U.S. patent application number 10/008745 was filed with the patent office on 2003-03-13 for nickel-based superalloy having 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 | 20030047252 10/008745 |
Document ID | / |
Family ID | 8173964 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030047252 |
Kind Code |
A1 |
Caron, Pierre ; et
al. |
March 13, 2003 |
Nickel-based superalloy having 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: 11.5 to 12.5% Mo: 0.8 to 1.2% W: 3.75 to 4.25%
Al: 3.75 to 4.25% Ti: 4 to 4.8% Ta: 1.75 to 2.25% 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
350 WEST COLORADO BOULEVARD
SUITE 500
PASADENA
CA
91105
US
|
Family ID: |
8173964 |
Appl. No.: |
10/008745 |
Filed: |
November 30, 2001 |
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 |
00403362 |
Claims
We claim:
1. A nickel-based superalloy, suitable for monocrystalline
solidification, characterised in that its composition by weight is
as follows:
5 Co: 4.75 to 5.25% Cr: 11.5 to 12.5% Mo: 0.8 to 1.2% W: 3.75 to
4.25% Al: 3.75 to 4.25% Ti: 4 to 4.8% Ta: 1.75 to 2.25% 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
TECHNICAL FIELD
[0001] 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
[0002] 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.
[0003] 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
matrix gamma (.gamma.) based on nickel, in which hardening
particles of the phase gamma prime (.gamma.') are dispersed whose
base is the intermetallic compound Ni.sub.3Al. 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.
[0004] 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.
[0005] 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.
[0006] 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, in 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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
[0011] 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 IN792, 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 1000.degree. C.
[0012] 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.
[0013] Furthermore, this superalloy must demonstrate good
microstructural stability in respect of the precipitation of
fragile intermetallic phases which are rich in chrome when
maintained for sustained periods at elevated temperature.
[0014] More specifically, an alloy compound is sought which
ensures:
[0015] optimised resistance to hot-corrosion, in any case at least
equivalent to that of reference polycrystalline super-alloy IN792,
and this in various environments which are representative of that
for combustion gases of industrial turbines;
[0016] a maximum volume fraction of hardening precipitates of the
.gamma.' phase in order to promote resistance to creep at elevated
temperature;
[0017] resistance to creep up to 1000.degree. C. which is superior
to that of reference polycrystalline alloy IN792;
[0018] a tendency to homogeneity by completely placing in solution
particles of the .gamma.' phase, including the .gamma./.gamma.'
eutectic phases;
[0019] the absence of precipitation of fragile intermetallic phases
which are rich in chrome, starting from the a matrix, when
maintained for sustained periods at elevated temperature;
[0020] a density which is less than 8.4 g.cm.sup.-3 in order to
minimise 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;
[0021] a good tendency to monocrystalline solidification of turbine
blades whose height can reach several tens of centimeters and the
mass several kilograms.
[0022] 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: 11.5 to 12.5% Mo: 0.8 to 1.2% W: 3.75 to
4.25% Al: 3.75 to 4.25% Ti: 4 to 4.8 Ta: 1.75 to 2.25% C: 0.006 to
0.04% B: .gtoreq.0.01% Zr: .gtoreq.0.01% Hf: .gtoreq.1% Nb:
.gtoreq.1% Ni and any impurities: complement to 100%.
[0023] The alloy according to the invention is an excellent
compromise between resistance to creep and resistance to
hot-corrosion. It is suitable 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.
[0024] The invention also relates to an industrial turbine blade
which is produced by monocrystalline solidification of the above
superalloy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The features and advantages of the invention will be set
forth in greater detail in the description below with reference to
the appended drawings.
[0026] FIGS. 1 and 2 are graphs illustrating the properties of
different superalloys.
DETAILED DESCRIPTION
[0027] An alloy according to the invention designated SCB444 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 -- 3.5 3.5 3.5 -- SCB444 Base 5 12 1 4
4 4.4 2 --
[0028] 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 12% by weight
was necessary and sufficient in the alloy of the invention in order
to obtain resistance to hot-corrosion that is equivalent to that of
reference alloy IN792 under the conditions for hot-corrosion tests
described below, which conditions are representative of the
environment created by combustion gases of some industrial
turbines. A higher chrome content would not allow the volume
fraction of the .gamma.' phase, which is necessary for good creep
resistance of the alloy up to 1000.degree. C., to be reached
without the alloy becoming unstable in respect of the precipitation
of fragile intermetallic phases which are rich in chrome in the
.gamma. matrix. Chrome also contributes to the hardening of the
.gamma. matrix in which this element is preferentially
distributed.
[0029] 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.
[0030] 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 homogenising 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 homogenising and good resistance to creep.
[0031] 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.
[0032] The concentration of aluminium 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. Aluminium
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 aluminium. The respective concentrations of those two
elements in the alloy of the invention are in the order of 4.4% by
weight for titanium and 2% by weight for tantalum. Under the
conditions for hot-corrosion tests described below, corresponding
to the intended application, tests showed that the presence of
titanium was more favourable to the resistance to hot-corrosion
than was the case with tantalum. However, the concentration of
titanium has been limited, on the one hand, by the fact that the
element can have a negative effect on the resistance to oxidation
and, on the other hand, because an excessively high concentration
of titanium can lead to a destabilisation of the .gamma.' phase.
The total of the concentrations of tantalum, titanium and aluminium
roughly determines the volume fraction of the .gamma.' hardening
phase. The concentrations of those three elements have been
adjusted in order to optimise 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 12% by weight in order to achieve the
desired resistance to corrosion.
[0033] Alloy SCB444 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 8.22 g.cm.sup.-3.
[0034] After directional solidification, the alloy is substantially
constituted by two phases: the austenitic matrix .gamma., 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. A small fraction of the .gamma.' phase is also located
within solid particles resulting from a liquid eutectic
transformation.fwdarw..g- amma.+.gamma.' once solidification has
ended. The volume fraction of the .gamma./.gamma.' eutectic phase
is in the order of 1.4%.
[0035] Alloy SCB444 underwent homogenising heat treatment at a
temperature of 1270.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 1253.degree. C., and less than the
solidus temperature, which is 1285.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, to eliminate the solid .gamma./.gamma.'
eutectic particles and to reduce the chemical heterogeneities which
are associated with the dendritic solidification structure.
[0036] The interval between the .gamma.' solvus temperature of the
alloy SCB444 and its solidus temperature is very large, which
allows ready application of the homogenising treatment without any
risk of melting and with the certainty of obtaining a homogeneous
microstructure which allows optimised resistance to creep.
[0037] The cooling which follows the homogenising 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.
[0038] The homogenising 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.
[0039] Alloy SCB444 was tested after undergoing a homogenising
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 stabilised. A first annealing treatment
consisted in heating the alloy to 1100.degree. C. for 4 hours with
cooling in air, which leads to stabilisation 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 optimised. This volume
fraction of the .gamma.' phase is estimated at 57% in alloy SCB444.
Once all of the heat treatments are completed, the .gamma.' phase
has been precipitated in the form of cuboid particles whose size is
between 200 and 500 nm. Cyclical hot-corrosion tests were carried
out at 900.degree. C. on alloy SCB444 on an industrial corrosion
bench with a burner. The cycle was as follows: 1 hour at
900.degree. C. in the corrosive atmosphere produced by the burner,
then 15 minutes out of the oven at ambient temperature. The burner
operated with fuel loaded with 0.20% sulphur. A saline water
solution at 0.5 g.l.sup.-1 NaCl was vaporized on the test piece at
a rate of 2.2 m.sup.3.h.sup.-1. The test piece was coated every 100
hours with a deposit of 0.5 mg.cm.sup.-2 Na.sub.2SO.sub.4. For
comparison, alloys IN738 and IN792 were tested at the same time.
The criterion for corrosion resistance is the number of cycles for
which the first corrosion pits appear on the surface of the test
piece.
[0040] The results of the corrosion tests are illustrated by the
graph in FIG. 1. The start of corrosion at 900.degree. C. occurs
for cycle totals which are comparable for alloys SCB444 and IN792,
which fulfils the stated objective.
[0041] Tests for creep under tensile stress were carried out on
machined test pieces in monocrystalline bars of orientation
<001>. The bars were homogenised 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 SCB444 Temperature
(.degree. C.) Stress (MPa) Rupture time (h) 750 725 134 750 650 612
750 600 1152 850 500 43.1 850 425 168.5 850 300 3545/>3456 950
250 115/135 950 200 551/544 950 180 578 950 140 2109 950 120
3872
[0042] The graph in FIG. 2 allows a comparison of the rupture times
with creep obtained for alloys SCB444, IN738, 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).times.10.sup.-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 SCB444 is
distinctly superior to that of alloy IN792.
[0043] The inspection of the microstructure of the test pieces of
alloy SCB444 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 matrix is over-saturated with additive
elements.
[0044] Manufacturing tests on monocrystalline components of
super-alloy SCB444 demonstrated that it was possible to cast a
large range of components whose mass can range from a few grammes
to more than 10 kg, with various levels of complexity. The growth
of components according to the crystallographic orientation
<001>is promoted and dominant and the presence of grains that
are orientated in a random manner is minimised. 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 homogenising treatment at
elevated temperature is absent in the case of alloy SCB444.
* * * * *