U.S. patent application number 12/406863 was filed with the patent office on 2009-08-06 for method for substrate stabilization of diffusion aluminide coated nickel-based superalloys.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Paul J. FINK, Christine GOVERN, Joseph M. GREENE, Brian T. HAZEL.
Application Number | 20090197112 12/406863 |
Document ID | / |
Family ID | 36616883 |
Filed Date | 2009-08-06 |
United States Patent
Application |
20090197112 |
Kind Code |
A1 |
FINK; Paul J. ; et
al. |
August 6, 2009 |
Method for Substrate Stabilization of Diffusion Aluminide Coated
Nickel-Based Superalloys
Abstract
An article and method for stabilization of a nickel-based
superalloy coated with a diffusion aluminide coating. The region
below the aluminide coating is first carburized to form refractory
carbides. The article is cleaned and masked as required so that
regions that will not have an aluminide coating are not carburized.
After placing the article into a furnace and heating in a
non-oxidizing atmosphere to a carburizing temperature, a
carburizing gas is introduced, and the near surface region is
carburized to a depth of about 100 microns. Refractory carbides are
formed in this region. When a diffusion aluminide coating is formed
on the article, the refractory elements, being present as
refractory carbides, are not available to form detrimental TCP
phases.
Inventors: |
FINK; Paul J.; (Maineville,
OH) ; HAZEL; Brian T.; (West Chester, OH) ;
GOVERN; Christine; (Cincinnati, OH) ; GREENE; Joseph
M.; (Indianapolis, IN) |
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: |
36616883 |
Appl. No.: |
12/406863 |
Filed: |
March 18, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11359788 |
Feb 22, 2006 |
7524382 |
|
|
12406863 |
|
|
|
|
60656691 |
Feb 26, 2005 |
|
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Current U.S.
Class: |
428/627 |
Current CPC
Class: |
C23C 8/02 20130101; Y10T
428/12576 20150115; C23C 8/20 20130101; C23C 10/48 20130101; C23C
8/04 20130101; Y02T 50/60 20130101 |
Class at
Publication: |
428/627 |
International
Class: |
B32B 15/04 20060101
B32B015/04 |
Claims
1. A nickel based superalloy article, comprising: a nickel-based
superalloy substrate having a surface; refractory carbide
precipitates formed by carburizing to a preselected depth below the
surface of at least a portion of the superalloy substrate, the
remainder of the superalloy substrate being substantially free of
refractory carbide precipitates from carburizing at or below the
surface; and a diffusion aluminide coating formed on that portion
of the substrate surface having carbide precipitates extending a
preselected distance below the surface, the diffusion aluminide
extending below the substrate surface to a distance substantially
no greater than that of the refractory carbides.
2. The nickel-based superalloy article of claim 1 wherein the
nickel-based superalloy substrate is a turbine airfoil selected
from the group consisting of blades and vanes.
3. The superalloy article of claim 1 wherein the refractory carbide
precipitates are formed to a preselected depth below the substrate
surface of between about 10-100 microns.
4. The superalloy article of claim 3 wherein the diffusion
aluminide coating is formed to a distance of 10-50 microns below
the substrate surface and wherein the distance is of the diffusion
aluminide coating extends to substantially the same distance below
the substrate surface as the refractory carbide precipitates.
5. The superalloy article of claim 1 further characterized by a
substantial absence of TCP phases in a near-surface region.
6. An article comprising: a substrate comprising a nickel-based
superalloy, the substrate having a substrate surface; a carbide
precipitate region comprising refractory carbide precipitates, at
least some of the refractory carbide precipitates being diffused
into the substrate to a depth of about 10 to about 100 micrometers
below the substrate surface, the carbide precipitate region being
substantially free of acicular topologically close-packed (TCP)
phase forms; an aluminum-based layer; and an aluminum-based
diffusion region contiguous to the aluminum-based layer, the
aluminum-based diffusion region extending from the aluminum-based
layer into the carbide precipitate region, and into the substrate
to a depth of about 25 to about 50 micrometers below the substrate
surface.
7. The article of claim 6, wherein the aluminum-based layer is
contiguous to the carbide precipitate region.
8. The article of claim 6, wherein the aluminum-based layer is the
outermost layer.
9. The article of claim 6, wherein the at least some of the
refractory carbide precipitates diffused into the substrate are
diffused therein to a depth of about 25 to about 100 micrometers
below the substrate surface.
10. The article of claim 6, wherein the article is a turbine
airfoil blade.
11. The article of claim 6, wherein the article is a turbine
airfoil vane.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 11/359,788, filed Feb. 22, 2006, which is incorporated by
reference in its entirety and which claims the benefit of U.S.
Provisional Application No. 60/656,691, filed Feb. 26, 2005.
FIELD OF THE INVENTION
[0002] The present invention relates to the carburization of
nickel-based superalloys, and more particularly, to methods for
carburizing nickel-based superalloys that include refractory
elements for preventing the formation of secondary reaction
zones.
BACKGROUND OF THE INVENTION
[0003] In a gas turbine engine such as used for aircraft
applications, air is drawn into the front of the engine, compressed
by a compressor, and mixed with fuel. The compressed mixture is
burned in a combustor, and the hot combustion gases flow through a
turbine that turns the compressor. The hot gases then flow from the
rear of the engine.
[0004] The turbine includes stationary turbine vanes that deflect
the hot gas flow sideways, and turbine blades mounted on a turbine
wheel that turns as a result of the impingement of the hot gas
stream. The turbine vanes and blades experience extreme conditions
of high temperature, thermal cycling when the engine is turned on
and off, oxidation, corrosion, and, in the case of the turbine
blades, high stress and fatigue loadings. The higher the
temperature of the hot combustion gas, the greater the efficiency
of the engine. There is therefore an incentive to push the
materials of the engine to ever-higher temperatures and
loadings.
[0005] Nickel-based superalloys are widely used as the materials of
construction of gas turbine blades and vanes. These superalloys
contain primarily nickel, and a variety of alloying elements such
as cobalt and aluminum, as well as refractory elements such as
tantalum, tungsten, chromium, rhenium, hafnium, and others in
varying amounts carefully selected to provide good mechanical
properties and physical characteristics over the extremes of
operating conditions experienced by the engine. However, these
refractory elements, which provide the nickel-based superalloys
with superior mechanical properties, also make superalloy articles
susceptible to the formation of a secondary reaction zone ("SRZ")
in certain circumstances. In particular, gas turbine alloy
airfoils, such as the turbine blade and vanes discussed above,
typically require an aluminide coating treatment as part of a
thermal barrier coating system and/or to provide environmental
protection. Nickel-based superalloy articles that include
refractory elements and which undergo aluminiding treatments are
particularly susceptible to formation of an SRZ, wherein an
acicular topologically close-packed (TCP) phase forms, such as
disclosed in "A New Type of Microstructural Instability in
Superalloys--SRZ," Superalloys, 1996 by W. S. Walston, J. C.
Schaeffer and W. H. Murphy, ed. R. D. Kissinger, et al. TMS pp.
9-18. Within the SRZ, the TCP phases are brittle and contain a high
percentage of refractory elements. In particular, the presence of
the brittle phases, the formation of high angle grain boundaries
between the SRZ and the alloy, and to a lesser extent, the
depletion of the refractory elements weaken the SRZ, making the SRZ
essentially non-load-bearing. Because this portion of the article
is unable to sustain its share of the load, the applied load is
shifted to the remainder of the article, increasing the stress in
this portion of the article and shortening its service life.
[0006] The problem with refractory elements in nickel-based
superalloy articles forming SRZ is known, having been identified in
U.S. Pat. No. 5,334,262, entitled SUBSTRATE STABILIZATION OF
DIFFUSION ALUMINIDE COATED NICKEL-BASE SUPERALLOYS issued Aug. 2,
1994 to Schaeffer and assigned to the assignee of the present
invention. This patent also identifies forming carbide precipitates
which reduce the driving force for the formation of TCP phases
within the substrate, a method for avoiding the formation of SRZ,
by depositing a layer of carbon on the surface of the substrate by
chemical vapor deposition and diffusing the carbon onto the
surface. The presence of the carbon allows for the combination of
carbon with the refractory elements to form stable carbides,
substantially reducing the refractory elements available for the
formation of TCP phases. This patent, U.S. Pat. No. 5,334,262 is
incorporated in its entirety herein by reference, forming a part of
this specification.
[0007] Carbon can be introduced into the nickel-based superalloy
article by carburizing techniques, such as vacuum carburizing.
Vacuum carburizing of steel is a well-known technique. U.S. Pat.
No. 4,836,864 issued Jun. 6, 1989, entitled "Method of Gas
Carburizing and Hardening" discloses gas carburizing and hardening
a steel article in a carburizing atmosphere at atmospheric
pressure, heating the article in a vacuum for a predetermined
period of time and hardening the article. U.S. Pat. No. 5,702,540
issued Dec. 30, 1997 entitled "Vacuum Carburizing Method and
Device, and Carburized Products" teaches vacuum carburizing steel
workpieces in a vacuum furnace by introducing acetylene gas into
the chamber at a vacuum of 1 kPa or less to produce a hardened and
uniform case depth in the steel article. U.S. Pat. No. 6,187,111
Feb. 13, 2001 entitled "Vacuum Carburizing Method" divulges an
improved vacuum carburizing method for steel by heating the steel
material to about 900-1100.degree. C. and then introducing ethylene
gas while maintaining a vacuum of 1-10 kPa, thereby eliminating the
potentially explosive acetylene and replacing the expensing vacuum
pumps or mechanical booster pumps required to maintain vacuums at
or below 1 kPa.
[0008] Of course, it may also be desirable to prevent selected
portions of the article from being carburized by preventing contact
of the surface with carbon. It is known to mask all or a selected
portion of an article surface with a cover or coating to prevent it
from being carburized. These coatings or covers, also referred to
as a maskant, typically are platings and are usually very
effective. These coatings, however, must be easy to remove or must
be incorporated into the article. Typical maskants include nickel
plating and copper plating. However, such plating may be unsuitable
for articles that have precise shapes or include intricate details,
since removing such plating after carburization can be difficult or
impossible without damaging the article. However, a boron glass
coating used as a maskant containing magnesium silicon compounds
may be acceptable for use on intricate articles such as turbine
blades, as this material can provide protection from carburization
to selected, intricate areas of an airfoil, yet can be removed
without damaging the airfoil. This maskant system is described in
U.S. Published Application No. 20020020471 A1, published Feb. 21,
2002, and also is incorporated herein by reference.
[0009] Coatings typically are formed on the surfaces of the
superalloy articles to protect the article from degradation in
harsh, high temperature environments. One type of coating is an
aluminide coating. Aluminum is diffused into the surface of the
nickel-based superalloy article to form a nickel-aluminide layer,
which then oxidizes to form an aluminum oxide surface coating
during treatment or in service. (Optionally, noble metals such as
platinum may also be diffused into the surface). The aluminum oxide
surface coating renders the coated article more resistant to
oxidation and corrosion, desirably without impairing its mechanical
properties. Aluminide coating of turbine blades and vanes is well
known and widely practiced in the industry, and is described, for
example, in U.S. Pat. Nos. 3,415,672 and 3,540,878.
[0010] Recently it has been observed that, when some advanced
nickel-based superalloys are coated with an aluminide coating and
then exposed to service or simulated-service conditions, a
secondary reaction zone (SRZ) forms in the underlying superalloy.
This SRZ region is observed at a depth of from about 50 to about
250 micrometers (about 0.002-0.010 inches) below the original
superalloy surface that has received the aluminide coating. The
presence of the SRZ reduces the mechanical properties in the
affected region, because the material in the SRZ appears to be
brittle and weak, and forms a high angle grain boundary between SRZ
and the alloy.
[0011] The formation of the SRZ is a major problem in some types of
turbine components, because there are cooling channels located
about 750 micrometers (about 0.030 inches) below the surface of the
article. Cooling air is forced through the channels during
operation of the engine, to cool the structure. If the SRZ forms in
the region between the surface and the cooling channel, it
significantly weakens that region and can lead to reduced strength
and fatigue resistance of the article.
[0012] While the prior art prevents the formation of the TCP phases
that weaken the SRZ, the prior art relies solely on a diffusion
mechanism to diffuse inward the carbon deposited on the surface of
the superalloy substrate. While acceptable results can thus be
obtained, it is desirable to improve the methods of deposition to
control the depth of carburization while allowing the absorption of
carbon into the surface quickly.
SUMMARY OF THE INVENTION
[0013] The present invention provides methods for carburizing a
nickel-based superalloy that includes refractory elements using
alkynes or ethylene (C2H4) as the carburizing agent. In accordance
with the present invention, a nickel-based superalloy that includes
refractory elements is carburized, under controlled conditions,
using alkyne gases, propane or ethylene gas (C2H4) or combinations
thereof as the carburizing agent in order to form stable refractory
carbides at a controlled, preselected distance below the surface.
These stable refractory carbides reduce the driving force for the
formation of TCP phases that would otherwise produce a weak SRZ in
the controlled, preselected distance at and below the substrate
surface.
[0014] The present invention contemplates cleaning the article
surface. Cleaning the article surface entails removing all oxides
from the surface of the substrate and preventing the reformation of
oxides from the surface that is to be carburized. It is imperative
that the surface that is to be carburized is free of oxides.
Removing oxides can be accomplished by mechanical or chemical
methods which do not damage or otherwise adversely affect the
substrate surface. After such cleaning, the surfaces may be cleaned
with a suitable solvent, while avoiding the formation of new
oxides. While oxides are to be avoided, it may be desirable to mask
portions of the surface in order to prevent these portions from
being carburized. This may be desirable for any one of a number of
reasons, such as portions of the surface may not be exposed to an
aluminizing treatment or the stresses in the portion of the surface
are not determinative of part life in that portion of the article.
In this event, the portion which should not be carburized is
masked. The masking should prevent carburizing of the area masked,
should be stable at the elevated temperatures of operation, and
should be easy to remove after carburizing, or otherwise be capable
of being incorporated into the article.
[0015] The cleaned article is then loaded into a furnace suitable
for performing the carburization process while also preventing the
formation of oxides. Suitable furnaces include vacuum furnaces or
furnaces that can maintain a controlled atmosphere. When
maintaining a controlled atmosphere, the atmosphere must be
non-oxidizing, as oxidation of the article surface must be
prevented during heat-up to the carburizing temperature and during
carburizing. Once the carburizing temperature is approached, the
carburizing gas, alkyne, propane or ethylene, is introduced into
the furnace. These carburizing gases may be introduced below the
carburizing temperature with hydrogen or to gradually replace
hydrogen, but should not be added at a temperature or in a volume
that will result in excessive soot formation. The carburizing gas
is provided either on a continuous basis or by a pulse method.
Regardless of the method, the carburizing gas is provided to ensure
sufficient carbon is present at the surface for desired
carburization so that carbides are formed in a layer of sufficient
thickness, so that the layer will not form TCP phases after
subsequent exposure to aluminum as a result of aluminizing. The
article will thus be free of the SRZ. The duration of the
carburization process itself is controlled to limit the depth of
carbide layer formation, since carbide layers that are too thick
also can adversely affect the mechanical properties of the article.
Clearly, over-carburization that produces a layer that is too thick
results in a substrate that is devoid of the beneficial effects of
the refractory elements, as the refractory elements are tied up in
the stable carbides.
[0016] The carburization process is completed by purging the
chamber of the carburizing gas. This can be accomplished by
stopping the flow of the carburizing gas and introducing an inert
gas, nitrogen or hydrogen into the chamber. This also serves to
quickly cool the article. Any masking that has been applied may now
be removed. The article can now be heat treated to in any way, such
as by aging, to cause the precipitation of desirable strengthening
phases such as gamma prime, if these phases have not previously
been formed. The article can now also be aluminided. The
aluminiding treatment can be accomplished by the addition of an
additive aluminide layer. Alternatively, the aluminide may be by a
thermally grown aluminide method, in which the aluminide layer is
grown into the top surface. While the method of aluminiding does
not matter, it is important that the aluminum from the aluminide
process does not penetrate significantly (a few microns) below the
layer of carbides.
[0017] An advantage of the present invention is that the process
can be carried out quickly and the carburization depth, the depth
of formation of the refractory carbides, can be closely controlled,
because the use of controlled flow of the reactive alkyne gases or
ethylene provides the necessary supply of carbon at the surface of
the article, increasing the chemical activity of carbon at the
surface as compared to prior art methods of introducing carbon to
the surface.
[0018] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a perspective view of a superalloy article.
[0020] FIG. 2 is a schematic sectional view of the article of FIG.
1 taken generally along line 2-2, illustrating the near-surface
region during a diffusional platinum-aluminiding treatment when the
treatment of the present invention is not used.
[0021] FIG. 3 is an enlarged, diagrammatic sectional view of the
microstructure near the surface in the region depicted in FIG.
2.
[0022] FIG. 4 is a process flow chart for the treatment of the
invention.
[0023] FIG. 5 is an enlarged, diagrammatic sectional view of the
microstructure near the surface of an article like that depicted in
FIGS. 1-3, except that the process treatment of FIG. 4 has been
utilized.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The stabilization approach of the invention is used with
nickel-based superalloys, in applications such as a jet engine
airfoils including turbine vanes and the illustrated gas turbine
blade 10 of FIG. 1. The blade may be formed of any nickel-based
superalloy that has a tendency to form a surface secondary reaction
zone during and after an aluminide treatment. These superalloys
typically include reactive elements such as tantalum, tungsten,
molybdenum, chromium, rhenium, hafnium, ruthenium, iridium, osmium,
and in certain alloys, platinum, palladium and rhodium, and others
which are added to provide enhanced mechanical properties such as
strength. Examples of such nickel-based superalloys include Rene
162, whose composition is disclosed in U.S. Pat. No. 5,151,249
incorporated herein by reference and the alloy known as 4EPM-102C
whose composition is disclosed in Walston et al., Superalloys, TMS,
Warrendale. Pa., 15-34, 2004. Rene 162, for example, has a
composition in weight percent of about 12.5 percent cobalt, 4.5
percent chromium, 6.25 percent rhenium, 7 percent tantalum, 5.74
percent tungsten, 6.25 percent aluminum, 0.15 percent hafnium, 0.5
percent yttrium, minor amounts of other elements, and balance
nickel.
[0025] The blade 10 includes an airfoil section 12 against which
hot combustion gases are directed when the engine operates, and
whose surface is subjected to severe oxidation and corrosion attack
during service. The airfoil section 12 is anchored to a turbine
disk (not shown) through a dovetail or root section 14. In some
cases, cooling passages 16 are present in the airfoil section 12,
through which cool bleed air is forced to remove heat from the
blade 10. The blade is normally prepared by a casting and
solidification procedure well known to those skilled in the art,
such as investment casting, directional solidification, or single
crystal growth.
[0026] FIGS. 2 and 3 are sections through the blade 10 showing the
result of a conventional aluminide treatment without the benefit of
any carburizing treatment, such as the carburizing treatment of the
present invention. An aluminum-containing layer 20 is formed on a
surface 22 of the airfoil section 12, which acts as a substrate 24.
Optionally, in some cases a thin layer 26 of nickel or a noble
metal, such as a platinum-containing layer, may be deposited on the
surface 22 prior to deposition of the aluminum-containing layer 20.
After application of layer 20 over surface 22, the blade 10 is
heated to elevated temperature so that there is interdiffusion
between the layer 20 (and optional layer 26) and the substrate 24,
indicated generally by the arrows 28. The type, amount, and extent
of the interdiffusion depends upon a number of factors such as
time, temperature, substrate alloy, and activity of the aluminum
source. Either during or after this process, an upper surface 30 is
allowed to oxidize, forming an aluminum oxide layer (not shown). An
optional ceramic topcoat may be applied over this outer layer.
[0027] The acicular TCP phases will vary from alloy to alloy, as
the content and specific refractory elements will vary from alloy
to alloy. The composition of the TCP phases in Alloy Rene '162 are
discussed in U.S. Pat. No. 5,334,263. The actual chemical
composition is not important, as the phases typically include
refractory elements, depleting the surrounding matrix of these
elements and forming the brittle acicular structure, thereby
weakening the material matrix in secondary reaction zone 34 of FIG.
3. FIG. 3 depicts the resulting metallurgical microstructure in a
typical refractory element-containing nickel-based superalloy
turbine blade that has been aluminized. Two types of diffusion
zones are produced. A primary diffusion zone 32 containing TCP
phases, such as a sigma (.SIGMA.) phase, a mu (.eta.) phase or a
.rho.-phase, either alone or in combination, in a beta or gamma
prime matrix form just below the layer 20. The secondary reaction
zone (SRZ) 34 forms between the primary diffusion zone 32 and the
substrate 24. The SRZ 34 has been determined to result in reduced
mechanical properties of the blade 10, particularly when it
occupies a substantial fraction of the material below the surface
22. This situation is exacerbated when there is also a subsurface
cooling passage 16 (FIG. 2), which almost invariably is
present.
[0028] The present approach reduces the amount of available
refractory element reactants available to form TCP phases in the
near-surface, aluminum-rich regions by utilizing a carburizing
process to form stable refractory carbide compounds in near surface
regions 32 and 34 of the article within a preselected distance from
the surface, while not reducing the refractory element
concentration in other regions remote from the surface.
[0029] A preferred process for reducing the amount of available
refractory element reactants that would otherwise be available to
form the TCP phase is depicted in FIG. 4. A refractory containing
nickel-based superalloy article, such as Rene 162, containing at
least one element from the group consisting of rhenium, chromium,
tantalum, molybdenum, tungsten ruthenium, iridium, osmium, and in
certain alloys, platinum, palladium and rhodium, is provided. This
alloy would form TCP phases following the aluminide process, unless
processed to avoid such TCP phase formation.
[0030] According to the present invention, the article surface is
cleaned to remove oxides. This can be achieved by mechanical or
chemical means. However, it is preferred that the surface be
cleaned using grit blasting techniques having adequate pressure and
grit size to remove the surface oxides. Acceptable grit size which
do not otherwise alter the surface include 80-600 mesh grit, and
preferably 80-220 mesh grit, at a pressure of 20-90 psi, preferably
40 psi, have been found to be acceptable. Alternatively, vapor
honing may be used. Chemical etching is an alternate method that is
acceptable to remove oxides.
[0031] After the surface has been cleaned to remove the oxides, the
article surface must be carefully protected to prevent the
reformation of oxides. This is probably best accomplished by
quickly placing the workpiece into the working zone of the
carburizing unit.
[0032] However, if there are portions of the article which are not
to be carburized, an optional step of masking should be performed.
While any suitable maskant may be utilized, certain maskants pose
particular problems as they can be difficult to remove after
carburizing. For example, nickel plating or copper plating, each
excellent as a maskant, may be difficult or impossible to remove
from an article surface having precise shapes or intricate details
such as a turbine blade. However, a compound of boron glass
containing magnesium-silicon, such as described in U.S. Patent
Application Publication No. 20020020471, published Feb. 21, 2002,
incorporated herein by reference, exhibits exceptional capabilities
as a suitable maskant. This maskant is particularly preferred for
use in vacuum carburization processes in which processing is
performed at high temperatures. This maskant exhibits the three
principal properties of a maskant in this application: it is stable
at the elevated temperatures of carburizing, prevents
carburization, and can be easily removed after carburization.
[0033] After any optional maskants are applied and while
maintaining the cleanliness of the article surface, the article is
placed in a working zone of a suitable furnace. The furnace must be
capable of preventing oxidation of the article surface as the
article is heated to the carburization temperature. Thus vacuum
furnaces or a furnace that can maintain a protective atmosphere are
preferred. The atmosphere may be an inert atmosphere or a reducing
atmosphere, maintained by the introduction of an inert gas or
hydrogen into the furnace or by achieving a vacuum preferably with
a pressure of less than 1 Torr. However, a reducing atmosphere,
obtained by introducing at least a partial pressure of hydrogen
into the furnace is most preferred. A partial pressure of hydrogen
maintained at about 0.0005-10 Torr is preferred during this stage,
but a partial pressure of 0.05-1.0 Torr is more preferred. As an
option, even when the carburizing is to be performed in a vacuum
furnace, a low partial pressure of hydrogen can be introduced into
the vacuum furnace. Even though the hydrogen is ultimately removed
as the vacuum is pulled by the vacuum pump, the reducing atmosphere
assists in preventing the oxidation of the surface. For example,
the vacuum is pulled as low as possible, less than about 1 Torr and
preferably to 1 milli-Torr or lower. Then hydrogen gas is
introduced into the working zone at a partial pressure of 0.0005-10
Torr, preferably at a partial pressure of 0.05-1.0 Torr, and most
preferably at a pressure of less than about 0.001 Torr, the
pressure being maintained.
[0034] After the preselected carburizing temperature is reached,
the protective atmosphere is removed by stopping the flow of the
protective gases. Optionally, as previously noted, the carburizing
gas can be introduced at lower temperatures in conjunction with
hydrogen or as a replacement for hydrogen as its partial pressure
is reduced, provided that it is added at a temperature and in a
volume so as not to form soot on the article surface. The
carburizing temperature is in the range of
1800.degree.-2250.degree. F., preferably 1800.degree.-2100.degree.
F., more preferably 1900.degree.-2050.degree. F., and most
preferably in the range of 1925.degree.-2000.degree. F. Carburizing
gas is then introduced into the working zone of the furnace.
Hydrocarbons having triple bonded carbon atoms, generally known as
alkynes, the simplest being acetylene (also known as ethyne) and
represented by the chemical formula C.sub.2H.sub.2,
H--C.ident.C--H, ethylene (C.sub.2H.sub.4) and propane are
preferred and are believed to be the most effective carburizing
agent for carburizing nickel-based superalloys that include
refractory elements in order to prevent the formation of TCP phases
near the surface of the substrate after aluminizing the article
substrate, when the carburizing is performed under carefully
controlled conditions of the present invention. Prior art methods
that utilize chemical vapor deposition (CVD) techniques are time
consuming and very limited by the size of the chambers used for the
CVD process. While larger CVD chambers can be developed, this
equipment is very expensive. The alkynes and ethylene are more
reactive and chemically unstable at the temperatures of
carburization of nickel-based superalloys than other carbon gases,
such as saturated hydrocarbons including such widely used gases as
methane (CH.sub.4) and propane (C.sub.2H.sub.6). Thus the alkynes,
in particular acetylene, and ethylene, decompose into their
constituent elements more readily than the saturated hydrocarbons,
thereby making free carbon readily available at the substrate
surface in the working zone of the furnace. Because of their
reactivity, care must be taken in the carburizing process to
prevent the introduction of oxidizing agents, such as oxygen, as an
explosive mixture can result.
[0035] The carburizing gases may be introduced into the carburizing
device by any method that prevents the introduction of oxygen. The
preferred methods are continuously flowing methods and pulse
methods.
[0036] In the continuously flowing method, carburizing gas was
introduced into the working zone of the furnace at the preselected
elevated carburizing temperature. In this method, the carburizing
gas was introduced at a partial pressure of about 2-3 Torr and this
partial pressure was maintained for the duration of the carburizing
operation, which was in the range of about 1 to about 240 minutes,
but preferably is about 10 minutes. The preferred carburizing gas
was acetylene with a carburizing time in the range of 1-20 minutes.
The carburizing time will vary depending upon the reactivity of the
carburizing gas mixture and the temperature of operation.
[0037] In the pulse method, which is most effective in a vacuum
furnace, as will become obvious, a pulse of carburizing gas is
introduced into the furnace at a preselected flow rate or to
achieve a preselected partial pressure, for example, in the range
of 0.1-10 Torr. The gas supply is then closed to prevent any
additional flow of carburizing gas. After a period of time, which
will vary depending upon a number of factors including but not
limited to size of the working zone, size of the work load,
temperature, vacuum pressure, etc. the working zone will be
depleted of carburizing gas and hence of carbon. At this point,
additional carburizing gas is introduced into the furnace and the
process is repeated. The period of time required will vary as
previously noted, but a typical period is about 5 minutes. The
process is repeated until carburization is completed. Thus, if it
takes 10 minutes of carburization to achieve the desired depth, it
is expected that two pulse cycles will be required.
[0038] Carburization is continued until the desired carburization
depth is reached at which time the operation is stopped by
introducing an inert gas at about 1800.degree. F. to cool the
article rapidly. Carburization ceases when the article passes a
critical temperature of less than 1800.degree. F. The desired or
target carburization depth is approximately equal to the depth that
aluminum penetrates below the substrate during the aluminizing
process. Small deviations (a few microns), either slightly greater
than or slightly less than the target depth will not seriously
impact the properties of the article. Because the carburizing
process is performed before the aluminizing process, it is
necessary to estimate the depth of penetration of the aluminum. Of
course, the depth of penetration of aluminum also will vary
depending on a number of factors such as activity of aluminum,
whether the process is thermally grown into the surface or an
additive layer, processing temperature, the aluminizing process
itself. However, experience indicates that the required carburizing
depth is between about 10 microns to about 100 microns.
[0039] As will be recognized by those skilled in the art, several
operating parameters can be varied, therefore these parameters must
be controlled to control the desired carbide layer thickness. These
parameters include, but are not limited to gas flow rate, which
determines partial gas pressure, temperature, type of furnace,
working zone size, work load and time. Gas flow rates of acetylene
of about 100 liters/per hour for the current furnace have been
found to be acceptable, with flow rates from as low as 50
liters/per hour to 100 liters per hour also likely to be
acceptable. Of course, flow rates will vary with furnace type,
furnace size and work load.
[0040] After processing and cooling, the work load, which typically
will comprise a plurality of articles, can be removed from the work
zone. Any optional masking may be removed before or after the
aluminide treatment, depending upon whether or not the masked areas
require aluminizing. Masking may be removed by any suitable means
that does not adversely affect the substrate surface, such as
chemical stripping, mechanical means such as blasting, or other
known methods consistent with the masking material. The articles
may also be heat treated as required, either before or after
aluminizing, to age or otherwise develop the final desired
microstructure. These aging treatments are related to precipitation
hardening mechanisms of nickel-based superalloys, and have little
or no effect on the stable carbide particles.
[0041] FIG. 5 illustrates the microstructure of the near-surface
region of the blade 10 when the approach of the invention, just
described in relation to FIG. 4, is followed. The structure is
similar to that of FIG. 3, but no TCP phase is present and
therefore no secondary reaction zone is present. Instead, an array
of fine carbon-rich precipitates (carbides) 36 are present in the
region to which the deposited carbon atoms have diffused in
sufficient quantity to form carbides. These carbides typically
contain refractory elements, such as rhenium, chromium, tantalum,
tungsten, molybdenum, ruthenium, iridium, osmium and in certain
alloys, palladium, rhodium and platinum, reducing the amount of
these elements available to react to form the TCP phase in a
depleted region 38, which may equivalently be described as a
carbide-precipitate region. The carbides typically form within the
gamma phase channels and are typically equal to or less than the
size of the gamma prime precipitates, less than 1 micron diameter.
The term "depleted region" means that the concentration of TCP
phase-forming elements in a form suitable for reacting to form the
TCP phase is reduced. The term should not be taken to mean that
those elements have been completely removed from the depleted
region 38. Instead, the TCP phase forming refractory elements are
present, but in a substantially reacted form such that they cannot
form the TCP phase.
[0042] As the article is aluminided, aluminum diffuses from the
layer 20 into the substrate to an extent indicated by an aluminide
depth 40. This diffusion, like the diffusion of carbon during
carburization, is governed by the well known Fick's Second Law of
Diffusion, being dependent on time and temperature. The depleted
region 38 extends to a depth which is preferably approximately
equal to the aluminide depth 40, but may be slightly greater than
or less than the aluminide depth 40. The depleted region 38 extends
to a depth of from about 10 to about 100 microns, preferably 25-100
microns below the surface of the substrate, and the aluminide layer
40 extends to a depth of from about 10 to about 50 micrometers
below the surface of the substrate. If the depleted region 38 is
substantially greater than the aluminide depth 40, the excess
volume of material is unnecessarily depleted of the solid-solution
strengthening refractory elements and includes unnecessary carbide
precipitates. The carbide precipitates may cause premature failure
of the superalloy if the depth of the region 38 is too great. If
the depleted region 38 is substantially less than the aluminide
depth 40, there will be a small region where the TCP phase may
form. The result is a secondary reaction zone that is smaller than
would otherwise be present, but its presence is still
detrimental.
[0043] Even though the near surface portion of the article includes
carbides, typically tantalum carbides, that increase the hardness
from about 40-45 Rc to 55-60 Rc, these nickel-based superalloy
articles can still be subject to traditional manufacturing
processes such as drilling, coating, shot peening etc.
Carburization does not inhibit such traditional manufacturing
processes.
EXAMPLE 1
[0044] Articles were carburized in a Turbotreater.RTM. horizontal
vacuum carburizing furnace, Model H3636 AvaC.TM. Ipsen
International Furnace having multiple nozzles for introducing
gases. Such a furnace is available from Ipsen International of
Rockford, Ill. The furnace has a working zone of
3'.times.2'.times.2' (l.times.w.times.h). The furnace utilizes
carbon heating elements that do not react with the gases
introduced. The working zone was loaded with a plurality of turbine
blades, after cleaning, about 10-50. These turbine blades are small
commercial engine blades made of refractory-containing superalloy
and having a size (overall blade length) of about 1.5''. The blades
were maintained under a reducing atmosphere, as hydrogen was
introduced into the furnace to a pressure of about 0.150 Torr until
the carburizing temperature of 1975.degree. F. was reached. Once at
1975.degree. F., the hydrogen was evacuated from the furnace work
zone and acetylene gas was then introduced into the furnace at a
flow rate of approximately 100 liters per hour to maintain a
pressure of about 2 Torr for a time of about 10 minutes. After the
10 minutes of carburization, the acetylene was evacuated from the
furnace work zone and nitrogen gas was introduced to allow for
rapid cooling of the load below about 1800.degree. F. A zone of
submicron carbide particles was formed in the near surface region
of the blades, the depth of which was about 66 microns for the
blades observed. A carburized blade was coated with platinum
modified beta nickel-aluminide coating and then exposed to
2000.degree. F. for about 400 hours. The aluminide coated blade
after thermal exposure showed no formation of SRZ where an
aluminide coated non-carburized control sample produced about
0.004'' depth of SRZ that covered greater than 50% of the surface
area.
EXAMPLE 2
[0045] A Turbotreater.RTM. horizontal vacuum carburizing furnace
Model H3636 AvaC.TM. Ipsen International Furnace was used, as
described in Example 1. 1'' diameter by 0.125'' thick specimen were
maintained under a vacuum atmosphere of less than 0.001 Torr until
the carburizing temperature of 1975.degree. F. was reached. Once
the temperature was stabilized at 1975.degree. F., acetylene gas
was introduced into the furnace at a flow rate of about 100 liters
per hour to maintain a pressure of about 2 Torr for a time of about
10 minutes. After about 10 minutes of carburization, the acetylene
was evacuated from the furnace work zone and argon gas was
introduced to allow for rapid cooling of the furnace load below
about 1800.degree. F. A zone of submicron carbide particles was
formed in the near surface region of the blades, the depth of which
was about 74 microns for the samples. A sample was coated with a
platinum modified beta nickel-aluminide coating ant then exposed to
2000.degree. F. for about 400 hours. The aluminide coated specimen
after thermal exposure showed no formation of SRZ.
[0046] The present invention provides an improved structure to
nickel-base superalloys that would otherwise be susceptible to
formation of a secondary reaction zone. Such superalloys with
aluminide, platinum aluminide (or other noble metal aluminide), and
overlay coatings, including MCrAlY overlay coatings or beta nickel
aluminide overlay coatings such as disclosed in U.S. Pat. No.
6,261,084, incorporated herein by reference, benefit from the
approach of the invention. The process used to form the stable
carbides can be performed using the preferred temperatures and
carburization gases more quickly than other known methods. Despite
the shorter times required, the precise depths of carbide formation
can still be carefully controlled.
[0047] While the invention has been described with reference to a
preferred embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments.
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