Note: Descriptions are shown in the official language in which they were submitted.
CA 02520700 2005-09-22
Specification
A method of producing a Ni based alloy
Technical Field
The present invention relates to a method of producing
a Ni-based alloy, which elutes little Ni even when used in
a high-temperature water environment for a long period, and
particularly relates to a method of producing the Ni-based
alloy suitable for use in a member for a nuclear power plant.
Background Art
A Ni-based alloy is used for various kinds of members
due to its superior mechanical properties. Particularly, for
the member in a nuclear reactor, which is exposed to
high-temperature water, the Ni-based alloy superior in
corrosion resistance is used. For instance, for a steam
generator of a pressurized water reactor (PWR), a
60gNi-30~Cr-lO~Fe alloy is used.
These members are used in the environment of
high-temperature water on the order of 300°C in a nuclear
reactor for several years to several tens of years . A Ni-based
alloy is superior in corrosion resistance and has a low
corrosion rate. However, when used for a long period, a very
small amount of Ni elutes from the alloy.
Eluted Ni is carried to a reactor core part along with
circulating furnace water and is irradiated with neutrons in
the proximity of the fuel. Then, the irradiated Ni with
CA 02520700 2005-09-22
neutrons is converted to radioactive Co through a nuclear
reaction. Radioactive Co has a very long half-life, and
incessantly emits radioactive rays for a long term. Therefore,
as the elution amount of Ni increases, the exposed dose of
an operator conducting a periodic inspection increases.
The reduction of the exposed dose is a very important
subject for using a light water reactor for a long period.
Accordingly, the countermeasures have been taken until now
in order to prevent Ni from eluting from a Ni-based alloy,
through improving the corrosion resistance of materials and
controlling the water quality of the nuclear reactor water.
The Patent Document 1 discloses a method for improving
uniform corrosion resistance of a heat exchanger tube made
of a Ni-based alloy, by annealing it in an atmosphere having
the degree of vacuum of 10-z to 10-4 Torr at 400 to 750°C, and
forming an oxide film mainly containing chromium oxide on the
surface of it.
The Patent Document 2 discloses a method of manufacturing
a member for a nuclear power plant by solution-treating a
Ni-based precipitation-strengthened alloy, and then
heat-treating it in an oxidizing atmosphere of air with 10-3
Torr to ambient pressure, while combining the treatment with
at least one part of aging treatment and oxide film-forming
treatment.
The Patent Document 3 discloses a method of producing
a Ni-based alloy product through heat-treating a Ni-based alloy
product in an atmosphere of hydrogen or a mixed gas of hydrogen
with argon, having a dew point of -60 to +20°C.
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The Patent Document 4 discloses a method for forming a
chromium enriched layer on an alloy workpiece containing Ni
and Cr by exposing the work-piece with a gaseous mixture
consisting of water vapor and at least one of non-oxidizing
gases.
Patent Document 1: JP S64-55366A
Patent Document 2: JP H08-29571A
Patent Document 3: JP 2002-121630A
Patent Document 4: JP 2002-322553A
Disclosure of the Invention
Subject to be Solved by the Invention
The film formed by the method disclosed in the Patent
Document 1 has such insufficient thickness that it tends to
be damaged during service for a long period, and may lose the
effect of preventing the elution.
The method disclosed in the Patent Document 2 has such
a problem that oxidized Ni is easily taken into a film and
the Ni elutes during use.
Methods for forming an oxide film by controlling the amount
of water vapor (a dew point) such as the methods disclosed
in the Patent Documents 3 and 4 have difficulty in forming
the oxide film consistent from the inlet side to the outlet
side of water vapor. This is for the following reason.
In the case of continuous treatment for forming an oxide
film in a long tube, for instance, the growth rate of the oxide
film is limited not only by oxygen potential but also by the
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diffusibility of an oxidizing gas to the surface of a material
to be treated through a concentration boundary layer . Here,
the concentration boundary layer means a boundary layer having
a concentration gradient of a gas, from the surface of the
material to a portion apart from the surface ( for instance,
the vicinity of the medial axis inside of the tube). The
diffusibility is affected by physical properties such as
diffusion coefficient and coefficient of kinematic viscosity
of a gas, and oxidation treatment conditions such as the
concentration and flow rate of the gas. Water vapor (Hzo)
has the higher diffusibility than other oxidizing gases such
as C02 have, hence it is hard to form the oxide film consistent
from the inlet side to outlet side on the tube by the oxidation
treatment in a water vapor atmosphere.
The present invention was accomplished for the purpose
of solving these problems, and is directed at providing a method
of producing a Ni-based alloy having a uniform oxide film
comprising chromium oxide inexpensively formed on thesurface.
Method to Solve the Subject
The present invention is summarized into the method of
producing a Ni-based alloy described in the following items
(1) to (14).
( 1 ) A method of producing a Ni based alloy having an oxide
film comprising chromium oxide on its surface,
characterized by subjecting the alloy to a heat treatment
in an atmosphere consisting of carbon dioxide gas or in
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an atmosphere consisting of 0 . 0001 Vol . % or more carbon
dioxide gas and 99 . 9999 vol. % or less non-oxidizing gas .
( 2 ) A method of producing a Ni based alloy according to
(1), characterized in that the atmosphere consists of
carbon dioxide gas and at least one of hydrogen gas and
rare gas.
( 3 ) A method of producing a Ni based alloy according to ( 1 ) ,
characterized in that the atmosphere consists of carbon
dioxide gas and hydrogen gas.
( 4 ) A method of producing a Ni based alloy according to any
one of ( 1 ) to ( 3 ) , characterized in that the atmosphere
contains 5 vol.$ or less oxygen gas.
( 5 ) A method of producing a Ni based alloy according to any
one of ( 1 ) to ( 4 ) , characterized in that the atmosphere
contains 50 vol.% or less carbon dioxide gas.
( 6 ) A method of producing a Ni based alloy according to any
one of ( 1 ) to ( 4 j , characterized in that the atmosphere
contains 10 vol.% or less carbon dioxide gas.
( 7 ) A method of producing a Ni based alloy according to any
one of ( 1 ) to ( 6 ) , characterized in that the temperature
of the heat treatment ranges from 500°C to 1,250°C.
( 8 ) A method of producing a Ni based alloy according to any
one of ( 1 ) to ( 7 ) , characterized in that the period of
the heat treatment ranges from 10 seconds to 35 hours.
( 9 j A method of producing a Ni based alloy according to any
one of (1j to (8), characterized in that the Ni based
alloy consists of, by mass %, C:0.15% or less, Si:1.00%
or less, Mn:2.0% or less, P:0.030% or less, S:0.030% or
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less, Cr:10.0-40.0%, Fe:15.0% or less, Ti:0.5% or less,
Cu : 0 . 50% or less, A1:2 . 00% or less, and the balance Ni
and impurities.
( 10 ) A method of producing a Ni based alloy according to any
one of ( 1 ) to ( 8 ) , wherein the Ni based alloy consists
of, by mass %, C:O. 15% or less, Si: 1.00% or less, Mn:2.0%
or less, P: 0 . 030% or less, S :0 .030% or less, Cr:14 .0-17 . 0%,
Fe : 6 . 0-10. 0%, Ti:O .5~ or less, Cu : 0 .50% or less, A1:2 . 00%
or less, and the balance Ni and impurities.
( 11 ) A method of producing a Ni based alloy according to any
one of (1) to (8), characterized in that the Ni based
alloy consists of, by mass %, C:0.06% or less, Si: 1.00%
or less, Mn:2.0% or less, P:0.030% or less, S:0.030% or
less, Cr:27.0-31.0%, Fe:7.0-11.0%, Ti:0.5% or less,
Cu : 0 . 50% or less, A1: 2 . 00% or less, and the balance Ni
and impurities.
( 12 ) A method of producing a Ni based alloy according to any
one of ( 9 ) to ( 11 ) , characterized in that the Ni based
alloy contains, by mass %, Nb and/or Ta: 3.15-4.15% each
element alone or in total in lieu of part of Ni.
( 13 ) A method of producing a Ni based alloy according to any
one of ( 9 ) to ( 12 ) , characterized in that the Ni based
alloy contains, by mass %, Mo: 8-10% in lieu of part of
Ni.
( 14 ) A method of producing a Ni based alloy according to any
one of ( 1 ) to ( 13 ) , characterized in that the Ni based
alloy is preferably used for a material of a member for
a nuclear power plant.
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Here, "oxide film comprising chromium oxides" means an
oxide film mainly containing Cr203, but may contain oxides
other than Cr203, such as MnCr204, Ti02, A1203 and SiOz . In
addition, so far as the Ni-based alloy has the oxide film
comprising chromium oxides on the surface, the alloy may have
other oxides layer formed as the upper layer ( the outside layer )
and/or the lower layer ( the inside layer ) of the chromium oxide
layer.
Advantageous Effect of the Invention
The method of producing Ni based alloy according to the
present invention can form chromium oxide on the surface of
a Ni-based alloy inexpensively and uniformly. Therefore,
manufactured Ni-based alloy elutes very little Ni even when
used in high-temperature water such as in a nuclear power plant
for a long time. Accordingly, the Ni-based alloy is most
suitable for a member for a nuclear power plant, such as steam
generator tubing, and a spacer spring, a coil spring, a finger
spring, a channel fastener and a nozzle stub for a lid used
in high-temperature water.
Best Mode for Carrying Out the Invention
1. Atmosphere for heat treatment
In a method of producing a Ni-based alloy according to
the present invention, a Ni-based alloy is subjected to a heat
treatment in the atmosphere consisting of carbon dioxide gas
or the atmosphere consisting of 0 . 0001 vol . ~ or more carbon
dioxide gas and 99.9999 vol.~ or less non-oxidizing gas in
CA 02520700 2005-09-22
order to form an oxide film composed of chromium oxide on the
surface of the alloy. In other words, the present invention
is characterized by a heating atmosphere containing carbon
dioxides of 0.0001 vol.~ or more for forming an oxide film
comprising chromium oxide on the surface of the Ni-based alloy
through the oxidative effect. When the atmosphere contains
less than 0.0001 vol.~ carbon dioxide, the oxide film
comprising chromium oxide may be insufficiently formed.
There is no particular upper limit on the concentration of
carbon dioxide contained in an atmosphere for heat treatment
and it could be 100 vol.~. However, from the viewpoint of
manufacturing cost reduction, it is preferable that
non-oxidizing gas, to be described in later paragraphs, is
added in an atmosphere for heat treatment in order to set the
concentration of carbon dioxide to be 50 vol . ~ or less, and
more preferably 10 vol.~ or less.
Carbon dioxide gas in a high-temperature atmosphere has
an effect of forming an oxide film comprising chromium oxide
on the surface of a Ni-based alloy. More specifically, in
an atmosphere comprising carbon dioxides, as shown in the
following reaction formula, COz adsorbs to a Ni-based alloy,
and then a Ni-based alloy directly takes O ( oxygen ) therein
from COz to form chromium oxide.
COz+Metal --~ CO + Metal-O
As described above, though the Patent Documents 3 and
4 disclose a method for forming an oxide film by heating a
Ni alloy under a water vapor atmosphere, the method has
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difficulty in forming an oxide film consistent from the inlet
side to the outlet side of water vapor.
However, because the diffusibility of carbon dioxide is
lower than that of water vapor, the thickness of a formed oxide
film is hardly affected by oxidation conditions such as the
concentration and flow rate of a supplied gas. As a result
of this, oxidation treatment in the carbon dioxide atmosphere
can form a more consistent oxide film on the surface of an
alloy than that in a conventional atmosphere of water vapor.
Amerit of using carbon dioxide gas includes that it can produce
a desired oxidation atmosphere more inexpensively than a method
of controlling the concentration of water with a conventional
dew point controller.
An atmosphere for heat treatment may contain 99 . 9999 vol. ~
or less non-oxidizing gases, that do not promote Cr oxide,
in addition to carbon dioxide gas . Such gases involve hydrogen
gas, rare gas (Ar, He and so on) , carbon monoxide gas, nitrogen
gas and hydrocarbon gas . Among these gases, carbon monoxide
gas, nitrogen gas and hydrocarbon gas may cause
carburisation or nitriding when they are contained in an
atmosphere for heat treatment. Therefore, it is preferable
to use at least either or both of hydrogen gas and rare gas.
By adjusting the concentration of the non-oxidizing gas,
the concentration of the carbon dioxide gas can be
appropriately controlled.
An atmosphere for heat treatment may contain 5 vol . ~ or
less oxygen gas that cause the oxidation of a Ni-based alloy
in addition to carbon dioxide gas, or dioxide gas and
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non-oxidizing gas. However, from the viewpoint of safety,
it is preferable to avoid an atmosphere for heat treatment
wherein both hydrogen gas and oxygen gas are contained so that
these gases do not react with each other to cause an explosion.
Hydrogen gas is industrially often used as an atmospheric
gas for heat treatment, so that the use of it for diluting
carbon dioxides can reduce a manufacturing cost. Therefore,
it is most preferable to perform heat treatment under the
atmosphere of the mixed gas consisting of carbon dioxides and
hydrogen gas.
2. Heating temperature and heating time
Heating temperature: 500 to 1,250°C
A heating temperature has only to be in a range for
producing the appropriate thickness and composition of an oxide
film on a surface of the alloy being heated, and imparting
the appropriate strength to the alloy itself . When the heating
temperature is lower than 500°C, the oxidation of chromium
can be insufficient, but when exceeding 1, 250°C, the strength
of the Ni-based alloy may not be ensured. Consequently, the
heating temperature is preferably in a range of 500 to 1, 250°C .
Heating time: 10 seconds to 35 hours
A heating time has only to be set into a range capable
of giving an alloy the appropriate thickness and composition
of an oxide film. More specifically, the allay is preferably
heated for 10 seconds or longer in order to form the oxide
film mainly containing chromium oxide, but the oxide film does
not grow any more by heating for longer than 35 hours.
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Consequently, the heating time is preferably in a range of
seconds to 35 hours.
The higher is the heating temperature, the shorter is
the heating time. Accordingly, when a heating temperature
is set in a range from 1, 000 to 1, 200°C for instance, a heating
time may be in a range of 10 seconds to 60 minutes.
As described above, by appropriately adjusting the
conditions of a heating temperature, a heating time and a gas
concentration, the thickness and composition of an oxide film
can be adjusted.
3. Ni-based alloy to be treated
An example of the Ni-based alloy used in a producing method
of the present invention comprises, by mass%, 0.15% or less
C, 1.00% or less Si, 2.0% or less Mn, 0.030% or less P, 0.030%
or less S, 10.0-40.0% Cr, 15.0% or less Fe, 0.5% or less Ti,
0.50% or less Cu, 2.00% or less A1 and the balance Ni and
impurities. Reasonsfor limiting each element are as follows.
In addition, "%" on content means "mass%" in the following
discussion.
C: 0.15% or less
C content exceeding 0 .15% may cause the lowering of stress
corrosion cracking resistance. Accordingly, when C is added,
the content is preferably controlled to 0.15% or less, and
further preferably to 0.06% or less. C has an effect of
increasing the strength of grain boundaries in an alloy. In
order to acquire the effect, the content of C is preferably
0.01% or more.
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Si: 1.00 or less
Si is used as a deoxidizing material in refining process
and remains as an impurity in an alloy. The content needs
to be limited to 1.00 or less. When Si content exceeds
0.50, the cleanliness factor of the alloy can be decreased.
Accordingly, Si content is preferably limited to 0.50 or
less.
Mn: 2.0% or less
Mn content exceeding 2 . 0~ lowers the corrosion resistance
of an alloy. Accordingly, the content is preferably controlled
to 2 . 0 0 or less . Mn has a lower free energy of formation for
the oxide than Cr, hence, Mn is precipitated as MnCr204 by
heating. In addition, CrZ03 normally forms in the vicinity
of a base metal by heating with precedence, and MnCr2o4 forms
as an upper layer on the outside of it because Mn has
comparatively high rate of dif fusion . I f MnCrz04 layer exists,
it protects a Cr203 layer in a use environment, and even when
the Cr203 layer is disrupted by some reason, MnCr204 promotes
the restoration of the Cr2o3 layer. Such an effect becomes
remarkable when Mn content is more than 0 .1% . Consequently,
desirable Mn content is 0.1 to 2.0%, and further desirably
is 0.1 to 1.0~.
P: 0.030°s or less
P is an element existing as an impurity in an alloy. P
content exceeding 0.030 may exert an adverse effect on
corrosion resistance. Accordingly, P content is preferably
limited to 0.030 or less.
S: 0.030 or less
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S is an element existing as an impurity in the alloy.
When the content exceeds 0.030, S may exert adverse effect
on corrosion resistance. Accordingly, S content is
preferably limited to 0.030 or less.
Cr: 10.0 to 40.0
Cr is a necessary element for forming an oxide film
comprising chromium oxide. In order to form such an oxide
film on the surface of the alloy, the content is preferably
10.0 or more. However, Cr content exceeding 40.0 may
deteriorate the corrosion resistance of the alloy, because
Ni content becomes relatively low. Accordingly, Cr content
is preferably 10.0 to 40.0%. In particular, when Cr content
is l4.Og to 17 .0~, the alloy has superior corrosion resistance
in a chloride-containing environment, and when Cr content is
27.Oo to 31.0, the corrosion resistance not only in a
chloride-containing environment, but also in
high-temperature pure water and an alkaline environment
becomes superior.
Fe: 15.0 or less
When Fe content is more than 15.0, it may impair the
corrosion resistance of the Ni-based alloy, and therefore Fe
content should be set at 15.0 or less. In addition, Fe is
an element that is dissolved in Ni and is usable as a substitute
for a part of expensive Ni, so that 4 .0~ or more Fe is desirably
contained. The content of Fe can be decided in terms of the
balance between Ni and Cr. When Cr content is 14.0 to 17.0$,
preferable Fe content is 6.0 to 10.0$, and when Cr content
is 27.0 to 31.0, preferable Fe content is 7.0 to 11Ø
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Ti: 0.5°s or less
Ti content exceeding 0.5~ may reduce cleanliness of an
alloy. Accordingly, the content is desirably controlled to
be 0 .5~ or less, and further desirably to 0 . 4~ or less . However,
from the viewpoint of improving the workability of the alloy
and inhibiting grain growth in welding, 0.1~ or more Ti is
preferably contained.
Cu: 0.50$ or less
Cu is an element existing as an impurity in the alloy.
If the content exceeds 0.50, the corrosion resistance of the
alloy can be lowered. Accordingly, Cu content is desirably
limited to 0.50 or less.
A1: 2.00 or less
A1 is used as a deoxidizing material in refining process
and remains as an impurity in an alloy. Remaining A1 forms
an oxide-based inclusion in the alloy, reduces the cleanliness
of the alloy, and may exert an adverse effect on the corrosion
resistance and mechanical properties of the alloy.
Accordingly, A1 content is desirably limited to 2.00 or less.
The above-described Ni-based alloy has only to include
the above-described elements and the ba lance Ni and impurities ,
however one or more elements among Nb, Ta and Mo may be added
in appropriate amount in order to improve characteristics such
as corrosion resistance and strength.
Nb and/or Ta : 3 .15-4 .15~ each element alone or in total
Nb and Ta are effective for improving the strength of
the alloy, because they easily form carbides . In addition,
they have an effect of fixing C in the alloy, hence they inhibit
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a shortage of Cr in grain boundaries and improve the corrosion
resistance of grain boundaries . Accordingly, one or both of
these elements are desired to be contained . The effect becomes
remarkable, when the content of either element is 3.15% or
more in the case of the alloy containing either one, or when
the total content of these is 3.15% or more in the case of
the alloy containing both. However, the excessive content
of Nb and/or Ta may impair hot workability and cold workability,
and may increase susceptibility to heating embrittlement.
Accordingly, the content of either element in the case of the
alloy containing either one, or the total content of these
in the case of the alloy containing both, is preferably
controlled to 4.15% or less. Consequently, the content of
one element or both elements of Nb and Ta is desirably controlled
to 3.15 to 4.15%.
Mo: 8-10%
Mo iseffective in improving pitting corrosion resistance,
so that it may be contained as needed. The above described
effect becomes remarkable when the content is 8% or more, but
when it exceeds 10%, intermetallic compounds precipitate and
may lower corrosion resistance. Accordingly, the content
of Mo, when added, is desirably controlled to 8 to 10%.
The above described Ni-based alloys are typically two
kinds described below.
(a) A Ni-based alloy comprising 0.15% or less C, 1.00%
or less Si, 2.0% or less Mn, 0.030% or less P, 0.030% or less
S, 14.0-17 .0% Cr, 6 .0-10.0% Fe, 0.5% or less Ti, 0.50 % or less
Cu, 2.00% or less Al, and the balance Ni and impurities.
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(b) A Ni-based alloy comprising 0.06 or less C, 1.00$
or less Si, 2.0% or less Mn, 0.030$ or less P, 0.030$ or less
S, 27.0-31.0 Cr, 7.0-11.0 Fe, 0.5~ or less Ti, 0.50 or less
Cu, 2.00 or less Al, and the balance Ni and impurities.
The alloy ( a ) includes Cr of 14 . 0 to 17 . 0~ and Ni of about
75~ and has superior corrosion resistance in a
chloride-containing environment. In the alloy, Fe content
is desirably controlled to 6.0 to 10.0, from the viewpoint
of a balance between the contents of Ni and Cr.
The alloy (b) includes 27.0 to 31.0 Cr and about 60~
Ni, therefore, it has superior corrosion resistance not only
in a chloride-containing environment, but also in
high-temperature pure water and an alkaline environment. In
this alloy as well, Fe content is desirably controlled to
7 .0-11.Og from the viewpoint of a balance between the contents
of Ni and Cr.
Embodiments
A tube was manufactured into the dimension with a diameter
of 20 mm a wall thickness of 1.5 mm and a length of 20 m with
the use of an alloy-A shown in Table 1, and a tube was
manufactured into the dimension with a diameter of 20 mm, a
wall thickness of 1.5 mm and a length of 10 m with the use
of alloys B to G shown in Table 1. Then, the tubes were
continuously heat-treated under the conditions shown in table
2.
The film compositions of both ends cut out from the tube
after heat treatment were examined with EDX ( Energy Dispersive
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,.
X-ray micro-analyzer), and the result proved that an oxide
film comprising chromium oxide was formed on any tube. The
thickness of an oxide film is measured by observing the cross
section with a scanning electron microscope (SEM; Scanning
Electron Microscope) , and the dispersion of the thickness was
evaluated as I tin-tout ~ /t;,n, where t;,n is the oxide film thickness
in the upstream side of a gas flow and to"t is the oxide film
thickness in the downstream side. The evaluation results are
shown in Table 2 as "O" when dispersions are 1.00 or under,
or as "X" when dispersions are over 1.00.
Table 1
Chemical
Composition
(mass
!6)
the
bafanca
Ni
and
impurfies
Alloy
C Si Mn P S Cr Fe Ti Gu AI Othars
A 0.0190.320,310.0110.00129.89.10.210.010.15
B 0.0220.330.280.012O.OOi16.28,90.230.180.13
C 0.0190.380.27O.OiO.OOi20.54.70.240.050.15Nb:3_5
2
D 0.0200.400.230.0150.00120.74.50.220.030.18Ta:3.7
E 0.0180.370.250.0130.00120.64.70.220.030.19Nh:3.3 Ta:0.3
F 0.0190.380.280.0110.00120.84.60.260.070.13Mo:8.5
G 0.0200.330.290.0130.00120.64.90.210.090.11Nb:3.3 Ta:0.3
Mo:8.7
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Table 2
Gas Heatingbleating
Content
ih
the
atmosphere
of
tha
Heat
Treatment
~Vol.%)
No. AlloyOxidizing Non-oxidizing temp.time evaluation
Gas Gas
COZ 02 Hz0 Hz Ar He (C)
1 A 0.3000- - 99.7000- - 1100300s O
2 A 0.1500- - 99.8500- - 1100300s O
3 A 0.1000- - 99.9000- - 1100300s O
4 A 100.0000- - - - - 1100300s O
5 A 98.9000- - 1.1000- - 1100300s O
6 A 50.0000- - 50.0000- - 1100300s O
7 A 10.OOOD- - 90.0000- - 1100300s O
8 A 0.1000- - - 99.9000- 1100300s O
9 A 0.1000- - - -r 99.900D1100300s O
0 10A 0, - - 98.80001.1000- 1100300s O
i
000
11A 0.1000- - 97.70001.10001.10001100300s O
c
+~ i A 0.1000- - 99.9000-- - 650 15h O
2
C
N
m 13A 98.90001.1000- - - - 1100300s O
a
14A 49.95000.0500- - 50.0000- 1100300s O
0 15B 100.0000- - - - - 1100300s O
16B 0.1000- - 99.9000--- - 1100300s O
x
17B 0.1000- - - 99.9000- 1100300s O
18B 0.1000- - 98.80001.1000- 1100300s O
198 0.1000- -- 99.9000- - 650 15h O
20B 98.80001.1000- 0.1000- - 1100300s O
21B 49.95000.0500- 0, 49.9000- 1100300s O
i
000
22C 0.1000- - 99.9000- - i 300s O
100
23D 0.1000- - 99.9000- - 1100300s O
24E 0.1000- - 99.9000- - 1100300s O
25F 0.1000- --- 99.9000- - 1100300s O
26G 0.10D0- - 99.9000- - 1100300s O
27A - - 0.861099.1390- - 1100300s x
A 2BA - - 0.861099.1390- - 1100300s x
E
0
29A - - 0.861098.03901.1000- 1100300s x
"s° means ~~seconds" and "h° means °hours" in the section
of the heating time.
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As shown in Table 2, oxide films formed in the conditions
shown in No. 1 and 2 with the use of COZ as an oxidizing gas,
showed as small dispersions as 0.05 and 0. 17, whereas the oxide
film formed in a condition shown in No. 27 with the use of
Hz0 showed the dispersion of 3.00, which was quite large
compared to the case with the use of COZ . Other examples treated
by the producing method of the present invention showed the
dispersions evaluated as o, whereas examples treated by a
comparative method using H20 showed large dispersions.
Industrial Applicability
The method according to the present invention can
inexpensively form uniform chromium oxide on the surface of
a Ni-based alloy, Therefore, it can manufacture a Ni-based
alloy which elutes extremely little Ni even when used in a
high-temperature water environment such as in a nuclear power
plant, for a long time. Accordingly, the Ni-based alloy is
most suitable for members of a nuclear power plant, such as
steam generator tubing, and a spacer spring, a coil spring,
a finger spring, a channel fastener and a nozzle stub for a
lid used in high-temperature water.
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