Technical Field
[0001] The present invention relates to a heat-resistant tube having an alumina barrier
layer, and more specifically to a heat-resistant tube having an alumina barrier layer
with a stable structure on the tube inner surface.
Background Art
[0002] An austenite-based heat-resistant alloy having excellent high-temperature strength
is used in heat-resistant tubes to be exposed to a high-temperature atmosphere, such
as reaction tubes for production of ethylene or propylene and decomposition tubes
used for thermal decomposition of hydrocarbons.
[0003] While this type of austenite-based heat-resistant alloy is used in a high-temperature
atmosphere, a portion of components (e.g., Cr, Si, Al, and Fe) contained in the base
material is oxidized, and thus a metal oxide layer is formed on the surface. This
oxide layer serves as a barrier and suppresses further oxidation of the base material.
[0004] However, when the metal oxide layer made of Cr-oxides (mainly constituted by Cr
2O
3 (chromia)) is formed, a function for preventing the entry of oxygen and carbon is
insufficient due to the oxides having a low denseness, thus causing the internal oxidation
of the base material in a high-temperature atmosphere and the thickening of the oxide
layer. Moreover, the thickened oxide layer is likely to be removed during repeated
cycles of heating and cooling. Even in a case where the oxide layer is not removed,
since the function for preventing the entry of oxygen and carbon from an outside atmosphere
is insufficient, there is a disadvantageous situation in which oxygen and carbon pass
through the oxide layer and cause the internal oxidation or carburization of the base
material.
[0005] For this reason, an attempt has been made to increase the Al content compared with
that in a general austenite-based heat-resistant alloy for the purpose of forming
a useful oxide layer made of alumina (Al
2O
3) in order to prevent carburization or internal oxidation. It is known that Al-oxides
have a high denseness, which makes it difficult for oxygen and carbon to pass therethrough,
and therefore, it is proposed to form an oxide layer including alumina (Al
2O
3) as a main component (i.e., "alumina barrier layer") on the tube inner surface (see
Patent Documents 1 and 2, for example).
[0006] Patent Document 3 discloses a coated stainless steel tube, the steel tube consisting
of 18-38% Cr, 18-48 Ni, the balance being Fe and alloying additives, usable for thermal
decomposition of hydrocarbons such as ethane. An inner protective coating comprises
predominantly alumina.
[0007] Patent Document 4 discloses a bimetallic tube for the transport of hydrocarbon feedstocks
through the radiant coils of refinery process furnaces, comprising an outer tube layer
being formed from carbon steels or low chromium steels, an inner tube layer being
formed from alumina forming bulk alloy and an oxide layer formed on the surface of
the inner tube layer, wherein the oxide layer comprises alumina.
Citation List
Patent Document
Summary of Invention
Technical Problem
[0009] As a result of increasing the Al content, a barrier function can be expected to be
improved by the alumina barrier layer. However, Al is a ferrite-forming element, and
therefore, an increase in the Al content causes a problem in that the mechanical characteristics
of the heat-resistant tube, such as creep rupture strength and tensile ductility,
deteriorate and a problem in that weldability deteriorates.
[0010] It is an object of the present invention to provide a heat-resistant tube in which
an alumina barrier layer is favorably provided on the tube inner surface and that
is excellent in mechanical characteristics such as creep rupture strength and tensile
ductility.
Solution to Problem
[0011] A heat-resistant tube according to the present invention is defined in claim 1. It
comprises a heat-resistant tube having an alumina barrier layer to be used for thermal
decomposition of hydrocarbons, the alumina barrier layer including an Al oxide and
being provided on an inner surface of a tube body,
wherein, in the tube body, an Al content on an inner diameter side is larger than
that on an outer diameter side.
[0012] It should be noted that the outer diameter side refers to an outer circumferential
side of the cross-sectional thickness of a heat-resistant tube shown in FIG. 1, and
the inner diameter side refers to an inner circumferential side thereof. A portion
near the center of the cross-sectional thickness is taken as the center in a thickness
direction (middle diameter side).
[0013] In the tube body, the Al content on the inner diameter side is larger than that on
the outer diameter side by a factor of 2 or more.
[0014] It is desirable that, in the tube body, the Al content on the inner diameter side
is larger by 1.3 mass% or more than that on the outer diameter side.
Advantageous Effects of the Invention
[0015] With the heat-resistant tube having an alumina barrier layer of the present invention,
the Al content on the inner diameter side of the tube body is larger than that on
the outer diameter side thereof, and therefore, an alumina barrier layer can be favorably
formed on the inner surface of the tube body by heating. Accordingly, the tube inner
surface to be brought into contact with a high-temperature hydrocarbon gas during
thermal decomposition of hydrocarbon can be provided with excellent oxidation resistance,
carburization resistance, nitridation resistance, corrosion resistance, and the like.
[0016] On the other hand, the Al content on the outer diameter side of the tube body is
small, and therefore, the deterioration of mechanical characteristics such as creep
rupture strength and tensile ductility due to the contained Al can be prevented. Moreover,
reducing the Al content on the outer diameter side of the tube body makes it possible
to prevent the deterioration of weldability on the outer diameter side of the tube.
[0017] Accordingly, since the heat-resistant tube having an alumina barrier layer of the
present invention includes the tube body in which an oxide layer including alumina
(Al
2O
3) as a main component is provided on the tube inner surface, and is provided with
improved oxidation resistance, carburization resistance, and the like and excellent
mechanical characteristics such as the creep rupture strength, it is preferable to
apply the heat-resistant tube to a heating furnace to be used in a high-temperature
environment.
[0018] In addition, in the heat-resistant tube having an alumina barrier layer of the present
invention, the Al content on the inner diameter side of the tube body is increased,
and therefore, the alumina barrier layer can be favorably regenerated by action of
the contained Al even if a portion of the alumina barrier layer inside the tube is
removed during the operation or the like.
Brief Description of Drawings
[0019]
FIG. 1 shows a heat-resistant tube including an alumina barrier layer according to
an embodiment of the present invention and a cross-sectional view thereof.
FIG. 2 is an explanatory diagram of a centrifugal casting apparatus for manufacturing
a heat-resistant tube including an alumina barrier layer according to an embodiment
of the present invention.
FIG. 3 shows SEM photographs showing regeneration states of alumina barrier layers
of an inventive example and a comparative example. FIGS. 3(a) and 3(a') are SEM photographs
of Inventive Example 7 and Comparative Example 1, respectively, after alumina barrier
layer removing processing, and FIGS. 3(b) and 3(b') are SEM photographs of Inventive
Example 7 and Comparative Example 1, respectively, after the alumina barrier layer
regenerating processing.
Description of Embodiments
[0020] Hereinafter, embodiments of the present invention will be described in detail.
[0021] A heat-resistant tube of the present invention is used as a reaction tube for manufacturing
ethylene, a decomposition tube for thermal decomposition of hydrocarbons, and the
like, and is to be provided in a heating furnace for manufacturing hydrocarbons such
as ethylene, for example.
[0022] As shown in FIG. 1, in a heat-resistant tube 10 of the present invention, an alumina
barrier layer 14 that contains an Al-oxide including alumina as a main component is
formed on the inner surface of a tube body 12. The heat-resistant tube 10 may have
an inner diameter of 30 to 300 mm, a length of 1000 to 6000 mm, and a thickness of
5 to 30 mm, for example. It will be appreciated that there is no limitation to these
dimensions.
Centrifugal Casting
[0023] The heat-resistant tube 10 can be manufactured using a centrifugal casting apparatus
20 as shown in FIG. 2. For example, the centrifugal casting apparatus 20 may have
a configuration in which a tubular metal framework 22 that is rotated at a high speed
by casting machine rollers 21 is provided, and a molten alloy 23 is poured into the
metal framework 22 from a ladle 24 via a casting pail 25.
[0024] The heat-resistant tube 10 of the present invention is characterized in that the
Al content on the inner diameter side (see FIG. 1) of the tube body 12 is larger than
that on the outer diameter side (see FIG. 1).
[0025] The Al content in the molten alloy poured into the metal framework from the casting
pail is changed over time in order to increase the Al content on the inner diameter
side of the tube body compared with that on the outer diameter side, thus making it
possible to manufacture the heat-resistant tube of the present invention. For example,
the pouring time is divided into the early stage, the middle stage, and the last stage,
and the Al content in the molten alloy at the middle stage and/or the last stage of
casting is increased compared with that at the early stage of casting, thus making
it possible to manufacture the heat-resistant tube of the present invention. The early
stage, the middle stage, and the last stage of casting can be set by dividing the
pouring time into substantially equal three stages, for example. It will be appreciated
that the pouring time may be divided into the first half and the latter half of casting,
and the Al content in the molten alloy at the latter half may be increased.
[0026] The Al content in the molten alloy in the casting pail can be adjusted by preparing
a ladle containing a molten alloy including a small amount of Al or no Al and a ladle
containing a molten alloy including a large amount of Al. Alternatively, at the middle
stage or the last stage, molten Al may be directly added to the ladle or the casting
pail using a dipper, or a lump of Al or an Al alloy may be charged into the ladle.
[0027] Increasing the Al content in the molten alloy poured into the metal framework at
the middle stage and/or the last stage of casting as described above makes it possible
to increase the Al content on the inner diameter side of the tube body in the heat-resistant
tube casted compared with that on the outer diameter side.
[0028] It should be noted that the Al content on the inner diameter side of the tube body
casted through centrifugal casting can also be increased by pouring the molten alloy
including a large amount of Al at only the middle stage, not at the middle and last
stages or only the last stage. The reason for this is that the molten alloy poured
at the middle stage is mixed with the molten alloy poured at the last stage by convection
of the molten alloy.
[0029] The tube body is made of a heat-resistant alloy containing at least Cr in an amount
of 15 to 50%, Ni in an amount of 18 to 70%, and Al in an amount of 1 to 6%.
[0030] It is desirable that the tube body is made of a heat-resistant alloy containing C
in an amount of 0.05 to 0.7%, Si in an amount of more than 0% to 2.5% or less, Mn
in an amount of more than 0% to 5% or less, Cr in an amount of 15 to 50%, Ni in an
amount of 18 to 70%, Al in an amount of 1 to 6%, a rare earth element in an amount
of 0.005 to 0.4%, and W in an amount of 0.5 to 10% and/or Mo in an amount of 0.1 to
5%, and
[0031] Fe and inevitable impurities as the balance.
[0032] It is desirable that the above heat-resistant alloy contains at least one selected
from the group consisting of Nb in an amount of 0.1 to 3%, Ti in an amount of 0.01
to 0.6%, and Zr in an amount of 0.01 to 1%.
[0033] At least one rare earth element selected from La, Y, and Ce can be used.
[0034] It is desirable that the above heat-resistant alloy contains B in an amount of 0.001
to 0.5%.
[0035] Furthermore, it is desirable that the above heat-resistant alloy contains N in an
amount of 0.005 to 0.2%.
[0036] In addition, it is desirable that the above heat-resistant alloy contains Ca in an
amount of 0.001 to 0.5%.
Explanation of Reasons for Component Restrictions
Cr: 15 to 50%
[0037] The Cr content is set to 15% or more for the purpose of contribution to the improvement
of high-temperature strength and cyclic oxidation resistance. However, if the content
is too large, high-temperature creep rupture strength deteriorates, and therefore,
the upper limit is set to 50%. It should be noted that the Cr content of 20 to 45%
is more desirable.
Ni: 18 to 70%
[0038] Ni is an element that is necessary to secure cyclic oxidation resistance and the
stability of a metal structure. If the Ni content is small, the Fe content relatively
becomes large. As a result, a Cr-Fe-Mn-oxide is likely to be formed on the surface
of the cast body, thus inhibiting the formation of the alumina barrier layer. Therefore,
the Ni content is set to at least 18%. Even if the Ni content exceeds 70%, it is impossible
to obtain the efficacy corresponding to the increasing amount, and therefore, the
upper limit is set to 70%. It should be noted that the Ni content of 20 to 50% is
more desirable.
Al: 1 to 6%
[0039] The Al content refers to an average content in the entire tube body. That is, in
the present invention, the Al content on the inner diameter side of the tube body
in the heat-resistant tube is increased compared with that on the outer diameter side
as described above, and therefore, when the Al content is 3%, for example, the Al
content on the inner diameter side is larger than 3%, whereas the Al content on the
outer diameter side is smaller than 3%.
[0040] The reason for adding Al is to form an alumina barrier layer having excellent oxidation
resistance, carburization resistance, coking resistance, and the like on the inner
surface of the tube body. On the other hand, an increase in the Al content causes
the deterioration of the mechanical characteristics such as creep rupture strength
and a tensile characteristic, and the deterioration of weldability. Therefore, in
the present invention, the Al content on the inner diameter side of the tube body
is larger than that on the outer diameter side as described above.
[0041] The Al content is set to at least 1% in order to favorably form the alumina barrier
layer on the inner diameter side of the tube body. However, the Al content exceeds
6%, an effect of forming the alumina barrier layer on the inner diameter side of the
tube body becomes substantially saturated, and therefore, the upper limit is set to
6% in the present invention. It should be noted that the Al content of 2.0 to 4.0%
is more desirable.
[0042] In the tube body, the Al content on the inner diameter side is preferably set to
be larger than that on the outer diameter side by a factor of 2 or more, desirably
2.5, and more preferably 4.0. Adjusting the Al content in this manner makes it possible
to favorably form the alumina barrier layer on the inner surface of the tube body
and to prevent the deterioration of the mechanical characteristics of the tube body.
[0043] An adjustment is performed on the tube body such that the Al content on the inner
diameter side is preferably larger by 1.3 mass% or more, and more desirably larger
by 2.0 mass% or more than that on the outer diameter side. It should be noted that,
in this specification, "%" refers to "mass%" unless otherwise stated. Adjusting the
Al content in this manner makes it possible to favorably form the alumina barrier
layer on the inner surface of the tube body and to prevent the deterioration of the
mechanical characteristics of the tube body.
[0044] Furthermore, it is preferable that the Al content on the inner diameter side of the
tube body is set to 1.5% or more, and the Al content on the outer diameter side thereof
is set to 5% or less. When the Al content on the inner diameter side is smaller than
the lower limit, a favorable alumina barrier layer is not formed, and when the Al
content on the outer diameter side exceeds the upper limit, it is difficult to maintain
the mechanical characteristics.
C: 0.05 to 0.7%
[0045] C acts to improve castability and enhance high-temperature creep rupture strength.
Therefore, the C content is set to at least 0.05%. However, if the content is too
large, a primary carbide of Cr
7C
3 is likely to be extensively formed, and thus the movement of Al, which forms the
alumina barrier layer, in the base material is suppressed. As a result, Al is insufficiently
supplied to the surface portion of a cast body and the alumina barrier layer locally
splits, and thus the continuity of the alumina barrier layer is impaired. Moreover,
a secondary carbide is excessively deposited to cause the deterioration of tensile
ductility and toughness. Therefore, the upper limit is set to 0.7%. It should be noted
that the C content of 0.2 to 0.6% is more desirable.
Si: more than 0% to 2.5% or less
[0046] Si is contained for the purpose of using Si as a deoxidizer for molten alloy and
enhancing the fluidity of the molten alloy. If the content is too large, high-temperature
creep rupture strength deteriorates, or Si is oxidized to form an oxide layer having
a low denseness, and therefore, the upper limit is set to 2.5%. It should be noted
that the Si content of 2% or less is more desirable.
Mn: more than 0% to 5% or less
[0047] Mn is contained for the purpose of using Mn as a deoxidizer for molten alloy and
fixing S in the molten alloy. If the content is too large, high-temperature creep
rupture strength deteriorates, and therefore, the upper limit is set to 5%. It should
be noted that the Mn content of 1.6% or less is more desirable.
Rare earth element: 0.005 to 0.4%
[0048] The term "rare earth element" 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 at least one rare earth element selected from the group consisting of La, Y and
Ce is contained in the heat-resistant alloy of the present invention. The rare earth
element contributes to the formation of the alumina barrier layer and the enhancement
of stability thereof.
[0049] When the alumina barrier layer is formed by heating in a high-temperature oxidizing
atmosphere, the rare earth element that is contained in an amount of 0.005% or more
effectively contributes to the formation of the alumina barrier layer.
[0050] On the other hand, if the content is too large, the tensile ductility and toughness
deteriorate, and therefore, the upper limit is set to 0.4%.
W: 0.5 to 10% and/or Mo: 0.1 to 5%
[0051] W and Mo form a solid solution in a matrix and strengthen an austenite phase in the
matrix, thus improving creep rupture strength. At least one of W and Mo is contained
in order to achieve this efficacy. The W content is set to 0.5% or more, and the Mo
content is set to 0.1% or more.
[0052] However, if the W content and the Mo content are too large, tensile ductility deteriorates
and carburization resistance deteriorates. Moreover, as in the case where the C content
is large, a primary carbide of (Cr, W, Mo)
7C
3 is likely to be extensively formed, and thus the movement of Al, which forms the
alumina barrier layer, in the base material is suppressed. As a result, Al is insufficiently
supplied to the surface portion of the cast body and the alumina barrier layer locally
splits, and thus the continuity of the alumina barrier layer is likely to be impaired.
Furthermore, since W and Mo have a large atomic radius, when they form a solid solution
in the matrix, the movement of Al in the base material is suppressed and the formation
of the alumina barrier layer is inhibited. Therefore, the W content is set to 10%
or less, and the Mo content is set to 5% or less. It should be noted that when both
elements are contained, the total content is preferably set to 10% or less.
[0053] In addition, the following components may be contained.
[0054] At least one selected from the group consisting of Nb in an amount of 0.1 to 3%,
Ti in an amount of 0.01 to 0.6%, and Zr in an amount of 0.01 to 1%
[0055] Nb, Ti, and Zr are elements that are likely to form carbides, and form less solid
solutions in the matrix than W and Mo. Therefore, Nb, Ti, and Zr do not exhibit any
particular action of forming the alumina barrier layer, but improve creep rupture
strength. At least one of Ti, Zr and Nb may be contained as needed. The Nb content
is set to 0.1% or more, and the Ti content and the Zr content are set to 0.01% or
more.
[0056] However, if they are excessively added, tensile ductility deteriorates. Furthermore,
Nb reduces the removing resistance of the alumina barrier layer. Therefore, the upper
limit of the Nb content is set to 1.8%, and the upper limits of the Ti content and
the Zr content are set to 0.6%.
B: 0.001 to 0.5%
[0057] Since B exhibits an action of strengthening the particle boundaries of the cast body,
B may be contained as needed. It should be noted that if the B content is large, creep
rupture strength deteriorates, and therefore, the B content is set to 0.5% or less
even in the case where B is added.
N: 0.005 to 0.2%
[0058] N forms a solid solution in an alloy matrix and improves high-temperature tensile
strength. However, the N content is large, N binds to Al to form AlN, and tensile
ductility deteriorates. Therefore, the N content is set to 0.2% or less. The N content
of 0.06 to 0.15% is preferable.
Ca: 0.001 to 0.5%
[0059] Ca serves as a desulfurizing element or a deoxidizing element. Therefore, Ca contributes
to the improvement of the yields of Ti and Al. This effect can be obtained when Ca
is added in an amount of 0.001% or more. However, if a large amount of Ca is added,
weldability is impaired, and therefore, Ca is added in an amount of 0.5% or less.
[0060] In the heat-resistant tube of the present invention, the heat-resistant alloy constituting
the tube body 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 present as long as the contents of such impurities are within a range that
is usually allowable to this type of alloy material.
[0061] In the obtained tube body, the Al content on the inner diameter side is larger than
that on the outer diameter side.
Machining Processing
[0062] An unsound layer that has protrusions and depressions or ununiformly includes impurities
is present on the inner surface of the tube body obtained through centrifugal casting,
and therefore, machining processing is performed on this unsound layer. It should
be noted that the machining processing preferably includes polishing processing that
is performed such that the surface roughness (Ra) of the inner surface of the tube
body is 0.05 to 2.5 µm. Setting the surface roughness (Ra) as mentioned above makes
it possible to suppress the formation of Cr-oxides (e.g., Cr
2O
3) on the inner surface of the tube body.
Heat Processing
[0063] The alumina barrier layer is formed on the inner surface of the tube body by heating
the tube body in an oxidizing atmosphere after the machining processing is performed
on the inner surface. It should be noted that this heat processing can also be performed
as an independent step or performed in a high-temperature atmosphere in which the
tube body installed in a heating furnace is used.
[0064] The heat processing is performed in an oxidizing atmosphere. The "oxidizing atmosphere"
refers to an oxidizing environment in which an oxidizing gas containing oxygen in
an amount of 20 vol% or more, steam, and CO
2 are mixed. The heat processing is performed at a temperature of 900°C or higher,
preferably 1000°C or higher, and more preferably 1050°C or higher, and heating time
is one hour or more.
[0065] When the heat processing is performed, the inner surface of the tube body comes into
contact with oxygen to oxidize Al, Cr, Ni, Si, and Fe that have diffused on the surface
of the matrix, and an oxide layer is thus formed. When the heat processing is performed
within the above temperature range, Al forms oxides prior to Cr, Ni, Si, and Fe.
[0066] In the present invention, the Al content on the inner diameter side of the tube body
is large, and therefore, Al located near the inner surface of the tube body favorably
binds to oxygen by being heated as described above to form, as the oxide layer, an
alumina barrier layer including an Al-oxide (Al
2O
3) as a main component.
[0067] When the tube body is heated as described above, an alumina barrier layer is favorably
formed on the inner surface due to the Al content on the inner diameter side being
large, whereas the tube body forms a heat-resistant tube that is excellent in mechanical
characteristics such as creep rupture strength and tensile ductility due to the Al
content on the outer diameter side being small.
[0068] Al is a component that causes defective welding and reduces weldability. However,
in the heat-resistant tube of the present invention, the Al content on the outer diameter
side is small, thus making it possible to suppress the deterioration of the weldability
when the heat-resistant tube is installed in a heating furnace.
[0069] When the heat-resistant tube of the present invention is used in a high-temperature
atmosphere, excellent oxidation resistance, carburization resistance, nitridation
resistance, and corrosion resistance can be maintained for a long period of time due
to the alumina barrier layer formed on the inner surface, and the mechanical characteristics
are excellent. Furthermore, when the heat-resistant tube is installed in a heating
furnace, weldability is also excellent. Accordingly, the lifetime of the heat-resistant
tube can be improved significantly, and the operation efficiency can be enhanced to
a level as high as possible.
Examples
[0070] Molten alloy was produced through atmospheric melting in a highfrequency induction
melting furnace, and the centrifugal casting apparatus shown in FIG. 2 was used to
form tube bodies having alloy compositions shown in Table 1 below (unit: %; it should
be noted that an average content is used for Al) in the following conditions, followed
by machining processing. The tube bodies each had an inner diameter of 80 mm, an outer
diameter of 100 mm, and a length of 250 mm prior to the machining processing. It should
be noted that "-" shown in Table 1 means that the component is not contained in the
tube body or is inevitably contained in the tube body.
Table 1
|
C |
Si |
Mn |
Cr |
Ni |
Mo |
W |
Al |
Nb |
Ti |
N |
Zr |
La |
Y |
B |
Ce |
Ca |
Inv. Ex. 1 |
0.6 |
0.3 |
0.5 |
30 |
42 |
0.5 |
- |
1 |
1.5 |
- |
- |
- |
- |
0.1 |
- |
- |
- |
Inv. Ex. 2 |
0.5 |
0.6 |
0.6 |
37 |
40 |
2.5 |
- |
4 |
0.5 |
0.1 |
- |
- |
0.1 |
- |
0.01 |
- |
- |
Inv. Ex. 3 |
0.45 |
0.5 |
0.3 |
24 |
35 |
3.3 |
- |
6 |
0.8 |
0.15 |
- |
0.2 |
0.03 |
- |
- |
- |
0.05 |
Inv. Ex. 4 |
0.2 |
0.3 |
0.5 |
45 |
33 |
0.3 |
- |
2 |
- |
- |
0.01 |
- |
- |
- |
- |
- |
- |
Inv. Ex. 5 |
0.4 |
0.8 |
0.8 |
25 |
20 |
0.5 |
2 |
5 |
0 |
0 |
- |
- |
0.1 |
- |
- |
- |
- |
Inv. Ex. 6 |
0.4 |
0.3 |
0.5 |
20 |
46 |
1.4 |
- |
2 |
0.8 |
0.1 |
- |
- |
0.05 |
0.05 |
- |
0.05 |
- |
Inv. Ex. 7 |
0.5 |
0.5 |
3 |
24 |
34 |
- |
1 |
3 |
- |
0.15 |
- |
0.1 |
0.03 |
- |
- |
- |
- |
Comp. Ex. 1 |
0.3 |
0.8 |
0.4 |
20 |
37 |
0.1 |
- |
1 |
- |
- |
- |
0.1 |
- |
0.05 |
- |
- |
- |
Comp. Ex. 2 |
0.5 |
0.6 |
0.6 |
24 |
34 |
0.2 |
1 |
4 |
- |
0.1 |
0.05 |
0.2 |
- |
0.1 |
0.01 |
- |
- |
[0071] Each of the tube bodies of inventive examples and comparative examples was produced
by setting the total weight of molten alloy to be poured into a casting pail to 40
kg, preparing three types of molten alloy including an early-stage molten alloy, an
middle-stage molten alloy, and the last-stage molten alloy in which the Al contents
(Al inputs) were different or the same as shown in Table 2 below, and pouring the
early-stage molten alloy, followed by pouring the middle-stage molten alloy and the
last-stage molten alloy in this order. It should be noted that the reason why the
composition of the manufactured tube body is inconsistent with the total weight of
the alloy and the Al input is that a portion of Al adhered to a dipper or a melting
pot and remained thereon.
Table 2
Al input (kg) |
|
Early-stage molten alloy |
Middle-stage molten alloy |
Last-stage molten alloy |
Inv. Ex. 1 |
0 |
0 |
0.5 |
Inv. Ex. 2 |
0 |
1.0 |
0 |
Inv. Ex. 3 |
0 |
1.0 |
0.5 |
Inv. Ex. 4 |
0 |
0 |
0.8 |
Inv. Ex. 5 |
0 |
1.3 |
0 |
Inv. Ex. 6 |
0 |
0.8 |
0 |
Inv. Ex. 7 |
0 |
1.0 |
0 |
Comp. Ex. 1 |
0.5 |
0 |
0 |
Comp. Ex. 2 |
1.5 |
0 |
0 |
[0072] Regarding the pouring time, the total time of the early stage, the middle stage,
and the last stage was set to 14 to 16 seconds. Zero second to fifth second was the
early stage, fifth second to seventh second was the middle stage, and seventh second
and onward was the last stage.
[0073] After the centrifugal casting, 2.5-mm inner surface processing was performed on the
unsound layer on the inner surface side of each obtained tube bodies such that the
thickness was 7.5 mm, and paper polishing was performed such that the surface roughness
(Ra) of the inner surface was 2.0 µm.
[0074] Then, regarding Inventive Examples 1 to 7 and Comparative Examples 1 and 2, the Al
contents at three points that were respectively located on the outer diameter side,
at the center in a thickness direction (middle diameter side), and on the inner diameter
side were measured. The measurement was performed using a fluorescent X-ray analysis
apparatus after the tube body was cut, the surfaces on the outer diameter side and
the inner diameter side were polished away so as to reduce the thickness by 1 to 2
mm, and the the middle diameter side was polished after the cutting. The measurement
was performed at six positions in total, namely two positions at each of the three
points that were portions near the two ends and the center in the longitudinal direction.
Table 3 shows the average Al contents (unit: %) in Inventive Examples 1 to 3 and Comparative
Example 1 out of the tube bodies measured.
Table 3
|
Inv. Ex. 1 |
Inv. Ex. 2 |
Inv. Ex. 3 |
Comp. Ex. 1 |
Outer diameter side |
0.48 |
1.40 |
2.88 |
1.00 |
Middle diameter side |
1.38 |
5.24 |
6.36 |
1.00 |
Inner diameter side |
1.85 |
6.36 |
7.34 |
1.00 |
[0075] The Al contents obtained through the above measurements were used to calculate a
ratio of the Al content on the inner diameter side with respect to that on the outer
diameter side (inner-outer content ratio), and a ratio of the Al content on the middle
diameter side with respect to that on the outer diameter side (middle-outer content
ratio). Table 4 shows the results.
Table 4
|
Inv. Ex. 1 |
Inv. Ex. 2 |
Inv. Ex. 3 |
Inv. Ex. 4 |
Inv. Ex. 5 |
Inv. Ex. 6 |
Inv. Ex. 7 |
Comp. Ex. 1 |
Comp. Ex. 2 |
Inner-outer content ratio |
3.85 |
4.56 |
2.55 |
3.20 |
2.93 |
2.65 |
4.87 |
1.00 |
1.00 |
Middle-outer content ratio |
2.88 |
3.75 |
2.21 |
2.53 |
2.30 |
2.09 |
3.63 |
1.00 |
0.99 |
Layer regeneration |
B |
A |
A |
B |
A |
B |
A |
C |
B |
Tensile ductility |
7.2% |
6.4% |
3.6% |
10.4% |
4.0% |
11.2% |
8.4% |
9.2% |
2.8% |
Comprehensive evaluation |
A |
A |
A |
A |
A |
A |
A |
B |
B |
[0076] As shown in Tables 3 and 4, the Al contents on the inner diameter side and the middle
diameter side were larger than that on the outer diameter side in all of Inventive
Examples 1 to 7. The reason for this is that molten alloy containing a large amount
of Al was used in the middle stage and/or the last stage of the casting in the inventive
examples. On the other hand, in Comparative Examples 1 and 2, the Al contents on the
inner diameter side and the middle diameter side were the same as that on the outer
diameter side, or the Al content on the middle diameter side was smaller than that
on the outer diameter side. The reason for this is that Al was poured at the early
stage of the casting in the comparative examples, and Al was uniformly diffused in
the molten alloy in the casting pail.
[0077] For example, the Al contents in Inventive Example 1 and Comparative Example 1 were
1%, but Tables 3 and 4 show that the Al content on the outer diameter side in Inventive
Example 1 was smaller than that in Comparative Example 1, and the Al contents on the
middle diameter side and the inner diameter side could be increased. The same applies
to Inventive Example 2 and Comparative Example 1.
Alumina Barrier Layer Forming Processing
[0078] The tube bodies of Inventive Examples 1 to 7 and Comparative Examples 1 and 2 were
heated in the atmosphere (containing oxygen in an amount of about 21%) at 950°C for
24 hours and then cooled in the furnace.
[0079] The cross sections of the inner surfaces of the obtained tube bodies were observed
using a scanning electron microscope (SEM). The results show that, in all of Inventive
Examples 1 to 7 and Comparative Example 2, the alumina barrier layer of 80 area% or
more was formed. On the other hand, in Comparative Example 1, the alumina barrier
layer of smaller than 80 area% was formed. The reason for this is that, in all of
Inventive Examples 1 to 7 and Comparative Example 2, the Al content on the inner diameter
side of the tube body could be increased, and in Comparative Example 1, the Al content
on the inner diameter side of the tube body was as small as 1%.
[0080] It should be noted that when Inventive Examples 1 to 7 and Comparative Example 2
were compared, the alumina barrier layer was formed on substantially the entire surface
in Inventive Examples 2, 3, 5, and 7.
Alumina Barrier Layer Removing Processing
[0081] Regarding the inventive examples and comparative examples, the alumina barrier layer
formed on the inner surface of the tube body was removed in the following conditions
in order to determine whether or not a favorable alumina barrier layer was formed
again at a position where the alumina barrier layer had been removed.
[0082] The removing conditions were as follows: all the tube bodies were heated in the atmosphere
(containing oxygen in an amount of about 21%) at 1200°C (which is higher than the
operation temperature of a heating furnace for manufacturing ethylene) for 60 hours
and then cooled in the furnace. As a result, while the tube body was being cooled,
the alumina barrier layer was removed from the inner surface of the tube body due
to the difference in heat shrinkage percentage between the tube body and the alumina
barrier layer.
[0083] FIGS. 3(a) and 3(a') are SEM photographs of the tube bodies 12 of Inventive Example
7 and Comparative Example 1, respectively, after alumina barrier layer removing processing.
These photographs show that an Al-oxide (Al
2O
3) on the inner surface of the tube body 12 did not take a layered form, and only a
portion of the Al-oxide remained on the inner surface of the tube body 12.
Alumina Barrier Layer Regenerating Processing
[0084] Subsequently, the tube bodies on which the above alumina barrier layer removing processing
had been performed were heated in the atmosphere (containing oxygen in an amount of
about 21%) at 950°C for 24 hours and then cooled in the furnace. The inner surface
of the tube body was observed to check whether or not an alumina barrier layer was
formed again thereon.
[0085] Table 4 (Layer regeneration) above shows the results. In Table 4, "A" shows that
an alumina barrier layer was regenerated on substantially the entire inner surface
(90 area% or more) of the tube body, "B" means that an Al-oxide of 80 area% or more
and less than 90 area% was formed, and an Al-oxide was not regenerated or Cr-oxides
were formed on the remaining area, and "C" means that an Al-oxide of less than 80
area% was regenerated, and an Al-oxide was not regenerated or Cr-oxides were formed
on the remaining area.
[0086] As shown in Table 4, Inventive Examples 2, 3, 5, and 7 were evaluated as "A" for
layer regeneration, meaning that substantially the entire alumina barrier layer was
regenerated. This was due to the Al contents on the inner diameter side of the tube
bodies of these inventive examples being 4.0% or more. Al contained in a large amount
on the inner diameter side bound to oxygen taken in through heat processing, and thus
a favorable alumina barrier layer was regenerated. Inventive Examples 1, 4, and 6
and Comparative Example 2 were inferior to the above inventive examples, and were
evaluated as "B" for layer regeneration, meaning that an alumina barrier layer of
80 area% or more could be regenerated. On the other hand, Comparative Example 1 was
evaluated as "C" for layer regeneration due to the Al content on the inner diameter
side of the tube body being small, meaning that an alumina barrier layer was regenerated
insufficiently.
[0087] FIGS. 3(b) and 3(b') are SEM photographs of the inner surfaces of the tube bodies
12 of Inventive Example 7 and Comparative Example 1, respectively, after alumina barrier
layer regenerating processing. In Inventive Example 7, the alumina barrier layer 14
constituted by an Al-oxide (Al
2O
3) was observed on substantially the entire surface of the tube body 12, and the formation
of Cr-oxides was not observed. On the other hand, as shown in FIG. 3(b'), in Comparative
Example 1, an Al oxide was partially regenerated, and Cr-oxides were also formed.
It is thought that the Al content on the inner diameter side of the tube body of Comparative
Example 1 was as small as 1%, and therefore, Cr, Ni, Si, Fe, and the like formed oxides
while an Al oxide was formed.
[0088] Discussions of the above alumina barrier layer regenerating processing will be given.
It is found that even when the alumina barrier layer is removed for one reason or
another while each of the inventive examples is used in an ethylene manufacturing
apparatus, the alumina barrier layer can be regenerated immediately, and oxidation
resistance, carburization resistance, nitridation resistance, corrosion resistance,
coking resistance, and the like can be provided.
Tensile Testing
[0089] Test pieces were produced from the tube bodies of Inventive Examples 1 to 7 and Comparative
Examples 1 and 2, and tensile testing was performed thereon to measure tensile ductility.
[0090] The tube body was cut in the thickness direction, and the test piece was produced
based on JIS Z 2201 (flat test piece). The distance between marks in the thickness
direction of the test piece is 5.65√S (S: cross-sectional area). The tensile testing
was performed in conformity with JIS Z 2241 (metallic materials tensile testing method).
It should be noted that the testing was performed at room temperature because a clear
difference can be observed compared with a case where the testing is performed at
a high temperature. Table 4 (Tensile ductility) above shows the results.
[0091] Table 4 shows that Inventive Examples 1, 2, 4, 6, and 7 and Comparative Example 1
had a tensile ductility of higher than 6% and thus were favorable. Inventive Examples
3 and 5 had a tensile ductility of higher than 3% and thus were also favorable. On
the other hand, Comparative Example 2 had a tensile ductility of lower than 3%.
[0092] The reason for this is that the Al contents on the outer diameter side could be reduced
in the inventive examples and Comparative Example 1. On the other hand, in Comparative
Example 2, the Al content on the outer diameter side was large. Therefore, Al acted
as a ferrite-forming element, and in addition, a compound of Ni and Al was deposited,
causing the deterioration of the tensile ductility.
[0093] It is found from these results that reducing the Al contents on the outer diameter
side of the tube bodies of the inventive examples made it possible to prevent the
deterioration of the mechanical characteristics such as creep rupture strength and
tensile ductility.
Comprehensive Evaluation
[0094] The inventive examples and comparative examples were evaluated comprehensively. Comprehensive
evaluation was determined as follows: in a case where an alumina barrier layer of
80 area% or larger was formed through the alumina barrier layer forming processing,
the layer of 80 area% or more was regenerated (evaluation for layer regeneration was
"A" or "B") through the alumina barrier layer regenerating processing, and the tensile
ductility measured in the tensile testing was 3% or more, the comprehensive evaluation
was "A", and in a case where at least one of the above criteria was not satisfied,
the comprehensive evaluation was "B".
[0095] As shown in Table 4 (Comprehensive evaluation), the comprehensive evaluation was
"A" for all the inventive examples, and these results show that the inventive examples
had a high ability to form and regenerate the alumina barrier layer and a high tensile
ductility. The reason why the ability to form and regenerate the alumina barrier layer
could be enhanced is that the Al content on the inner diameter side of the tube body
could be increased. In addition, the reason why excellent mechanical characteristics
could be provided is that the Al content on the outer diameter side of the tube body
could be reduced. It is found from the description above that the Al content on the
inner diameter side of the tube body is preferably larger than that on the outer diameter
side by a factor of 2 or more, and the Al content on the inner diameter side is preferably
larger by 1.3 mass% or more than that on the outer diameter side.
[0096] On the other hand, regarding Comparative Example 1 in which the Al content in the
tube body was merely reduced, the mechanical characteristics could be secured, but
the ability to form and regenerate the alumina barrier layer deteriorated, and therefore,
the comprehensive evaluation was "B". Regarding Comparative Example 2 in which the
Al content in the tube body was merely increased, the ability to form and regenerate
the alumina barrier layer could be enhanced, but the mechanical characteristics deteriorated,
and therefore, the comprehensive evaluation was "B". In addition, regarding Comparative
Example 2, the Al content on the outer diameter side was large, and therefore, the
weldability was not favorable. Accordingly, when comprehensively evaluated as heat-resistant
tubes to be used in a high-temperature environment, these comparative examples were
inferior to the inventive examples.
[0097] As described above, with the heat-resistant tube having an alumina barrier layer
of the present invention, the alumina barrier layer is less likely to be removed even
when subjected to repeated cycles of heating and cooling, Even if the alumina barrier
layer is removed, the alumina barrier layer is regenerated immediately. Accordingly,
even when used in a high-temperature atmosphere, the heat-resistant tube having an
alumina barrier layer of the present invention can exhibit excellent oxidation resistance,
carburization resistance, nitridation resistance, corrosion resistance, coking resistance,
and the like for a long period of time, and is excellent in mechanical characteristics
such as creep rupture strength and tensile ductility. Furthermore, the Al content
on the outer diameter side is small, and therefore, the heat-resistant tube also exhibits
excellent weldability when installed in a heating furnace. Accordingly, the lifetime
of the heat-resistant tube can be improved significantly, and the operation efficiency
can be enhanced to a level as high as possible because a time and a frequency of maintenance
such as a coking removing operation can be reduced.
[0098] The foregoing description is intended to illustrate the present invention, and should
not be construed as limiting the invention defined in the claims. Also, the configuration
of each element of the invention is not limited to the foregoing examples, and various
modifications can be made within the technical scope of the claims.
List of Reference Numerals
[0099]
- 10
- Heat-resistant tube
- 12
- Tube body
- 14
- Alumina barrier layer