U.S. patent application number 14/365236 was filed with the patent office on 2014-12-25 for nickel-based heat-resistant superalloy.
The applicant listed for this patent is NATIONAL INSTITUTE FOR MATERIAL SCIENCE. Invention is credited to Yuefeng Gu, Hiroshi Harada, Toshio Osada, Tadaharu Yokokawa, Yong Yuan.
Application Number | 20140373979 14/365236 |
Document ID | / |
Family ID | 48612657 |
Filed Date | 2014-12-25 |
United States Patent
Application |
20140373979 |
Kind Code |
A1 |
Gu; Yuefeng ; et
al. |
December 25, 2014 |
NICKEL-BASED HEAT-RESISTANT SUPERALLOY
Abstract
Disclosed herein is a nickel-based heat-resistant superalloy
produced by a casting and forging method, the nickel-based
heat-resistant superalloy comprising 2.0 mass % or more but 25 mass
% or less of chromium, 0.2 mass % or more but 7.0 mass % or less of
aluminum, 19.5 mass % or more but 55.0 mass % or less of cobalt,
[0.17.times.(mass % of cobalt content-23)+3] mass % or more but
[0.17.times.(mass % of cobalt content-20)+7] mass % or less and 5.1
mass % or more of titanium, and the balance being nickel and
inevitable impurities, and being subjected to solution heat
treatment at 93% or more but less than 100% of a .gamma.' solvus
temperature.
Inventors: |
Gu; Yuefeng; (Ibaraki,
JP) ; Osada; Toshio; (Ibaraki, JP) ; Yuan;
Yong; (Ibaraki, JP) ; Yokokawa; Tadaharu;
(Ibaraki, JP) ; Harada; Hiroshi; (Ibaraki,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NATIONAL INSTITUTE FOR MATERIAL SCIENCE |
Ibaraki |
|
JP |
|
|
Family ID: |
48612657 |
Appl. No.: |
14/365236 |
Filed: |
December 14, 2012 |
PCT Filed: |
December 14, 2012 |
PCT NO: |
PCT/JP2012/082467 |
371 Date: |
June 13, 2014 |
Current U.S.
Class: |
148/428 ;
148/442 |
Current CPC
Class: |
C22C 30/00 20130101;
F05D 2300/175 20130101; F01D 5/02 20130101; C22F 1/10 20130101;
C22C 19/056 20130101; C22C 19/07 20130101; C22F 1/00 20130101; B21J
5/02 20130101; C22C 19/05 20130101; F01D 5/28 20130101 |
Class at
Publication: |
148/428 ;
148/442 |
International
Class: |
C22C 19/05 20060101
C22C019/05; C22C 30/00 20060101 C22C030/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 15, 2011 |
JP |
2011-274604 |
Claims
1. A nickel-based heat-resistant superalloy produced by a casting
and forging method, the nickel-based heat-resistant superalloy
comprising 2.0 mass % or more but 25.0 mass % or less of chromium,
0.2 mass % or more but 7.0 mass % of less of aluminum, 19.5 mass %
or more but 55.0 mass % or less of cobalt, [0.17.times.(mass % of
cobalt content-23)+3] mass % or more but [0.17.times.(mass % of
cobalt content-20)+7] mass % or less and 5.1 mass % or more of
titanium, and the balance being nickel and inevitable impurities,
and being subjected to solution heat treatment at 93% or more but
less than 100% of a .gamma.' solvus temperature.
2. The nickel-based heat-resistant superalloy according to claim 1,
wherein the cobalt is contained in an amount of 21.8 mass % or more
but 55.0 mass % or less.
3. The nickel-based heat-resistant superalloy according to claim 1,
wherein the titanium is contained in an amount of 5.5 mass % or
more but 12.44 mass % or less.
4. The nickel-based heat-resistant superalloy according to claim 3,
wherein the titanium is contained in an amount of 6.1 mass % or
more but 12.44 mass % or less.
5. The nickel-based heat-resistant superalloy according to claim 1,
which is subjected to solution heat treatment at 94% or more but
less than 100% of the .gamma.' solvus temperature.
6. The nickel-based heat-resistant superalloy according to claim 1,
which contains one or both of 10 mass % or less of molybdenum and
10 mass % or less of tungsten.
7. The nickel-based heat-resistant superalloy according to claim 6,
wherein the molybdenum is contained in an amount of less than 4
mass %.
8. The nickel-based heat-resistant superalloy according to claim 6,
wherein the tungsten is contained in an amount of less than 3 mass
%.
9. The nickel-based heat-resistant superalloy according to claim 1,
which contains one or both of 10 mass % or less of tantalum and 5.0
mass % or less of niobium.
10. The nickel-based heat-resistant superalloy according to claim
1, which contains at least one of 2 mass % or less of vanadium, 5
mass % or less of rhenium, 0.1 mass % or less of magnesium, 2 mass
% or less of hafnium, and 3 mass % or less of ruthenium.
11. The nickel-based heat-resistant superalloy according to claim
1, which comprises 12 mass % or more but 14.9 mass % or less of
chromium, 2.0 mass % or more but 3.0 mass % or less of aluminum,
20.0 mass % or more but 27.0 mass % or less of cobalt, 5.5 mass %
or more but 6.5 mass % or less of titanium, 0.8 mass % or more but
1.5 mass % or less of tungsten, 2.5 mass % or more but 3.0 mass %
or less of molybdenum, at least one of 0.01 mass % or more but 0.2
mass % or less of zirconium, 0.01 mass % or more but 0.15 mass % or
less of carbon, and 0.005 mass % or more but 0.1 mass % or less of
boron, and the balance being nickel and inevitable impurities.
Description
TECHNICAL FIELD
[0001] The present invention relates to a nickel-based
heat-resistant superalloy used for heat-resistant members of
aircraft engines, power-generating gas turbines, etc., especially
for turbine disks or turbine blades.
BACKGROUND ART
[0002] For example, turbine disks, which are heat-resistant members
of aircraft engines, power-generating gas turbines, etc., are
rotary members that support turbine blades, and are subjected to
much higher stress than turbine rotor blades. Therefore, turbine
disks require a material excellent in mechanical characteristics,
such as creep strength or tensile strength in a high-temperature
and high-stress region and low-cycle fatigue characteristics, and
forgeability. On the other hand, in order to improve fuel
efficiency or performance, an increase in engine gas temperature
and a reduction in the weight of turbine disks are required, and
therefore the material is required to have higher heat resistance
and higher strength.
[0003] In general, nickel-based forged alloys are used for turbine
disks. For example, Inconel 718 (which is a registered trademark of
The International Nickel Company, Inc.) using a .gamma.'' (gamma
double prime) phase as a strengthening phase and Waspaloy (which is
a registered trademark of United Technoligies, Inc.) using, as a
strengthening phase, about 25 vol % of a precipitated .gamma.'
(gamma prime) phase stabler than a .gamma.'' phase are frequently
used. Further, Udimet 720 (which is a registered trademark of
Special Metals, Inc.) has been introduced since 1986 from the
viewpoint of dealing with higher temperatures. Udimet 720 has about
45 vol % of a precipitated .gamma.' phase and tungsten added for
solid-solution strengthening of a .gamma. phase, and is therefore
excellent in heat-resistant characteristics.
[0004] On the other hand, the structural stability of Udimet 720 is
not always sufficient, and a harmful TCP (Topologically close
packed) phase is formed during use. Therefore, Udimit 720Li
(U720Li/U720LI) has been developed by making improvements, such as
a reduction in the amount of chromium, to Udimet 720. However, the
formation of a TCP phase still occurs also in improved Udimit
720Li, and therefore the use of Udimit 720Li for a long time or at
high temperature is limited.
[0005] Powder metallurgical alloys typified by AF115, N18, and
Rene88DT are sometimes used for high-pressure turbine disks
required to have high strength. The powder metallurgical alloys
have a merit that homogeneous disks having no segregation can be
obtained in spite of the fact that many strengthening elements are
contained. On the other hand, the powder metallurgical alloys have
a problem that their production process needs to be highly
controlled, e.g., vacuum melting needs to be performed at a high
cleaning level or a proper mesh size needs to be selected for
powder classification, to suppress the mixing of inclusions and
therefore their production cost is significantly increased.
[0006] In addition, many proposals have been made to improve the
chemical compositions of conventional nickel-based heat-resistant
superalloys. All of them contain cobalt, chromium, molybdenum or
molybdenum and tungsten, aluminum, and titanium as their major
constituent elements, and typical ones contain one or both of
niobium and tantalum as their essential constituent element(s). The
presence of niobium and/or tantalum is suitable for the
above-described powder metallurgy, but is a factor making casting
and forging difficult.
[0007] Titanium is added for its function of strengthening a
.gamma.' phase and improving tensile strength or crack propagation
resistance. However, the amount of titanium added is limited to up
to about 5 mass %, because excess addition of only titanium results
in an increase in .gamma.' solvus temperature and formation of a
harmful phase, which makes it difficult to obtain a sound
.gamma./.gamma.' two-phase structure.
[0008] Under the circumstances, the present inventors have made a
study of optimization of the chemical composition of a nickel-based
heat-resistant superalloy and have found that a harmful TCP phase
can be suppressed by actively adding cobalt in an amount of up to
55 mass %. Further, the present inventors have found that a
.gamma./.gamma.' two-phase structure can be stabilized by
increasing both a cobalt content and a titanium content so that
cobalt and titanium are contained in a predetermined ratio. Based
on these findings, the present inventors have proposed a
nickel-based heat-resistant superalloy that can withstand higher
temperatures for a long time than conventional alloys and that has
excellent workability (Patent Literature 1).
[0009] Further, some proposals focused on the microstructure of a
nickel-based heat-resistant alloy have been made to improve the
performance of the nickel-based heat-resistant superalloy (Patent
Literatures 2, 3, and 4).
[0010] In a nickel-based heat-resistant superalloy produced by
powder metallurgy, crystal grains are less likely to become too
large even after solution heat treatment performed in a temperature
region exceeding a .gamma.' solvus temperature (at a supersolvus
temperature), and therefore crystal grain size and grain size
distribution are generally controlled by performing aging heat
treatment after solution heat treatment performed in a temperature
region exceeding a solvus temperature (e.g., Patent Literature 7).
However, while crystal grains are less likely to become too large,
it is often the case that the control of crystal grains is poor.
Therefore, in order to avoid harmful growth of crystal grains
during solution heat treatment performed in a temperature region
exceeding a solvus temperature, the importance of strain rate
control during forging has also been proposed (e.g., Patent
Literatures 5 and 6). Further, in order to promote proper growth of
crystal grains, a method has also been proposed in which a
nickel-based heat-resistant alloy having a high carbon content is
forged at a high local strain rate (Patent Literature 8).
[0011] However, the alloys described in the above Patent
Literatures are powder alloys whose production process is
complicated and production cost is high. The powder alloys vary in
optimum microstructure according to their chemical composition, and
are therefore considered to be applicable only to some limited
materials and production methods.
[0012] On the other hand, when a nickel-based heat-resistant
superalloy produced by a casting and forging method is subjected to
solution heat treatment in a temperature region exceeding a solvus
temperature, crystal grains become too large and therefore
heat-resistant characteristics are significantly impaired.
Therefore, in general, solution heat treatment is performed at 90%
or less of a solvus temperature, and then aging heat treatment is
performed.
[0013] At present, however, no nickel-based heat-resistant
superalloy has been found which is produced by a conventional
casting and forging method and has heat-resistant characteristics
significantly higher than those of nickel-based heat-resistant
superalloys produced by powder metallurgy. Therefore, there is a
strong demand for development of a nickel-based heat-resistant
superalloy that is produced by a casting and forging method capable
of significantly simplifying its production process and that is
superior also in terms of heat-resistant characteristics and cost
to nickel-based heat-resistant superalloys produced by powder
metallurgy.
[0014] Patent Literature 1: WO 2006/059805
[0015] Patent Literature 2: Japanese Patent No. 2666911
[0016] Patent Literature 3: Japanese Patent No. 2667929
[0017] Patent Literature 4: JP 2003-89836 A
[0018] Patent Literature 5: U.S. Pat. No. 4,957,567
[0019] Patent Literature 6: U.S. Pat. No. 5,529,643
[0020] Patent Literature 7: JP 2011-12346 A
[0021] Patent Literature 8: JP 2009-7672 A
SUMMARY OF INVENTION
Technical Problem
[0022] In order to achieve an improvement in energy efficiency,
there has recently been an urgent need for development of a
material of heat-resistant members of aircraft engines,
power-generating gas turbines, etc. to allow the heat-resistant
members to be used at higher temperatures. For example, there has
been a strong demand for development of a novel alloy for turbine
disks which is superior in mechanical characteristics such as
fatigue strength, high-temperature creep strength, fracture
toughness, and high-temperature fatigue crack resistance.
[0023] Under circumstances where no nickel-based heat-resistant
superalloy has been found which is produced by a conventional
casting and forging method and has heat-resistant characteristics
significantly higher than those of nickel-based heat-resistant
superalloys produced by powder metallurgy, the present inventors
have made an intensive study to develop a nickel-based
heat-resistant superalloy that is superior in terms of
heat-resistant characteristics and cost to those produced by powder
metallurgy. It is an object of the present invention to provide a
nickel-based heat-resistant superalloy that is produced by a
casting and forging method capable of significantly simplifying its
production process and that is superior in heat-resistant
characteristics to nickel-based superalloys produced by powder
metallurgy.
Solution to Problem
[0024] The present inventors have intensively studied the solution
heat treatment conditions of a nickel-based heat-resistant
superalloy produced by a casting and forging method and having a
specific alloy composition, and have found that a nickel-based
heat-resistant superalloy excellent in both tensile strength and
creep life at high temperature can be obtained by properly
controlling especially a solution heat treatment temperature, which
has led to the completion of the present invention. A casting and
forging method is generally known as an inexpensive production
process, and the present inventors have found that a nickel-based
heat-resistant superalloy superior in high-temperature
heat-resistant characteristics, which can be achieved only by
powder metallurgy requiring high production cost, can be produced
by a casting and forging method.
[0025] More specifically, the present invention is directed to a
nickel-based heat-resistant superalloy produced by a casting and
forging method, the nickel-based heat-resistant superalloy
comprising 2.0 mass % or more but 25 mass % or less of chromium,
0.2 mass % or more but 7.0 mass % or less of aluminum, 19.5 mass %
or more but 55.0 mass % or less of cobalt, [0.17.times.(mass % of
cobalt content-23)+3] mass % or more but [0.17.times.(mass % of
cobalt content-20)+7] mass % or less and 5.1 mass % or more of
titanium, and the balance being nickel and inevitable impurities,
and being subjected to solution heat treatment at 93% or more but
less than 100% of a .gamma.' solvus temperature.
[0026] It is preferred that in the nickel-based heat-resistant
superalloy, the cobalt is contained in an amount of 21.8 mass % or
more but 55.0 mass % or less.
[0027] Further, it is also preferred that in the nickel-based
heat-resistant superalloy, the titanium is contained in an amount
of 5.5 mass % or more but 12.44 mass % or less.
[0028] Further, it is also preferred that in the nickel-based
heat-resistant superalloy, the titanium is contained in an amount
of 6.1 mass % or more but 12.44 mass % or less.
[0029] Further, it is also preferred that the nickel-based
heat-resistant superalloy is subjected to solution heat treatment
at 94% or more but less than 100% of the .gamma.' solvus
temperature.
[0030] Further, it is also preferred that the nickel-based
heat-resistant superalloy contains one or both of 10 mass % or less
of molybdenum and 10 mass % or less of tungsten.
[0031] Further, it is also preferred that in the nickel-based
heat-resistant superalloy, the molybdenum is contained in an amount
of less than 4 mass %.
[0032] Further, it is also preferred that in the nickel-based
heat-resistant superalloy, the tungsten is contained in an amount
of less than 3 mass %.
[0033] Further, it is also preferred that the nickel-based
heat-resistant superalloy contains one or both of 10 mass % or less
of tantalum and 5.0 mass % or less of niobium.
[0034] Further, it is also preferred that the nickel-based
heat-resistant superalloy contains at least one of 2 mass % or less
of vanadium, 5 mass % or less of rhenium, 0.1 mass % or less of
magnesium, 2 mass % or less of hafnium, and 3 mass % or less of
ruthenium.
[0035] Further, it is also preferred that the nickel-based
heat-resistant superalloy comprises 12 mass % or more but 14.9 mass
% or less of chromium, 2.0 mass % or more but 3.0 mass % or less of
aluminum, 20.0 mass % or more but 27.0 mass % or less of cobalt,
5.5 mass % or more but 6.5 mass % or less of titanium, 0.8 mass %
or more but 1.5 mass % or less of tungsten, 2.5 mass % or more but
3.0 mass % or less of molybdenum, at least one of 0.01 mass % or
more but 0.2 mass % or less of zirconium, 0.01 mass % or more but
0.15 mass % or less of carbon, and 0.005 mass % or more but 0.1
mass % or less of boron, and the balance being nickel and
inevitable impurities.
[0036] The nickel-based heat-resistant superalloy according to the
present invention that satisfies the following three requirements
is excellent in both tensile strength and creep life at high
temperature:
[0037] 1) being a nickel-based heat-resistant superalloy produced
by a casting and forging method;
[0038] 2) comprising 2.0 mass % or more but 25 mass % or less of
chromium, 0.2 mass % or more but 7.0 mass % or less of aluminum,
19.5 mass % or more but 55.0 mass % or less of cobalt,
[0.17.times.(mass % of cobalt content-23)+3] mass % or more but
[0.17.times.(mass % of cobalt content-20)+7] mass % or less and 5.1
mass % or more of titanium, and the balance being nickel and
inevitable impurities; and
[0039] 3) be subjected to solution heat treatment in a temperature
region of 93% or more but less than 100% of a .gamma.' solvus
temperature.
BRIEF DESCRIPTION OF DRAWINGS
[0040] FIG. 1 shows a relationship between creep life (hr) and the
ratio of solution heat treatment temperature (T) to .gamma.' solvus
temperature (Ts), which was determined by a creep test performed
under conditions of 725.degree. C. and 630 MPa.
[0041] FIG. 2 shows a comparison of creep life among Inventive
alloys 1 to 3 and Reference alloy 1 (test temperature: 725.degree.
C., applied stress: 630 MPa) when the ratio of solution heat
treatment temperature (T) to .gamma.' solvus temperature (Ts) was
set to a constant value of 99%.
[0042] FIG. 3 shows a relationship between 0.2% proof stress (test
temperature: 750.degree. C.) and creep life (test temperature:
725.degree. C., applied stress: 630 MPa) of Inventive alloys 1 to 3
and Reference alloys 1 to 5.
DESCRIPTION OF EMBODIMENTS
[0043] As described above, when a nickel-based heat-resistant
superalloy produced by a casting and forging method is subjected to
solution heat treatment in a temperature region exceeding a solvus
temperature, crystal grains generally become too large and
therefore heat-resistant characteristics are significantly
impaired. Particularly, it is said that tensile strength (0.2%
proof stress) is significantly reduced. Further, it is said that
even when the solution heat treatment is performed in a temperature
region equal to or less than a solvus temperature (at a subsolvus
temperature), crystal grains become coarse with an increase in
solution heat treatment temperature, and therefore tensile strength
(0.2% proof stress) is significantly reduced (e.g., J. C. Williams
et al. Acta Mater, 51 (2003) 5775). However, the present inventors
have found that even when produced by a casting and forging method,
a nickel-based heat-resistant superalloy that is subjected to
solution heat treatment not at a solution heat treatment
temperature commonly used but at a high temperature of 93% or more
but less than 100% of a .gamma.' solvus temperature is excellent in
both tensile strength (0.2% proof stress) and creep life even in a
temperature region, in which excellent tensile strength and
excellent creep life cannot conventionally be achieved, as long as
the nickel-based heat-treatment superalloy is a high-cobalt and
high-titanium alloy containing 19.5 mass % or more but 55.0 mass %
or less of cobalt and [0.17.times.(mass % of cobalt content-23)+3]
mass % or more but [0.17.times.(mass % of cobalt content-20) +7]
mass % or less and 5.1 mass % or more of titanium.
[0044] A nickel-based heat-resistant superalloy according to the
present invention contains, as major constituent elements,
chromium, cobalt, titanium, aluminum, and nickel and may contain an
addition ingredient and an inevitable impurity element.
[0045] Chromium is added to improve environment resistance or
fatigue crack propagation characteristics. If a chromium content is
less than 1.0 mass %, a desired improvement in these
characteristics cannot be achieved, and if the chromium content
exceeds 30.0 mass %, a harmful TCP phase is likely to be formed.
Therefore, the chromium content is 2.0 mass % or more but 25.0 mass
% or less, preferably 5.0 mass % or more but 20.0 mass % or less,
more preferably 12 mass % or more but 14.9 mass % or less.
[0046] Cobalt is a component useful for controlling a .sub.7' phase
solvus temperature. An increase in cobalt content reduces the
.gamma.' solvus temperature and widens a process window (ranges of
various conditions in which a process such as forging can be
industrially performed), and therefore a forgeability-improving
effect can also be obtained. Particularly, when titanium is
contained in a large amount, cobalt can be added in a slightly
larger amount to suppress a TCP phase and improve high-temperature
strength. The cobalt content is usually 19.5 mass % or more but
55.0 mass % or less. Based on the result of a high-temperature
compression test, the compressive strength of a nickel-based
heat-resistant superalloy whose cobalt content exceeds 55.0 mass %
tends to reduce in a temperature region from room temperature to
750.degree. C. Therefore, the upper limit of the cobalt content is
generally 55.0 mass %. The cobalt content is more preferably 19.5
mass % or more but 35.0 mass % or less, even more preferably 21.8
mass % or more but 27.0 mass % or less.
[0047] Titanium is an addition element preferably used to
strengthen a .gamma.' phase to improve strength. A titanium content
is usually 2.5 mass % or more but 15.0 mass % or less. When
titanium is added in combination with cobalt, a more beneficial
effect can be obtained by adding 5.1 mass % or more but 15.0 mass %
or less of titanium. The addition of titanium in combination with
cobalt makes it possible to achieve a nickel-based heat-resistant
superalloy having excellent phase stability and high strength.
Basically, a nickel-based heat-resistant superalloy that is stable
in structure and has high strength even at a high alloy
concentration can be achieved by selecting a heat-resistant
superalloy having a .gamma./.gamma.' two-phase structure and adding
a Co--Co.sub.3Ti alloy having a .gamma./.gamma.' two-phase
structure just like the heat-resistant superalloy. In this case,
the titanium content is within a range represented by the following
formula.
[0048] That is, the titanium content is 0.17.times.(mass % of
cobalt-23)+3 or more but 0.17.times.(mass % of cobalt-20)+7 or
less.
[0049] However, if the titanium content exceeds 15.0 mass %, it is
often the case that the formation of an phase that is a harmful
phase becomes conspicuous. Therefore, the upper limit of the
titanium content is preferably 12.44 mass %. The titanium content
is more preferably 5.5 mass % or more but 12.44 mass % or less,
even more preferably 6.1 mass % or more but 11.0 mass % or
less.
[0050] Aluminum is an element that forms a .gamma.' phase, and an
aluminum content is adjusted to form a .gamma.' phase in a proper
amount. The aluminum content is 0.2 mass % or more but 7.0 mass %
or less. Further, the ratio between the titanium content and the
aluminum content is strongly linked to the formation of an phase,
and therefore in order to suppress the formation of a TCP phase
that is a harmful phase, the aluminum content is preferably high to
some extent. Further, aluminum is directly involved in the
formation of an aluminum oxide on the surface of a nickel-based
heat-resistant superalloy and is also involved in oxidation
resistance. The aluminum content is preferably 1.0 mass % or more
but 6.0 mass % or less, more preferably 2.0 mass % or more but 3.0
mass % or less.
[0051] Further, the nickel-based heat-resistant superalloy
according to the present invention may contain the following
elements as addition ingredients.
[0052] Molybdenum mainly has the effect of strengthening a .gamma.
phase and improving creep characteristics. Molybdenum is a
high-density element, and therefore if its content is too high, the
density of a nickel-based heat-resistant superalloy is increased,
which is not preferred from a practical viewpoint. The molybdenum
content is usually 10 mass % or less, preferably less than 4 mass
%, more preferably 2.5 mass % or more but 3.0 mass % or less.
[0053] Tungsten is an element that is dissolved in a .gamma. phase
and a .gamma.' phase and strengthens both the phases, and is
therefore effective at improving high-temperature strength. If a
tungsten content is low, there is a case where creep
characteristics are poor. On the other hand, if the tungsten
content is high, there is a case where the density of a
nickel-based heat-resistant superalloy is increased because
tungsten is a high-density element just like molybdenum. The
tungsten content is usually 10 mass % or less, preferably less than
3 mass %, 0.8 mass % or more but 1.5 mass % or less.
[0054] Tantalum is effective as a strengthening element. On the
other hand, if a tantalum content is high to some extent, a
nickel-based heat-resistant superalloy has a high specific gravity
and becomes expensive. The tantalum content is usually preferably
10 mass % or less.
[0055] Niobium is effective as a strengthening element and is also
effective at controlling a specific gravity. On the other hand, if
its content is high to some extent, there is a possibility that an
undesirable phase is formed or cracks occur during hardening at
high temperature. The niobium content is usually 5.0 mass % or
less, preferably 0.1 mass % or more but 4.0 mass % or less.
[0056] The nickel-based heat-resistant superalloy according to the
present invention may also contain, as another element, at least
one element selected from vanadium, rhenium, magnesium, hafnium,
and ruthenium as long as its characteristics are not impaired. For
example, a vanadium content is 2 mass % or less, a rhenium content
is 5 mass % or less, a magnesium content is 0.1 mass % or less, a
hafnium content is 2 mass % or less, and a ruthenium content is 3
mass % or less. Ruthenium is effective at improving heat resistance
and workability.
[0057] Further, the nickel-based heat-resistant superalloy
according to the present invention may contain, as another element,
at least one element selected from zirconium, carbon, and boron as
long as its characteristics are not impaired. Zirconium is an
element effective at improving ductility, fatigue characteristics,
etc. Usually, a zirconium content is preferably 0.01 mass % or more
but 0.2 mass % or less.
[0058] Carbon is an element effective at improving ductility and
creep characteristics at high temperature. Usually, a carbon
content is 0.01 mass % or more but 0.15 mass % or less, preferably
0.01 mass % or more but 0.10 mass % or less, more preferably 0.01
mass % or more but 0.05 mass % or less. Boron can improve creep
characteristics, fatigue characteristics, etc. at high temperature.
Usually, a boron content is 0.005 mass % or more but 0.1 mass % or
less, preferably 0.005 mass % or more but 0.05 mass % or less, more
preferably 0.01 mass % or more but 0.03 mass % or less. If the
carbon content and boron content exceed their respective ranges
described above, there is a case where creep strength is reduced or
a process window becomes narrow.
[0059] The nickel-based heat-resistant superalloy according to the
present invention is produced by melting a blended raw material
having the above-described composition to prepare an ingot and
forging this ingot. The nickel-based heat-resistant superalloy
according to the present invention having a high cobalt content and
a high titanium content has a wide process window and excellent
forgeability and therefore can be produced efficiently. The
prepared forged material is subjected to solution heat treatment
and then to aging heat treatment so that the nickel-based
heat-resistant superalloy according to the present invention is
obtained. The nickel-based heat-resistant superalloy according to
the present invention having a high cobalt content and a high
titanium content and treated in the process of solution heat
treatment in a high temperature region of 93% or more but less than
100%, preferably 94% or more but less than 100% of a .gamma.'
solvus temperature is excellent in both tensile strength and creep
life even in a high temperature region in which excellent tensile
strength and excellent creep life cannot conventionally be
achieved.
[0060] A nickel-based heat-resistant superalloy is generally forged
at a solvus temperature or higher at which the nickel-based
heat-resistant superalloy has a single phase, because if a .gamma.'
phase that is a precipitation strengthening phase is present,
ductility is reduced. On the other hand, the nickel-based heat
resistant superalloy according to the present invention having a
high cobalt content and a high titanium content exhibits excellent
forgeability even in a temperature region less than a .gamma.'
solvus temperature. Therefore, the nickel-based heat-resistant
superalloy according to the present invention forged in such a
temperature region is excellent in both creep life and tensile
strength and is very suitable for practical use.
[0061] Hereinbelow, the nickel-based heat-resistant superalloy
according to the present invention will be described in more detail
with reference to examples. As a matter of course, the present
invention is not limited to the following examples.
EXAMPLES
[0062] Ingots of three kinds of inventive alloys (Inventive alloys
1 to 3) having compositions shown in Table 1 were prepared by
triple melting in which three different melting processes, that is,
vacuum induction melting, electroslag remelting, and vacuum arc
remelting were performed, and were then subjected to homogenization
heat treatment at about 1200.degree. C. Then, the ingots were
forged at 1100.degree. C. on average to produce simulated turbine
disks. Further, as comparative samples, simulated turbine disks
were produced using typical existing alloys (Reference alloys 1 to
5) in the same manner as described above. The chemical compositions
of the reference alloys are also shown in Table 1.
TABLE-US-00001 TABLE 1 .gamma.' solvus Alloy composition (mass %)
temperature Alloy number Ni Cr Mo W Co Ti Al (.degree. C.) Notes
Inventive alloy 1 Balance 13.5 2.8 1.2 25.0 6.2 2.3 .apprxeq.1162
TMW alloy Inventive alloy 2 Balance 13.8 2.6 1.1 25.0 5.6 2.2
.apprxeq.1150 TMW alloy Inventive alloy 3 Balance 14.4 2.7 1.1 21.8
6.2 2.3 .apprxeq.1166 TMW alloy Reference alloy 1 Balance 16.0 3.0
1.25 15.0 5.0 2.5 .apprxeq.1158 U720Li Reference alloy 2 Balance
15.0 5.0 -- 19.0 3.3 4.3 -- Udimet700 Reference alloy 3 Balance
18.0 4.0 -- 18.0 3.0 2.9 -- Udimet500 Reference alloy 4 Balance
19.0 10.0 -- 11.0 3.2 1.5 -- Rene41 Reference alloy 5 Balance 19.5
4.25 -- 13.5 3.0 1.3 -- Waspaloy
[0063] The simulated turbine disks obtained by casting and forging
Inventive alloys 1 to 3 and Reference alloy 1 (U720Li) were
subjected to heat treatment in air for 4 hours at different
solution heat treatment temperatures and then subjected to aging
heat treatment. After the treatment, the samples were subjected to
a creep life test. FIG. 1 shows a relationship between the ratio of
solution heat treatment temperature (T) to .gamma.' solvus
temperature (Ts) (T/Ts) and creep life. As can be seen from FIG. 1,
the creep life was excellent when the ratio of solution heat
treatment temperature (T) to .gamma.' solvus temperature (Ts)
(T/Ts) was set to about 0.93 or more but less than 1.0. When the
solution heat treatment temperature (T) was equal to or higher than
the .gamma.' solvus temperature (Ts), the creep life was rapidly
reduced. Further, in the case of Reference alloy 1 (U720Li) showing
the best performance among the existing nickel-based heat-resistant
superalloys, a significant improvement in creep life was not
observed even when the ratio of solution heat treatment temperature
(T) to .gamma.' solvus temperature (Ts) was brought close to 1.0,
and its creep life was shorter than those of Inventive alloys 1 to
3. It has been found from these results that the nickel-based
heat-resistant superalloys according to the present invention
produced by a casting and forging method and having a high cobalt
content and a high titanium content specifically exhibit excellent
creep life when the ratio of solution heat treatment temperature
(T) to .gamma.' solvus temperature (Ts) (T/Ts) is set to about 0.93
or more but less than 1.0.
[0064] FIG. 2 shows a comparison of creep life among Inventive
alloys 1 to 3 and Reference alloy 1 when the ratio of solution heat
treatment temperature (T) to .gamma.' solvus temperature (Ts) was a
constant value of 99% (test temperature: 725.degree. C., applied
stress: 630 MPa). As can be seen from FIG. 2, the nickel-based
heat-resistant superalloys according to the present invention
having a high cobalt content and a high titanium content have a
creep life about three to five times that of the
commercially-available reference alloy (U720Li).
[0065] FIG. 3 shows a relationship between 0.2% proof stress (test
temperature: 750.degree. C.) and creep life (test temperature:
725.degree. C., applied stress: 630 MPa) of Inventive alloys 1 to 3
and Reference alloys 1 to 5. As can be seen from FIG. 3, the
nickel-based heat-resistant superalloys according to the present
invention have not only significantly-improved creep life as
compared to the existing nickel-based heat-resistant superalloys
but also excellent tensile strength.
[0066] The above test results demonstrate that a nickel-based
heat-resistant superalloy that satisfies the following three
requirements is excellent in both creep life and tensile strength
and is very suitable for practical use:
[0067] 1) being a nickel-based heat-resistant superalloy produced
by a casting and forging method;
[0068] 2) comprising 2.0 mass % or more but 25 mass % or less of
chromium, 0.2 mass % or more but 7.0 mass % or less of aluminum,
19.5 mass % or more but 55.0 mass % or less of cobalt,
[0.17.times.(mass % of cobalt content-23)+3] mass % or more but
[0.17.times.(mass % of cobalt content-20)+7] mass % or less and 5.1
mass % or more of titanium, and the balance being nickel and
inevitable impurities; and
[0069] 3) be subjected to solution heat treatment at 93% or more
but less than 100% of a .gamma.' solvus temperature.
INDUSTRIAL APPLICABILITY
[0070] It is possible to provide a nickel-based heat-resistant
superalloy mainly having significantly-improved heat-resistant
characteristics. The nickel-based heat resistant superalloy is
useful for heat-resistant members of aircraft engines,
power-generating gas turbines, etc., especially for
high-temperaturehigh-pressure turbine disks, compressor blades,
shafts, turbine cases, etc.
* * * * *