FIELD OF THE INVENTION
[0001] The present invention relates to nickel base superalloy castings and, more particularly,
to a method of heat treating single crystal superalloy castings in a manner to reduce
or localize deleterious extraneous grain recrystallization during heat treatment.
BACKGROUND OF THE INVENTION
[0002] U.S. Patent 4 643 782 describes single crystal castings made from a nickel base superalloy
having a composition consisting essentially of, in weight %, of 6.4% to 6.8% Cr, 9.3%
to 10.0% Co, 0.5% to 0.7% Mo, 6.2% to 6.6% W, 6.3% to 6.7% Ta, 5.45% to 5.75% Al,
0.8% to 1.2% Ti, 2.8% to 3.2% Re, 0.07 to 0.12% Hf and balance essentially nickel.
Carbon is held to incidental impurity levels of for example 60 ppm maximum C in the
alloy.
[0003] U.S. Patent 5 759 303 describes addition of carbon to a nickel base superalloy including
the alloy of the first-discussed patent above to reduce the amount of non-metallic
inclusions (e.g. oxide inclusions) in the microstructure of single crystal investment
castings produced therefrom.
SUMMARY OF THE INVENTION
[0004] In attempts to manufacture investment cast gas turbine engine single crystal blades
from a nickel base superalloy of the first-discussed patent above, applicants discovered
that such single castings were prone to develop deleterious extraneous grain recrystallization
at the airfoil and/or root of the gas turbine engine blade during a subsequent conventional
heat treatment to develop alloy mechanical properties wherein the castings are initially
subjected to a high temperature solution heat treatment. Such grain recrystallization
is to be avoided or localized to non-critical regions of a casting that are subsequently
removed therefrom. Grain recrystallization is a cause for rejection of single crystal
castings if present beyond a preset maximum for recrystallized grains and can result
in quite low yields of acceptable heat treated single crystal castings.
[0005] The present invention provides a method of making of superalloy single crystal castings,
such as gas turbine engine single crystal blades and vanes (airfoils), in a manner
to address the problem of grain recrystallization during heat treatment of the single
crystal castings. The invention involves the discovery that grain recrystallization
can be reduced by solution heat treating the single crystal castings in the presence
of gaseous species carburizing relative to the superalloy castings so as to introduce
carbon into the castings in an effective amount to reduce recrystallized grains during
heat treatment. For example, a carburizing atmosphere can be provided by introducing
a mixture of carbon monoxide and an inert gas, such as argon, into the heat treatment
furnace or by heat treating in a furnace having a component, such as heating elements,
that inherently provides a carburizing atmosphere during heat treatment. Such a furnace
to this end typically comprises heating elements, heat shields and/or other furnace
components or inserts comprising graphite or other carbon-bearing material as a source
of carbon for reaction with oxygen to form a carbon-bearing gas, such as carbon monoxide,
in-situ in the furnace that is carburizing relative to the castings.
[0006] The carbon concentration of at least the outer surface region of the superalloy single
crystal castings is locally increased during heat treatment as compared to the nominal
carbon concentration of the bulk superalloy casting as evidenced, for example, by
the presence of blocky carbides of one or more alloying elements, which carbides are
not present when the superalloy casting is heat treated under vacuum or inert gas
atmosphere only in the absence of a carburizing gas species in the furnace. The carbides
form in the microstructure in a manner to pin any recrystallized grain boundaries
during solution heat treatment and retard, limit and localize their growth in a manner
to improve the yield of acceptable heat treated single crystal castings.
DESCRIPTION OF THE DRAWINGS
[0007]
Figures 1 and 2 are photomicrographs at 500X and 200X of single crystal nickel base
superalloy castings after solution heat treatment pursuant to an embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0008] The present invention involves heat treating nickel base superalloys formulated for
single crystal casting in a manner to unexpectedly and surprisingly substantially
reduce or localize grain recrystallization after heat treatment of the casting at
elevated temperature, such as a high temperature solution heat treatment to dissolve
or solution most of the eutectic and coarse gamma prime phases present in the as-cast
microstructure. Improved yields of acceptable heat treated single crystal castings
are thereby achieved.
[0009] In general, the present invention can be practiced on a variety of low carbon nickel
base superalloys that are formulated for single crystal casting and include W, Ta,
Mo, Co, Al and Cr as important alloying elements as well as optionally Ti, Re, Y,
Hf, one or more rare earth elements such as La, B, and Mg as intentional alloying
elements and that suffer undesirable grain recrystallization upon heat treatment.
Such grain recrystallization prone nickel base superalloys typically have carbon concentration
less than about 200 ppm by weight (about 0.02 weight % C) with some less than about
100 ppm C (about 0.01 weight % C), although the invention may be practiced with superalloys
having other carbon concentrations to reduce grain recrystallization in a particular
nickel base superalloy. Nickel base superalloys formulated for casting single crystal
castings such as single crystal airfoils (blades and vanes), and heat treatable pursuant
to the invention include, but are not limited to, those described in U.S. Patents
4 643 782 and 5 366 695 the teachings of which are incorporated herein by reference
with respect to particular alloy compositions.
[0010] An illustrative nickel base superalloy casting composition heat treatable pursuant
to the present invention consists essentially of, in weight % or parts per million
(ppm) by weight, of about 6% to 6.8% Cr, about 8% to 10% Co, about 0.5% to 0.7% Mo,
about 6.2% to 6.6% W, about 6.3% to 7% Ta, about 5.4% to 5.8% Al, about 0.6% to 1.2%
Ti, about 0.10% to 0.3% Hf, up to about 200 ppm by weight B, up to about 50 ppm by
weight Mg, up to about 200 ppm by weight carbon, and balance essentially Ni and castable
to provide a single crystal microstructure, especially for gas turbine engine blades
and vanes (i.e. airfoils).
[0011] An illustrative low carbon, high Re nickel base superalloy casting composition heat
treatable pursuant to the present invention consists essentially of, in weight %,
of about 1.5% to 5% Cr, about 1.5% to 10% Co, about 0.25% to 2% Mo, about 3.5% to
7.5% W, about 7% to 10% Ta, about 5% to 7% Al, up to about 1.2% Ti, about 5% to 7%
Re, up to about 0.15% Hf, up to about 0.5% Nb, C less than about 0.02% or at incidental
impurity level, and balance essentially Ni and castable to provide a single crystal
microstructure, especially for gas turbine engine blades and vanes (i.e. airfoils).
An illustrative low carbon, high Cr nickel base superalloy casting composition heat
treatable pursuant to the present invention consists essentially of, in weight %,
of about 11% to 16% Cr, about 2% to 8% Co, about 0.2% to 2% Mo, about 3.5% to 7.5%
W, about 4% to 6% Ta, about 3% to 6% Al, about 2% to about 5% Ti, up to about 0.2%
Nb, C less than about 0.02% or at incidental impurity level, and balance essentially
Ni and castable to provide a single crystal microstructure, especially for gas turbine
engine blades and vanes (i.e. airfoils).
[0012] The following example is offered for purposes of illustrating but not limiting the
invention. Single crystal gas turbine engine blades were conventionally cast using
the Bridgeman withdrawal technique from commercially available CMSX-4 nickel base
superalloy, described in US Patent 4 643 782, and were subjected in the as-cast condition
after removal of a ceramic shell mold and a ceramic core to solution heat treatments
in various atmospheres. The nominal composition, in weight %, of the single crystal
blades was 6.4% Cr, 9.7% Co, 0.6% Mo, 6.4% W, 6.5% Ta, 5.6% Al, 1.0% Ti, 2.9% Re,
0.10% Hf, 30 ppm by weight C and balance essentially Ni and impurities. After casting,
the ceramic shell mold and core were removed completely from the castings in conventional
manner using a mechanical knock-out procedure and chemical leaching. The single crystal
blade castings then were solution heat treated in various furnaces using various atmospheres.
After heat treatment, the castings were examined for the presence of recrystallized
grains on the casting surfaces.
[0013] In particular, the cast single crystal blades were solution heat treated in various
heat treatment furnaces. One type of furnace included graphite electrical resistance
heating element and graphite sides or heat shield liners. Another type of furnace
included molybdenum electrical resistance heating elements and graphite sides or heat
shield liners. Different heat treatment atmospheres were provided for different heat
treatment runs in the different types of furnaces.
[0014] For example, one heat treatment run involved providing a vacuum of less than 5 microns
in a furnace having molybdenum heating elements and graphite heat shields or liners
and then introducing a mixture of argon and 10% by volume CO at a flow rate during
continued vacuum pump evacuation of the furnace to maintain 400 microns partial pressure
of argon plus CO in the furnace as the atmosphere during heat treatment. The argon/10%
by volume CO gas mixture was introduced from a conventional gas cylinder having a
mixture of argon and 10% by volume CO therein. The mixture was introduced after the
furnace temperature reached 1900 degrees F so as to reduce chromium vaporization from
the castings. A total of 10 cast single crystal blades were solution heat treated
in this furnace by slowly heating the castings to a solutioning temperature of 2400
degrees F plus or minus 15 degrees F over 11 hours. The solutioning temperature was
held for 6 hours, and the castings were cooled to room temperature over a time of
1 hour.
[0015] The solution heat treatment dissolved most of the eutectic and coarse gamma prime
phases in the as-cast microstructure. The only recrystallized grains observed were
initiated proximate a core print (at a blade tip) with the recrystallized grains localized
to an extent that they existed outside the finished casting dimensions for the particular
blade involved; i.e. such that the localized amount of recrystallized grains on the
heat treated blade would be removed by subsequent finish machining of the blade. In
microstructural examination, blocky carbides rich in Ta and Ti having a lateral dimension
(e.g. diameter) of less than 0.5 mil (0.0005 inch) were observed to exist throughout
the airfoil and pinning the recrystallized grain boundaries as illustrated in Figure
1 by the arrow. The carbides were determined to include Ta in the approximate range
of 71-77 weight % Ta, Ti in approximate range of 9-10 weight % Ti, Hf in approximate
range of 2-7 weight % Hf, Ni in approximate range of 3-4 weight % Ni with other elements
such as Co, W, Cr, Fe, also present in lesser amounts. The carbides were formed predominantly
along cast surfaces, providing a high population of carbides in thin sections of the
castings where grain growth is more likely to be a problematic. The carbides were
attributed to the carburization of the castings, resulting in introduction of carbon
into the castings during heat treatment. For example, typical carbon concentration
at the airfoil surface and of the bulk airfoil was twice as high (at least 100% higher)
as the as-cast carbon content at the airfoil surface. For purposes of illustration
only, for a particular cast single crystal blade measured for carbon, the as-cast
carbon concentration of the bulk airfoil and bulk root were increased from about 38
ppm by weight C to 113 ppm and 89 ppm by weight carbon for the airfoil and root, respectively.
The carbon content at the airfoil surface was even higher, being about 171 ppm by
weight C after the above heat treatment.
[0016] Similar results were achieved when the cast single crystal blades were heat treated
under similar parameters in a furnace having graphite heating elements and graphite
heat shields or liners and an atmosphere comprising 400 microns argon only introduced
in a manner similar to the above described argon/CO mixture when the furnace reached
1900 degrees F and at a flow rate to provide the 400 microns argon partial pressure
during heat treatment. Blocky carbides were observed throughout the airfoil and also
throughout the thickness (35 mils) of the platform, and 25 mils inwardly from the
root surface. Recrystallized grains were small and confined near the core print by
the carbides (see arrows in Figure 2) pinning the recrystallized grain boundaries.
For a particular cast single crystal blade measured for carbon, the as-cast carbon
concentrations of the bulk airfoil and bulk root were increased to 200 and 124 ppm
by weight carbon, respectively.
[0017] Similar results were achieved when the cast single crystal blades were heat treated
under similar parameters in a furnace having molybdenum heating elements and graphite
heat shields or liners with introduction of the above described mixture of argon/10%
by volume CO when the furnace temperature reached 1900 degrees F to provide 400 microns
partial pressure argon plus CO as described above during heat treatment. Blocky carbides
were observed to be present interdendritically in clusters throughout the airfoil
and approximately 10 mils below the root surface. Recrystallized grains were small
and confined near the core print by carbide pinning the recrystallized grain boundaries.
[0018] In contrast, similar results were not achieved when the cast single crystal blades
were heat treated under similar parameters in a furnace having molybdenum heating
elements and graphite heat shields or liners in a vacuum of less than 1 micron or
when argon gas only was introduced to the furnace after a furnace temperature of 1900
degrees F to provide 400 microns partial pressure argon only during heat treatment.
Blocky carbides were not observed to be present throughout the airfoil or other regions
of the castings heat treated under vacuum or under 400 microns partial pressure argon
only using the molybdenum heating elements. The castings exhibited unacceptable recrystallized
grains.
[0019] The present invention provides single crystal castings having carbon concentrations
increased by the heat treatment in an amount discovered to form carbides in-situ in
the heat treated microstructure that pin recrystallized grain boundaries and retard,
limit and localize their growth to reduce recrystallized grains that are cause for
rejection of the single crystal castings and increase yield of acceptable heat treated
castings. Practice of the invention as described above produced a six times increase
in yield of acceptable heat treated single crystal turbine blade castings. The present
invention envisions use of carburizing atmospheres or gaseous carburizing species
other than carbon monoxide that are effective to introduce carbon to single crystal
nickel base superalloy castings during their heat treatment in amounts effective to
reduce or localize recrystallized grains.
[0020] While the invention has been described in terms of specific embodiments thereof,
it is not intended to be limited thereto but rather only to the extent set forth in
the following claims.
1. A method of making a single crystal casting, comprising providing a nickel base superalloy
single crystal casting and heat treating the casting in presence of a gaseous species
effective to introduce carbon into the casting and form carbides therein that reduce
grain recrystallization in the single crystal casting during heat treating.
2. The method of claim 1 wherein said heat treating forms blocky carbides rich in Ta
in said casting.
3. The method of claim 1 wherein said atmosphere includes an inert gas and carbon-bearing
gas constituent in a heat treatment furnace.
4. The method of claim 3 wherein the carbon-bearing as comprises carbon monoxide.
5. The method of claim 3 wherein said carbon-bearing gas is introduced into said furnace
from a source external of said furnace.
6. The method of claim 3 including generating said carbon-bearing gas in-situ in said
furnace using carbon from a heating element comprising graphite.
7. A method of making a single crystal casting, comprising providing a low carbon nickel
base superalloy single crystal casting including W, Ta, Mo, Co, Al and Cr as alloying
elements and heat treating the casting in presence of a gaseous species effective
to introduce carbon into the casting and form carbides therein that reduce grain recrystallization
in the single crystal casting during heat treating.
8. The method of claim 7 wherein the nickel base superalloy includes one or more of Ti,
Re, Y, Hf, rare earth element, B, and Mg as intentional alloying elements.
9. The method of claim 7 wherein carbon concentration of the superalloy is less than
about 200 ppm by weight.
10. A method of making a single crystal casting, comprising providing a nickel base superalloy
single crystal casting consisting essentially of, in weight %, about 6% to 6.8% Cr,
about 8% to 10% Co, about 0.5% to 0.7% Mo, about 6.2% to 6.6% W, about 6.3% to 7%
Ta, about 5.4% to 5.8% Al, about 0.6% to 1.2% Ti, about 0.10% to 0.3% Hf, up to about
100 ppm by weight B, up to 50 ppm by weight Mg, up to about 200 ppm by weight C and
balance essentially Ni and heat treating the casting in presence of gaseous species
effective to introduce carbon into the casting and form carbides therein that reduce
grain recrystallization in the single crystal casting during heat treating.
11. The method of claim 10 wherein said heat treating forms blocky carbides rich in Ta
in said casting.
12. The method of claim 10 wherein said atmosphere includes an inert gas and carbon-bearing
gas constituent in a heat treatment furnace.
13. The method of claim 12 wherein the carbon-bearing gas comprises carbon monoxide.
14. The method of claim 12 wherein said carbon-bearing gas is introduced into said furnace
from a source external of said furnace.
15. The method of claim 12 including generating said carbon-bearing gas in-situ in said
furnace using carbon from a heating element comprising graphite.
16. A method of making a single crystal casting, comprising providing a nickel base superalloy
single crystal casting consisting essentially of, in weight %, about 1.5% to 5% Cr,
about 1.5% to 10% Co, about 0.25% to 2% Mo, about 3.5% to 7.5% W, about 7% to 10%
Ta, about 5% to 7% Al, up to about 1.2% Ti, about 5% to 7% Re, up to about 0.15% Hf,
up to about 0.5% Nb, C less than about 0.02%, and balance essentially Ni and heat
treating the casting in presence of gaseous species effective to introduce carbon
into the casting and form carbides therein that reduce grain recrystallization in
the single crystal casting during heat treating.
17. A method of making a single crystal casting, comprising providing a nickel base superalloy
single crystal casting consisting essentially of, in weight %, of about 11% to 16%
Cr, about 2% to 8% Co, about 0.2% to 2% Mo, about 3.5% to 7.5% W, about 4% to 6% Ta,
about 3% to 6% Al, about 2% to about 5% Ti, up to about 0.2% Nb, C less than about
0.02%, and balance essentially Ni and heat treating the casting in presence of gaseous
species effective to introduce carbon into the casting and form carbides therein that
reduce grain recrystallization in the single crystal casting during heat treating.
18. A heat treated nickel base superalloy single crystal casting, said heat treated single
crystal casting having carbides formed in the microstructure at a recrystallized grain
boundary.
19. The casting of claim 18 including W, Ta, Mo, Co, Al and Cr as alloying elements.
20. The casting of claim 18 further including at least one of Ti, Re, Y, Hf, rare earth
element, B, and Mg as intentional alloying elements
21. A heat treated single crystal nickel base alloy casting consisting essentially of,
in weight %, of about 6% to 6.8% Cr, about 8% to 10% Co, about 0.5% to 0.7% Mo, about
6.2% to 6.6% W, about 6.3% to 7% Ta, about 5.4% to 5.8% Al, about 0.6% to 1.2% Ti,
about 0.1% to 0.3% Hf, up to about 100 ppm by weight B, up to 50 ppm by weight Mg,
up to about 200 ppm by weight C, and balance essentially Ni, said heat treated single
crystal casting having carbides at a recrystallized grain boundary.