U.S. patent application number 10/618054 was filed with the patent office on 2004-01-15 for casting of single crystal superalloy articles with reduced eutectic scale and grain recrystallization.
This patent application is currently assigned to Howmet Research Corporation. Invention is credited to Corrigan, John, Gratti, Gilbert M., Mihalisin, John R., Vogt, Russell G..
Application Number | 20040007296 10/618054 |
Document ID | / |
Family ID | 23058365 |
Filed Date | 2004-01-15 |
United States Patent
Application |
20040007296 |
Kind Code |
A1 |
Mihalisin, John R. ; et
al. |
January 15, 2004 |
Casting of single crystal superalloy articles with reduced eutectic
scale and grain recrystallization
Abstract
A single crystal casting is cast from a nickel base superalloy
including Cr, Co, Mo, W, Ta, Al, Ti, Re and Hf as alloying elements
with C increased effective to substantially reduce formation of a
solidification-driven, as-cast eutectic/secondary phase scale
metallurgically bonded to the casting when the alloy is cast as a
single crystal and to reduce recrystallized grains when the casting
is solution heat treated.
Inventors: |
Mihalisin, John R.; (North
Caldwell, NJ) ; Corrigan, John; (Yorktown, VA)
; Gratti, Gilbert M.; (Smithfield, VA) ; Vogt,
Russell G.; (Yorktown, VA) |
Correspondence
Address: |
Edward J. Timmer
Walnut Woods Centre
5955 W. Main Street
Kalamazoo
MI
49009
US
|
Assignee: |
Howmet Research Corporation
|
Family ID: |
23058365 |
Appl. No.: |
10/618054 |
Filed: |
July 11, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10618054 |
Jul 11, 2003 |
|
|
|
09276858 |
Mar 26, 1999 |
|
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Current U.S.
Class: |
148/555 ;
148/428; 148/562; 420/448 |
Current CPC
Class: |
C22C 19/057
20130101 |
Class at
Publication: |
148/555 ;
148/562; 148/428; 420/448 |
International
Class: |
C22F 001/10 |
Claims
We claim
1. A method of making a single crystal casting, comprising
providing a nickel base superalloy that includes Cr, Co, Mo, W, Ta,
and Al as alloying elements and that exhibits as-cast metallic
surface scale when the superalloy is cast as a single crystal
casting with a C concentration effective to substantially reduce
formation of said as-cast metallic surface scale, and solidifying
the superalloy in a mold to form a single crystal casting.
2. The method of claim 1 wherein said nickel base superalloy
includes at least one of Ti, Re, Hf, Y, a rare earth element, Mg,
and B.
3. The method of claim 1 wherein the superalloy as-cast is free of
said scale.
4. The method of claim 1 including solution heat treating the
casting wherein recrystallized grains are reduced after heat
treatment.
5. A method of making a single crystal casting, comprising
providing a nickel base superalloy consisting essentially of, in
weight %, about 6% to 6.8% Cr, about 8% to 10% Co, about 0.5% to
0.7% Mo, about 5.0% 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 0.05% to 0.3% Hf, up to about
100 ppm by weight B, up to 50 ppm by weight Mg, and balanc
essentially Ni that exhibits as-cast metallic surface scale when
the superalloy is cast as a single crystal casting, including
providing said superalloy with a C concentration greater than 0.04
weight % effective to substantially reduce formation of an as-cast
metallic scale when the superalloy is cast as a single crystal and
solidifying the superalloy in a mold to form a single crystal
casting.
6. The method of claim 5 wherein the superalloy as-cast is free of
said scale.
7. The method of claim 5 including solution heat treating the
casting wherein recrystallized grains are reduced after heat
treatment.
8. The method of claim 5 wherein C is included in an amount of
greater than 0.04% to about 0.1% by weight.
9. A m thod of making a single crystal casting, comprising
providing a nickel base superalloy including Cr, Co, Mo, W, Ta, and
Al as alloying elements that exhibits as-cast metallic surface
scale when the superalloy is cast as a single crystal casting,
including providing said superalloy with a C concentration
controlled in accordance with the equation, % area fraction scale
=-0.193 .times. carbon content in ppm +86 effective to
substantially reduce formation of an as-cast metallic scale when
the superalloy is cast as a single crystal and solidifying the
superalloy in a mold to form a single crystal casting.
10. The method of claim 9 including heat treating the casting
wherein recrystallized grains are reduced after heat treatment.
11. The method of claim 9 wherein said superalloy includes at least
one of Ti, Re, Hf, Y, a rare earth element, B, Mg, and B.
12. As cast, metallic scale-free 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 5.0% 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 0.05% to 0.3% Hf, up to about 100 ppm by weight B, up to
50 ppm by weight Mg, up to 100 ppm Y and balance essentially Ni and
a C concentration greater than 0.04 weight %, said casting being
substantially free of as-cast metallic scale.
13. The casting of claim 12 wherein the Hf content is from about
0.15 to about 0.30 weight %.
14. The casting of claim 12 including Re.
15. Single crystal nickel base alloy 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 5.0% 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 0.15% to 0.3% Hf, up to about
100 ppm by weight B, up to 50 ppm by weight Mg, up to 100 ppm Y and
balance essentially Ni and a C concentration greater than 0.04
weight % to produce a single crystal casting substantially free of
as-cast metallic scale when the alloy is cast as a single
crystal.
16. The casting of claim 15 including Re.
17. A method of making a single crystal casting, comprising
providing a nickel base superalloy that includes Cr, Co, Mo, W, Ta,
and Al as alloying elements and that exhibits grain
recrystallization when the superalloy is solution heat treated,
including providing said superalloy with a C concentration
effective to substantially reduce grain recrystallization during
heat treating, and solution heat treating the superalloy.
18. The method of claim 17 wherein said nickel base superalloy
includes at least one of Ti, Re, Hf, Y, a rare earth element, Mg,
and B.
19. The method of claim 17 wherein said nickel base superalloy
consisting essentially of, in weight %, about 6% to 6.8% Cr, about
8% to 10% Co, about 0.5% to 0.7% Mo, about 5.0% 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
0.05% to 0.3% Hf, up to about 100 ppm by weight B, up to 50 ppm by
weight Mg, greater than 0.4% C, and balance essentially Ni.
20. The method of claim 17 wherein the Hf content is from about
0.15 to about 0.30 weight %.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to nickel base superalloy
castings and, more particularly, to a method of making single
crystal superalloy castings in a manner to reduce deleterious
as-cast eutectic/secondary phase scale and extraneous grain
recrystallization during heat treatment.
BACKGROUND OF THE INVENTION
[0002] U.S. Pat. No. 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 60 ppm maximum in the
patented alloy.
[0003] U.S. Pat. No. 5,759,301 describes single crystal castings
made from a nickel base superalloy having a composition consisting
essentially of, in weight %, of 6.0% to 6.8% Cr, 8.0% to 10.0% Co,
0.5% to 0.7% Mo, 6.2% to 6.6% W, 6.3% to 7.0% Ta, 5.4% to 5.8% Al,
0.6% to 1.2% Ti, 2.7% to 3.2% Re, 0.15% to 0.3% Hf, 0.02% to 0.04%
C, 40 ppm to 100 ppm B, 15 ppm to 50 ppm Mg and balance essentially
nickel wherein the alloying elements C, B, Hf, and Mg are said to
have a beneficial effect on small angle grain boundaries.
[0004] U.S. Pat. No. 5,549,765 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
[0005] In attempts to investment cast gas turbine engine single
crystal blades from a nickel base superalloy of the first-discussed
patent above, applicants discovered that there was formation of an
as-cast metallic scale extensively over the airfoil surfaces of the
single crystal cast blades and observable after the ceramic shell
mold was removed from the castings. The surface scale was
discovered to include, among other constituents, one or more low
melting point alloy eutectics and secondary alloy phases rich in
one or more of such alloy elements as W, Ta, Re, Mo, Cr, Co, Ti and
Hf, with the scale being metallurgically bonded to the casting.
Formation of the surface scale appeared to be solidification driven
by segregation of alloying elements and eutectic and secondary
phase reactions occurring during single crystal solidification. The
scale was extensively present on the as-cast airfoil surfaces of
the single crystal castings, occurring over as much as 80% of the
airfoil surface. The presence of the scale rendered the castings
unacceptable for use and required a post-cast abrasive belt or
other mechanical finishing operation to remove the scale.
[0006] Applicants also discovered that such single crystal castings
were prone to develop deleterious extraneous grain nucleation and
growth at the airfoil and/or root of the gas turbine engine blade
during a subsequent conventional solution heat treatment that is a
part of a heat treatment schedule to develop alloy mechanical
properties. Such recrystallized grain regions are to be avoided and
can be the cause for r jection of singl crystal castings if present
beyond a preset maximum for recrystallized grains.
[0007] The present invention provides a method of making of
superalloy single crystal airfoil castings, such as gas turbine
engine single crystal blades and vanes, that suffer from the
problem of solidification-driven scale formation in the as-cast
condition and extraneous recrystallized grains in the heat treated
condition.
[0008] The present invention involves the further discovery that
the problem of formation of such surface scale on surfaces of
as-cast single crystal nickel base superalloy castings can be
reduced or prevented by increasing the carbon concentration of the
superalloy beyond the specified alloy carbon level to this end.
[0009] The present invention involves the additional discovery that
the problem of formation of recrystallized grains after heat
treatment of the single crystal castings also can be reduced or
prevented by increasing the carbon concentration of the superalloy
beyond the specified alloy carbon level to this end.
[0010] In particular, the carbon concentration of the superalloy is
increased to an amount effective to substantially reduce or
eliminate (1) formation of the solidification-driven non-oxide
scale on the surfaces of single crystal castings in the as-cast
condition and (2) recrystallized grains in the heat treated
condition.
[0011] The above objects and advantages of the present invention
will become more readily apparent from the following detailed
description taken with the following drawings.
DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1A, 1B and 1C are photographs of the airfoil regions
of cast single crystal nickel base superalloy turbine blades after
removal of a ceramic shell mold showing the effect of carbon level
of the nickel base superalloy on the amount of metallic surface
scale present on the airfoil surfaces after mold removal.
[0013] FIG. 2 is a graph of the relationship of percent scale
coverage of the casting surfaces versus carbon concentration of the
nickel base superalloy.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The present invention involves increasing the carbon
concentration of nickel base superalloys formulated for single
crystal casting in an amount discovered to unexpectedly and
surprisingly substantially reduce or eliminate formation of the
solidification-driven metallic as-cast scale discovered to be
formed on the surfaces of single crystal castings of the
superalloys under single crystal casting conditions and to
unexpectedly and surprisingly eliminate recrystallized grains after
heat treatment of the castings to develop mechanical properties.
Generally, the present invention can be practiced with 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 including Ti, Re, Hf, Y, one or more rare earth
elements such as La, B, Mg and other intentional alloying elements
and that suffer from the problem of solidification-driven scale
formation in the as-cast condition and extraneous recrystallized
grains in th heat treated condition. Particular nickel base
superalloys which can be modified pursuant to the present invention
to have increased carbon to this end include, but are not limited
to, those described in U.S. Pat. Nos. 4,643,782, 5,759,301 and
5,366,695, the teachings of which are incorporated herein by
reference with respect to particular alloy compositions.
[0015] A particular nickel base superalloy casting composition
modified in accordance with the present invention offered for
purposes of illustration and not limitation consists essentially
of, in weight %, of about 6% to 6.8% Cr, about 8% to 10% Co, about
0.5% to 0.7% Mo, about 5.0% 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 0.05% to 0.3% Hf, up
to about 100 ppm by weight B, up to 50 ppm by weight Mg, balance
essentially Ni and, C and castable to provide a single crystal
microstructure, especially for gas turbine engine blades and vanes
(i.e. airfoils). One embodiment includes about 0.05 to about 0.12
weight % Hf in the alloy composition, while another embodiment
includes higher hafnium from about 0.15 to about 0.30 weight % Hf.
The carbon concentration of the alloy composition is controlled to
reduce or eliminate solidification-driven metallic scale formation
in the as-cast condition and recrystallized grains after heat
treatment of the castings to develop mechanical properties. These
nickel base superalloys are modified in accordance with the
invention to include increased carbon concentrations of greater
than 0.04 weight %, more preferably from 0.04% to 0.1 weight %
C.
[0016] Other illustrative nickel base superalloys formulated for
casting as single crystal airfoils which can be modified pursuant
to the present invention to have increased carbon to this end are
high-Re nickel base alloys described below and in U.S. Pat. No.
5,366,695, the teachings of which are incorporated herein by
reference with respect to particular alloy compositions, and
high-Cr nickel base superalloys.
[0017] Generally, a high-Re nickel base superalloy which can be
modified to benefit from practice of the invention consists
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, 0 to about 1.2% Ti, about 5% to 7% Re, up to
about 0.15% Hf, up to about 0.5% Nb, and balance essentially Ni and
C. Generally, a high-Cr nickel base superalloy which can be
modified to benefit from practice of the invention consists
essentially of, in weight %, 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 0.5% Nb and
balance essentially Ni and C. Other nickel base superalloys which
can be modified to benefit from practice of the invention consists
essentially of, in weight %, about 4% to 10% Cr, about 4% to 12%
Co, about 1% to 4% Mo, about 4% to 10% W, about 5% to 10% Ta, about
4% to 8% Al, up to about 2% Ti, up to about 0.5% Hf, up to about 5%
Re (preferably about 3% Re), and balance essentially Ni and C. For
example, one such alloy has a nominal composition, in weight %, of
7% Cr, 8% Co, 2% Mo, 5% W, 7% Ta, 3% Re, 6.2% Al, 0.2% Hf and
balance essentially Ni and C. Another such alloy has a nominal
composition, in weight %, of 8% Cr, 5% Co, 2% Mo, 8% W, 6% Ta, 5.0%
Al, 1.5% Ti, and balance essentially Ni and C. Still another such
alloy has a nominal composition, in weight %, of 5% Cr, 10% Co, 2%
Mo, 5% W, 3% Re, 8.5% Ta, 5.2% Al, 1.0% Ti, 0.1% Hf, and balance
essentially Ni and C. Still a further such alloy has a nominal
composition, in weight %, of 5% Cr, 10% Co, 2% Mo, 6% W, 3% Re, 9%
Ta, 5.6% Al, 0.1% Hf and balance essentially Ni and C. The C
concentrations of these superalloys can be intentionally increased
above normal carbon impurity levels to an amount, for example only
greater than 0.04 weight % C, effective to substantially reduce
formation of an as-cast metallic scale when the alloy is cast as a
single crystal.
[0018] The following single crystal casting tests were conducted
and are offered to further illustrate, but not limit, the present
invention. Heats #1, #2, and #3 having a nickel base superalloy
composition in weight percents as set forth in Table I were
prepared.
1TABLE I Heat Cr Co Mo W Ta Al Ti C Re Hf Ni #1 6.4 9.6 0.6 6.4 6.5
5.7 1.03 .0025 2.9 .10 balance #2 6.4 9.6 0.6 6.4 6.5 5.6 1.03 .02
2.9 .21 balance #3 6.3 9.5 0.6 6.5 6.5 5.7 1.0 .039 2.97 .10
balance
[0019] Each heat was made using conventional vacuum melting
practice wherein carbon was controlled by small additions to the
master alloy melt. Each heat was remelted and cast to form single
crystal cored IGT blade castings having an airfoil region and a
root r gion. The single crystal castings were produced using the
conventional Bridgeman mold withdrawal directional solidification t
chnique with a crystal selector passage (pigtail) to propagate a
single crystal through the mold cavity. For example, each heat was
melted in a crucible of a conventional casting furnace under a
vacuum of less than 1 micron and superheated to 1482 degrees C.
(2700 degrees F.). The superheated melt was poured into investment
casting mold having a mold facecoat comprising zirconia backed by
additional slurry/stucco layers comprising various forms of alumina
and zirconia. Each mold cluster was preheated to 1510 degrees C.
(2750 degrees F.) and mounted on a chill plate to effect
unidirectional heat removal from the molten alloy in the mold. The
melt-filled mold on the chill plate was withdrawn from the furnace
into a solidification chamber of the casting furnace at a vacuum of
1 micron at a withdrawal rate of 2 to 12 inches per hour. The
single crystal castings were cooled to room temperature and removed
from the shell mold in conventional manner using a mechanical
knock-out procedure, and then solution heat treated at 1310 degrees
C. (2390 degrees F.) for 6 hours. After mold knock-out, the
castings were observed visually for the presence of surface scale
on th casting surfaces. After heat treatment, the castings were
observed visually for the presence of recrystallize grains on the
casting surfaces.
[0020] The results of casting tests with respect to scale coverage
(dark areas) are illustrated in FIGS. 1A, 1B, 1C. In FIG. 1A wh re
the alloy had a carbon level of 0.0025 w ight % C, approximately
80% of the as-cast airfoil surface of the single crystal casting
after mold removal was cov red with a as-cast non-oxide scale
discovered to include, among other constituents, one or more low
melting point alloy eutectics and secondary alloy phases rich in
one or more of such alloy elements as W, Ta, Re, Mo, Cr, Co, Ti and
Hf and located predominantly at interdendritic areas of the
microstructure proximate the casting surface. For example, the
as-cast scale included as constituents various TCP (topologically
close packed) type phases including sigma phases found by TEM
(transmission electron microscopy) to be rich in W, Ta, Re, Mo, Cr,
Co with some to be rich in W, Ta, Re, Mo, Cr, Co, and Hf. Eutectic
phases rich in titanium and tantalum also were present at some
regions of the surface scale. Also present were spherical particles
rich in Cr and Ni. Formation of the surface scale appeared to be
solidification driven by segregation of alloying elements (solute
segregation) and eutectic and phase reactions occurring during
single crystal solidification. The as-cast eutectic/secondary phase
surface scale had a widely variable thickness and was
metallurgically bonded to the casting and very adherent, requiring
a separate mechanical abrasive belt operation finishing to remove.
The metallic scale is detrimental in that important alloying
elements are depleted from the alloy proximate the metallic scale.
Use of such mechanical methods to remove surface scale can cause
rejection of castings due to the alteration of the dimensional and
aerodynamic integrity of the airfoil.
[0021] The as-cast scale can occur without or with the presence of
oxide products, such as layers and/or particles, resulting from
reaction between the shell mold and nickel base superalloy melt,
corrosion of crucible and shell mold ceramics, and pull-out of
ceramic particles from the shell mold. If the oxide products are
present, they typically overlie the solidification-driven as-cast
surface scale. The oxide products can comprise such oxides
zirconium oxide, aluminum oxide and zirconium-aluminum-silicon
oxide particles and layers depending upon the ceramic materials
used in mold and crucible manufacture.
[0022] In FIG. 1B where the alloy had a carbon level of 0.02 weight
% C, approximately 48% of the airfoil surface of the single crystal
casting was covered with the as-cast scale.
[0023] In FIG. 1C where the alloy had a carbon level of 0.039
weight % C, approximately 10% of the airfoil surface of the single
crystal casting was covered with the as-cast scale.
[0024] FIG. 2 illustrates graphically the relationship between
percent scale coverage (% of airfoil) versus the carbon content of
the single crystal castings. From FIG. 2, it is apparent that
carbon concentrations over 0.04 weight %, preferably from greater
than 0.04 weight % to about 0.1 weight % C will eliminate or
substantially reduce to less than 10% coverage of as-cast scale on
the airfoil surfaces of the single crystal castings. The carbon
content can be adjusted as necessary to achieve the benefits of the
invention with respect to reduction or elimination of the as-cast
surface scale for different nickel base superalloys. The higher
alloy carbon concentration of Heat #3 appears to form alloy
carbides, such as carbides of Ta and Ti as well as Mo, W, Hf, in th
as-cast microstructure that reduce formation of the as-cast
metallic scale and also reduce or localize recrystallized grains
during heat treatment by virtue of pinning recrystallized grain
boundaries and retarding their growth during heat treatment.
[0025] From FIG. 2, it is apparent that the reduction in scale
coverage with carbon concentration of the alloy can be expressed by
the equation (1):
% area fraction scale =-0.193 .times. carbon content in ppm +86
(1)
[0026] The measurements of percent scale coverage were made after
the ceramic shell mold was removed from the castings. Each
measurement was made by overlaying a grid on photographs of the
single crystal blades cast as described above and counting the
boxes containing scale versus boxes without scale. A comparison was
generated by averaging three photographs for three blades cast from
each of the three heats.
[0027] The results of casting and heat treatment tests with respect
to extraneous recrystallized grains was made by visual rating over
the surfaces of 100 single castings at the aforementioned carbon
contents of 0.0025 weight % and 0.02 weight % and 12 single crystal
castings at the carbon content of 0.039 weight % after the above
described solution heat treatment. The single crystal castings were
visually observed for presence of any extraneous recrystallized
grains at the single crystal casting surface, which recrystallized
grains typically form and grow during solution heat treatment as
residual casting stresses at locations of the airfoil casting, e.g.
at the airfoil tip or other locations, are relieved. The single
crystal castings having a carbon lev 1 of 0.0025 weight % C were
observed to have about 10% occurrence of recrystallized grains
(i.e. 10 of the 100 single crystal castings having the carbon
content of 0.0025 weight % C exhibited recrystallized grains to an
extent to cause rejection of the casting pursuant to customer
specifications). The single crystal castings having a carbon level
of 0.02 weight % C were observed to have about 30% occurrence of
recrystallized grains (i.e. 30 of the 100 sample single crystal
castings exhibited recrystallized grains to an extent to cause
rejection of the casting pursuant to customer specifications).
However, at an alloy carbon concentration of 0.039 weight %, none
of the sample single crystal castings exhibited any visually
observed recrystallized grains on the single crystal castings after
the solution heat treatment. Thus, the invention envisions
increasing carbon concentration of the nickel base superalloy to an
amount effective to eliminate recrystallized grains on single
crystal castings made therefrom. Alloy carbides, such as carbides
of Ta and Ti as well as Ni, Hf, Mo, W, are formed in the as-cast
microstructure that appear to pin any recrystallized grain
boundaries and retard and limit their growth during solution heat
treatment.
[0028] The present invention provides single crystal castings
having carbon concentrations increased in an amount discovered to
substantially reduce or eliminate formation of as-cast metallic
scale on the surfaces of single crystal castings of the superalloys
and recrystallized grains after solution heat treatment of the
castings to develop mechanical properties.
[0029] The present invention provides single crystal blade and vane
(airfoils) castings which are substantially devoid of surface scale
in the cast condition and recrystallized grains in the as solution
heat treated condition. The present invention can be practiced in
manufacture of myriad small and large sizes of airfoils, such large
airfoils comprising large industrial gas turbine (IGT) blades which
have a length of about 20 centimeters to about 60 centimeters and
above, such as about 90 centimeters length, used throughout the
stages of the turbine of stationary industrial gas turbine
engines.
[0030] 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.
* * * * *