Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
TITLE OF INVENTION
NiCrFe Alloy
TECHNICAL FIELD
[0001]
The present invention relates to an austenitic heat resistant alloy, and more
specifically a NiCrFe alloy.
BACKGROUND ART
[0002]
Conventionally, facilities such as thermal power generation boilers and
chemical plants are operated in high temperature environments (such as 400 to
800 C) and, in addition, they are brought into contact with process fluids
including
sulfides and/or chlorides. Therefore, materials to be used in such facilities
require
their creep strength and corrosion resistance at high temperatures.
[0003]
Examples of the material for use in such facilities include 18-8 stainless
steel
such as SUS304H, SU5316H, SUS321H, and SUS347H, and NiCrFe alloys
represented by Alloy 800H, which is specified as NCF800H by the JIS standard.
[0004]
A NiCrFe alloy excels in corrosion resistance and high temperature strength
compared to an 18-8 stainless steel. Further, a NiCrFe alloy excels in
economic
efficiency compared to a Ni-base alloy represented by Alloy617. Therefore,
NiCrFe alloys are widely used in regions of severe use environments.
[0005]
NiCrFe alloys used in such severe use environments are proposed in Japanese
Patent Application Publication No. 2013-227644 (Patent Literature 1), Japanese
Patent Application Publication No. 06-264169 (Patent Literature 2), Japanese
Patent
Application Publication No. 2002-256398 (Patent Literature 3), and Japanese
Patent
Application Publication No. 08-13104 (Patent Literature 4).
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[0006]
An austenitic heat resistant alloy disclosed in Patent Literature 1 consists
of,
in mass%, C: less than 0.02%, Si: not more than 2%, Mn: not more than 2%, Cr:
not
less than 20% and less than 28%, Ni: more than 35% and not more than 50%, W:
2.0
to 7.0%, Mo: less than 2.5% (including 0%), Nb: less than 2.5% (including 0%),
Ti:
less than 3.0% (including 0%), Al: not more than 0.3%, P: not more than 0.04%,
S:
not more than 0.01%, and N: not more than 0.05%, with the balance being Fe and
impurities, wherein fl = (1/2)W + Mo is 1.0 to 5.0, 12 = (1/2)W + Mo + Nb +
2Ti is
2.0 to 8.0, and 13 = Nb + 2Ti is 0.5 to 5Ø
[0007]
A heat resistant and corrosion resistant alloy disclosed in Patent Literature
2
consists of, in weight%, 55 to 65% of Nickel, 19 to 25% of Chromium, 1 to 4.5%
of
Aluminum, 0.045 to 0.3% of Yttrium, 0.15 to 1% of Titan, 0.005 to 0.5% of
Carbon,
0.1 to 1.5% of Silicon, not more than 1% of Manganese, a total of 0.005% of at
least
one element selected from the group consisting of Magnesium, Calcium, and
Cerium,
a total of less than 0.5% of Magnesium and Calcium, less than 1% of Cerium,
0.0001
to 0.1% of Boron, not more than 0.5% of Zirconium, 0.0001 to 0.2% of Nitrogen,
and not more than 10% of Cobalt, with the balance being Fe and accompanying
impurities.
[0008]
An austenitic alloy disclosed in Patent Literature 3 contains, in mass%, C:
0.01 to 0.1%, Mn: 0.05 to 2%, Cr: 19 to 26%, and Ni: 10 to 35%, with the Si
content
satisfying a formula of 0.01 < Si < (Cr + 0.15 x Ni - 18)/10.
[0009]
A heat resistant alloy disclosed in Patent Literature 4 consists of, in
weight%,
C: 0.02 to 0.15%, Si: 0.70 to 3.00%, Mn: not more than 0.50%, Ni: 30.0 to
40.0%,
Cr: 18.0 to 25.0%, Al: 0.50 to 2.00%, and Ti: 0.10 to 1.00%, with the balance
being
Fe and inevitable impurities.
CITATION LIST
PATENT LITERATURE
[0010]
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Patent Literature 1: Japanese Patent Application Publication No. 2013-227644
Patent Literature 2: Japanese Patent Application Publication No. 06-264169
Patent Literature 3: Japanese Patent Application Publication No. 2002-256398
Patent Literature 4: Japanese Patent Application Publication No. 08-13104
NON PATENT LITERATURE
[0011]
Non Patent Literature 1: Hans van Wortel: "Control of Relaxation Cracking in
Austenitic High Temperature Components", CORROSION2007 (2007), NACE,
Paper No.07423
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0012]
The austenitic heat resistant alloy disclosed in Patent Literature 1 controls
the
formation of Laves phase by specifying the contents of W, Mo, Nb, and Ti,
thereby
improving creep strength and toughness. The heat resistant and corrosion
resistant
alloy disclosed in Patent Literature 2 improves high-temperature oxidation
resistance
by causing y' to be precipitated during creep. The austenitic alloy disclosed
in
Patent Literature 3 improves carburizing properties by suppressing exfoliation
of the
oxide film dominantly composed of Cr203 and formed on the material surface.
The
heat resistant alloy disclosed in Patent Literature 4 contains a specific
amount of Cr,
a reduced amount of Mn, and a fixed amount of Si, thereby making it possible
to
obtain excellent oxidation resistance even in a case in which the Ni content
is
reduced.
[0013]
On the other hand, Non Patent Literature 1 discloses that a NiCrFe alloy has a
high susceptibility to stress relaxation cracking. This means that a NiCrFe
alloy
requires stress relief heat treatment, after working, for a bent part and
welded part, in
which residual stress is present. Therefore, a NiCrFe alloy requires not only
excellent creep strength but also excellent stress relaxation cracking
resistance.
[0014]
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An objective of the present invention is to provide a NiCrFe alloy which
excels in creep strength and stress relaxation cracking resistance.
SOLUTION TO PROBLEM
[0015]
A NiCrFe alloy according to the present invention has a chemical composition
consisting of, in mass%, C: 0.03 to 0.15%, Si: not more than 1.00%, Mn: not
more
than 2.00%, P: not more than 0.040%, S: not more than 0.0050%, Cr: 18.0 to
25.0%,
Ni: 25.0 to 40.0%, Ti: 0.10 to 1.60%, Al: 0.05 to 1.00%, N: not more than
0.020%,
0: not more than 0.008%, rare earth metal (REM): 0.001 to 0.100%, B: 0 to
0.010%,
Ca: 0 to 0.010%, Mg: 0 to 0.010%, V: 0 to 0.5%, Nb: 0 to 1.0%, Ta: 0 to 1.0%,
Hf: 0
to 1.0%, Mo: 0 to 1.0%, W: 0 to 2.0%, Co: 0 to 3.0%, and Cu: 0 to 3.0%, with
the
balance being Fe and impurities, the chemical composition satisfying Formulae
(1) to
(3):
0.50 Ti + 48A1/27 5_ 2.20 (1)
0.40 Ti/(Ti + 48A1/27) 5_ 0.80 (2)
E[REM/(A(REM))] - S/32 - 2/3.0/16 0 (3)
where, each symbol of element in the above described formulae is substituted
by the content (mass%) of the corresponding element. A(REM) in Formula (3) is
substituted by the atomic weight of each rare earth metal.
ADVANTAGEOUS EFFECT OF INVENTION
[0016]
A NiCrFe alloy according to the present invention excels in creep strength
and stress relaxation cracking resistance.
BRIEF DESCRIPTION OF DRAWINGS
[0017]
[FIG. 1] FIG. 1 is a diagram to show a relation between fn2 of each Reference
mark
of Examples, and a sum (mass%) of y' and ti phase after aging treatment.
DESCRIPTION OF EMBODIMENTS
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[0018]
The present inventors have conducted detailed study on the creep strength and
the stress relaxation cracking resistance of NiCrFe alloys. As a result, the
present
inventors have obtained the following findings.
[0019]
(A) To obtain excellent creep strength, the precipitation amount of y'
(intermetallic compound: Ni3(Ti, Al)), which precipitates during creep under a
high-
temperature environment, may be increased. If precipitates sufficiently during
creep under a high-temperature environment, the creep strength of the alloy is
increased by precipitation hardening. However, if y' precipitates excessively,
deformability within an austenite grain deteriorates, thus causing stress
concentration
in grain boundary surfaces. As a result, the stress relaxation cracking
resistance of
the alloy deteriorates. Therefore, to achieve excellent creep strength and
excellent
stress relaxation cracking resistance at the same time, it is necessary to
adjust the
amount of y' which precipitates during creep under a high temperature
environment.
To obtain an appropriate amount of 7' precipitation, the contents of Ti and
Al, which
constitute 7', may be adjusted.
[0020]
Specifically, the chemical composition of NiCrFe alloy satisfies Formula (1)
to maintain stress relaxation cracking resistance while ensuring creep
strength:
0.50 Ti + 48A1/27 2.20 (1)
where, each symbol of element in Formula (1) is substituted by the content
(mass%) of the corresponding element.
[0021]
Now define fnl as fnl = Ti + 48A1/27. fnl is an index to indicate the
amount of y' that precipitates during creep. fnl is a total content of Al and
Ti, the
content of Al being converted into the amount of Ti. When fnl is less than
0.50, a
sufficient precipitation amount of y' will not be obtained. For that reason,
the
NiCrFe alloy cannot obtain excellent creep strength. On the other hand, when
fnl
is more than 2.20, the stress relaxation cracking resistance of the NiCrFe
alloy will
deteriorate due to a large amount of precipitation of y'.
[0022]
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(B) The y' that has precipitated during creep under a hot-temperature
environment may change in its form over time. Specifically, while fine y'
precipitates in an early stage of creep, the y' may change to a coarse and
acicular 11
phase (Ni3Ti) during creep under a high-temperature environment over time.
Formation of T1 phase will decrease the creep strength of the NiCrFe alloy.
[0023]
Then, the present inventors have investigated in detail on a case in which 7'
phase changes to ri phase under a high-temperature environment. As a result,
they
supposed that the Ti content with respect to the total content of Al and Ti,
the content
of Al being converted into the amount of Ti is related to the change from y'
phase to
11 phase. Accordingly, the present inventors have investigated in detail on
the Ti
content with respect to the total content of Al and Ti, the content of Al
being
converted into the amount of Ti, and the microstructure during creep.
[0024]
Now define fn2 as fn2 = Ti/(Ti + 48A1/27). fn2 is a ratio of the Ti content
with respect to the total content of Al and Ti, the content of Al being
converted into
the amount of Ti. FIG. 1 shows a relation between fn2 and a sum of y' and 11
phase
after aging treatment. FIG. 1 is obtained by the following method. It is
created by
using fn2, and Ti, Al, and Ni contents in the y' and ri phase after aging
treatment,
which are obtained by the below described method, for NiCrFe alloys whose
chemical compositions are within the range of the present invention, and in
which
the above described Formula (1) and the below described Formula (3) are within
the
range of the present invention. Further, y' and 11 phase are discriminated by
using a
method to be described below. The symbol "0" in FIG. 1 indicates an Example in
which the number density of 11 phase after aging treatment is less than 5/100
[tm2.
On the other hand, the symbol "=" in FIG. 1 indicates an Example in which the
number density ofil phase after aging treatment is not less than 5/1001.1m2.
[0025]
Referring to FIG. 1, when fn2 is less than 0.40, a sufficient precipitation
amount of y' will not be obtained. In this case, the NiCrFe alloy cannot
obtain
excellent creep strength. On the other hand, when fn2 is more than 0.80, 7'
changes
to ri phase. As a result, the NiCrFe alloy cannot obtain excellent creep
strength.
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Therefore, when fn2 is 0.40 to 0.80, it is possible to increase the creep
strength of the
NiCrFe alloy.
[0026]
As described so far, if the chemical composition of the NiCrFe alloy of the
present invention satisfies Formula (2), y' precipitates in an appropriate
amount, and
the precipitation of 11 phase will be suppressed even after time has passed so
that
excellent creep strength will be obtained:
0.40 Ti/(Ti + 48A1/27) 0.80 (2)
where, each symbol of element in Formula (2) is substituted by the content
(mass%) of the corresponding element.
[0027]
(C) One cause of stress relaxation cracking is segregation of S in grain
boundaries. Therefore, it is possible to increase the stress relaxation
cracking
resistance of the NiCrFe alloy by decreasing an impurity S. which segregates
in grain
boundaries, thereby causing grain boundary embrittlement. On the other hand,
rare
earth metals (REM) combine with a minute amount of S, which cannot be removed
by refining, in the alloy thereby forming inclusions. In other words, a REM
can
immobilize S as inclusions.
[0028]
Therefore, adjusting the content of REM to be an appropriate amount will
allow improving the stress relaxation cracking resistance of the NiCrFe alloy.
REM
combines with S and is also likely to combine with 0 easily. Therefore, to
immobilize S by REM, the REM content should be adjusted while the amount of
REM that combines with 0 is taken into consideration.
[0029]
If the chemical composition of the NiCrFe alloy of the present invention
satisfies Formula (3), S will be sufficiently immobilized by REM, and
excellent
stress relaxation cracking resistance will be obtained:
E[REM/(A(REM))] - S/32 - 2/3.0/16 0 (3)
where, each symbol of element in Formula (3) is substituted by the content
(mass%) of the corresponding element, and A(REM) is substituted by the atomic
weight of each rare earth metal.
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[0030]
E[REM/(A(REM))] is substituted by an addition sum of values which are
obtained by dividing each REM content (mass%) contained in the NiCrFe alloy by
the atomic weight of the REM.
[0031]
Now define fn3 as fn3 = E[REM/(A(REM))] - S/32 - 2/3.0/16. REM is a
generic name of a total of 17 elements of Sc, Y, and lanthanoids. When fn3 is
not
less than 0, REM can sufficiently immobilize S as inclusions, thereby
improving the
stress relaxation cracking resistance.
[0032]
The NiCrFe alloy according to the present invention, which has been
completed based on the above described findings, has a chemical composition
consisting of, in mass%, C: 0.03 to 0.15%, Si: not more than 1.00%, Mn: not
more
than 2.00%, P: not more than 0.040%, S: not more than 0.0050%, Cr: 18.0 to
25.0%,
Ni: 25.0 to 40.0%, Ti: 0.10 to 1.60%, Al: 0.05 to 1.00%, N: not more than
0.020%,
0: not more than 0.008%, rare earth metal (REM): 0.001 to 0.100%, B: 0 to
0.010%,
Ca: 0 to 0.010%, Mg: 0 to 0.010%, V: 0 to 0.5%, Nb: 0 to 1.0%, Ta: 0 to 1.0%,
Hf: 0
to 1.0%, Mo: 0 to 1.0%, W: 0 to 2.0%, Co: 0 to 3.0%, and Cu: 0 to 3.0%, with
the
balance being Fe and impurities, the chemical composition satisfying Formulae
(1) to
(3):
0.50 5. Ti + 48A1/27 5_ 2.20 (1)
0.40 5 Ti/(Ti + 48A1/27) 5_ 0.80 (2)
E[REM/(A(REM))] - S/32 - 2/3.0/16 0 (3)
where, each symbol of element in Formulae (1) to (3) is substituted by the
content (mass%) of the corresponding element. A(REM) in Formula (3) is
substituted by an atomic weight of each rare earth metal.
[0033]
The above described chemical composition may contain B: 0.0001 to 0.010%.
[0034]
The above described chemical composition may contain one or two types
selected from the group consisting of Ca: 0.0001 to 0.010%, and Mg: 0.0001 to
0.010%.
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[0035]
The above described chemical composition may contain one or more types
selected from the group consisting of V: 0.01 to 0.5%, Nb: 0.01 to 1.0%, Ta:
0.01 to
1.0%, and Hf: 0.01 to 1.0%.
[0036]
The above described chemical composition may contain one or more types
selected from the group consisting of Mo: 0.01 to 1.0%, W: 0.01 to 2.0%, Co:
0.01 to
3.0%, and Cu: 0.01 to 3.0%.
[0037]
The NiCrFe alloy according to the present invention has excellent creep
strength and excellent stress relaxation cracking resistance. To be more
specific,
the NiCrFe alloy will not rupture for 300 or more hours even if it is
subjected to
tensile strain of 10% at a strain rate of 0.05 min-I and is kept as is under
air
atmosphere of 650 C after being subjected to cold rolling at a reduction of
area of
20%.
[0038]
Hereinafter, the NiCrFe alloy according to the present invention will be
described in detail. The symbol "%" regarding elements means, unless otherwise
stated, mass%.
[0039]
[Chemical composition]
The chemical composition of NiCrFe alloy of the present invention contains
the following elements.
[0040]
C: 0.03 to 0.15%
Carbon (C) stabilizes austenite, and increases high temperature creep strength
of the alloy. When the C content is too low, these effects cannot be obtained.
On
the other hand, when the C content is too high, coarse carbide will
precipitate in large
amount, thus deteriorating the ductility of grain boundaries. Further, the
toughness
and creep strength of the alloy decrease. Therefore, the C content is 0.03 to
0.15%.
The lower limit of the C content is preferably 0.04%, more preferably more
than
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0.04%, and further preferably 0.05%, and further preferably 0.06%. The upper
limit of the C content is preferably 0.12%, and more preferably 0.10%.
[0041]
Si: not more than 1.00%
Silicon (Si) is inevitably contained. Si deoxidizes the alloy, and improves
the corrosion resistance and oxidation resistance at high temperatures of the
alloy.
However, when the Si content is too high, the stability of austenite
deteriorates, and
toughness and creep strength of the alloy decrease. Therefore, the Si content
is not
more than 1.00%. The upper limit of the Si content is preferably 0.80%, more
preferably 0.60%, and further preferably less than 0.60%. Excessive reduction
of
the Si content deteriorates deoxidization effect, thus deteriorating the
corrosion
resistance and oxidization resistance at high temperatures of the alloy. And
further,
the production cost is significantly increased. Therefore, the lower limit of
the Si
content is preferably 0.02%, and more preferably 0.05%.
[0042]
Mn: not more than 2.00%
Manganese (Mn) is inevitably contained. Mn deoxidizes the alloy, and
stabilizes austenite. However, when the Mn content is too high, embrittlement
is
caused and the toughness and creep ductility of the alloy deteriorate.
Therefore, the
Mn content is not more than 2.00%. The upper limit of the Mn content is
preferably 1.80%, and more preferably 1.50%. Excessive reduction of the Mn
content deteriorates the deoxidization effect and stabilization of austenite,
and further
causes significant increase of the production cost. Therefore, the lower limit
of the
Mn content is preferably 0.10%, more preferably 0.30%, and further preferably
more
than 0.50%.
[0043]
P: not more than 0.040%
Phosphorous (P) is an impurity. P deteriorates hot workability and
weldability of the alloy, and also deteriorates creep ductility of the alloy
after long
hours of usage. Therefore, the P content is not more than 0.040%. The upper
limit of the P content is preferably 0.035%, and more preferably 0.030%. The P
content is preferably as low as possible. However, excessive reduction of the
P
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content will increase the production cost. Therefore, the lower limit of the P
content is preferably 0.0005%, and more preferably 0.0008%.
[0044]
S: not more than 0.0050%
Sulfur (S) is an impurity. S deteriorates the stress relaxation cracking
resistance of the alloy, and also deteriorates the hot workability,
weldability, and
creep ductility of the alloy. Therefore, the S content is not more than
0.0050%.
The upper limit of the S content is preferably 0.0030%. The S content is
preferably
as low as possible. However, excessive reduction of the S content will
increase the
production cost. Therefore, the lower limit of the S content is preferably
0.0002%,
and more preferably 0.0003%.
[0045]
Cr: 18.0 to 25.0%
Chromium (Cr) improves the oxidation resistance and corrosion resistance at
high temperatures of the alloy. When the Cr content is too low, these effects
cannot
be obtained. On the other hand, when the Cr content is too high, the stability
of
austenite at high temperatures deteriorates and the creep strength of the
alloy
decreases. Therefore, the Cr content is 18.0 to 25.0%. The lower limit of the
Cr
content is preferably 18.5%, and more preferably 19.0%. The upper limit of the
Cr
content is preferably 24.5%, and more preferably 24.0%.
[0046]
Ni: 25.0 to 40.0%
Nickel (Ni) stabilizes austenite structure. Further, Ni forms yi, thereby
increasing the creep strength of the alloy. When the Ni content is too low, y'
is not
likely to be formed, and the aforementioned effect cannot be obtained. On the
other
hand, when the Ni content is too high, the production cost increases.
Therefore, the
Ni content is 25.0 to 40.0%. The lower limit of the Ni content is preferably
26.0%,
and more preferably 27.0%. The upper limit of the Ni content is preferably
37.0%,
and more preferably 35.0%.
[0047]
Ti: 0.10 to 1.60%
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Titanium (Ti) combines with Ni to form y'. Further, Ti combines with C to
form TiC, thereby increasing the creep strength and tensile strength of the
alloy at
high temperatures. When the Ti content is too low, such effects cannot be
obtained.
On the other hand, when the Ti content is too high, y' precipitates
excessively,
thereby deteriorating the stress relaxation cracking resistance of the alloy.
Therefore, the Ti content is 0.10 to 1.60%. The lower limit of the Ti content
is
preferably 0.20%, more preferably 0.30%, and further preferably more than
0.60%.
The upper limit of the Ti content is preferably 1.50%, more preferably less
than
1.50%, and further preferably 1.40%.
[0048]
Al: 0.05 to 1.00%
Aluminum (Ai) deoxidizes the alloy. Further, Al combines with Ni to form
y' and increases the creep strength and tensile strength of the alloy at high
temperatures. When the Al content is too low, such effects cannot be obtained.
On the other hand, when the Al content is too high, 7' precipitates in a large
amount,
thereby deteriorating the stress relaxation cracking resistance, creep
ductility, and
toughness of the alloy. Therefore, the Al content is 0.05 to 1.00%. The lower
limit of the Al content is preferably 0.08%, and more preferably 0.10%. The
upper
limit of the Al content is preferably 0.90%, and more preferably 0.80%.
[0049]
N: not more than 0.020%
Nitrogen (N) is an impurity. N precipitates as coarse TiN, and decreases the
amount of dissolved Ti, thereby decreasing the creep strength of the alloy.
Further,
N deteriorates toughness and hot workability of the alloy. Therefore, the N
content
is not more than 0.020%. The upper limit of the N content is preferably
0.017%,
and more preferably 0.015%. The N content is preferably as low as possible.
However, excessive reduction thereof will increase production cost. Therefore,
the
lower limit of the N content is preferably 0.002%, and more preferably 0.004%.
[0050]
0: not more than 0.008%
Oxygen (0) is an impurity. Oxygen deteriorates the hot workability of the
alloy, and also deteriorates the toughness and ductility of the alloy.
Therefore, the
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0 content is not more than 0.008%. The upper limit of the 0 content is
preferably
0.006%, and more preferably 0.005%. The 0 content is preferably as low as
possible. However, excessive reduction thereof will increase the production
cost.
Therefore, the lower limit of the 0 content is preferably 0.0005%, and more
preferably 0.0008%.
[0051]
REM: 0.001 to 0.100%
Rare earth metal (REM) forms a compound with S, thereby decreasing the
content of S which has dissolved into the matrix, and improving the stress
relaxation
cracking resistance of the alloy. Further, REM improves the hot workability
and
oxidization resistance of the alloy. When the REM content is too low, these
effects
cannot be obtained. On the other hand, when the REM content is too high, the
hot
workability and weldability of the alloy will deteriorate. Therefore, the REM
content is 0.001 to 0.100%. The lower limit of the REM content is preferably
0.003%, and more preferably 0.005%. The upper limit of the REM content is
preferably 0.090%, and more preferably 0.080%.
[0052]
REM is a generic name of a total of 17 elements of Sc, Y, and lanthanoids,
and the REM content refers to a total content of one or more elements of REM.
Moreover, REM is generally contained in misch metal. For that reason, REM may
be added to molten metal as misch metal, and may be adjusted such that the REM
content is within the above described range.
[0053]
The balance of the chemical composition of the NiCrFe alloy according to the
present invention consists of Fe and impurities. Here, the term impurity means
an
element which is introduced from ores and scraps as the raw material, or from
a
production environment, etc., when the NiCrFe alloy is industrially produced,
and
which is permitted within a range not adversely affecting the NiCrFe alloy of
the
present embodiment.
[0054]
[Optional elements]
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The NiCrFe alloy according to the present invention may contain B in place
of part of Fe.
[0055]
B: 0 to 0.010%
Boron (B) is an optional element and may not be contained. When
contained, B increases the creep strength of the alloy by causing grain
boundary
carbides to be finely dispersed. Further, B segregates in grain boundaries to
assist
the effects of REM. When B is contained in any small amount, the
aforementioned
effects will be obtained to some degree. However, when the B content is too
high,
the weldability and hot workability of the alloy will deteriorate. Therefore,
the B
content is 0 to 0.010%. The upper limit of the B content is preferably 0.008%.
The lower limit of the B content to effectively obtain the aforementioned
effects is
preferably 0.0001%, and more preferably 0.0005%.
[0056]
The NiCrFe alloy according to the present invention may contain one or two
types selected from the group consisting of Ca and Mg in place of part of Fe.
Each
of these elements forms a compound with S, thereby assisting the effects of
REM.
[0057]
Ca: 0 to 0.010%
Calcium (Ca) is an optional element and may not be contained. When
contained, Ca forms a compound with S, thereby assisting the S immobilizing
effect
of REM. If Ca is contained in any small amount, the aforementioned effect will
be
obtained to some degree. However, when the Ca content is too high, Ca forms
oxide, and deteriorates the hot workability of the alloy. Therefore, the Ca
content is
0 to 0.010%. The upper limit of the Ca content is preferably 0.008%. The lower
limit of the Ca content to effectively obtain the aforementioned effect is
preferably
0.0001%, more preferably 0.0002% and further preferably 0.0003%.
[0058]
Mg: 0 to 0.010%
Magnesium (Mg) is an optional element and may not be contained. When
contained, Mg forms a compound with S, thereby assisting the S immobilizing
effect
of REM. When Mg is contained in any small amount, the aforementioned effect
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will be obtained to some degree. However, when the Mg content is too high, Mg
forms oxide, thereby deteriorating the hot workability of the alloy.
Therefore, the
Mg content is 0 to 0.010%. The upper limit of the Mg content is preferably
0.008%.
The lower limit of the Mg content to effectively obtain the aforementioned
effect is
preferably 0.0001%, more preferably 0.0002% and further preferably 0.0003%.
[0059]
The NiCrFe alloy according to the present invention may contain one or more
types selected from the group consisting of V, Nb, Ta, and Hf in place of part
of Fe.
Each of these elements forms carbide and carbonitride, thereby increasing the
creep
strength of the alloy.
[0060]
V: 0 to 0.5%
Vanadium (V) is an optional element and may not be contained. When
contained, V forms fine carbide and carbonitride with C and N, thereby
increasing
the creep strength of the alloy. When V is contained in any small amount, the
aforementioned effect will be obtained to some degree. However, when the V
content is too high, a large amount of carbide and carbonitride will
precipitate,
thereby deteriorating the creep ductility of the alloy. Therefore, the V
content is 0
to 0.5%. The upper limit of the V content is preferably 0.4%. The lower limit
of
the V content to effectively obtain the aforementioned effect is 0.01%.
[0061]
Nb: 0 to 1.0%
Niobium (Nb) is an optional element and may not be contained. When
contained, Nb forms fine carbide and carbonitride with C and N, thereby
increasing
the creep strength of the alloy. When Nb is contained in any small amount, the
aforementioned effect will be obtained to some degree. However, when the Nb
content is too high, a large amount of carbide and carbonitride will
precipitate,
thereby deteriorating the creep ductility and toughness of the alloy.
Therefore, the
Nb content is 0 to 1.0%. The upper limit of the Nb content is preferably 0.4%.
The lower limit of the Nb content to effectively obtain the aforementioned
effect is
0.01%.
[0062]
CA 03039043 2019-04-01
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Ta: 0 to 1.0%
Tantalum (Ta) is an optional element and may not be contained. When
contained, Ta forms fine carbide and carbonitride with C and N, thereby
increasing
the creep strength of the alloy. When Ta is contained in any small amount, the
aforementioned effect will be obtained to some degree. However, when the Ta
content is too high, a large amount of carbide and carbonitride will
precipitate,
thereby deteriorating the creep ductility and toughness of the alloy.
Therefore, the
Ta content is 0 to 1.0%. The upper limit of the Ta content is preferably 0.4%.
The
lower limit of the Ta content to effectively obtain the aforementioned effect
is 0.01%.
[0063]
Hf: 0 to 1.0%
Hafnium (Hf) is an optional element and may not be contained. When
contained, Hf forms fine carbide and carbonitride with C and N, thereby
increasing
the creep strength of the alloy. When Hf is contained in any small amount, the
aforementioned effect will be obtained to some degree. However, when the Hf
content is too high, a large amount of carbide and carbonitride will
precipitate,
thereby deteriorating the creep ductility and toughness of the alloy.
Therefore, the
Hf content is 0 to 1.0%. The upper limit of the Hf content is preferably 0.4%.
The
lower limit of the Hf content to effectively obtain the aforementioned effect
is 0.01%.
[0064]
The NiCrFe alloy according to the present invention may contain one or more
types selected from the group consisting of Mo, W, Co, and Cu in place of part
of Fe.
[0065]
Mo: 0 to 1.0%
Molybdenum (Mo) is an optional element and may not be contained. When
contained, Mo dissolves into the alloy, thereby increasing the creep strength
of the
alloy at high temperatures. When Mo is contained in any small amount, such
effect
will be obtained to some degree. However, when the Mo content is too high, the
stability of austenite will be lost, thereby deteriorating the toughness of
the alloy.
Therefore, the Mo content is 0 to 1.0%. The upper limit of the Mo content is
preferably 0.9%. The lower limit to effectively obtain the aforementioned
effect is
preferably 0.01%.
CA 03039043 2019-04-01
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[0066]
W: 0 to 2.0%
Tungsten (W) is an optional element and may not be contained. When
contained, W dissolves into the alloy, thereby increasing the creep strength
of the
alloy at high temperatures. When W is contained in any small amount, such
effect
will be obtained to some degree. However, when the W content is too high, the
stability of austenite will be lost, thereby deteriorating the toughness of
the alloy.
Therefore, the W content is 0 to 2.0%. The upper limit of the W content is
preferably 1.8%. The lower limit of the W content to effectively obtain the
aforementioned effect is preferably 0.01%.
[0067]
Co: 0 to 3.0%
Cobalt (Co) is an optional element and may not be contained. When
contained, Co stabilizes austenite and dissolves into the alloy, thereby
increasing the
creep strength of the alloy at high temperatures. When Co is contained in any
small
amount, such effects will be obtained to some degree. However, when the Co
content is too high, the production cost increases. Therefore, the Co content
is 0 to
3.0%. The upper limit of the Co content is preferably 2.8%. The lower limit of
the Co content to effectively obtain the aforementioned effects is preferably
0.01%.
[0068]
Cu: 0 to 3.0%
Cupper (Cu) is an optional element and may not be contained. When
contained, Cu stabilizes austenite and suppresses precipitation of brittle
phase such
as a phase during use at high temperatures. When Cu is contained in any small
amount, such effects will be obtained to some degree. However, when the Cu
content is too high, the hot workability of the alloy deteriorates. Therefore,
the Cu
content is 0 to 3.0%. The upper limit of the Cu content is preferably 2.5%,
and
more preferably less than 2.0%. The lower limit of the Cu content to
effectively
obtain the aforementioned effects is preferably 0.01%.
[0069]
[Formula (1)]
CA 03039043 2019-04-01
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The NiCrFe alloy according to the present invention further satisfies Formula
(1):
0.50 Ti + 48A1/27 2.20 (1)
where, each symbol of element in Formula (1) is substituted by the content
(mass%) of the corresponding element.
[0070]
fnl = Ti + 48A1/27 is an index to indicate the precipitation amount of y'. fnl
indicates a total amount of Ti when the amount of Al is converted into the
amount of
Ti. When fnl is less than 0.50, a sufficient precipitation amount of y'
will not be
obtained, so that the NiCrFe alloy cannot obtain excellent creep resistance.
On the
other hand, when fnl is more than 2.20, the stress relaxation cracking
resistance,
creep ductility, and toughness of the alloy will deteriorate due to an
excessive
precipitation amount of y'. Therefore, fnl is 0.50 to 2.20. In this range, an
appropriate amount of" is precipitated, and excellent creep resistance is
obtained.
The upper limit of the fnl is preferably 2.00. The lower limit of fnl is
preferably
0.65.
[0071]
[Formula (2)]
The above described chemical composition further satisfies Formula (2):
0.40 Ti/(Ti + 48A1/27) 0.80 (2)
where, each symbol of element in Formula (2) is substituted by the content
(mass%) of the corresponding element.
[0072]
fn2 = Ti/(Ti + 48A1/27) is a ratio of the Ti content with respect to the total
content of Al and Ti, the content of Al being converted into the amount of Ti.
When fn2 is less than 0.40, the Ti content is too low with respect to the Al
content,
and the precipitation amount of y' decreases. As a result, the NiCrFe alloy
cannot
obtain excellent creep strength. On the other hand, when fn2 is more than
0.80, the
Ti content becomes excessive with respect to the Al content so that although
fine y'
precipitates in an early stage of creep, the y' changes to coarse and acicular
r phase
over time. As a result, the creep strength and toughness of the alloy
deteriorate.
Therefore, fn2 is 0.40 to 0.80. In this range, y' precipitates in an
appropriate amount,
CA 03039043 2019-04-01
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and will not change to 11 phase even when further time passes so that
excellent creep
strength is obtained. The upper limit of fn2 is preferably 0.75.
[0073]
[Formula (3)]
The above described chemical composition further satisfies Formula (3):
E[REM/(A(REM))] - S/32 - 2/3.0/16 0 (3)
where, each symbol of element in Formula (3) is substituted by the content
(mass%) of the corresponding element, and A(REM) is substituted by the atomic
weight of each REM.
[0074]
fn3 = E[REM/(A(REM))] - S/32 - 2/3.0/16 is an index to indicate the amount
of S that segregates in grain boundaries. When fn3 is a negative value, S
segregates
in grain boundaries, thereby resulting in grain boundary embrittlement so that
the
stress relaxation cracking resistance of the alloy deteriorates. On the other
hand,
when fn3 is not less than 0, REM immobilizes S as inclusions, thereby
decreasing the
S content in the matrix. As a result, it is possible to improve the stress
relaxation
cracking resistance of the alloy. Therefore, fn3 is not less than 0.
[0075]
[Production method]
One example of production method of the NiCrFe alloy of the present
embodiment will be described. The production method of the present embodiment
comprises a process of producing an ingot (steelmaking process), and a process
of
producing a hot-rolled plate (hot working process). Hereinafter, each process
will
be described in detail.
[0076]
[Steelmaking process]
First, alloys having the above described chemical compositions are melted.
The melting is performed by using, for example, the high-frequency induction
vacuum melting. Next, an ingot is produced by an ingot-making method.
[0077]
[Hot working process]
CA 03039043 2019-04-01
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In the hot working process, normally, hot working is performed once or
multiple times. First, the ingot is heated, and thereafter hot working is
performed.
The hot working refers to, for example, hot forging and hot rolling. The hot
working may be performed by a well-known method.
[0078]
Further, the hot-worked NiCrFe alloy may be subjected to cold working.
The cold working is, for example, cold rolling.
[0079]
Further, the NiCrFe alloy, which has been subjected to the above described
working, may be subjected to heat treatment. The heat treatment temperature is
preferably 1050 to 1200 C. Further, after being heated and held, the NiCrFe
alloy
is preferably water cooled.
[0080]
In the above described exemplary production method, a production method of
a NiCrFe alloy plate has been described. However, the NiCrFe alloy May be a
bar
or an alloy pipe. In other words, the shape of the product will not be
limited.
Moreover, in the case of the alloy pipe, it is preferable that hot working by
hot
extrusion is performed.
[0081]
The NiCrFe alloy produced by the processes described so far has excellent
creep strength and excellent stress relaxation cracking resistance.
[0082]
[Microstructure]
In the NiCrFe alloy according to the present invention, y' and i phase
precipitate in a use environment at high temperatures. In other words, the
microstructure of the NiCrFe alloy according to the present invention after
being
kept at 650 C for 3000 hours contains a total of 2 to 6 mass% of y' and i
phase,
wherein the number density of ri phase is less than 5/100 m2. Note that they'
and
ii phase are herein also collectively referred to as "aging precipitates".
[0083]
In a case where the NiCrFe alloy according to the present invention is
subjected to aging treatment for keeping the alloy at 650 C for 3000 hours and
then a
CA 03039043 2019-04-01
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total of y' and ri phase is less than 2 mass%, the precipitation amount of y'
in the alloy
will be decreased. As a result, the NiCrFe alloy cannot obtain excellent creep
strength. On the other hand, in a case where the same aging treatment is
performed
and then a total of y' and ri phase is more than 6 mass%, the precipitation
amount of
y' may excessively increase. In that case, the alloy cannot obtain excellent
stress
relaxation cracking resistance. Therefore, the total of y' and rl phase after
aging
treatment is 2 to 6 mass%.
[0084]
Specifically, the total of y' and ri phase can be measured by the following
method. The NiCrFe alloy according to the present invention is subjected to
aging
treatment for keeping the alloy at 650 C for 3000 hours. A test specimen of 10
mm
x 5 mm x 50 mm is sampled from the NiCrFe alloy after the aging treatment.
When the alloy is an alloy plate, the test specimen is sampled from a middle
part of
plate thickness of the alloy pipe. On the other hand, when the alloy is an
alloy pipe,
the test specimen is sampled from a middle part of wall thickness. Note that
the
weight of the test specimen is measured in advance.
[0085]
The sampled test specimen is electrolyzed in a 1% tartaric acid-1%
(NH4)2504-water solution to sample the residue from the electrolyte. The
sampled
residue is melted by HC1 (1+4)-20% tartaric acid solution of 60 C and the
solution is
filtered. The filtrate is measured by ICP emission spectrometry to determine
Ti, Al,
and Ni concentrations in the residue. From the determined Ti, Al, and Ni
concentrations in the residue, and the weight of the test specimen, Ti, Al,
and Ni
contents in they' and i phase of the test specimen are determined. The sum of
Ti,
Al, and Ni contents, which have been determined by the method described so
far, is
defined as a sum of y' and ri phase (mass%).
[0086]
In a case where the NiCrFe alloy according to the present invention is
subjected to aging treatment for keeping the alloy at 650 C for 3000 hours and
then
the number density of ri phase is not less than 5/100 lArn2, part of y' has
changed to ii
phase. For that reason, the NiCrFe alloy cannot obtain excellent creep
strength.
CA 03039043 2019-04-01
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Therefore, the number density of ri phase after aging treatment is less than
5/100
2
VIII =
[0087]
Specifically, the number density of 11 phase can be measured by the following
method. The NiCrFe alloy according to the present invention is subjected to
aging
treatment for keeping the alloy at 650 C for 3000 hours. Microscopic
observation
is performed on the NiCrFe alloy after aging treatment. Specifically, a
microscopic
test specimen is sampled from the NiCrFe alloy after aging treatment. When the
alloy is an alloy plate, the test specimen is sampled from a middle part of
the plate
thickness. On the other hand, when the alloy is an alloy pipe, the microscopic
test
specimen is sampled from a middle part of wall thickness of the alloy pipe.
The
sampled microscopic test specimen is subjected to mechanical polishing. The
surface of the microscopic test specimen after mechanical polishing is
electrolytically corroded by 10% oxalic acid. The microscopic test specimen
after
electrolytic corrosion is observed by a scanning electron microscope (SEM) in
5
visual fields, and an SEM image is created for each visual field. The
observation
magnification is 10000 times, and observation field is, for example, 12 pitn x
9 tim.
[0088]
The 7 and ri phase differ in their shapes. Specifically, 7' is observed to be
spherical and 11 phase be acicular. More specifically, an aspect ratio of 7'
is less
than 3, and an aspect ratio of phase is not less than 3. Here, the term
"aspect
ratio" means a value obtained by dividing the major axis length by the minor
axis
length for each aging precipitate.
[0089]
In the above described SEM image of each visual field, aging precipitates (7'
and 11 phase) are identified from contrast. Further, by image processing,
aspect
ratios are calculated for the identified aging precipitates. To calculate an
aspect
ratios, general purpose application software may be used. When a calculated
aspect
ratio is not less than 3, the aging precipitate is identified to be ri phase.
[0090]
For an SEM image of each visual field, the number of identified r phase is
counted to determine a sum of the numbers in all visual fields. By using the
CA 03039043 2019-04-01
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number of ri phase in all visual fields and the area of the all visual fields,
the number
density of ri phase in an observation field of 100 vim2 (number/100 pm2) is
determined.
EXAMPLES
[0091]
Alloys having chemical compositions indicated by Reference marks 1 to 15
shown in Table 1 were melted by the high-frequency induction vacuum melting
method.
[0092]
[Table 1]
TABLE 1
Chemical composition (in mass%, with the balance being Fe and impurities)
REM
Reference mark
fnl fn2 fn3
C Si Mn P S Cr Ni Ti Al N 0 REM Others
element
1 0.06 0.41 0.85 0.013 0.0020 20.4 34.8 0.85 0.37 0.008 0.003 0.028 -
1.51 0.56 0.0000069 Nd
2 0.07 0.38 1.04 0.011 0.0006 23.5 35.0 1.35 0.36 0.012 0.002 0.015
B:0.002,Mo:0.5 1.99 0.68 0.0000021 Nd
_
3 0.07 0.25 0.98 0.014 0.0010 19.8 31.4 0.95 0.43 0.007 0.005 0.035
Ca:0.003 1.71 0.55 0.0000035 Nd
4 0.08 0.29 1.48 0.008 0.0003 19.3 29.5 0.84 0.55 0.006 0.002 0.019
W:1.2 1.82 0.46 0.0000392 Nd
0.07 0.25 0.78 0.011 0.0007 21.7 30.7 1.19 0.43 0.009 0.005 0.044
Mg:0.004,V:0.3 1.95 0.61 0.0000753 Nd
6 0.05 0.36 0.89 0.010 0.0006 23.6 33.0 0.64 0.35 0.009 0.004 0.031
Nb:0.3,Co:2.3 1.26 0.51 0.0000376 La
_
7 0.05 0.38 0.95 0.012 0.0010 20.5 27.3 0.72 0.45 0.003 0.003 0.030
Ta:0.1,Hf:0.4 1.52 0.47 0.0000580 Ce
_
P
8 0.08 0.22 0.99 0.015 0.0010 22.1 32.9 0.77 0.65 0.004 0.003 0.018
Cu:1.3,Co:2.1 1.93 0.40 0.0000460 Y 0,
Lo
_
.
9 0.08 0.55 1.45 0.013 0.0010 21.8 31.6 0.28 0.11 0.013 0.001 0.028
Co:2.5 0.48 0.59 0.0001215 Nd Lo
0.09 0.21 0.64 0.016 0.0010 21.8 29.8 1.33 0.70 0.008 0.002 0.032 Nb:0.3
2.57 0.52 0.0001076 Nd ..
Iv
L,
_
11 0.10 0.25 1.12 0.008 0.0008 21.3 34.5 0.45 0.84 0.008 0.005 0.042 -
1.94 0.23 0.0000583 Nd
1'
12 0.08 0.18 1.51 0.009 0.0004 19.5 29.9 0.65 0.57 0.008 0.002 0.024
W:1.5 1.66 0.39 0.0000708 Nd 0
..
,
13 0.10 0.33 0.88 0.011 0.0005 20.9 38.2 1.67 0.11 0.011 0.002 0.065
B:0.002,Cu:1.3 1.87 0.90 0.0003524 Nd
,
14 0.10 0.25 1.07 0.009 0.0010 19.9 32.8 0.88 0.41 0.009 0.005 0.026
B:0.003 1.61 0.55 -0.0000590 Nd
0.07 0.42 0.88 0.013 0.0010 20.8 34.5 0.88 0.36 0.007 0.003 - -
1.52 0.58 -0.0001563 -
CA 03039043 2019-04-01
- 25 -
[0093]
An ingot of 50 kg was produced by using an alloy of each Reference mark.
The ingot was subjected to hot forging and hot rolling to obtain a plate
material
having a thickness of 15 mm. Each plate material was kept at 1150 C for 30
minutes, and thereafter the plate material was rapidly cooled (water cooling)
and
subjected to solution treatment. By the production processes described so far,
NiCrFe alloy plate materials were produced. Using thus produced NiCrFe alloy
plate materials, the following tests were conducted.
[0094]
[Creep rupture test]
A test specimen was fabricated from the produced alloy plate material. The
test specimen was sampled from a central part of thickness of the alloy plate
material
in parallel with the longitudinal direction (rolling direction). The specimen
was a
round bar test specimen, whose parallel part had a diameter of 6 mm, and which
had
a gauge length of 30 mm. By using the test specimen, a creep rupture test was
conducted. The creep rupture test was performed under a tensile load of 70 MPa
in
the air atmosphere of 750 C. A test specimen whose rupture time was not less
than
3000 hours was evaluated as "E" (Excellent), and those whose rupture time was
less
than 3000 hours as "NA" (Not Acceptable).
[0095]
[Table 2]
TABLE 2
Sum of Stress
Creep
Reference Microstructure y' and r relaxation
rupture
mark evaluation phase test cracking
(mass%) test
1 E 3.0
2 E 5.2
3 E 3.7
4 E 3.3
E 5.0
6 E 3.4
7 E 2.9
8 E 2.8
9 L 0.0 NA
TM 7.6 E NA
CA 03039043 2019-04-01
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11 L 0.8 NA
12 L 1.9 NA
13 TM, 7.0 NA
14 E 3.6 E NA
15 E 3.8 E NA
[0096]
[Microstructure observation]
From thus produced alloy plate materials, test specimens were fabricated by
the above described method. The fabricated test specimens were subjected to
aging
treatment to keep them at 650 C for 3000 hours, and the sum (mass%) of y' and
phase of each test specimen was determined by the above described method.
Further, the number density of ri phase (number/100 pim2) was determined by
the
above described method. A sum of y' and ri phase of less than 2 mass% was
evaluated as "L" (Less), that of 2 to 6 mass% as "E" (Excellent), and that of
more
than 6 mass% as "TM" (Too Much). Further, those showed a number density of
phase of not less than 5/100 [tm2 were evaluated as "1".
[0097]
[Stress relaxation cracking test]
The produced alloy plate material was further subjected to cold working.
Specifically, cold rolling was performed on the alloy plate material until its
thickness
became 12 mm. The reduction of area of this cold rolling was 20%. A test
specimen was fabricated from this alloy plate material. The test specimen was
sampled from a central part of thickness of the alloy plate material in
parallel with
the longitudinal direction (rolling direction). The specimen was a round bar
test
specimen, whose parallel part had a diameter of 6 mm, and which had a gauge
length
of 30 mm. By using the specimen, a stress relaxation cracking test was
conducted.
The stress relaxation cracking test was conducted such that the test specimen
is
subjected to tensile strain 10% at a strain rate of 0.05 min' and is kept as
is for 300
hours in air atmosphere of 650 C. A specimen which did not rupture after being
kept for 300 hours was evaluated as "E" (Excellent), and one which ruptured as
"NA" (Not Acceptable).
[0098]
CA 03039043 2019-04-01
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[Test results]
Test results are shown in Table 2.
[0099]
Referring to Table 2, the chemical compositions of Reference marks 1 to 8
were appropriate, so that fnl was 0.50 to 2.20, fn2 was 0.40 to 0.80, and fn3
was not
less than 0. For that reason, in the microstructure, the sum of y' and ri
phase was 2
to 6 mass%. Further, the number density of ti phase was less than 5/100 pim2.
As
a result, the creep rupture time was not less than 3000 hours, thus exhibiting
excellent creep strength. Further, none of the specimens ruptured in the
stress
relaxation cracking test, exhibiting excellent stress relaxation cracking
resistance.
[0100]
On the other hand, in Reference mark 9, the value of fnl was too low. For
that reason, in the microstructure, the sum of y' and ri phase was less than 2
mass%,
which was too low. As a result, the creep rupture time was less than 3000
hours,
not exhibiting excellent creep strength.
[0101]
In Reference mark 10, the value of fnl was too high. For that reason, in the
microstructure, the sum of y' and ri phase was more than 6 mass%. Further, the
number density of1 phase was less than 5/100 pm2. In other words, in the
microstructure, y' was more than 6 mass%, which was too high. As a result, the
test
specimen ruptured in the stress relaxation cracking test, thus not exhibiting
excellent
stress relaxation cracking resistance.
[0102]
In Reference marks 11 and 12, the value of fn2 was too low. For that reason,
in the microstructure, the sum of y' and ri phase was less than 2 mass%, which
was
too low. As a result, the creep rupture time was less than 3000 hours, not
exhibiting
excellent creep strength.
[0103]
In Reference mark 13, the value of fn2 was too high. For that reason, in the
microstructure, the number density of i phase was not less than 5/100 Rm2. As
a
result, the creep rupture time was less than 3000 hours, not exhibiting
excellent creep
strength.
CA 03039043 2019-04-01
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[0104]
In Reference mark 14, the value of fn3 was too low. As a result, the
specimen ruptured in the stress relaxation cracking test, thus not exhibiting
excellent
stress relaxation cracking resistance. This is considered because S in the
matrix
could not be immobilized.
[0105]
In Reference mark 15, the REM content was too low. Further the value of
fn3 was too low. As a result, the test specimen ruptured in the stress
relaxation
cracking test, not exhibiting excellent stress relaxation cracking resistance.
This is
considered because S in the matrix could not be immobilized.
[0106]
So far, the embodiments of the present invention have been described.
However, the above described embodiments are merely illustration for
practicing the
present invention. Therefore, the present invention will not be limited to the
above
described embodiments, and can be practiced by appropriately modifying the
above
described embodiments within a range not departing from the spirit of the
invention.
INDUSTRIAL APPLICABILITY
[0107]
The present invention can be widely applied to uses for which high creep
strength and stress relaxation cracking resistance are demanded. Particularly,
the
present invention can be suitably used for high temperature members of thermal
power generation boilers, petroleum refining and chemical industry plants, or
the like.
