JP6434306B2 - Heat resistant tube with an alumina barrier layer - Google Patents

Description

  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 having a stable structure on the inner surface of the tube.

  Since heat-resistant tubes such as reaction tubes for producing ethylene and propylene and cracked tubes used for hydrocarbon pyrolysis are exposed to a high-temperature atmosphere, austenitic heat-resistant alloys having excellent high-temperature strength are used.

  In this type of austenitic heat-resistant alloy, a part of components (Cr, Si, Al, Fe, etc.) contained in the base material is oxidized during use in a high temperature atmosphere, and a metal oxide layer is formed on the surface. The oxide layer serves as a barrier and suppresses further oxidation of the base material.

However, if Cr oxide (mainly composed of Cr ₂ O ₃ (chromia)) is formed as these metal oxide layers, the oxide has low density, so that the function of preventing oxygen and carbon from entering is not sufficient. However, the base material undergoes internal oxidation in a high temperature atmosphere, and the oxide layer is enlarged. In addition, the enlarged oxide layer easily peels off in repeated heating and cooling cycles, and even if it does not reach peeling, the function of preventing oxygen and carbon from entering from the outside atmosphere is not sufficient. There is an inconvenience of passing through the layer and causing internal oxidation and carburization in the base material.

Therefore, in order to form alumina (Al ₂ O ₃ ) which is a useful oxide layer so as not to cause carburization and internal oxidation, the content of Al is increased more than that of a general austenitic heat resistant alloy. ing. Al oxides are known to be dense and difficult to permeate oxygen and carbon, and an oxide layer mainly composed of alumina (Al ₂ O ₃ ) (so-called “alumina barrier layer”) is provided in the pipe. It has been proposed to form the surface (for example, see Patent Document 1 and Patent Document 2).

JP-A-52-78612 JP-A-57-39159

  Increasing the Al content can be expected to improve the barrier function by the alumina barrier layer. However, since Al is a ferrite-forming element, there is a problem that when the content increases, mechanical properties such as creep rupture strength and tensile ductility of the heat-resistant pipe are lowered, and weldability is lowered.

  An object of the present invention is to provide a heat-resistant pipe excellent in mechanical properties such as creep rupture strength and tensile ductility while forming an alumina barrier layer on the pipe inner surface.

The heat-resistant tube according to the present invention is
A heat-resistant pipe having an alumina barrier layer containing an Al oxide on the inner surface of a pipe body used for thermal decomposition of hydrocarbons,
The pipe body has an Al content on the inner diameter side that is higher than an Al content on the outer diameter side.

  The outer diameter side refers to the outer peripheral side of the cross-sectional thickness of the heat-resistant tube shown in FIG. 1, and the inner diameter side refers to the inner peripheral direction side. To do.

  The tube main body preferably has an Al content on the inner diameter side that is at least twice the Al content on the outer diameter side.

  In addition, it is desirable that the tube body has an Al content on the inner diameter side increased by 1.3% or more in mass% as compared with the Al content on the outer diameter side.

  According to the heat-resistant pipe having the alumina barrier layer of the present invention, the Al content on the inner diameter side of the pipe body is larger than the Al content on the outer diameter side. An alumina barrier layer can be formed satisfactorily. Therefore, excellent oxidation resistance, carburization resistance, nitridation resistance, corrosion resistance, and the like can be provided on the inner surface of the tube that comes into contact with the high-temperature hydrocarbon gas in the thermal decomposition of the hydrocarbon.

  On the other hand, since the outer diameter side of the tube body has a small Al content, it is possible to prevent deterioration of mechanical properties such as creep rupture strength and tensile ductility due to Al content. Further, by reducing the Al content on the outer diameter side of the pipe main body, it is possible to prevent a decrease in weldability on the outer diameter side of the pipe.

Therefore, the heat resistant pipe having the alumina barrier layer of the present invention has an oxide layer mainly composed of alumina (Al ₂ O ₃ ) formed on the inner surface of the pipe to improve oxidation resistance and carburization resistance, and at the same time creep rupture. Since the tube main body having excellent mechanical properties such as strength is provided, it is suitable to be applied to a heating furnace used in a high temperature environment.

  In addition, the heat-resistant tube having the alumina barrier layer of the present invention contains the Al content on the inner diameter side of the tube body, even if the alumina barrier layer in the tube is partially peeled off during operation. The alumina barrier layer can be regenerated satisfactorily by the action of Al.

FIG. 1 is 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 view of a centrifugal casting apparatus for producing a heat-resistant tube having an alumina barrier layer according to an embodiment of the present invention. FIG. 3 is an SEM photograph showing the regenerated states of the alumina barrier layers of the inventive example and the comparative example, and FIGS. 3 (a) and 3 (a ′) show the alumina barrier layers of the inventive example 7 and the comparative example 1, respectively. FIGS. 3B and 3B are SEM photographs after the regeneration treatment of the alumina barrier layer of Invention Example 7 and Comparative Example 1, respectively.

Hereinafter, embodiments of the present invention will be described in detail.
The heat-resistant tube of the present invention is used as a reaction tube for ethylene production, a cracking tube for thermal decomposition of hydrocarbons, etc., and is installed in a heating furnace for producing hydrocarbons such as ethylene, for example.

  As shown in FIG. 1, in the heat-resistant tube 10 of the present invention, an alumina barrier layer 14 containing an Al oxide mainly composed of alumina is formed on the inner surface of a tube body 12. The heat-resistant tube 10 can have an inner diameter of 30 mm to 300 mm, a length of 1000 mm to 6000 mm, and a wall thickness of 5 mm to 30 mm, for example. Of course, it is not limited to these dimensions.

<Centrifuge casting>
The heat resistant tube 10 can be manufactured by a centrifugal casting apparatus 20 as shown in FIG. The centrifugal casting apparatus 20 includes a cylindrical metal frame 22 that is rotated at high speed by casting machine rollers 21 and 21, and has a configuration in which a molten alloy 23 is poured from a ladle 24 into a metal frame 22 through a casting rod 25. It can be illustrated.

  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 made larger than the Al content on the outer diameter side (the same).

  In order to increase the Al content on the inner diameter side of the tube main body more than the Al content on the outer diameter side, the heat-resistant pipe of the present invention has an Al content for the molten alloy poured from the casting rod to the metal frame. Can be manufactured by changing the aging with time. For example, it can be manufactured by dividing the pouring time into the first half, the middle stage, and the second half, and increasing the Al content of the cast middle stage and / or the second half molten alloy as compared with the first half of the molten alloy. The first half, the middle stage, and the second half of casting can be set, for example, by dividing the pouring time into approximately three equal parts. Of course, the pouring time may be divided into the first half and the second half of casting to increase the Al content of the molten alloy in the second half of casting.

  In addition, adjustment of the Al content of the molten alloy in the pouring gutter includes, for example, a ladle containing a molten alloy containing a small amount of Al or not containing Al and a molten alloy containing a large amount of Al. This can be done by preparing a ladle. In addition, molten Al may be directly inoculated into the ladle or cast iron in the middle or the latter half of the ladle, or Al or an Al alloy lump may be charged into the ladle.

  As described above, by increasing the Al content of the molten alloy poured into the metal frame in the middle of the casting and / or in the second half, the heat-resistant pipe to be cast has the Al content on the inner diameter side of the pipe body as the outer diameter. It can be increased compared with the Al content on the side.

  It should be noted that the Al content on the inner diameter side of the pipe body cast by centrifugal casting can be increased by pouring the molten alloy having a high Al content not only in the middle plate, the second half, or the second half, but also only in the middle plate. This is because the molten alloy poured into the middle plate by the convection of the molten alloy is stirred with the latter half molten metal.

  For example, the tube body is preferably made of a heat-resistant alloy containing at least Cr: 15% to 50%, Ni: 18% to 70%, and Al: 1 to 6%.

Further, the tube main body is C: 0.05% to 0.7%, Si: more than 0% to 2.5% or less, Mn: more than 0% to 5% or less, Cr: 15% to 50%, Ni: 18% to 70%, Al: 1% to 6%, rare earth element: 0.005% to 0.4%, and W: 0.5% to 10% and / or Mo: 0.1% to Containing 5%,
It is desirable to use a heat-resistant alloy composed of the remaining Fe and inevitable impurities.

  The heat-resistant alloy includes at least one selected from the group consisting of Nb: 0.1% to 3%, Ti: 0.01% to 0.6%, and Zr: 0.01% to 1%. It is desirable to contain.

  The rare earth element can be at least one of La, Y, and Ce.

  The heat-resistant alloy preferably contains B: 0.001% to 0.5%.

  Furthermore, it is desirable that the heat-resistant alloy contains N: 0.005% to 0.2%.

  Further, the heat-resistant alloy preferably contains Ca: 0.001% to 0.5%.

<Description of reasons for limiting ingredients>
Cr: 15% to 50%
Cr is contained in an amount of 15% or more for the purpose of improving the high temperature strength and the repeated oxidation resistance. However, if the content is too high, the high temperature creep rupture strength is lowered, so the upper limit is made 50%. The Cr content is more preferably 20% to 45%.

Ni: 18% to 70%
Ni is an element necessary for ensuring repeated oxidation resistance and stability of the metal structure. In addition, when the Ni content is small, the Fe content is relatively increased, and as a result, Cr-Fe-Mn oxide is easily generated on the surface of the cast body, so that the formation of the alumina barrier layer is hindered. . For this reason, it shall contain at least 18% or more. Since even if it contains exceeding 70%, the effect corresponding to the increase cannot be obtained, the upper limit is made 70%. The Ni content is more preferably 20% to 50%.

Al: 1% to 6%
The Al content is the average content of the entire tube body. That is, in the present invention, as described above, the heat-resistant pipe is configured such that the Al content on the inner diameter side of the pipe body is increased as compared with the Al content on the outer diameter side. When it is 3%, the Al content on the inner diameter side is larger than 3%, and the actual Al content on the outer diameter side is smaller than 3%.

  The reason for adding Al is to form an alumina barrier layer excellent in oxidation resistance, carburization resistance, coking resistance, and the like on the inner surface of the pipe body. On the other hand, an increase in Al leads to a decrease in mechanical properties such as creep rupture strength and tensile properties, and a decrease in weldability. Therefore, in the present invention, the Al content is increased on the inner diameter side of the tube body as compared with the outer diameter side as described above.

  Al is contained at least 1% in order to satisfactorily form an alumina barrier layer on the inner diameter side of the tube body. However, if the Al content exceeds 6%, the effect of forming the alumina barrier layer on the inner diameter side of the tube main body is almost saturated, so the upper limit is defined as 6% in the present invention. The Al content is more preferably 2.0% to 4.0%.

  The pipe body preferably has an Al content on the inner diameter side that is at least twice as large as the Al content on the outer diameter side, preferably 2.5 times, and more preferably 4.0 times. . By adjusting the Al content in this way, an alumina barrier layer can be suitably formed on the inner surface of the tube body, and deterioration of the mechanical properties of the tube body can be prevented.

  Further, it is preferable that the pipe body is adjusted so that the Al content on the inner diameter side is 1.3% or more in mass% as compared with the Al content on the outer diameter side. It is more desirable to increase it by more than%. In the present specification, “%” is “% by mass” unless otherwise indicated. By adjusting the Al content in this way, an alumina barrier layer can be suitably formed on the inner surface of the tube body, and deterioration of the mechanical properties of the tube body can be prevented.

  Furthermore, the Al content on the inner diameter side of the tube body is preferably 1.5% or more, and the Al content on the outer diameter side is preferably 5% or less. If the Al content on the inner diameter side is less than the lower limit, a good alumina barrier layer is not formed, and if the outer diameter side exceeds the upper limit, it is difficult to maintain the mechanical properties.

C: 0.05% to 0.7%
C has the effect of improving castability and increasing the high temperature creep rupture strength. For this reason, at least 0.05% is contained. However, if the content is too large, the primary carbide of Cr ₇ C ₃ is likely to be widely formed, and the movement of Al that forms the alumina barrier layer in the base material is suppressed. Al shortage of supply occurs, and local breakage of the alumina barrier layer occurs, and the continuity of the alumina barrier layer is impaired. Moreover, since secondary carbide precipitates excessively, the tensile ductility and toughness are reduced. For this reason, the upper limit is set to 0.7%. The C content is more preferably 0.2% to 0.6%.

Si: more than 0% to 2.5% or less Si is included as a deoxidizer for molten alloy and to increase the fluidity of the molten alloy. If the content is too high, the high temperature creep rupture strength may be reduced. The upper limit is set to 2.5% because the formation of an oxide layer with low density due to oxidation. The Si content is more preferably 2% or less.

Mn: more than 0% and 5% or less Mn is included as a deoxidizer for molten alloy and for fixing S in the molten metal, but if the content is too large, the high temperature creep rupture strength is reduced. The upper limit is 5%. The Mn content is more preferably 1.6% or less.

Rare earth elements: 0.005% to 0.4%
The rare earth element means 17 kinds of elements obtained by adding Y and Sc to 15 kinds of lanthanum series from La to Lu in the periodic table. The rare earth element contained in the heat-resistant alloy of the present invention preferably contains at least one or more members selected from the group consisting of La, Y, and Ce. This rare earth element contributes to the generation and stabilization of the alumina barrier layer.

When the production of the alumina barrier layer is carried out by heat treatment in a high-temperature oxidizing atmosphere, the rare earth element is contained in an amount of 0.005% or more, which contributes effectively to the production of the alumina barrier layer.
On the other hand, if the content is too large, the tensile ductility and toughness deteriorate, so the upper limit is made 0.4%.

W: 0.5% to 10% and / or Mo: 0.1% to 5%
W and Mo are dissolved in the matrix and strengthen the austenite phase of the matrix, thereby improving the creep rupture strength. In order to exert this effect, at least one of W and Mo is contained. In the case of W, 0.5% or more is contained, and in the case of Mo, 0.1% or more is contained.

However, when the content of W and Mo is too large, the tensile ductility is lowered and the carburization resistance is deteriorated. Further, as in the case where there is a large amount of C, primary carbides of (Cr, W, Mo) ₇ C ₃ are easily formed widely, and movement of Al in the base material forming the alumina barrier layer is suppressed. Insufficient supply of Al to the surface portion of the cast body occurs, the local breakage of the alumina barrier layer occurs, and the continuity of the alumina barrier layer tends to be impaired. In addition, since W and Mo have a large atomic radius, they dissolve in the matrix, thereby suppressing the movement of Al in the base material and preventing the formation of an alumina barrier layer. For this reason, W is 10% or less, and Mo is 5% or less. Even when both elements are contained, the total content is preferably 10% or less.

  Moreover, the following components can further be included.

Nb: 0.1% to 3%, Ti: 0.01% to 0.6%, and Zr: 0.01% to 1% and at least one selected from the group consisting of Nb, Ti, and Zr, Since it is an element that easily forms carbides and does not dissolve in the matrix as much as W and Mo, no special action is observed in the formation of the alumina barrier layer, but it has the action of improving the creep rupture strength. If necessary, at least one of Ti, Zr and Nb can be contained. The content of Nb is 0.1% or more, and Ti and Zr are 0.01% or more.
However, if added excessively, the tensile ductility is reduced. Nb further reduces the peel resistance of the alumina barrier layer. For this reason, the upper limit is 1.8% for Nb and 0.6% for Ti and Zr.

B: 0.001% to 0.5% or less B has an effect of strengthening the grain boundary of the cast body, and can be contained as necessary. In addition, since the fall of creep rupture strength will be caused when content increases, even when adding, it is made into 0.5% or less.

N: 0.005% to 0.2%
N has the effect of improving the high temperature tensile strength by dissolving in the alloy matrix. However, if the amount increases, it binds to Al to form AlN, and the tensile ductility is lowered. Preferably it is 0.06 to 0.15%.

Ca: 0.001% to 0.5%
Ca acts as a desulfurization / deoxidation element. Therefore, it contributes to the yield improvement of Ti and Al. This effect can be obtained by adding 0.001% or more. However, if added in a large amount, the weldability is impaired, so 0.5% or less.

  In the heat-resistant pipe of the present invention, the heat-resistant alloy constituting the pipe body contains the above components and the balance is Fe, but P, S and other impurities inevitably mixed during the melting of the alloy are this kind of alloy material. May be present as long as it is normally acceptable.

  The obtained tube main body has a larger Al content on the inner diameter side than the Al content on the outer diameter side.

<Machining>
Since the pipe body obtained by centrifugal casting has an unhealthy layer with irregularities and a large amount of impurities on the inner surface, the unhealthy layer is machined. The machining is preferably accompanied by a polishing treatment so that the surface roughness (Ra) of the inner surface of the tube main body is 0.05 μm to 2.5 μm. By setting the surface roughness (Ra) as described above, generation of Cr oxide (Cr ₂ O ₃ or the like) on the inner surface of the tube body can be suppressed.

<Heat treatment>
After the inner surface is machined, an alumina barrier layer is formed on the inner surface of the tube body by heat-treating the tube body in an oxidizing atmosphere. In addition, this heat processing can also be implemented as an independent process, and can also be implemented in the high temperature atmosphere at the time of installing and using a pipe | tube main body in a heating furnace.

The heat treatment is performed in an oxidizing atmosphere. The oxidizing atmosphere is an oxidizing environment in which an oxidizing gas containing 20% by volume or more of oxygen, steam or CO ₂ is mixed. The heat treatment is performed at a temperature of 900 ° C. or higher, preferably 1000 ° C. or higher, more preferably at a temperature of 1050 ° C. or higher, and the heating time is 1 hour or longer.

  By performing the heat treatment, the inner surface of the tube body comes into contact with oxygen, and Al, Cr, Ni, Si, and Fe diffused on the base surface are oxidized to form an oxide layer. By performing the heat treatment in the above temperature range, Al forms an oxide in preference to Cr, Ni, Si, and Fe.

In the present invention, since the tube body has a large Al content on the inner diameter side, Al in the vicinity of the inner surface of the tube body is suitably combined with oxygen by the heat treatment, and the oxide layer is formed of Al oxide (Al ₂ O ₃ ) is the main alumina barrier layer.

  The pipe body subjected to the heat treatment as described above has a large Al content on the inner diameter side, so that an alumina barrier layer is favorably formed on the inner surface, while the outer diameter side has a small Al content. It becomes a heat-resistant tube with excellent mechanical properties such as tensile ductility.

  In addition, Al is a component that causes welding defects and deteriorates weldability. However, the heat-resistant pipe of the present invention has a low Al content on the outer diameter side, so that the weldability decreases when installed in a heating furnace. Can also be suppressed.

  The heat-resistant tube of the present invention can maintain excellent oxidation resistance, carburization resistance, nitridation resistance, and corrosion resistance over a long period of time by using an alumina barrier layer formed on the inner surface, and mechanical properties. In addition, it has excellent weldability when installed in a heating furnace. Therefore, the life of the heat-resistant tube can be greatly improved, and the operation efficiency can be increased as much as possible.

  The molten alloy is melted by air melting in a high-frequency induction melting furnace, and the tube body having the alloy composition (unit:%, where Al is the average content) listed in Table 1 is prepared using the centrifugal casting apparatus shown in FIG. Fabricated and machined. The tube body before machining has an inner diameter of 80 mm, an outer diameter of 100 mm, and a length of 250 mm. In Table 1, “-” means not contained or unavoidably contained.

  The pipe bodies of the inventive example and the comparative example have a total weight of 40 kg of molten metal poured into the pouring gutter, respectively, and as shown in Table 2 below, the Al content (input amount) is different or the same. Three melts were prepared: a molten metal for the middle plate, a molten metal for the latter half, and a molten metal for the second half, and then the molten metal for the middle plate and the molten metal for the second half were poured in that order. In addition, although the component composition of the manufactured pipe | tube main body does not correspond about the total weight of an alloy, and the input amount of Al, this is because a part of Al adhered and remained in the crucible or the crucible.

  The pouring time was 14 to 16 seconds in total for the first half, the middle stage, and the second half, the first half was 0 to 5 seconds, the middle board was 5 to 7 seconds, and the second half was 7 seconds or later.

  After centrifugal casting, the obtained tube body has an unhealthy layer on the inner surface side, so that 2.5 mm inner surface processing is performed, the wall thickness is 7.5 mm, and the surface roughness of the inner surface by paper polishing (Ra) was 2.0 μm.

  And about the invention example 1 thru | or the invention example 7, the comparative example 1, and the comparative example 2, Al content of three points | pieces of an outer diameter side, thickness direction center (medium diameter side), and an inner diameter side was measured. The measurement was performed using a fluorescent X-ray analyzer by cutting the tube body, polishing the outer diameter side and inner diameter side by 1 mm to 2 mm from the surface, and polishing the inner diameter side after cutting. There are a total of six measurement locations, two at each of the three locations near both ends and in the center in the length direction. Table 3 shows the average Al content (unit:%) of Invention Examples 1 to 3 and Comparative Example 1 among the measured tube bodies.

  Regarding the Al content obtained by the above measurement, the ratio of the Al content on the inner diameter side to the Al content on the outer diameter side (inner / outer ratio) and the Al content on the outer diameter side with respect to the Al content on the medium diameter side, respectively. The ratio (the ratio between the inside and outside) was calculated. The results are shown in Table 4.

  Referring to Table 3 and Table 4, in each of Invention Examples 1 to 7, the Al content on the inner diameter side and the middle diameter side is larger than the outer diameter side. This is because the molten alloy having a high Al content was employed in the middle and / or second half of the casting for the inventive example. On the other hand, in Comparative Examples 1 and 2, the Al content on the inner diameter side and the middle diameter side are the same or less on the inner diameter side than the outer diameter side. In the comparative example, Al was introduced in the first half of casting, and Al was uniformly diffused in the molten alloy in the casting iron.

  For example, although both the inventive example 1 and the comparative example 1 have an Al content of 1%, referring to Tables 3 and 4, the inventive example 1 has an outer-diameter-side Al content as compared with the comparative example 1. It can be seen that the Al content is increased on the medium diameter side and the inner diameter side. The same applies to Invention Example 2 and Comparative Example 1.

<Alumina barrier layer formation treatment>
About the pipe | tube main body of the invention example 1 thru | or the invention example 7 and the comparative example 1 and the comparative example 2, the atmosphere (about 21% of oxygen) was heated at 950 degreeC for 24 hours, and the furnace cooling process was performed after the heating. .

  The cross section of the inner surface of the obtained tube body was observed with an SEM (scanning electron microscope). As a result, in each of Invention Examples 1 to 7 and Comparative Example 2, an alumina barrier layer of 80 area% or more was formed. On the other hand, the alumina barrier layer of Comparative Example 1 was less than 80 area%. This is because all of Invention Examples 1 to 7 and Comparative Example 2 were able to increase the Al content on the inner diameter side of the tube body, and Comparative Example 1 had an Al content of 1% on the inner diameter side of the tube body. It is because it is low.

  Inventive Example 1 to Inventive Example 7 and Comparative Example 2 were compared, and Inventive Example 2, Inventive Example 3, Inventive Example 5 and Inventive Example 7 had an alumina barrier layer formed on almost the entire surface.

<Alumina barrier layer peeling treatment>
In the case of the inventive example and the comparative example, when the alumina barrier layer is peeled off, it is formed on the inner surface of the tube body under the following conditions in order to check whether a good alumina barrier layer is generated again at the peeled portion. The alumina barrier layer was peeled off.

  Peeling treatment conditions are as follows: all pipe bodies are heated in the atmosphere (approximately 21% oxygen), 1200 ° C. (higher than the operating temperature in the heating furnace for ethylene production), 60 hours, and then cooled in the furnace. Processing was performed. Thereby, when heat fell, the alumina barrier layer peeled from the inner surface of the tube body due to the difference in thermal shrinkage between the tube body and the alumina barrier layer.

FIG. 3A and FIG. 3A ′ are SEM photographs after the release treatment of the alumina barrier layer of the tube body 12 of Invention Example 7 and Comparative Example 1, respectively. Referring to the figure, it can be seen that the Al oxide (Al ₂ O ₃ ) on the inner surface of the tube body 12 has lost its layered form, and only a part of the Al oxide remains on the inner surface of the tube body 12.

<Regeneration treatment of alumina barrier layer>
Subsequently, each of the tube main bodies subjected to the separation treatment of the alumina barrier layer was heated in the atmosphere (about 21% oxygen) at 950 ° C. for 24 hours, and after the heating, the furnace was cooled. It was observed whether an alumina barrier layer was formed again on the inner surface of the film.

  The results are shown in Table 4 above (layer regeneration). In Table 4, “◎” indicates that the alumina barrier layer has been regenerated on almost the entire inner surface of the tube body, and “◯” indicates that 80 area% or more is Al oxide and the remaining portion is unregenerated or Cr oxide. “×” means that the Al oxide was regenerated at less than 80 area%, and the remaining portion was not regenerated or Cr oxide was generated.

  Referring to Table 4, in Invention Example 2, Invention Example 3, Invention Example 5 and Invention Example 7, the alumina barrier layer was almost completely regenerated. This is due to the fact that the Al content on the inner diameter side of the tube main bodies of these invention examples was 4.0% or more, and as a result of the high Al content on the inner diameter side, it combined with the oxygen taken in by heat treatment. Thus, a good alumina barrier layer is regenerated. Inventive Example 1, Inventive Example 4, Inventive Example 6 and Comparative Example 2 are inferior to the above-described inventive example, but are capable of realizing regeneration of an alumina barrier layer of 80 area% or more. On the other hand, in Comparative Example 1, since the Al content on the inner diameter side of the tube main body was small, the alumina barrier layer could not be sufficiently regenerated.

FIG. 3B and FIG. 3B ′ are SEM photographs of the inner surface of the tube body 12 of Invention Example 7 and Comparative Example 1 after the regeneration treatment of the alumina barrier layer. In Invention Example 7, the alumina barrier layer 14 made of Al oxide (Al ₂ O ₃ ) is observed on almost the entire surface of the tube body 12, and the formation of Cr oxide is not recognized. On the other hand, in Comparative Example 1, as shown in FIG. 3 (b ′), Al oxide is partially regenerated, but Cr oxide or the like is also formed. In Comparative Example 1, since the Al content on the inner diameter side of the tube main body is as low as 1%, Al oxide is also produced, but it is considered that Cr, Ni, Si, Fe, etc. formed the oxide.

  Considering the above-mentioned regeneration treatment of the alumina barrier layer, any of the invention examples can be promptly regenerated even if the alumina barrier layer is peeled off for some reason during use in the ethylene production apparatus, It can be seen that oxidation resistance, carburization resistance, nitridation resistance, corrosion resistance, coking resistance, and the like can be provided.

<Tensile test>
Test pieces were produced from the tube bodies of Invention Examples 1 to 7, Comparative Examples 1 and 2, respectively, and subjected to a tensile test to measure the tensile ductility.

  The test piece was prepared based on JIS Z 2201 (plate test piece) by cutting the tube body in the thickness direction. The distance between the gauge marks in the thickness direction of the test piece is 5.65√S (S: cross-sectional area). Further, the tensile test was performed in accordance with JIS Z 2241 (metal material tensile test method). The test was carried out at room temperature because the difference appears more clearly than at high temperature. The results are shown in Table 4 (Tensile ductility).

  Referring to Table 4, it can be seen that Invention Example 1, Invention Example 2, Invention Example 4, Invention Example 6, Invention Example 7 and Comparative Example 1 have particularly excellent tensile ductility exceeding 6%. Inventive Example 3 and Inventive Example 5 are also good because the tensile ductility exceeds 3%. On the other hand, Comparative Example 2 has a tensile ductility of less than 3%.

  This is because the Al content on the outer diameter side of the pipe body was reduced in the inventive example and the comparative example 1. On the other hand, Comparative Example 2 has a large Al content on the outer diameter side, Al acts as a ferrite-forming element, Ni and Al compounds are precipitated, and the tensile ductility is lowered.

  From these, it can be seen that by reducing the Al content on the outer diameter side of the tube body, the invention example could prevent the deterioration of mechanical properties such as creep rupture strength and tensile ductility.

<Comprehensive evaluation>
When the inventive examples and comparative examples are comprehensively evaluated, as shown in Table 4 (overall), the inventive examples all have high alumina barrier layer generation and regeneration capabilities and high tensile ductility. The reason why the generation and regeneration ability of the alumina barrier layer could be increased is that the Al content on the inner diameter side of the tube body could be increased. Moreover, the reason why the mechanical properties were excellent was that the Al content on the outer diameter side of the tube body could be reduced. From the above, the pipe body preferably has an Al content on the inner diameter side that is at least twice the Al content on the outer diameter side, and the Al content on the inner diameter side contains Al on the outer diameter side. It can be seen that it is preferable that the content is 1.3% or more in mass% as compared with the amount.

  On the other hand, Comparative Example 1 in which the Al content in the tube body was simply reduced can ensure the mechanical characteristics, but the production and regeneration ability of the alumina barrier layer decreased. Further, Comparative Example 2 in which the Al content of the tube body was simply increased can increase the generation and regeneration ability of the alumina barrier layer, but the mechanical properties were lowered. In Comparative Example 2, the weldability is not good because the Al content on the outer diameter side is high. Therefore, these comparative examples are inferior as heat resistant pipes used in a high temperature environment as compared to the invention examples when comprehensively evaluated.

  As described above, the heat resistant tube having the alumina barrier layer of the present invention is difficult to peel off even if it undergoes repeated heating and cooling cycles, and even if it peels off, the alumina barrier layer is quickly regenerated. It is. Therefore, it can have excellent oxidation resistance, carburization resistance, nitriding resistance, corrosion resistance, caulking resistance, etc. for a long period of time when used in a high temperature atmosphere, and it also has a machine such as creep rupture strength and tensile ductility. It also has excellent mechanical properties, and further has good weldability when installed in a heating furnace because the Al content on the outer diameter side is small. Accordingly, the life of the heat-resistant pipe can be greatly improved, and maintenance time and frequency of coking removal work and the like can be reduced, so that the operation efficiency can be increased as much as possible.

  The above description is for explaining the present invention, and should not be construed as limiting the invention described in the claims or limiting the scope thereof. Further, the configuration of each part of the present invention is not limited to the above-described embodiment, and various modifications can be made within the technical scope described in the claims.

10 Heat-resistant tube 12 Tube body 14 Alumina barrier layer

Claims (10)

A heat-resistant pipe having an alumina barrier layer containing an Al oxide on the inner surface of a pipe body used for thermal decomposition of hydrocarbons,
The tube body is in mass%,
C: 0.05% to 0.7%, Si: more than 0% to 2.5% or less, Mn: more than 0% to 5% or less, Cr: 15% to 50%, Ni: 18% to 70 %, Al: 1% to 6%, rare earth element: 0.005% to 0.4%, and
W: 0.5% to 10% and / or Mo: 0.1% to 5%,
It consists of the balance Fe and inevitable impurities,
The tube body, the Al content of the inner diameter side at the position of 1 mm to 2 mm from the inner diameter side surface is greater than the Al content of the outer diameter side from the outer diameter side surface at the position of 1 mm to 2 mm,
A heat-resistant tube having an alumina barrier layer. The tube body, the Al content of the inner diameter side is more than twice the Al content of the outer diameter side,
A heat-resistant tube having the alumina barrier layer according to claim 1. The tube body, the Al content of the inner diameter side, as compared with the Al content of the outer diameter side, with more 1.3% by mass or more%,
A heat-resistant tube having the alumina barrier layer according to claim 1. The Al content on the inner diameter side and the Al content on the outer diameter side are averages of values measured at two points for each of three points at both ends of the tube body and the center in the length direction.
A heat-resistant tube having the alumina barrier layer according to any one of claims 1 to 3. The tube body is in mass%,
Containing at least one selected from the group consisting of Nb: 0.1% to 3%, Ti: 0.01% to 0.6%, and Zr: 0.01% to 1%,
A heat-resistant tube having the alumina barrier layer according to any one of claims 1 to 4 . The rare earth element is at least one of La, Y, and Ce.
Heat pipe having an alumina barrier layer according to any one of claims 1 to 5. The heat-resistant tube is in mass%,
B: 0.001% to 0.5% is contained,
Heat pipe having an alumina barrier layer according to any one of claims 1 to 6. The heat-resistant tube is in mass%,
N: 0.005% to 0.2% is contained,
Heat pipe having an alumina barrier layer according to any one of claims 1 to 7. The heat-resistant tube is in mass%,
Ca: 0.001% to 0.5% is contained,
Heat pipe having an alumina barrier layer according to any one of claims 1 to 8. A heating furnace having a heat-resistant tube having the alumina barrier layer according to any one of claims 1 to 9 .