comprises injecting argon.
7. A method according to claim 2, wherein said gas injecting step
comprises injecting a reducing gas.
8. A method according to claim 1, wherein said diffusion heat
treatment step is carried out at a temperature in the range of 1900
degrees Fahrenheit to 2500 degrees Fahrenheit for a time period in
the range of 1 to 24 hours.
9. A method according to claim 1, wherein said diffusion heat
treatment step comprises injecting said gas into said workpiece
center location at a rate sufficient to carry away contaminants in
said workpiece but less than a rate at which a door to said furnace
is caused to open.
10. A method according to claim 9, wherein said diffusion heat
treatment step comprises injecting said gas into said workpiece
center location at a partial pressure of at least 0.8 Torr.
11. A method according to claim 9, wherein said gas is injected
into said furnace at a flow rate of 30 liters per minute to 70
liters per minute.
12. A method according to claim 9, wherein said diffusion heat
treatment comprises injecting an inert gas.
13. A method according to claim 9, wherein said diffusion treatment
comprises injecting argon.
14. A method according to claim 9, wherein said diffusion heat
treatment comprises injecting a reducing gas.
15. A method for providing at least one workpiece having a coating
comprising the steps of: diffusion heat treating said at least one
workpiece in gas atmosphere within a furnace with said gas being
injected at a workpiece center location; removing said workpiece
from said furnace; and subjecting said coated workpiece to a
surface finishing operation.
16. A method according to claim 15, wherein said diffusion heat
treatment step is carried out at a temperature in the range of 1900
degrees Fahrenheit to 2500 degrees Fahrenheit for a time period in
the range of 1 to 24 hours.
17. A method according to claim 15, wherein said diffusion heat
treatment step comprises injecting said gas into said workpiece
center location at a rate sufficient to carry away contaminants in
said workpiece but less than a rate at which a door to said furnace
is caused to open.
18. A method according to claim 17, wherein said diffusion heat
treatment step comprises injecting said gas into said workpiece
center location at a partial pressure of at least 0.8 Torr.
19. A method according to claim 17, wherein said gas is injected
into said furnace at a flow rate of 30 liter per minute to 70
liters per minute.
20. A method according to claim 15, wherein said surface finishing
step comprising subjecting said coated workpiece to a peening
operation.
21. A method according to claim 1S, wherein said diffusion heat
treating step comprises injecting an inert gas into said workpiece
center location.
22. A method according to claim 15, wherein said diffusion heat
treating step comprises injecting argon into said workpiece center
location.
23. A method according to claim 15, wherein said diffusion heat
treating step comprises injecting a reducing gas into said
workpiece center location.
24. A system for heat treating a coated workpiece comprising: a
furnace having a chamber; and means for injecting a gas into an
interior of said furnace chamber at a workpiece center
location.
25. A system according to claim 24, wherein said gas injecting
means comprises means for injecting said gas at a flow rate
sufficient to carry any contaminants from said workpiece center
location toward an exit.
26. A system according to claim 24, wherein said injecting means
comprises means for injecting at least one of an inert gas or a
reducing gas.
27. A system according to claim 24, wherein said injecting means
comprises means for injecting argon gas.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method for heat treating
workpieces, such as coated turbine components, and to an improved
system for performing the heat treat method of the present
invention.
[0002] Overlay type metallic coatings (i.e. NiCoCrAlY, CoCrAlY,
etc.) are mostly characterized by their oxidation resistant
sub-alloy protection properties and improved life span within the
turbine engine environment. These overlay metallic coatings may be
applied to substrate surfaces by thermal spray processes, such as
low pressure plasma spray and atmosphere pressure plasma spray, or
by vapor deposition processes such as electron beam physical vapor
deposition or cathodic arc. The density of the coating plays an
important role in the oxidation resistance characteristics as well
as the life span at which the coating will protect the substrate
from the corrosive environment in which it operates. A coating free
of open pockets, voids, fissures, cracks, or leaders provides
significantly longer oxidation life protection than a coating
containing such aforementioned characteristics. The state-of-the
art technology used today to ensure that such coatings are close to
100% dense as possible is to apply the coating as dense as
possible, then diffusion heat treat the coating, followed by
subjecting the overlay coating to energy from processes such as
peening. The peening process transfers enough kinetic energy at
impact from the peen media velocity into the coating surface to
increase the coating density by compaction and to improve the
coating surface finish. The extent to which the peening process can
improve the coating density and surface finish is related to the
amount of kinetic energy that can be transferred from the peening
media impact event onto and into the coating surface (often
measured with almen strip intensity) in conjunction with the
coating's ductility. It should be noted that to apply coatings
which are excessively ductile will not provide the proper
protection within the hot corrosive environments in which they
operate. Also, if one applies a coating that is excessively hard,
the coating will not react well to the peening process and will
leave excessive porosity within the coating structure, ultimately
resulting in a poor life oxidation resistance coating.
SUMMARY OF THE INVENTION
[0003] Accordingly, it is an object to provide an improved method
for heat treating coated workpieces, such as coated turbine engine
components.
[0004] It is a further object of the present invention to provide
an improved system for heat treating at least one coated
workpiece.
[0005] The foregoing objects are attained by the present
invention.
[0006] In accordance with the present invention, a method for heat
treating workpieces is provided. The method broadly comprises the
steps of cleaning a furnace to be used during the heat treating
method, the cleaning method comprising injecting an inert gas, such
as argon, or a reducing gas, such as hydrogen, at a workpiece
center location and applying heat, and thereafter diffusion heat
treating the at least one coated workpiece in a gas atmosphere,
such as an inert gas or a reducing gas atmosphere, with the gas
again being injected at the workpiece center location. After the
heat treatment, the coated workpiece may be subjected to a surface
finishing operation.
[0007] Further, in accordance with the present invention, there is
provided a system for heat treating a coated workpiece broadly
comprising a furnace and means for injecting a gas into an interior
of the furnace at a workpiece center location.
[0008] Other details of the clean atmosphere heat treat for coated
turbine components, as well as other advantages and objects
attendant thereto, are set forth in the following detailed
description and the accompanying drawings wherein like reference
numerals depict like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic representative of a heat treatment
system in accordance with the present invention;
[0010] FIG. 2 is a photomicrograph showing an as-deposited and
diffused coating on a workpiece;
[0011] FIG. 3 is a photomicrograph showing a coating which has been
subjected to the clean atmosphere diffusion heat treatment of the
present invention after surface finishing; and
[0012] FIG. 4 is a photomicrograph showing a coating which has not
been subjected to the clean atmosphere diffusion heat treatment of
the present invention after surface finishing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0013] Overlay coatings are subjected to a diffusion heat treatment
process followed by high energy impact events from processes such
as peening to improve the coating density. The extent that a
coating can be made 100% dense is related to the coating ductility
as well as the surface finishing energy that can be obtained.
[0014] It has been found by the inventors that the cleanliness of
the diffusion heat treatment environment plays a significant role
in coating ductility and the coating's final quality acceptability.
A coating that has extensive open pockets, voids, fissures, cracks
or leaders and has been exposed to a typical heat treat furnace
atmosphere (vacuum or inert gas) can result in a coating that is
impossible to bring to an acceptable density and acceptable quality
condition. The contamination that affects the coating quality
occurs within the furnace, from vacuum leaks and/or contamination
from various elements within the furnace itself.
[0015] Previous practice within the coating industry to correct a
contaminated furnace has been to ensure the furnace is adequately
free from vacuum leaks (a leak-up rate of 20 microns an hour or
less) and perform a vacuum burn-out heat treat cycle a few hundred
degrees higher than the highest temperature production heat treat
cycle previously used within the furnace.
[0016] It has been found that in cases where a coating that has
been applied at a less than optimum deposition angle or in cases of
a normally deposited coating that has an abundance of extensive
open pockets, voids, fissures, cracks, or leaders followed by a
diffusion heat treat cycle in a standard, normally acceptable and
high temperature thermally cycled furnace, the coating generally
cannot be transformed by surface finishing processes to an
acceptable density/quality level.
[0017] The solution to improving coatings so they can be better
transformed by surface finishing processes to a desirable
density/quality level/surface finish begins with cleaning a furnace
to be used in the diffusion heat treatment using a high temperature
burnout heat-treat cycle with a gas, such as inert gas, preferably
argon, and/or a reducing gas, such as hydrogen, being injected at
the center of the work piece location area at a partial pressure
preferably of 0.8 Torr or greater. It has been found that this
creates a significantly cleaner furnace than the standard burn-out
heat treat cycle used throughout the industry.
[0018] FIG. 1 illustrates a modified heat treatment system 10 in
accordance with the present invention. The system 10 includes a gas
source 12, a furnace 13 having a chamber 14 in which workpieces
(not shown), such as coated turbine engine components, to be
treated are placed, a manifold 18 for delivering the gas to the
center 20 of the work piece location area, a feed line 22 between
the manifold 18 and the gas source 12, and a valve 24 for
controlling the flow rate of the gas. The inventive furnace 13 is
different from prior art furnaces where a gas is injected into the
furnace through nozzles positioned about the exterior surface of
the chamber 14. It has been found that nozzles positioned in such
locations actually increase the contamination which appears in the
workpieces and the coatings. This is because when heat treating a
workpiece and coating within such a furnace, any contaminants which
are present on or in the furnace walls are mostly turned into a
vapor state once the furnace reaches adequate temperature. These
contaminants are deposited onto the workpieces and the coating,
changing the coating ductility by tying up grain boundaries within
the coating. Once the ductility of the coating is decreased, the
coating and workpiece cannot be surface finished with enough energy
to adequately improve coating density to an acceptable level
without damaging the work piece. It should be understood that when
heat treating a coating within a furnace, any vacuum leaks which
are present within the furnace leak in air which contains oxygen.
The oxygen often oxidizes the workpieces as well as contaminates
them, which changes the coating ductility by tying up grain
boundaries within the coating. Once the ductility of the coating is
decreased, the coating and the workpieces cannot be surface
finished with enough energy to adequately improve coating density
to an acceptable level without damaging the workpieces.
[0019] The system 10 of the present invention with the improved
furnace design avoids such contamination of the workpieces and the
coatings.
[0020] In accordance with the present invention, the furnace
chamber 14 is first cleaned by heating the furnace to a temperature
which is 200-300.degree. F. greater than the diffusion heat
treatment temperature, typically greater than 2000.degree. F., for
a time period of 30 minutes or more. During the heating cycle, the
gas is introduced at a flow rate which creates movement of
contaminants from the center 20 of the workpiece location towards
low pressure areas 26 about the furnace chamber 14 created by one
or more vacuum pumps 30 and the exit area 28. Suitable gas flow
rates are within the range of those sufficient to carry the
contaminants away from the center 20 to those which would cause the
door of the furnace chamber 14 to open. A preferred flow rate for
the gas is in the range of 30 liters per minute to 70 liters per
minute. The gas is introduced at a partial pressure sufficient to
create a pressure differential which carries the contaminants away
from the center 20. A particularly useful gas partial pressure is
0.8 Torr or greater.
[0021] After cleaning the furnace in the above manner, the
diffusion heat treatment of the coated workpieces is carried out in
the same gas environment under the same gas flow rate and partial
pressure conditions. As before, an inert gas, with argon being a
preferred gas, and/or a reducing gas, such as hydrogen, is injected
into the chamber 14 at the center 20 of the workpiece location at
the flow rate and partial pressures mentioned hereinabove. It has
been found that by flowing the gas at a rate of 30 liters per
minute to 70 liters per minute, the vacuum level during the
diffusion heat treatment is in the range of 800 microns to 2000
microns. While partial pressures of 0.8 Torr or greater are useful,
the beneficial range of partial pressure depends on the
configuration of the heat treat furnace as well as the quantity and
condition of the coated workpieces being heat treated. The
diffusion heat treatment may be carried out at a temperature in the
range of 1900 degrees Fahrenheit to 2500 degrees Fahrenheit for a
time period in the range of 1 to 24 hours. It has been found that
workpieces subjected to the diffusion heat treatment described
herein were able to be surface finished to produce an acceptable
density and quality part.
[0022] After the diffusion heat treatment step, the workpieces with
the coatings can be subjected to any surface finishing operation
known in the art, such as a peening operation, to form a coating
having an acceptable coating density and quality level.
[0023] The physics of producing an acceptable coating density and
quality level through heat treating and surface finishing using the
method of the present invention is as follows. When heat treating a
workpiece and coating within a furnace, any vacuum leaks or
elemental contamination which are present during the heat treat
process will effectually reach the parts resulting in a decrease in
coating ductility which cannot be further surface finished
adequately to produce an acceptable density level coating. The
method of first cleaning the furnace by performing a partial
pressure heat treat with the gas, preferably argon, injected at the
workpiece center location (typically the furnace center) results in
the gas sweeping from the center of the furnace outward carrying
(by means of random molecule collisions) all contaminates away from
the furnace center which are removed by the vacuum pump(s) 30. The
second step of actually performing the diffusion heat treatment of
the coating and workpieces within the partial pressure gas
atmosphere with the gas, preferably argon, being injected at the
work pieces' center location results in a high pressure clean area
within the vacuum furnace where the parts are located. All
contaminates, whether from inside the furnace or as a result of
vacuum leaks, are forced away from the high-pressure protective
area (where the parts are located) by means of random molecule
collisions where the high pressure area always seeks low pressure
areas. This method results in a clean diffusion heat treatment that
allows the coatings to adequately diffuse into the base alloy
without changing the coating ductility.
[0024] The method of the present invention has been found to have
particular utility in the diffusion heat treatment of turbine
engine components having an overlay coating applied thereto. The
method of the present invention can be used with any workpiece
coated with any overlay coating known in the art.
[0025] FIG. 2 illustrates a workpiece with an as deposited and
diffused coating. FIG. 3 illustrates a coating which has been
formed using the method described herein and which was surface
finished by shot peening. As can be seen from FIG. 3, the coating
is free of pores, voids, and other bad features. In fact, the
coating is homogeneous and has very good ductility. FIG. 4
illustrates a coating which was not formed using the heat diffusion
treatment of the present invention. After surface finishing, a poor
quality coating was produced. As can be seen from FIG. 4, the
coating has voids and fissures which makes it quite brittle.
[0026] While it is preferred to use a single gas for the furnace
cleaning and diffusion heat treating steps, it is possible to use a
mixture of gases, such as a mixture of inert gases or a mixture of
an inert gas with a reducing gas.
[0027] It is apparent that there has been provided in accordance
with the present invention a clean heat treat for coated turbine
components which fully satisfies the objects, means, and advantages
set forth hereinbefore. While the present invention has been
described in the context of specific embodiments thereof, other
alternatives, modifications, and variations will become apparent to
those skilled in the art having read the foregoing detailed
description. Accordingly, it is intended to embrace those
alternatives, modifications, and variations as fall within the
broad scope of the appended claims.
* * * * *