U.S. patent application number 11/068085 was filed with the patent office on 2005-09-08 for nickel-based superalloy having very high resistance to hot-corrosion for monocrystalline blades of industrial turbines.
Invention is credited to Blackler, Michael, Caron, Pierre, Escale, Andre Marcel, Lelait, Laurent, McColvin, Gordon Malcolm, Wahi, Rajeshwar Prasad.
Application Number | 20050194068 11/068085 |
Document ID | / |
Family ID | 8173963 |
Filed Date | 2005-09-08 |
United States Patent
Application |
20050194068 |
Kind Code |
A1 |
Caron, Pierre ; et
al. |
September 8, 2005 |
Nickel-based superalloy having very high resistance to
hot-corrosion for monocrystalline blades of industrial turbines
Abstract
Nickel-based superalloy, suitable for monocrystalline
solidification, having the following composition by weight: 1 Co:
4.75 to 5.25% Cr: 15.5 to 16.5% Mo: 0.8 to 1.2% W: 3.75 to 4.25%
Al: 3.75 to 4.25% Ti: 1.75 to 2.25% Ta: 4.75 to 5.25% C: 0.006 to
0.04% B: .ltoreq.0.01% Zr: .ltoreq.0.01% Hf: .ltoreq.1% Nb:
.ltoreq.1% Ni and any impurities: complement to 100%.
Inventors: |
Caron, Pierre; (Les Ulis,
FR) ; Blackler, Michael; (Exeter, GB) ;
McColvin, Gordon Malcolm; (Lincoln, GB) ; Wahi,
Rajeshwar Prasad; (Berlin, DE) ; Escale, Andre
Marcel; (Omex, FR) ; Lelait, Laurent;
(Darvault, FR) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
PO BOX 7068
PASADENA
CA
91109-7068
US
|
Family ID: |
8173963 |
Appl. No.: |
11/068085 |
Filed: |
February 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11068085 |
Feb 28, 2005 |
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|
10460860 |
Jun 12, 2003 |
|
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|
10460860 |
Jun 12, 2003 |
|
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09999167 |
Nov 29, 2001 |
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Current U.S.
Class: |
148/428 |
Current CPC
Class: |
C22C 19/056
20130101 |
Class at
Publication: |
148/428 |
International
Class: |
C22C 019/05 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2000 |
EP |
00403361 |
Claims
What is claimed is:
1. A nickel-based superalloy, suitable for monocrystalline
solidification, characterized in that its composition by weight is
as follows:
5 Co: 4.75 to 5.25% Cr: 15.5 to 16.5% Mo: 0.8 to 1.2% W: 3.75 to
4.25% Al: 3.75 to 4.25% Ti: 1.75 to 2.25% Ta: 4.75 to 5.25% C:
0.006 to 0.04% B: .ltoreq.0.01% Zr: .ltoreq.0.01% Hf: .ltoreq.1%
Nb: .ltoreq.1% Ni and any impurities: complement to 100%.
2. Industrial turbine blade produced by monocrystalline
solidification of a superalloy according to claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of U.S. application Ser.
No. 10/460,860, filed Jun. 12, 2003, which is a continuation of
U.S. application Ser. No. 09/999,167, filed Nov. 29, 2001, which
claims priority of European Patent Application Number EP00403361,
filed Nov. 30, 2000.
TECHNICAL FIELD
[0002] The invention relates to a nickel-based superalloy which is
adapted to the manufacture of fixed and movable monocrystalline
blades of industrial gas turbines by directional
solidification.
BACKGROUND OF THE INVENTION
[0003] Nickel-based superalloys are the most high-performance
materials used today in the manufacture of movable and fixed blades
of industrial gas turbines. The two principal features required
until now of these alloys for those specific applications have been
good resistance to creep at temperatures of up to 850.degree. C.
and very good resistance to hot-corrosion. Some reference alloys
currently used in this field are designated IN738, IN939 and
IN792.
[0004] Blades manufactured using those reference alloys are
produced by conventional casting using the lost-wax process and
have a polycrystalline structure, that is to say, they are
constituted by the juxtaposition of crystals which are orientated
in a random manner relative to each other and which are called
grains. Those grains are themselves constituted by an austenitic
gamma (.gamma.) matrix based on nickel, in which hardening
particles of the gamma prime (.gamma.') phase are dispersed whose
base is the intermetallic compound Ni.sub.3A1. This specific
structure of the grains gives those alloys a high level of creep
resistance up to temperatures in the order of 850.degree. C., which
ensures the longevity of the blades, for which service lives of
from 50,000 to 100,000 hours are generally sought. The chemical
composition of alloys IN939, IN738 and IN792 has further been
determined to give them excellent resistance to the combustion gas
environment, in particular in respect of hot-corrosion, a
phenomenon which is particularly aggressive in the case of
industrial gas turbines. Significant additions of chrome, typically
of from 12 to 22% by weight, are thus necessary to give those
alloys the necessary resistance to hot-corrosion for the
applications concerned. From the point of view of resistance to
creep, the order of the alloys is: IN939<IN738<IN792. From
the point of view of resistance to hot-corrosion, the order is the
reverse, that is: IN792<IN738<IN939.
[0005] In order to improve the performance of industrial gas
turbines in terms of output and consumption, one method consists in
increasing the temperature of the gases at the turbine inlet. This
consequently makes it necessary to be able to provide alloys for
turbine blades which can tolerate operating temperatures which are
higher and higher, whilst retaining the same mechanical features,
in particular in terms of creep, in order to be able to achieve the
same service lives.
[0006] The same type of problem has been posed in the past in the
case of gas turbines for turbo-jets and turbo-engines for
aeronautical applications. In this case, the selected solution
consisted in changing from blades, known as polycrystalline blades,
which are produced by conventional casting to blades, known as
monocrystalline blades, that is to say, which are constituted by a
single metallurgical grain.
[0007] Those monocrystalline blades are manufactured by directional
solidification with lost-wax casting. The elimination of grain
boundaries, which are preferential locations for creep deformation
at elevated temperature, has allowed the performance of
nickel-based superalloys to be increased spectacularly.
Furthermore, the process of monocrystalline solidification allows
the preferred orientation of growth of the monocrystalline
component to be selected and, that manner, the orientation
<001> which is optimum from the point of view of resistance
to creep and thermal fatigue to be chosen, those two types of
mechanical stress being the most disadvantageous for turbine
blades.
[0008] However, the chemical superalloy compounds developed for
monocrystalline turbine blades for aeronautical applications are
not suitable for blades for terrestrial or marine applications,
known as industrial applications. Those alloys are determined in
order to promote their mechanical resistance up to temperatures
greater than 1100.degree. C., and this to the detriment of their
resistance to hot-corrosion. In that manner, the concentration of
chrome of the superalloys for aeronautical monocrystalline turbine
blades is generally less than 8% by weight, which allows volume
fractions of the .gamma.' phase in the order of 70% to be achieved,
which levels are advantageous for resistance to creep at elevated
temperature.
[0009] A nickel-based superalloy which is rich in chrome and which
is suitable for the monocrystalline solidification of components of
industrial gas turbines is known by the designation SC16 and is
described in FR 2 643 085 A. Its concentration of chrome is
equivalent to 16% by weight. The features concerning the creep
resistance of alloy SC16 are such that the alloy provides, relative
to the polycrystalline reference alloy IN738, an increase in
operating temperature ranging from approximately 30.degree. C.
(830.degree. C. instead of 800.degree. C.) to approximately
50.degree. C. (950.degree. C. instead of 900.degree. C.).
Comparative tests for cyclical corrosion at 850.degree. C. in air
at atmospheric pressure with Na.sub.2SO.sub.4 contamination showed
that the resistance to hot-corrosion of alloy SC16 was at least
equivalent to that of the reference polycrystalline alloy
IN738.
[0010] Hot-corrosion tests have been carried out on alloy SC16 by
the manufacturers of industrial turbines on their own test benches.
In very severe environments, which are representative of extreme
operating conditions, it has been shown that the resistance to
hot-corrosion of that alloy remained inferior to that of alloy
IN738.
[0011] Furthermore, the increasing demand from those manufacturers
for an increase in the operating temperature of gas turbines gives
rise to the need for superalloys for blades to have a resistance to
creep which is increased still further.
SUMMARY OF THE INVENTION
[0012] The problem addressed by the invention is to provide a
nickel-based superalloy having a resistance to hot-corrosion in the
aggressive combustion gas environment of industrial gas turbines
which is at least equivalent to that of reference polycrystalline
superalloy IN738, and having a resistance to creep which is greater
than or equal to that of reference alloy IN792 within a temperature
range of up to 950.degree. C.
[0013] This superalloy must in particular be suitable for
manufacture of fixed and movable monocrystalline blades having
large dimensions (up to several tens of centimeters in height) of
industrial gas turbines by directional solidification.
[0014] Furthermore, this superalloy must demonstrate good
micro-structural stability in respect of the precipitation of
fragile intermetallic phases which are rich in chrome when
maintained for sustained periods at elevated temperature.
[0015] More specifically, an alloy compound is sought which
ensures:
[0016] optimized resistance to hot-corrosion, in any case at least
equivalent to that of reference polycrystalline superalloy IN738,
and this in an environment which is representative of that for
combustion gases of industrial turbines;
[0017] a maximum volume fraction of hardening precipitates of the
.gamma.' phase in order to promote resistance to creep at elevated
temperature;
[0018] resistance to creep up to 950.degree. C. which is at least
equivalent to that of reference polycrystalline alloy IN792;
[0019] a tendency to homogeneity by completely placing in solution
particles of the .gamma.' phase, including the .gamma./.gamma.'
eutectic phases;
[0020] the absence of precipitation of fragile intermetallic phases
which are rich in chrome, starting from the .gamma. matrix, when
maintained for sustained periods at elevated temperature;
[0021] a density which is less than 8.4 g.multidot.cm.sup.-3 in
order to minimize the mass of the monocrystalline blades and,
consequently, to limit the centrifugal stress acting on the blades
and on the turbine disc to which they are fixed;
[0022] a good tendency to monocrystalline solidification of turbine
blades whose height can reach several tens of centimeters and the
mass several kilograms.
[0023] The superalloy according to the invention, which is suitable
for monocrystalline solidification, has the following composition
by weight:
2 Co: 4.75 to 5.25% Cr: 15.5 to 16.5% Mo: 0.8 to 1.2% W: 3.75 to
4.25% Al: 3.75 to 4.25% Ti: 1.75 to 2.25% Ta: 4.75 to 5.25% C:
0.006 to 0.04% B: .ltoreq.0.01% Zr: .ltoreq.0.01% Hf: .ltoreq.1%
Nb: .ltoreq.1% Ni and any impurities: complement to 100%.
[0024] The alloy according to the invention is an excellent
compromise between resistance to creep and resistance to
hot-corrosion. It is for the manufacture of monocrystalline
components, that is to say, components which comprise a single
metallurgical grain. This specific structure is obtained, for
example, by means of a conventional directional solidification
process at a thermal gradient, using a helical or chicane-like
device for selecting a grain, or a monocrystal nucleus.
[0025] The invention also relates to an industrial turbine blade
which is produced by monocrystalline solidification of the above
superalloy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The features and advantages of the invention will be set
forth in greater detail in the description below with reference to
the appended drawings.
[0027] FIGS. 1 to 4 are graphs illustrating the properties of
different superalloys.
DETAILED DESCRIPTION
[0028] An alloy according to the invention designated SCA425 has
been produced with reference to the nominal composition listed in
Table I. In this table, the nominal concentrations of major
elements of reference alloys IN939, IN738, IN792 and SC16 are also
listed.
3TABLE I Concentrations by weight of major elements (%) Alloy Ni Co
Cr Mo W Al Ti Ta Nb IN939 Base 19 22.5 -- 2 1.9 3.7 1.4 1 IN738
Base 8.5 16 1.7 2.6 3.4 3.4 1.7 0.9 IN792 Base 9 12.4 1.9 3.8 3.1
4.5 3.9 -- SC16 Base -- 16 3 -- 35 3.5 3.5 -- SCA425 Base 5 16 1 4
4 2 5 --
[0029] Chrome has an advantageous and dominant effect on the
resistance to hot-corrosion of nickel-based superalloys. Thus,
tests have shown that a concentration in the order of 16% by weight
was necessary in the alloy of the invention in order to obtain
resistance to hot-corrosion that is equivalent to that of reference
alloy IN738 under the conditions for hot-corrosion tests described
below, which conditions are representative of the environment
created by combustion gases of some industrial turbines. Chrome
also contributes to the hardening of the .gamma. matrix in which
this element is preferentially distributed.
[0030] Molybdenum greatly hardens the .gamma. matrix in which the
element is preferentially distributed. The quantity of molybdenum
which can be introduced to the alloy is limited, however, because
the element has a disadvantageous effect on the resistance to
hot-corrosion of nickel-based superalloys. A concentration in the
order of 1% by weight in the alloy of the invention is not
detrimental to the corrosion resistance and contributes
significantly to its hardening.
[0031] Cobalt also contributes to the hardening in the form of a
solid solution of the .gamma. matrix. The concentration of cobalt
has an effect on the dissolution temperature of the .gamma.'
hardening phase (.gamma.' solvus temperature). Thus, it is
advantageous to increase the concentration of cobalt in order to
decrease the solvus temperature of the .gamma.' phase and to
facilitate the homogenizing of the alloy by means of heat treatment
without any risk of causing melting to start. Furthermore, it can
also be advantageous to reduce the concentration of cobalt in order
to increase the solvus temperature of the .gamma.' phase and to
benefit in that manner from greater stability of the .gamma.' phase
at elevated temperature, which promotes resistance to creep. A
concentration in the order of 5% by weight of cobalt in the alloy
of the invention leads to an optimum compromise between a good
capacity for homogenizing and good resistance to creep.
[0032] Tungsten, whose concentration is in the order of 4% by
weight in the alloy of the invention, is distributed in a
substantially equal manner between the .gamma. and .gamma.' phases
and, in that manner, contributes to the respective hardening
processes thereof. Its concentration in the alloy is, however,
limited because the element is heavy and has a negative effect on
the resistance to hot-corrosion.
[0033] The concentration of aluminum is in the order of 4% by
weight in the alloy of the invention. The presence of the element
causes the precipitation of the .gamma.' hardening phase. Aluminum
also promotes resistance to oxidation. The elements titanium and
tantalum are added to the alloy of the invention in order to
reinforce the .gamma.' phase in which they are substituted for the
element aluminum. The respective concentrations of those two
elements in the alloy of the invention are in the order of 2% by
weight for titanium and 5% by weight for tantalum. Under the
conditions for hot-corrosion tests described below, corresponding
to the intended application, tests showed that the presence of
tantalum was more favorable to the resistance to hot-corrosion than
was the case with titanium. However, tantalum is heavier than
titanium, which is disadvantageous in respect of the density of the
alloy. The total of the concentrations of tantalum, titanium and
aluminum roughly determines the volume fraction of the .gamma.'
hardening phase. The concentrations of those three elements have
been adjusted in order to optimize the volume fraction of the
.gamma.' phase, while keeping the .gamma. and .gamma.' phases
stable when maintained for long periods at elevated temperature,
and taking into consideration the fact that the concentration of
chrome has been fixed at approximately 16% by weight in order to
achieve the desired resistance to corrosion.
[0034] Alloy SCA425 has been produced in the form of monocrystals
having orientation <001>. The density of that alloy has been
measured and found to be equal to 8.36 g.multidot.cm.sup.3.
[0035] After directional solidification, the alloy is substantially
constituted by two phases: the austenitic .gamma. matrix, which is
a solid nickel-based solution, and the .gamma.' phase, which is an
intermetallic compound whose basic formula is Ni.sub.3Al and which
precipitates mainly within the .gamma. matrix in the form of fine
particles measuring less than 1 micrometer during cooling to the
solid state. Contrary to what is generally found in monocrystalline
superalloys for turbine blades, alloy SCA425 does not contain any
interdentritic solid particles of the .gamma.' phase resulting from
a eutectic transformation of the residual liquid once
solidification has ended.
[0036] Alloy SCA425 underwent homogenizing heat treatment at a
temperature of 1285.degree. C. for 3 hours with cooling in air.
This temperature is higher than the solvus temperature of the
.gamma.' phase (dissolution temperature of the precipitates of the
.gamma.' phase), which is 1198.degree. C., and less than the
solidus temperature, which is 1300.degree. C. The treatment is
intended to dissolve all of the precipitates of the .gamma.' phase,
whose distribution of sizes is very wide in the coarse state of
directional solidification, and to reduce the chemical
heterogeneities which are associated with the dendritic
solidification structure.
[0037] The interval between the .gamma.' solvus temperature of the
alloy SCA425 and its solidus temperature is very large, which
allows ready application of the homogenizing treatment without any
risk of melting and with the certainty of obtaining a homogeneous
microstructure which allows optimized resistance to creep.
[0038] The cooling which follows the homogenizing treatment
described above was carried out by hardening in air. In practice,
the rate of this cooling must be so high that the size of the
particles precipitated during the cooling operation is less than
500 nm.
[0039] The homogenizing heat treatment procedure which has just
been described is an example which allows the intended result to be
achieved, that is to say, a homogeneous distribution of fine
particles of the .gamma.' phase whose size is no greater than 500
nm. This does not exclude the possibility of obtaining a similar
result by using a different treatment temperature provided that the
temperature lies within the range separating the .gamma.' solvus
temperature and the solidus temperature.
[0040] Alloy SCA425 was tested after undergoing a homogenizing
treatment as described above, then two annealing treatments which
allow the size and the volume fraction of the precipitates of the
.gamma.' phase to be stabilized. A first annealing treatment
consisted in heating the alloy to 1100.degree. C. for 4 hours with
cooling in air, which leads to stabilization of the size of the
precipitates of the .gamma.' phase. A second annealing treatment at
850.degree. C. for 24 hours, followed by cooling in air, allows the
volume fraction of the .gamma.' phase to be optimized. This volume
fraction of the .gamma.' phase is estimated at 50% in alloy SCA425.
Once all of the heat treatments are completed, the majority of the
.gamma.' phase has been precipitated in the form of cuboid
particles whose size is between 200 and 500 nm. A small fraction of
fine particles of the phase .gamma.' whose size does not exceed 50
nm is present between the large precipitates.
[0041] Hot-corrosion tests were carried out at different
temperatures on alloy SCA425 using the following procedure: samples
are partially immersed in a container containing a mixture of
combustion residues whose composition by weight is as follows: 4.3%
Na.sub.2SO.sub.4+22.7% CaSO.sub.4+22.3% Fe.sub.2O.sub.3+20.6%
ZnSO.sub.4+10.4% K.sub.2SO.sub.4+2.8% MgO+6.5%
Al.sub.2O.sub.3+10.4% SiO.sub.2. A mixture of air+0.15% SO.sub.2 by
volume passes through the mixture of combustion residues at a rate
of 6 liters per hour. The mixture of combustion residues is renewed
every 500 hours. This environment is representative of the very
aggressive environment of combustion gases for some industrial
turbines. For comparison purposes, samples of alloys IN738, IN939,
IN792 and SC16 were tested at the same time.
[0042] The samples were cut into sections and the depth of metal
destroyed by the corrosion phenomenon was measured. The graphs in
FIGS. 1 to 3 show the mean depths of penetration of the corrosion
for the different alloys at 700.degree. C., 800.degree. C. and
850.degree. C., respectively, as a function of the test duration.
The resistance to corrosion is even better since the depth of
penetration is low. At 700.degree. C. and 800.degree. C., the alloy
SCA425 demonstrates a resistance to corrosion equivalent to that of
alloy IN738 and better than that of alloy SC16. At 850.degree. C.,
the resistance to corrosion of alloy SCA425 is comparable to that
of reference alloys IN738 and IN939.
[0043] Tests for creep under tensile stress were carried out on
machined test pieces in monocrystalline bars of orientation
<001>. The bars were homogenized beforehand then annealed
according to the procedures described above. Values for rupture
times obtained at 750, 850 and 950.degree. C. for different levels
of stress applied are listed in Table II.
4TABLE II Service lives with creep of alloy SCA425 Temperature
(.degree. C.) Stress (MPa) Rupture time (h) 750 650 216/321.1 750
575 984 850 400 201/276 850 300 2121/2945/3220 850 250 6161 950 250
73/76 950 200 261/291 950 180 578 950 160 1098 950 140 2109 950 120
3872
[0044] The graph in FIG. 4 allows a comparison of the rupture times
with creep obtained for alloys SCA425, IN792 and SC16. The stress
applied is plotted on the abscissa. The value of the Larson-Miller
parameter is marked on the ordinate. This parameter is given by the
formula P=T(20+log t).sub.x 10-3, where T is the creep temperature
in Kelvin and t is the rupture time in hours. This graph shows that
the creep resistance of alloy SCA425 is at least equivalent to that
of alloy IN792, which is the stipulated objective, and greater than
that of reference alloy SC16.
[0045] The inspection of the microstructure of the test pieces of
alloy SCA425 at the end of the creep tests demonstrated the absence
of precipitation of fragile intermetallic particles which are rich
in chrome and which are capable of appearing when maintained for
sustained periods at elevated temperature in nickel-based
superalloys where the .gamma. matrix is over-saturated with
additive elements.
[0046] Manufacturing tests on monocrystalline components of
super-alloy SCA425 demonstrated that it was possible to cast a
large range of components whose mass can range from a few grams to
more than 10 kg, with various levels of complexity. The growth of
components according to the crystallo-graphic orientation
<001> is promoted and dominant and the presence of grains
that are orientated in a random manner is minimized. The liquid
metal is stable in the sense that it does not react with the
materials commonly used in the manufacture of moulds. The
phenomenon of recrystallisation which can occur during homogenizing
treatment at elevated temperature is absent in the case of alloy
SCA425.
* * * * *