Note: Descriptions are shown in the official language in which they were submitted.
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NICKEL-BASED SUPERALLOY, PROCESS THEREFOR,
AND COMPONENTS FORMED THEREFROM
CROSS REFERENCE TO RELATED APPLICATIONS
10001] This
application claims the benefit of U.S. Provisional Application No.
61/670,634, filed July 12, 2012, the contents of which are incorporated herein
by
reference.
BACKGROUND OF THE INVENTION
100021 The
present invention generally relates to alloy compositions, and more
particularly to superalloys suitable for components requiring a
polycrystalline
microstructure and both high temperature dwell and/or creep capabilities, for
example,
turbine disks of gas turbine engines. Such alloys may also be useful in a
multi-grain
directionally solidified form or single crystal form.
100031 The
turbine section of a gas turbine engine is located downstream of a
combustor section and contains a rotor shaft and one or more turbine stages,
each having
a turbine disk (rotor) mounted or otherwise carried by the shaft, and turbine
blades
mounted to and radially extending from the periphery of the disk. Components
within
the combustor and turbine sections are often formed of superalloy materials in
order to
achieve acceptable mechanical properties while at elevated temperatures
resulting from
the hot combustion gases. Higher compressor exit temperatures in modem high
pressure
ratio gas turbine engines can also necessitate the use of high performance
superalloys for
compressor disks, blisks, and other components. Suitable alloy compositions
and
microstructures for a given component are dependent on the particular
temperatures,
stresses, and other conditions to which the component is subjected. For
example, airfoil
components such as blades and vanes are often formed of equiaxed,
directionally
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solidified (DS), or single crystal (SX) superalloys, whereas turbine disks are
typically
formed of superalloys that must undergo carefully controlled forging, heat
treatments,
and surface treatments to produce a polycrystalline microstructure having a
controlled
grain structure and desirable mechanical properties.
100041 Turbine
disks are often formed of gamma prime (yN) precipitation-
strengthened nickel-base superalloys (hereinafter, gamma prime nickel-base
superalloys)
containing chromium, tungsten, molybdenum, rhenium and/or cobalt as principal
elements that combine with nickel to form the gamma (y) matrix, and contain
aluminum,
titanium, tantalum, niobium, and/or vanadium as principal elements that
combine with
nickel to form the desirable gamma prime precipitate strengthening phase,
principally
Ni3(AI,Ti). Particularly notable gamma prime nickel-base superalloys include
Rene
88DT (R88DT; U.S. Patent No. 4,957,567) and Ren6 104 (R104; U.S. Patent No.
6,521,175), as well as certain nickel-base superalloys commercially available
under the
trademarks Inconel , Nimonic , and Udimet . R88DT has a composition of, by
weight,
about 15.0-17.0% chromium, about 12.0-14.0% cobalt, about 3.5-4.5% molybdenum,
about 3.5-4.5% tungsten, about 1.5-2.5% aluminum, about 3.2-4.2% titanium,
about 0.5-
1.0% niobium, about 0.010-0.060% carbon, about 0.010-0.060% zirconium, about
0.010-
0.040% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, and about 0.0-
0.01
yttrium, the balance nickel and incidental impurities.
100051 Disks
and other critical gas turbine engine components are often forged from
billets produced by powder metallurgy (P/M), conventional cast and wrought
processing,
and spraycast or nucleated casting forming techniques. Gamma prime nickel-base
superalloys formed by powder metallurgy are particularly capable of providing
a good
balance of creep, tensile, and fatigue crack growth properties to meet the
performance
requirements of turbine disks and certain other gas turbine engine components.
In a
typical powder metallurgy process, a powder of the desired superalloy
undergoes
consolidation, such as by hot isostatic pressing (HIP) and/or extrusion
consolidation. The
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resulting billet is then isothermally forged at temperatures slightly below
the gamma
prime solvus temperature of the alloy to approach superplastic forming
conditions, which
allows the filling of the die cavity through the accumulation of high
geometric strains
without the accumulation of significant metallurgical strains. These
processing steps are
designed to retain the fine grain size originally within the billet (for
example, ASTM 10
to 13 or finer), achieve high plasticity to fill near-net-shape forging dies,
avoid fracture
during forging, and maintain relatively low forging and die stresses. Such
alloys may be
heat treated below or above the gamma prime solvus. In order to improve yield
strength
and ductility at moderately elevated temperature these alloys may be heat
treated below
their gamma prime solvus temperature (generally referred to as subsolvus heat
treatment)
to maintain fine uniform grains. In order to improve fatigue crack growth
resistance and
mechanical properties at even more elevated temperatures, these alloys are
heat treated
above their gamma prime solvus temperature (generally referred to as
supersolvus heat
treatment) to cause significant, uniform coarsening of the grains.
100061 Current
alloys, including R88DT, have provided significant improvements in
rotor performance capabilities. However, improvements in the economics and
feasibility
of producing these alloys are continuously sought. Key factors for realizing
low-cost
processing while maintaining premium quality alloy product include utilization
of a high
level of scrap and revert as input to melting processes and powder metallurgy
processes
often used in alloy production. . Revert material is often in the form of
solids or chips of
the nominal composition of a material, whereas the nominal composition of a
scrap
material may be of a different composition and may contain elements not
intended in the
composition of the desired alloy to be produced. Alloys of different
compositions may
be used as input materials for a melting batch, either as revert materials or
scrap
materials, along with other elemental input materials. The requirement is that
the
aggregate chemistry of the input materials meets the allowable composition
ranges for the
alloy desired to be produced. Such combinations of input materials are guided
by rule of
mixtures and are well established in standard melting practices and currently
utilized as
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state of the art.
100071 Rotor-
grade superalloys may be produced by a variety of processes including
powder processing and melt processing. R88DT is typically manufactured using
powder
metal processing. Some other disk alloys, such as the nickel-base superalloy
IN718 are
typically produced using conventional melting processes. To improve cost
effectiveness,
it is desirable that R88DT and similar alloys be produced using conventional
melting
processes utilizing scrap and revert with the same melting and billet
conversion
equipment used to produce 1N718. Nominal elemental composition ranges reported
for
I1N718 are, by weight: 50-55% nickel, 17-21% chromium, 2.8-3.33% molybdenum,
4.75-
5.5% niobium, 0-1.0% cobalt, 0.65-1.15 titanium, 0.2-0.8% aluminum, 0-0.35%
manganese, 0-0.3% copper, 0-0.08% carbon, 0-0.006% boron, the balance iron
(18.5%
nominal) and incidental impurities. Among superalloys IN718 stands out for its
pervasive use, reported to be at approximately 45% of total industrial
production of
wrought nickel base superalloys. With this level of usage, there is also the
practical
potential for IN718 to be mixed within many R88DT revert forms, especially
chips and
other superalloy scrap. However, the ability to tolerate the significant iron
content of
IN718 a limiting factor in cost-effective revert and scrap utilization for
R88DT, because
as described above, R88DT does not contain iron as a constituent. It is
generally
believed that R88DT when contaminated with iron can lead to formation of
observable
amounts of sigma phase which, in R88DT, is generally (Fe,Mo)õ(Ni,Co)y, where x
and y
= 1 to 7. Sigma phase is a well-known topological.ly close-packed (TCP) phase
which
can adversely affect the mechanical capabilities of a gamma prime nickel-base
alloy. In
the context of this discussion, an observable amount is considered to be any
amount that
can be seen in suitable etched metallographic samples at an optical
magnification of
500X. Hence, scrap or revert utilization for use in the production of R88DT
would carry
with it a high probability of iron contamination and sigma phase formation
that is not
desirable in R88DT.
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100081 Prior
attempts to prevent iron cross-contamination while utilizing high levels
of IN718 scrap or revert include alloy segregation by physically keeping chips
of
different alloys separate. Unfortunately, such methods have significant
limitations in
terms of additional personnel training and maintaining separate chips or
containers. In
addition, there can be contamination from melting equipment or melt-handling
or
machining equipment that may have been previously used for the production of
an iron-
containing alloy, in which case extensive cleaning is required of equipment
between
switching over to production of difkrent alloys.
100091 The
above mentioned methods of preventing iron contamination result in loss
of valuable material and/or production efficiency. Thus it would be desirable
if a gamma
prime nickel-base superalloy could be developed capable of having properties
similar to
R88DT, yet could tolerate the presence of iron, so that the use of scrap and
revert iron-
containing alloys or the use of revert containing inadvertent iron
contamination would be
permissible. It would be further desirable if superalloy compositions that
fall within the
composition space of R88DT could be identified that would lend themselves to
additions
of measurable amounts of iron that would not lead to formation of sigma phase
and hence
would not adversely affect the mechanical properties of the superalloy.
BRIEF DESCRIPTION OF THE INVENTION
100101 The
present invention provides gamma prim.e nickel-based superalloys
suitable for use in forming components such as a turbine disk, compressor
disk, blisk,
seal, shaft or retainer, and processes for producing such superalloys, wherein
the
processes allow scrap and revert usage. The superalloys accommodate limited
amounts
of iron in their compositions and are particularly well suited for achieving
physical and
chemical properties similar to those of R88DT, yet allow for an iron content
that was
previously considered excessive and unallowable in R88DT, including powder-
metallurgy processed versions of R88DT.
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10011.1
According to a first aspect of the invention, a gamma prime nickel-based
superalloy has a composition that falls within a compositional space defmed by
the
following ranges, by weight: 15.8-16.2% chromium, about 12.9-13.3% cobalt,
about
3.95-4.1% molybdenum, about 3.9-4.1% tungsten, about 2.01-2.24% aluminum,
about
3.6-3.9% titanium, about 0.5-1.0% niobium, about 0.010-0.060% carbon, about
0.02-
0.06% zirconium, about 0.010-0.040% boron, about 0.0-0.3% hafnium, about 0.0-
0.01
vanadium, and about 0.0-0.01 yttrium, the balance nickel and incidental
impurities,
wherein the superalloy further contains iron in an amount exceeding an
impurity level
and up to 2.0%, and the superalloy being free of an observable amount of sigma
phase.
1000121 According to a second aspect of the invention, structural components
can be
formed from the superalloy described above, particular examples of which
include
turbine disks, compressor disks and blisks, seals, shafts, and retainers of
gas turbine
engines.
1000131 A third aspect of the invention is a process for making the superalloy
described above which includes scrap and revert usage of alloys that contain
iron, either
intentionally or inadvertently, and/or using melting and melt-handling
equipment without
extensive cleaning after producing an iron-containing alloy.
100141 A
technical effect of the invention is that the superalloy described above is
capable of providing approximately the same properties and structural and
chemical
capabilities as R88DT or a similar superalloy designed and processed to a
consistent
microstructure for high temperature properties, and is obtainable by suitable
processing to
achieve a desirable microstructure, but allows for significant iron content.
In this
manner, the superalloy is capable of being produced more economically and
efficiently,
with less material waste, higher scrap and revert utilization, less cleaning
of machining,
scrap and chip handling equipment, melting and melt-handling equipment, and
lower
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personnel and time requirements.
100151 Other
aspects and advantages of this invention will be better appreciated from
the following detailed description.
BRIEF DESCRIPTION OF THE DRAWING
100161 FIG. 1
is a perspective view of a turbine disk of a type used in gas turbine
engines.
DETAILED DESCRIPTION OF THE INVENTION
100171 The
present invention is directed to nickel-base alloys, and particularly to
gamma prime nickel-base alloys suitable for components produced by a hot
working
(e.g., forging) operation. A particular but non-limiting example represented
in FIG. 1 is a
high pressure turbine disk 10 for a gas turbine engine. The invention will be
discussed in
reference to alloys suitable for a high-pressure turbine disk for a gas
turbine engine,
though those skilled in the art will appreciate that the teachings and
benefits of this
invention are also applicable to compressor disks, blades, and blisks of gas
turbine
engines, as well as numerous other components that are subjected to stresses
at high
temperatures and therefore benefit from a high temperature capability.
100181 Disks of the type shown in FIG. 1 are typically produced by
isothermally forging
a fine-grained billet formed by powder metallurgy (P/M), a cast and wrought
processing,
or a spraycast or nucleated casting type technique. In a particular embodiment
utilizing a
powder metallurgy process, the billet can be formed by consolidating a powder
of the
desired nickel-base alloy, such as by hot isostatic pressing (HIP), extrusion
consolidation,
or combinations thereof. In other forms the billet is formed by casting an
ingot and
working the material to a billet form. suitable for subsequent forging
operations. The
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billet is typically forged at a temperature at or near the recrystallization
temperature of
the alloy but less than the gamma prime solvus temperature of the alloy, and,
if the billet
is formed by powder metallurgy processes, under superplastic forming
conditions. After
forging, a subsolvus or supersolvus (solution) heat treatment is performed,
during which
grain growth occurs consistent with the proximity of the heat-treat
temperatures to the
gamma prime solvus temperature, as is well known in the art. A supersolvus
solution
heat treatment is performed at a temperature above the gamma prime solvus
temperature
(but below the incipient melting temperature) of the superalloy to
recrystallize the
worked grain structure and dissolve (solution) the gamma prime precipitates in
the
superalloy enabling significant grain growth to occur. Alternatively, a
subsolvus solution
heat treatment is performed at a temperature below the gamma prime solvus
temperature
(and below the incipient melting temperature) of the superalloy to partially
dissolve
(solution) the gamma prime precipitates in the superalloy so that a finer
grain size can be
maintained for some applications. Following the solution heat treatment, the
component
is cooled at an appropriate rate to re-precipitate gamma prime within the
gamma matrix
or at grain boundaries, so as to achieve the particular mechanical properties
desired. The
component may also undergo aging or stress relief using known techniques.
100191 The
present invention discloses a set of compositions that share certain
similarities with other nickel-base superalloys, including Rene 88DT (R88DT;
U.S.
Patent No. 4,957,567). The present invention is particularly intended to
maintain the
structural and mechanical attributes of R88DT most advantageously in the
subsolvus
fine-grained condition, especially as produced in a cast and wrought form.
However, in
several of the current compositional ranges allowed in commercial
formulations, R88DT
is considered to be unable to accommodate significant iron contamination while
maintaining desirable mechanical properties and avoiding an observable amount
of sigma
phase. More particularly, conventional wisdom has been that the introduction
of iron into
R88DT promotes the formation of sigma phase, which can be detrimental to
mechanical
properties of R88DT. Accordingly, conventional practice has been to avoid any
iron
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contamination of R88DT so as to ensure absence of an observable amount of
sigma phase
and so that mechanical properties of R88DT are not compromised. As such,
R88DT, in
its widest composition ranges, cannot be guaranteed to be processed
efficiently with the
scrap and revert utilization methods described previously if there is a
significant risk of
iron contamination. In
contrast, the superalloys of the current invention can
accommodate a significant amount of iron contamination without sacrificing
advantageous structural and chemical characteristics, particularly those of
R88DT
subsolv-us fine-grained condition and especially as produced in a cast and
wrought form.
As a result, the superalloys of this invention are capable of allowing scrap
and revert
usage of alloys that contain iron, either intentionally or inadvertently, and
can also
advantageously allow the use of machining equipment, scrap and revert handling
equipment, and melting and melt-handling equipment without extensive cleaning
after
producing an iron-containing alloy. A notable example of an iron-containing
alloy is
IN718.
100201 To
identify a gamma prime nickel-base superalloy composition that contains
iron but with properties similar to those of R88DT, a targeted allowable iron
content was
initially identified by modeling alloy phase stability. Phase stability
modeling showed
that the largest impact of increasing iron content is an increased sigma
solvus
temperature. Alloy phase stability modeling suggested that formation of sigma
phase is
thermodynamically possible at zero iron content. However, experience has shown
that
lower solvus temperatures with zero iron content make the kinetics of the
formation to be
a primary controlling factor and no observable sigma formation occurs. At an
addition of
2.0% iron, by weight, to R88DT, the sigma solvus temperature would be
approximately
1400 "F (760 'V), coincident with a preferred heat-treat aging temperature,
indicating
that sigma phase formation is thermodynamically possible. However it is known
that, at
a sufficiently low temperature, kinetics of phase formation are not often
favorable for the
formation of all thermodynamically predicted phases. This phenomenon suggested
that a
potential maximum iron content exists below which kinetics control the sigma
phase
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formation and are not favorable for an observable amount of sigma phase
formation.
100211 Several
heats were produced utilizing compositions in the R88DT
compositional space, which is defined herein to be superalloy compositions
that fall
within the ranges described above for R88DT, but were produced to further
contain
intentional amounts of iron, for example, iron contents of about 0.6, 1.3, or
1.34 weight
percent. The alloys were evaluated in the as heat-treated condition and
confirmed to be
free of observable amounts of sigma phase. Based on the analysis of this data,
1- three
standard deviations were taken to provide acceptable ranges for the elemental
compositions of these iron-containing alloys to be free of observable amounts
of sigma
phase. Experimentally, long-term exposure tests of up to 10,000 hours at
temperatures up
to 1400 F (760 C) confirmed the lack of observable amount of sigma formation
confirming the acceptability of this iron limit. Sigma phase formation was
evaluated by
an optical examination at a minimum of 500X utilizing suitably etched
metallographic
samples. Any microstructural features suspected of being sigma phase were
subjected to
additional chemical composition analysis and crystallographic analysis for
final
determination of the absence of sigma phase. Based on this analysis, a
suitable
composition comprises, and more preferably consists of, by weight, 15.8 to
16.2%
chromium 12.9 to 13.3% cobalt, 3.95 to 4.1% molybdenum, 3.9 to 4.1% tungsten,
2.01 to
2.24% aluminum, 3.6 to 3.9% titanium, 0.67 to 0.74% niobium, 0.012 to 0.02%
boron,
about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, and about 0.0-0.01 yttrium
0.005 to
0.011% carbon, 0.02 to 0.06% zirconium, and iron in an amount of about 0.6 to
about
1.3%, the balance nickel and incidental impurities. More broadly, on the basis
of the
investigations it was concluded that gamma prime nickel-base superalloys of
this
invention can have a composition that falls within a compositional space of,
by weight,
about 15.0-17.0% chromium, about 12.0-14.0% cobalt, about 3.5-4.5% molybdenum,
about 3.5-4.5% tungsten, about 1.5-2.5% aluminum, about 3.2-4.2% titanium,
about 0.5-
1.0% niobium, about 0.010-0.060% carbon, about 0.010-0.060% zirconium, about
0.010-
0.040% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium, and about 0.0-
0.01
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yttrium, the balance nickel and incidental impurities, the superalloy further
containing
iron in an amount exceeding an impurity level and up to 2.0%, the superalloy
being free
of an observable amount of sigma phase.
100221 It
should be noted that typical impurity levels for iron in R88DT can be up to
about 0.1%. In addition, the alloy may contain up to 0.0035% nitrogen. Also,
it is
generally recognized that carbon and nitrogen levels together influence the
degree of
carbo-nitride inclusions. Higher levels of carbon and nitrogen and higher
scrap input
may be tolerated if higher levels of carbo-nitride inclusions are acceptable
for a specific
application. A particular embodiment of the alloy, contains, by weight, about
13%
cobalt, 16% chromium, 4% molybdenum, 4% tungsten, 2.1% aluminum, 3.7%
titanium,
0.7% niobium, 0.008% boron, about 0.0-0.3% hafnium, about 0.0-0.01 vanadium,
about
0.0-0.01 yttrium 0.005 to 0.011% carbon, 0.03 to 0.06% zirconium, up to
0.0035%
nitrogen and more preferably up to 0.0018% nitrogen, the balance nickel,
incidental
impurities, and iron in an amount greater than impurity levels and up to about
1.3%. This
particular embodiment of the alloy, as well as other alloys of the invention
indicated
through the composition ranges above, can be produced using scrap and revert
usage of
alloys that contain iron, intentionally or in advertently. Further, these
alloys can be
advantageously produced in melt equipment previously used for the production
of iron-
containing alloys without the need for significant decontamination or
expensive alloy
segregation procedures.
100231 A
nonlimiting example of a method capable of producing a superalloy of this
invention includes combining at least one iron-containing alloy with raw
materials that do
not contain intentional additions of iron, wherein the iron-containing
alloy(s) and raw
materials are combined in appropriate amounts and then melted to produce the
desired
composition for the superalloy and its intentional but limited addition of
iron. At least
one iron-containing scrap alloy can be used in place of or in addition to the
iron-
containing alloy(s). Alternatively or in addition, intentional additions of
iron can be
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made present in a superalloy of this invention as a result of melting raw
materials, and/or
iron-containing alloys(s), and/or iron-containing scrap alloy(s) using melt
equipment that
had been immediately previously used to melt an iron-containing alloy and
without
cleaning the melt equipment to remove remnants of the iron-containing alloy.
It should
be apparent that the presence of iron can occur through any combination of the
above
techniques in a manner that may promote efficiencies and/or reduce material
and
processing costs.
100241 It
should be stressed that, while the alloys of the present invention, without
considering the iron content, fall into the compositional space of R88DT,
several alloys
in the R.88DI compositional space do not lend themselves to significant
introduction of
iron without compromising their mechanical properties. This phenomenon is
because
phases formed in multicomponent systems (for example, superal.loys) are a
complex
function of the elemental composition of the system. It should also be
stressed that, in
addition to the alloys of the present invention described above, certain other
compositions
within the general compositional space of R88DT (as defined by its
compositions
reported herein) may also lend themselves to iron addition without a
significant
compromise in properties compared to R88DT. This is due to the complex
thermodynamic interactions prevailing among the elements in a multicomponent
system
in an n-dimensional space, where n is the number of significant elements in
the
composition of the alloy. The effects of these interactions create situations
wherein, at
the same percentage content of an element, different phases can occur as the
percentage
contents of the other constituent elements vary, even when temperature and
pressure are
fixed. Due to this complex nature of the multicomponent systems, it is not
readily
apparent as to what compositional ranges within R88DT would lend themselves to
additions of iron and simultaneously the required phase stability and
properties for the
alloy. However, in investigations leading to the present invention, alloy
phase stability
modeling indicated the potential absence of an observable amount of sigma
phase in the
iron-containing superalloys having the compositions described above. These
alloy
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compositions were produced using known melting processes, the absence of an
observable mount of sigma phase was verified, and the observed phases were of
the same
chemistry as those observed in similarly-produced R88DT. The microstructural
features
and the structural and chemical properties of the iron-containing alloys of
the present
invention were also evaluated and found to be similar to those of R88DT. The
coefficient
of thermal expansion (CTE) was measured over a suitable temperature range and
shown
to be essentially the same among the iron-containing alloys and essentially
iron-free
forms of the alloys that were prepared. Also, Young's Modulus was measured
over a
suitable temperature range and found to be essentially the same among the iron-
containing and essentially iron-free forms of the alloys. Test samples were
also
processed in the subsolvus heat-treat condition and tested in tension over a
suitable
temperature range, yielding nominally equivalent values for 0.2% yield
strength and
tensile strength that demonstrated no loss in strength for the iron-containing
alloys
compared to that of the essentially iron-free forms of the alloys when
processed to yield
similar microstructures. These results evidenced that superalloys of the
invention can be
utilized to produce structural components and in particular, as non-limiting
examples,
turbine disks, compressor disks and blisks, seals and shaft retainers of gas
turbine
engines.
100251 In view of the above, the superalloys of this invention are capable
of
exhibiting comparable properties to similar high temperature superalloys,
including
R88DT, while accommodating significant iron content with negligible or no loss
in
advantageous properties. This ability to accommodate significant levels of
iron
contamination allows superalloys to be produced with scrap and revert usage of
alloys
that contain iron, either intentionally or inadvertently, and can also allow
for the
advantageous use of melting and melt-handling equipment without extensive
cleaning
after an iron-containing alloy, for example, 11N718, is produced with the
equipment. This
flexibility can lead to significant reduction in production costs of the
superalloy.
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100261
Additional potential benefits include the ability to reduce or eliminate the
need for special training of personnel and the expense of extensive cleaning
of melting
process equipment when the equipment is switched between iron-containing alloy
compositions and superalloys of this invention. As an example, the invention
can reduce
or eliminate the need to segregate iron from a recycling stream with
specialized
equipment capable of isolating an iron-containing material as well as reduce
or eliminate
the need for operator training in order to strictly maintain such isolation.
Additionally,
the invention can promote recycling economics by allowing superalloys to be
produced
with the use of machining chips or recycled material from articles having
multi-alloy
constructions that include iron-containing alloys, a notable example of which
are
compressor spools that often have multi-alloy constructions.
100271 While
the invention has been described in terms of specific embodiments,
including particular compositions and properties of the superalloys, it is
apparent that other
forms could be adopted by one skilled in the art.
Accordingly, it should be
understood that the invention is not limited to the specific disclosed
embodiments, and
the scope of the invention is to be limited only by the following claims.
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