Note: Descriptions are shown in the official language in which they were submitted.
CAST PRODUCT HAVING ALUMINA BARRIER LAYER
This application is a divisional of Canadian application serial No. 2940179
filed on December 17, 2014.
BACKGROUND OF THE INVENTION
Field of the Invention
[00011 The present invention relates to a cast product having an alumina
barrier layer, and more specifically, to a cast product having an alumina
barrier layer with a stable structure.
Description of the Related Art
[00021 Heat-resistant steel castings such as reaction tubes and
decomposition tubes for producing ethylene, and hearth rolls, radiant tubes
and metal dusting-resistant materials for use in carburizing heat-treatment
furnaces are exposed to a high-temperature atmosphere, and therefore are
made of an austenite-based heat-resistant alloy having superior high-
temperature strength.
[00031 A metal oxide layer is formed on the surface of this type of austenite-
based heat-resistant alloy during use in a high-temperature atmosphere.
This oxide layer serves as a barrier, and thus protects the base material
under a high-temperature atmosphere.
[00041 On the other hand, when Cr-oxides (mainly constituted by Cr2O3) are
formed as the metal oxide, the Cr-oxide layer has an insufficient function for
preventing the entry of oxygen and carbon due to its low denseness, thus
causing the internal oxidation under a high-temperature atmosphere and the
thickening of the oxide film. Moreover, the Cr-oxide layer is likely to become
detached during repeated cycles of heating and cooling. Even if the Cr-oxide
layer do not become detached, the Cr-oxide layer has an insufficient function
for preventing the entry of oxygen and carbon from an outside atmosphere,
and therefore, there is a disadvantageous situation in which oxygen and
1
Date Recue/Date Received 2022-07-12
carbon pass through the film and cause the internal oxidation or
carburization of the base material.
[00051 To address this, it is proposed that an oxide layer including alumina
(A1203) as a main component that has high denseness and makes it difficult
for oxygen and carbon to pass therethrough is formed on the surface of the
base material by increasing the content of Al compared with that in a
common austenite-based heat-resistant alloy (see Patent Documents 1 and 2,
for example).
[00061 However, Al is a ferrite-forming element, and therefore, when the
content of Al is increased, the ductility of the materials is deteriorated and
the high-temperature strength is reduced. This tendency of reduction of the
ductility is observed particularly when the content of Al exceeds 5%
For this reason, the austenite-based heat-resistant alloy of Patent
Documents 1 and 2 can be expected to have an enhanced barrier function due
to A1203, but has the disadvantage of causing a reduction of the ductility of
the base material.
[00071 Therefore, in order to provide a cast product that can secure the high-
temperature stability of A1203 and can achieve a superior barrier function
under a high-temperature atmosphere without reducing the ductility of the
materials, Patent Document 3 proposes a cast product in which an alumina
barrier layer including A1203 is formed on the inner surface of a cast body
and
Cr-based particles that contain Cr at a higher concentration than that of a
matrix of the base material are dispersed at an interface between the
alumina barrier layer and the cast body by performing heat treatment under
an oxidizing atmosphere after processing the inner surface such that a
surface roughness (Ra) of the cast body is 0.05 to 2.5 pm (see Patent
Document 3, for example).
[00081 Due to the presence of a stable alumina barrier layer, superior
oxidation resistance, carburization resistance, nitriding resistance,
corrosion
resistance, and the like of the cast product of Patent document 3 can be
2
Date Recue/Date Received 2022-07-12
maintained for a long period of time of use under a high-temperature
atmosphere.
PRIOR ART DOCUMENTS
PATENT DOCUMENTS
[00091
Patent Document 1: JP 552-78612A
Patent Document 2: JP 557-39159A
Patent Document 3: WO 2010/113830
[00101 It is an object of the present invention to provide a cast product that
can further enhance the stability of the alumina barrier layer and can exhibit
further superior oxidation resistance, carburization resistance, nitriding
resistance, corrosion resistance, and the like when used under a high-
temperature atmosphere.
SUMMARY OF THE INVENTION
[00111 The cast product according to the present invention is a cast product
having an alumina barrier layer including an aluminum oxide on a surface of
a matrix, wherein the aluminum oxide is (Al(i-x)M())203, where M is at least
one of Cr, Ni, Si, and Fe, and x satisfies a relationship 0<x<0.5.
[00121 Also, the cast product according to the present invention is a cast
product having an alumina barrier layer including an aluminum oxide on a
surface of a matrix, wherein at least one of Cr, Ni, Si, and Fe forms a solid
solution in the aluminum oxide, and the at least one of Cr, Ni, Si, and Fe
forming the solid solution with Al is contained so as to satisfy a
relationship
Al/(Cr+Ni+Si+Fe)>2.0 in an atomic % ratio.
EFFECTS OF THE INVENTION
[00131 With the cast product of the present invention, at least one of Cr, Ni,
Si, and Fe forms a solid solution in an alumina barrier layer formed on a
3
Date Recue/Date Received 2022-07-12
surface of a matrix, thus enabling an aluminum oxide phase to have a stable
structure. With the aluminum oxide, it is possible to suppress the coupling
between the matrix and oxygen and the formation of oxides containing Cr, Ni,
Si, Fe and the like as a main component on the surface of the matrix.
[00141 This makes it possible that the cast product of the present invention
exhibits further superior oxidation resistance, carburization resistance,
nitriding resistance, corrosion resistance, and the like when used under a
high-temperature atmosphere.
[00151 Accordingly, when the cast product of the present invention is used
for a reaction tube for producing ethylene, for example, it is possible to
suppress the occurrence of coking, to prevent the yield from being reduced by
the reduction of heat exchange rate and thermal conductivity due to the
occurrence of coking, and to extend continuous operating time. In addition,
since coking is unlikely to occur, it is possible to reduce the frequency and
time period for coking-removing operation and to enhance operation efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[00161 FIG. 1 is a cross-sectional view of a cast product before heat
treatment.
FIG. 2 is a schematic cross-sectional view illustrating a state where a
dilute-Al layer is formed by low-temperature heat treatment.
FIG. 3 is a schematic cross-sectional view illustrating a state where a
concentrated-Al layer is formed between the dilute-Al layer and a matrix by
high-temperature heat treatment.
FIG. 4 shows a TEM photograph of a film of Working Example 2 and
graphs illustrating the results of an EDX analysis.
FIG. 5 shows a TEM photograph of a film of Working Example 7 and
graphs illustrating the results of an EDX analysis.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
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Date Recue/Date Received 2022-07-12
[00171 Hereinafter, embodiments of the present invention will be described
in detail.
A cast product of the present invention has an alumina barrier layer
including an aluminum oxide on the surface of a matrix.
[00181 The aluminum oxide in the alumina barrier layer is (Al(i-x)M())203,
where M is at least one of Cr, Ni, Si, and Fe, and x is adjusted so as to
satisfy
the relationship 0<x<0.5.
[00191 Also, at least one of Cr, Ni, Si, and Fe forms a solid solution in the
aluminum oxide in the alumina barrier layer, and at least one of Cr, Ni, Si,
and Fe forming a solid solution with Al is contained so as to satisfy the
relationship Al/(Cr+Ni+Si+Fe)>2.0 in an atomic % ratio.
[00201 Explanation of reasons for limiting components
As long as the cast product of the present invention is made of a heat-
resistant alloy containing Cr in an amount of 15 mass% or more, Ni in an
amount of 18 mass% or more, and Al in an amount of 1 to 5 mass%, it is
possible to obtain the effects of the present invention. The cast product of
the present invention is made of the following components, for example. It
should be noted that in the following description, the term "%" refers to
"mass%" unless otherwise stated.
[00211 C: 0.05 to 0.7%
C acts to improve castability and enhance a high-temperature creep
rupture strength. Therefore, the content of C is set to at least 0.05%.
However, if the content is too large, a primary carbide of Cr7C3 is likely to
be
extensively formed and the movement of Al for forming the alumina barrier
layer is suppressed. As a result, Al is insufficiently supplied to the surface
portion of a cast body and the alumina barrier layer is locally divided, and
thus the continuity of the alumina barrier layer is impaired. Moreover, a
secondary carbide excessively deposits to reduce ductility and toughness.
Therefore, the upper limit is set to 0.7%. It should be noted that the content
of C is more desirably 0.3 to 0.5%.
5
Date Recue/Date Received 2022-07-12
[00221 Si: more than 0% to 2.5% or less
Si is contained to serve as a decoddizer for molten alloy and to
enhance the fluidity of molten alloy. If the content is too large, a high-
temperature creep rupture strength is reduced, and therefore, the upper limit
is set to 2.5%. It should be noted that the content of Si is more desirably
2.0% or less.
[00231 Mn: more than 0% to 3.0% or less
Mn is contained to serve as a deoxidizer for molten alloy and to fix S
in molten alloy. If the content is too large, high-temperature creep rupture
strength is reduced, and therefore, the upper limit is set to 3.0%. It should
be noted that the content of Mn is more desirably 1.6% or less.
[00241 Cr: 15.0 to 50.0%
Cr is contained in an amount of 15.0% or more in order to contribute
to the enhancement of high-temperature strength and cyclic oxidation
resistance. However, if the content is too large, high-temperature creep
rupture strength is reduced, and therefore, the upper limit is set to 50.0%.
It should be noted that the content of Cr is more desirably 23.0 to 35.0%.
[00251 Ni: 18.0 to 70.0%
Ni is an element that is necessary to secure cyclic oxidation resistance
and the stability of a metal structure. If the content of Ni is small, the
content of Fe relatively becomes large. As a result, a Cr-Fe-Mn oxide is
likely to be formed on the surface of the cast body, and thus the formation of
the alumina barrier layer is inhibited. Therefore, the content of Ni is set to
at least 18.0%. Even if the content of Ni exceeds 70.0%, it is impossible to
obtain the efficacy corresponding to the increasing amount, and therefore, the
upper limit is set to 70.0%. It should be noted that the content of Ni is more
desirably 28.0 to 45.0%.
[00261 Al: 1.0 to 5.0%
Al is an element that is effective for enhancing carburization
resistance and coking resistance. Also, in the present invention, Al is an
6
Date Recue/Date Received 2022-07-12
element that is essential for forming the alumina barrier layer on the surface
of the cast body. Therefore, the content of Al is set to at least 1.0%.
However, if the content of Al exceeds 5%, the ductility is deteriorated, and
therefore, the upper limit is set to 5.0% in the present invention. It should
be noted that the content of Al is more desirably 2.5 to 3.8%.
[00271 Rare earth elements: 0.005 to 0.4%
The term "rare earth elements" means 17 elements including 15
elements of the lanthanide series ranging from La to Lu in the periodic table,
and Y and Sc. It is preferable that rare earth elements to be contained in
the heat-resistant alloy of the present invention include at least one element
selected from the group consisting of Ce, La and Nd. Rare earth elements
contribute to the formation of the alumina barrier layer and the enhancement
of stability thereof.
When the alumina barrier layer is formed by heat treatment under a
high-temperature oxidizing atmosphere, rare earth elements that are
contained in an amount of 0.005% or more effectively contribute to the
formation of the alumina barrier layer.
On the other hand, if the content is too large, the ductility and
toughness are deteriorated, and therefore, the upper limit is set to 0.4%.
[00281 W: 0.5 to 10.0% and/or Mo: 0.1 to 5.0%
W and Mo enhance creep rupture strength by forming a solid solution
in a matrix and strengthening an austenite phase. At least one of W and Mo
is contained in order to achieve this efficacy. The content of W is set to
0.5%
or more, and the content of Mo is set to 0.1% or more.
However, if the contents of W and Mo are too large, ductility is
reduced and carburization resistance is deteriorated. Moreover, as in the
case where the content of C is large, a primary carbide of (Cr, W, Mo)7C3 is
likely to be extensively formed and the movement of Al for forming the
alumina barrier layer is suppressed. As a result, Al is insufficiently
supplied
to the surface portion of the cast body and the alumina barrier layer is
locally
7
Date Recue/Date Received 2022-07-12
divided, and thus the continuity of the alumina barrier layer is likely to be
impaired. Furthermore, since W and Mo have a large atomic radius, they
suppress the movement of Al and Cr and inhibit the formation of the alumina
barrier layer due to the formation of a solid solution in the matrix.
Therefore, the content of W is set to 10.0% or less, and the content of
Mo is set to 5.0% or less. It should be noted that when both elements are
contained, the total content is preferably set to 10.0% or less.
[00291 In addition, the following components can be contained.
[00301 At least one selected from the group consisting of Ti in an amount of
0.01 to 0.6%, Zr in an amount of 0.01 to 0.6%, and Nb in an amount of 0.1 to
1.8%
Ti, Zr and Nb are elements that are likely to form carbides, and form
less solid solutions in the matrix than W and Mo. Therefore, Ti, Zr and Nb
do not exhibit any particular action of forming the alumina barrier layer, but
enhance creep rupture strength. At least one of Ti, Zr and Nb can be
contained as needed. The content of Ti or Zr is set to 0.01% or more and the
content of Nb is set to 0.1% or more.
However, if they are excessively added, ductility is reduced.
Furthermore, Nb reduces the peeling resistance of the alumina barrier layer.
Therefore, the upper limit of the content of Ti or Zr is set to 0.6%, and the
upper limit of the content of Nb is set to 1.8%.
[00311 B: more than 0% to 0.1% or less
Since B exhibits an action of strengthening the particle boundaries of
the cast body, B can be contained as needed. It should be noted that if the
content of B is large, creep rupture strength is reduced, and therefore, the
content of B is set to 0.1% or less even in the case where B is added.
[00321 The heat-resistant alloy making up the cast body of the present
invention includes the above-described components and Fe as the balance. P,
S, and other impurities that are inevitably mixed in the alloy when melting
the alloy may be contained as long as such impurities are contained in an
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Date Recue/Date Received 2022-07-12
amount within a range that is usually allowable to this type of alloy
material.
[00331 Cast product
Molten metal having a composition including the above-described
components is produced and cast by centrifugal casting, static casting, or the
like into the cast product of the present invention having the above
composition.
[00341 The obtained cast product can be shaped depending on the intended
application.
One example of the cast product is a tube, in particular, a reaction
tube used under a high-temperature environment.
[00351 It is particularly preferable to produce the cast product of the
present
invention by centrifugal casting. This is because when using centrifugal
casting, a fine metal structure grows in the radial direction with orientation
due to the progress of cooling by a metal mold and thus an alloy structure in
which Al easily moves can be obtained.
[00361 Heat treatment, which will be described later, is performed on the
cast product. The alumina barrier layer having a stable phase structure is
formed by the heat treatment.
[00371 Heat treatment
The heat treatment is performed on the cast product of the present
invention under an oxidizing atmosphere. The heat treatment can be
divided into low-temperature heat treatment and high-temperature heat
treatment. It should be noted that the low-temperature heat treatment and
the high-temperature heat treatment can be performed in separate steps or
the high-temperature heat treatment may be performed subsequently to the
low-temperature heat treatment.
[00381 Low-temperature heat treatment
The low-temperature heat treatment is treatment in which an
aluminum oxide layer is formed on the surface of the matrix under an
oxidizing atmosphere. One example of a low temperature is a temperature
9
Date Recue/Date Received 2022-07-12
of less than 1050 C. It is desirably 600 to 900 C. It is desirable to perform
the low-temperature heat treatment for 5 to 15 hours.
[00391 By performing the low-temperature heat treatment, oxygen comes
into contact with a matrix 10 as shown in FIG. 1 to oxidize Al, Cr, Ni, Si,
and
.. Fe that has diffused from the matrix 10 to the surface of the matrix, and
an
oxide layer 22 is formed as shown in FIG. 2. Since this heat treatment is
performed at a low temperature, Al forms oxides prior to Cr, Ni, Si, and Fe.
Accordingly, an aluminum oxide layer 22 that contains Al as a main
component and in which at least one of Cr, Ni, Si, and Fe, which have
similarly diffused from the matrix, forms a solid solution is the oxide layer.
[00401 In the aluminum oxide formed by the low-temperature heat
treatment, at least one of Cr, Ni, Si, and Fe forming a solid solution with Al
is
contained so as to satisfy the relationship Al/(Cr+Ni+Si+Fe)>2.0 in an
atomic % ratio. In addition, it is desirable that the composition thereof is
(Al(i-x)M0203, where M is at least one of Cr, Ni, Si, and Fe, and x satisfies
the
relationship 0<x<0.5. Moreover, at least Cr forms a solid solution in the
aluminum oxide, and Cr forming a solid solution with Al is more preferably
contained so as to satisfy the relationship Al/Cr>10 in atomic % ratio, and
still more preferably the relationship A1/Cr>15. Furthermore, at least one of
Ni, Si, and Fe forms a solid solution, and it is more desirable that the total
atomic % of at least one of Ni, Si, and Fe forming a solid solution with Al is
10
atm% or less.
[00411 The aluminum oxide formed by the above-described low-temperature
heat treatment has a metastable y or 0 alumina structure, which is a porous
structure. Accordingly, the strength is not enough.
[00421 High-temperature heat treatment
The high-temperature heat treatment is heat treatment that is
performed after the low-temperature heat treatment, and in which, as
described later, the phase of the aluminum oxide formed by the low-
temperature heat treatment is transformed to an a alumina structure
Date Recue/Date Received 2022-07-12
(corundum structure), and an aluminum oxide layer containing Al at a high
concentration is formed between that aluminum oxide layer and the matrix.
[00431 The high-temperature heat treatment can be performed by heating
the cast product on which the low-temperature heat treatment has been
performed and the alumina barrier layer having a y or 0 alumina structure
has been formed, at a high temperature under an oxidizing atmosphere.
One example of a high temperature is a temperature of 1050 C or more. It
is desirable to perform the high-temperature heat treatment for 3 to 15 hours.
[00441 By performing the high-temperature heat treatment, the phase of the
aluminum oxide that was formed first and has a y or 0 alumina structure is
transformed to a stable a alumina structure (corundum structure). In the
present invention, at least one of Cr, Ni, Si, and Fe forms a solid solution
in
the aluminum oxide layer having a y or 0 alumina structure. This makes it
possible to promote the phase transformation from a y or 0 alumina structure
to an a alumina structure (corundum structure) compared with the case
where the aluminum oxide layer contains Al in a high ratio.
[00451 Then, the high-temperature heat treatment is further continuously
performed on the cast product having the aluminum oxide layer in which the
phase has been transformed to an a alumina structure (corundum structure),
and thus oxygen passes through the aluminum oxide layer 22 as shown in
FIG. 3.
[00461 The oxygen that has passed through the aluminum oxide layer 22
oxidizes Al that diffuses from the matrix, and an aluminum oxide layer 24
that contains Al at a high concentration is formed.
[00471 Here, as shown in FIG. 3, the aluminum oxide layer that is formed by
the low-temperature heat treatment and in which at least one of Cr, Ni, Si,
and Fe forms a solid solution is referred to as "dilute-Al layer", and the
aluminum oxide layer that is formed between the dilute-Al layer and the
surface of the matrix and contains Al at a high concentration is referred to
as
"concentrated-Al layer". Specifically, in the concentrated-Al layer 24,
11
Date Recue/Date Received 2022-07-12
Al/(Cr+Ni+Si+Fe) is larger than that in the dilute-Al layer 22.
[00481 It is thought that the reason why in terms of the alumina barrier
layer, the concentrated-Al layer formed between the matrix and the dilute-Al
layer contains Al at a higher concentration than the dilute-Al layer at the
surface is as follows.
[00491 The formed dilute-Al layer 22 allows a small amount of oxygen to
pass therethrough under an oxidizing atmosphere.
Al, Cr, Ni, Si, and Fe diffuse from the matrix 10 side to the matrix
surface side as shown in FIG. 3. However, since Al needs a smaller amount
of energy for binding to oxygen than Cr, Ni, Si, and Fe, Al priorly binds to
oxygen, and the aluminum oxide layer (concentrated-Al layer 24) containing
aluminum oxide at a high concentration is formed between the matrix 10 and
the dilute-Al layer 22.
[00501 The concentrated-Al layer 24 is formed by high-temperature heat
treatment, and thus has a stable a alumina structure (corundum structure).
It is desirable that 80 vol% or more of a crystal structure of each of the
dilute-
Al layer 22 and the concentrated-Al layer 24 is an a alumina structure
(corundum structure).
[00511 Since the alumina barrier layer 20 is constituted by the dilute-Al
layer 22 and the concentrated-Al layer 24 formed between the matrix 10 and
the dilute-Al layer 22, both of which have stable a alumina structures
(corundum structures), the alumina barrier layer 20 has high denseness,
serves as a barrier for preventing oxygen, carbon and nitrogen from entering
the base material from the outside during use under a high-temperature
atmosphere in the cast product provided therewith, and can maintain
superior oxidation resistance, carburization resistance, nitriding resistance,
corrosion resistance, and the like for a long period of time.
[00521 It should be noted that the concentrated-Al layer 24 is desirably
formed so as to be thicker than the dilute-Al layer 22, and it is preferable
to
form the concentrated-Al layer 24 such that the thickness of the
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Date Recue/Date Received 2022-07-12
concentrated-Al layer 24 is one fifth or more of that of the alumina barrier
layer 20.
[00531 It is more desirable that the dilute-Al layer 22 has a thickness of
0.04
to 8.0 pm and the concentrated-Al layer 24 has a thickness of 0.01 to 2.0 pm.
[00541 It is desirable to heat the cast product while rotating it in order to
preferably form the aluminum oxide layer in the above-described low-
temperature heat treatment and high-temperature heat treatment. This
makes it possible to heat the cast product uniformly and bring the cast
product into contact with oxygen in a good state. As a result, it is possible
to
reduce the surface roughness (Ra) of the formed alumina barrier layer 20.
[00551 Surface treatment
It is possible to perform surface treatment on the alumina barrier
layer of the cast product as needed. One example of the surface treatment is
polishing. For example, when the cast product is used for a reaction tube, Fe,
Ni and the like of the cast product come into contact with a hydrocarbon as a
raw material and coke (carbon) is likely to adhere to the inner surface of the
tube due to the catalytic action of Fe and Ni, but it is possible to suppress
the
adhesion of coke by performing the surface treatment to reduce the surface
roughness (Ra) of the alumina barrier layer.
[00561 It is desirable to perform the surface treatment such that the surface
roughness (Ra) of the alumina barrier layer is 15 pm or less. The surface
roughness (Ra) is more desirably 0.05 to 10 pm.
Example 1
[00571 Molten metal was produced by atmospheric melting in a high-
frequency induction melting furnace and cast by metal mold centrifugal
casting into tube bodies having alloy chemical compositions shown in Table 1
below. The tube body has an inner diameter of 80 mm, an outer diameter of
100 mm, and a length of 250 mm.
13
Date Recue/Date Received 2022-07-12
a
Ell
CD
[0058] Table 1
CD
0
Alloy chemical composition (the balance includes Fe and inevitable impurities)
(mass%)
No.
C Si Mn Cr Ni Al REM W Mo Ti Zr Nb B
CD Work.
0
CD 0.33 0.49 0.32 24.5 43.6 1.2 0.25 2.1 0.6
0.7
Ex. 1
CD
0- Work.
0.45 0.49 0.9 24.3 34.6 1.0 0.16 1.5 0.13
Ex. 2
Work.
Ex. 3 0.4 0.33 0.7 23.8 31.5 3.3 0.26
2.8 0.03
17.)'
Work.
0.46 1.5 1.2 25.2 35.0 2.8 0.21 4.2 0.09 0.12
Ex. 4
Work.
0.26 0.41 0.5 23.5 34.6 3.1 0.07 0.9
0.07
Ex. 5
Work.
0.31 0.4 0.2 18.3 67.1 4.7 0.01 0.4
1.6
Ex. 6
Work.
0.38 0.26 0.4 23.8 34.4 4.9 0.11 0.95
0.1
Ex. 7
Work.
0.67 1.5 1.1 23.9 40.1 4.9 0.19 2.9 0.03
Ex. 8
Comp.
0.33 1.78 0.17 25.0 33.4 0.0 0.11 0.83 0.12
Ex. 1
Comp.
0.40 1.3 0.9 25.4 15.0 0.9 0.29 2.9
Ex. 2
Comp.
0.27 1.02 0.2 23.8 33.6 1.1 0.19 11.7
Ex. 3
Comp.
0.34 0.6 0.2 25.0 45.4 2.9 0.09 1.5
1.3
Ex. 4
Comp.
0.45 1.43 1.3 22.9 34.7 3.2 0.24 3.15 0.23
Ex. 5
Comp.
0.45 0.54 0.7 23.8 29.7 5.1 0.15 1.5 0.21
Ex. 6
14
[00591 Two steps of heat treatment that differed in a heating temperature
were performed on each of Working Examples 1 to 8, which were obtained
examples of the present invention, and Comparative Examples 1 to 6 under
an oxidizing atmosphere. First, the low-temperature heat treatment was
performed, and the high-temperature heat treatment was subsequently
performed. The low-temperature heat treatment was performed for 5 hours,
and the high-temperature heat treatment was performed for 5 hours.
[00601 Table 2
N Low-temperature heat High-temperature heat
o.
treatment temperature ( C) treatment temperature ( C)
Work. Ex. 1 700 1050
Work. Ex. 2 900 1100
Work. Ex. 3 800 1050
Work. Ex. 4 800 1100
Work. Ex. 5 900 1100
Work. Ex. 6 600 1050
Work. Ex. 7 700 1100
Work. Ex. 8 600 1150
Comp. Ex. 1 800 900
Comp. Ex. 2 1000 1250
Comp. Ex. 3 1200 1300
Comp. Ex. 4 500 1150
Comp. Ex. 5 900 1000
Comp. Ex. 6 600 800
[00611 The atomic percentages of elements (Al, Cr, Fe, Ni, Si, 0) contained in
the alumina barrier layer formed on the surface of a sample tube of each of
Working Examples 1 to 8 and Comparative Examples 1 to 6 that had been
subjected to the heat treatment were measured by an EDX analysis (energy
dispersive X-ray spectrometry). Table 3 shows the results.
Date Recue/Date Received 2022-07-12
0
w
8-
x
cp
)
c [0062] Table 3
cp
0
w
Co
Fe+1\11 .
ncentrated-Al
ro' Al Cr Fe Ni Si 0 Cr+Fe Al/
X +Ni+Si
(Cr+Fe Al/Cr +Si layer thickness
No' atm%atm%atm%atm%atm%atm%1
om ( ) ( ) ( ) ( ) ( ) ( )
(atm%)
+Ni+Si) (atm%) /A barrier layer
co
thickness
co
ci Work.
N) 41.66 2.7 4.46 3.52 48.56 10.7 3.9
15.43 7.98 0.50
o Ex. 1
N)
N) Work' 39.2 O 3.6 1.4 3.2 52.6 8.2
4.8 10.89 4.6 0.30
'74 Ex. 2
N) Work' 46.6 0.1 0.1 53.2 0.2 233.0
466.0 0.1 0.80
Ex. 3
Work.
40.96 2.23 2.34 2.12 52.35 6.7 6.1 18.37 4.46
0.60
Ex. 4
Work.
38.4 1.5 10.1 6.4 1.2 42.4 19.2
2.0 25.60 17.7 0.40
Ex. 5
Work' 42.49 1.31 2.13 1.95 52.12 5.4
7.9 32.44 4.08 0.75
Ex. 6
Work' 42.69 1.28 55.93 1.3
33.4 33.35 0 0.70
Ex. 7
Work. 41.8 2.4 3.9 6.5 45.4 12.8
3.3 17.42 10.4 0.50
Ex. 8
Comp.
4.61 10.25 7.09 31 47.05 53.0 0.0 0.00 48.34
0.00
Ex. 1
Comp.
11.17 15.25 30.1 25.28 1.47 16.73 72.1 0.2 0.73
56.85 0.10
Ex. 2
Comp.
6.7 35.46 1.65 1.8 1.64 52.75 40.6 0.2 0.19 5.09
0.05
Ex. 3
Comp.
36.7 6.7 1.8 10.3 44.5 18.8
1.95 5.48 12.1 0.15
Ex. 4
Comp.
9.77 21.54 11.34 9.51 1.02 46.82 43.4 0.2 0.45
21.87 0.10
Ex. 5
Comp.
8.35 11 15.18 13.76 51.71 39.9 0.2 0.76 28.94
0.05
Ex. 6
16
[00631 All of Working Examples 1 to 8, which are examples of the present
invention, satisfy the relationship Al/(Cr+Ni+Si+Fe)>2.0 in atomic % ratio.
Furthermore, they satisfy the relationship A1/Cr>10. On the other hand,
Comparative Example 1 contains no Al in the matrix, and therefore, no
aluminum oxide is formed and both Al/(Cr+Ni+Si+Fe) and Al/Cr are zero.
[00641 Moreover, all of Comparative Examples 2 to 6 satisfy the
relationships A]/(Cr+Ni+Si+Fe)<2.0 and A1/Cr<10.
[00651 Furthermore, Fe+Ni+Si is 10 atm% or less in Working Examples 1 to
4, and 6 and 7, and Comparative Example 3, and more than 10 atm% in other
working examples and comparative examples.
[00661 The ratio of the thickness of the concentrated-Al layer with respect to
the thickness of the formed alumina barrier layer was measured in each of
obtained Working Examples 1 to 8 and Comparative Examples 1 to 6. Table
3 above shows the results.
[00671 Table 3 shows that the ratio of the thickness of the concentrated-Al
layer with respect to the thickness of the alumina barrier layer is 0.3 or
more,
that is, one fifth or more in each working example, but it is 0.15 at most in
the comparative examples. It should be noted that since Comparative
Example 1 contains no Al, no alumina barrier layer is formed.
[00681 This shows that since the low-temperature heat treatment was
performed on the working examples, which were examples of the present
invention, at a temperature of less than 1050 C and the high-temperature
heat treatment was performed thereon at a temperature of 1050 C or more,
the dilute-Al layer was formed on the surface of the matrix by the low-
temperature heat treatment and then the concentrated-Al layer could be
formed between the dilute-Al layer and the matrix by the high-temperature
heat treatment.
[00691 On the other hand, it is thought that the ratio of the thickness of the
concentrated-Al layer with respect to the thickness of the alumina barrier
layer was 0.15 at most in Comparative Examples 2 to 6 on which the alumina
17
Date Recue/Date Received 2022-07-12
barrier layer was formed for the following reasons.
In Comparative Example 2, the cast body contained Al in a small
amount of 0.9%, and Al for forming a film on the surface of the cast body was
insufficient. In Comparative Example 3, since the low-temperature heat
treatment was performed at a high temperature of 1200 C, oxides containing
Cr, Ni, Si, Fe and the like as a main component were formed before the
alumina barrier layer having a y or 0 alumina structure was formed. In
Comparative Example 4, since the low-temperature heat treatment was
performed at a low temperature of 500 C, no alumina barrier layer having a y
or 0 alumina structure was formed. In Comparative Examples 5 and 6, the
high-temperature heat treatment was performed at a low temperature of
1000 C. As a result, a small amount of oxygen passed through the dilute-Al
layer in the high-temperature heat treatment after the dilute-Al layer was
formed in the low-temperature heat treatment, and Al did not obtain
sufficient energy for binding to the oxygen taken in because the temperature
was low.
[00701 Next, a coking test was performed on the obtained sample tubes.
The coking test was performed by placing the sample tubes in an
electric furnace, supplying a hydrocarbon (ethane) to the sample tubes, and
then heating them at a high temperature (955 C) for a predetermined time
(12 to 24 hours). After the test, the degrees of carburization of the inner
surfaces of the sample tubes were compared, and the weight ratio of coke
(carbon) adhering to the inner surface of each sample tube was measured.
Table 4 shows the results.
18
Date Recue/Date Received 2022-07-12
[00711 Table 4
No. Carburization Weight ratio of Surface
resistance formed coke roughness (Ra)
Work. Ex. 1 Good 0.4 0.13
Work. Ex. 2 Fair 0.6 5.51
Work. Ex. 3 Good 0.8 7.63
Work. Ex. 4 Good 0.6 1.53
Work. Ex. 5 Fair 1.0 11.7
Work. Ex. 6 Good 0.6 1.54
Work. Ex. 7 Good 0.7 2.02
Work. Ex. 8 Good 1.1 13.8
Comp. Ex. 1 Poor 0.9 9.17
Comp. Ex. 2 Poor 0.7 4.5
Comp. Ex. 3 Poor 1.5 18.1
Comp. Ex. 4 Poor 0.7 8.32
Comp. Ex. 5 Poor 0.8 6.27
Comp. Ex. 6 Poor 1.4 16.47
[00721 Table 4 shows that all of Working Examples 1 to 8, which were
examples of the present invention, had favorable carburization resistance.
On the other hand, all of the comparative examples were carburized to the
inside of the sample tube.
[00731 It is because the alumina barrier layer having a stable a alumina
structure (corundum structure) that was constituted by the concentrated-Al
layer and the dilute-Al layer was preferably formed on the surface of the
matrix that Working Examples 1 to 8 had superior carburization resistance.
In particular, Working Examples 1, 3, 4, and 6 to 8 had extremely superior
carburization resistance compared with the other working examples. It is
thought that this is because a smaller amount of the concentrated-Al layer
was formed in Working Examples 2 and 5 than in the other working
examples.
[00741 Furthermore, the surface roughness (Ra) of each sample tube was
measured. Table 4 shows the results as well. Table 4 shows that the
weight ratio of the formed coke and the surface roughness (Ra) were
substantially in proportion to each other. Accordingly, the surface
roughness (Ra) is preferably 15 pm or less, and more preferably 10 pm or less.
19
Date Recue/Date Received 2022-07-12
[00751 It is possible to adjust the surface roughness (Ra) by performing the
heat treatment while rotating the cast product. It is thought that it is
because the heat treatment for forming a film was not properly performed
and the surface roughness was increased due to the peeling and restoration of
the film that the surface roughness of Comparative Examples 3 and 6
exceeded 15 pm.
Example 2
[00761 The alumina barrier layers of Inventive Examples 2 and 7 are
observed using a transmission electron microscope (TEM). Furthermore, the
EDX analysis was performed on the dilute-Al layer and the concentrated-Al
layer of each inventive example. FIG. 4 shows the results from Inventive
Example 2, and FIG. 5 shows the results from Inventive Example 7.
[00771 FIG. 4 shows that in Inventive Example 2, small amounts of Cr, Fe,
and Ni were detected in the dilute-Al layer 22 formed on the surface side,
which contained Al oxides as a main component. On the other hand, no Cr,
Fe, Ni, and the like were detected in the concentrated-Al layer 24 but AL
Accordingly, it is found that the concentrated-Al layer 24 was made of
aluminum oxide having a very high purity.
[00781 FIG. 5 shows that in Inventive Example 7, a small amount of Cr was
detected in the dilute-Al layer 22 formed on the surface side, which mainly
included Al oxides. On the other hand, nothing but Al was detected in the
concentrated-Al layer 24. Accordingly, it is found that the concentrated-Al
layer 24 was made of aluminum oxide having a very high purity.
20
Date Recue/Date Received 2022-07-12
