JP2016132019A - Welding structure of heat-resistant pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welding structure of a heat-resistant pipe capable of suppressing structure embrittlement of a weld peripheral part of a pipe outer surface or intergranular corrosion, and improving oxidation resistance.SOLUTION: In a welding structure of a heat-resistant pipe, which is a welding structure of a heat-resistant pipe obtained by welding each austenitic high Ni heat-resistant pipe 20, 20 containing at least Cr:15%-50%, Ni:18%-70%, Al:1%-6% in terms of mass%, an alumina coat 40 containing Al oxide is formed on a weld peripheral part 35 of a pipe outer surface of each heat-resistant pipe.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a welded structure in which heat-resistant pipes are welded together, and more specifically, to suppress the embrittlement and intergranular corrosion of the structure around the welded outer surface of the pipe and to improve the oxidation resistance. The present invention relates to a heat-resistant pipe welded structure.

  For heat-resistant pipes such as reaction pipes for ethylene and propylene production and cracking pipes used for hydrocarbon pyrolysis, straight pipes and / or pipes of straight pipes and curved pipes are butt welded from the outer peripheral surface side. Thus, a piping system is configured. Since this piping system is used in a high-temperature oxidizing atmosphere heated to 900 ° C. to 1150 ° C., an austenitic heat-resistant alloy having excellent high-temperature strength is used. In recent years, further high-temperature operation has been demanded in order to increase the efficiency of operation.

  This kind of austenitic heat-resistant alloy needs to suppress the base material from being oxidized during use in a high temperature atmosphere in order to maintain high temperature strength. Therefore, a part of the components (Cr, Si, Al, Fe, etc.) contained in the base material is oxidized to form a metal oxide layer on the inner surface of the tube, and this oxide layer serves as a barrier. It has been proposed to suppress the oxidation of (see, for example, Patent Document 1 and Patent Document 2).

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

  When heat-resistant tubes such as reaction tubes and decomposition tubes made of austenitic heat-resistant alloys are operated at a temperature of about 1000 ° C or higher, the structure becomes brittle, intergranular corrosion, and oxidation-resistant in the weld periphery on the outer surface of the tube. However, there is a disadvantage that the life of the reaction tube and the decomposition tube is shortened.

  And as a result of earnest research, in addition to the formation of Cr carbide at the grain boundary at the weld peripheral part of the pipe outer surface, the cause is Cr oxide (mainly composed of Cr2O3 (chromia)) as a metal oxide layer. I found out that it was due to the formation.

  If Cr oxide is formed as a metal oxide layer, the oxide density is low, so that the function of preventing the entry of oxygen and carbon is not sufficient, and the base material undergoes internal oxidation in a high temperature atmosphere, and the oxide The layer becomes 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.

  In order to solve the above problems, an object of the present invention is to provide a heat resistant pipe welding structure capable of preventing the embrittlement and intergranular corrosion of the structure around the welded outer surface of the pipe and preventing the deterioration of oxidation resistance. That is.

The welded structure of the heat-resistant pipe according to the present invention is as follows:
A welded structure of a heat-resistant pipe formed by welding austenitic high Ni heat-resistant pipes containing at least Cr: 15% to 50%, Ni: 18% to 70%, and Al: 1% to 6%. There,
An alumina coating containing Al oxide was formed on the periphery of the welded outer surface of the heat resistant tubes.

  The weld peripheral part may be a pipe outer surface that is affected by heat when welding heat-resistant pipes.

  According to the welded structure of the heat resistant pipe of the present invention, by forming an alumina coating on the weld peripheral part of the pipe outer surface between the heat resistant pipes, the embrittlement of the structure in the high temperature region is suppressed for the weld peripheral part of the pipe outer surface, Further, it is possible to prevent a decrease in oxidation resistance.

FIG. 1 is an explanatory view showing a main part of a heating furnace having a heat-resistant pipe welding structure of the present invention. FIG. 2 is an enlarged view of the vicinity of the welded portion of the heat-resistant pipes according to the present invention. FIG. 3 is a cross-sectional view taken along line AA in FIG. FIG. 4 is a cross-sectional view of the groove processing portion of the heat-resistant pipe before welding. FIG. 5 is a photograph of the vicinity of the welded portion after heat-resistant pipes of Invention Example 6 and before heat treatment. FIG. 6 is a photograph of the vicinity of a welded portion subjected to heat treatment after welding of heat-resistant tubes of Invention Example 6. FIG. 7 is a photograph of the vicinity of a welded portion subjected to heat treatment after welding of heat resistant pipes of Comparative Example 2.

Hereinafter, embodiments of the present invention will be described in detail.
In the cracking furnace 10 for producing ethylene, as shown in FIG. 1, straight tubular heat-resistant pipes 20, 20 are butt-welded 30, and these joint-shaped heat-resistant pipes (fitting) 12 are partially manufactured by an in-place casting method. As a result of being assembled by welding 14, it is used in a coil shape. The heat-resistant tube 20 has, for example, an outer diameter of 50 mm to 160 mm, a wall thickness of 5 mm to 15 mm, and a length of 3000 mm to 6000 mm.

  2 and 3 show a welded structure of the butt weld portion 31 of the heat-resistant tube 20 of the present invention. A bead 33 in which a welding rod having the same component as that of the heat-resistant tube 20 is melted is formed by TIG (Tungsten Inert Gas) welding or the like at the butt portion of the heat-resistant tubes 20, 20 or the welded portion or joint. Yes. In addition, welding is not limited to TIG welding, and covering arc welding, MIG (Metal Inert Gas) welding, electron beam welding, plasma arc welding, laser beam welding, and the like are also possible.

  As shown in FIG. 4, the heat-resistant pipe 20 is welded in a state in which a groove processing is applied to an end portion of the pipe outer surface 21. The groove processing portion 25 has a U-shaped groove portion 26 at the tip so as to ensure the design thickness, and from the parallel portion 27 and the parallel portion 27 that are continuous to the groove portion 26 in order to eliminate the difference in outer diameter. A tapered portion 28 is provided continuously to the as-cast portion 22 of the heat-resistant tube 20.

  The tube inner surface 23 is processed so that the inner diameter becomes uniform by machining or polishing.

  The groove processing part 25 is exposed in the furnace and exposed to a high-temperature oxidizing atmosphere. Therefore, the strength tends to decrease due to the embrittlement of the structure, intergranular corrosion, and deterioration of oxidation resistance. In particular, since the parallel part 27 and the taper part 28 are thinner than the as-cast part 22, they are susceptible to these effects.

  Therefore, in the present invention, the alumina coating 40 containing aluminum oxide is formed in a region around the bead 33 that becomes the welded portion 31, that is, in the welded peripheral portion 35. The weld peripheral part 35 means the parallel part 27 and the taper part 28. Note that the weld peripheral portion 35 includes at least a heat affected portion of the pipe outer surface 21 that is affected by heat during welding.

The aluminum oxide that forms the alumina coating is mainly composed of alumina (Al ₂ O ₃ ). Alumina coating is highly dense and difficult to permeate oxygen and carbon, so it can suppress oxidation around the weld exposed to high-temperature oxidizing atmosphere, prevent thinning due to oxidation and maintain strength, etc. To help.

  In addition, by forming an alumina coating, it is possible to suppress the formation of a chromia coating made of chromium oxide on the periphery of the weld, and the embrittlement of the structure, intergranular corrosion, and reduction in oxidation resistance caused by the chromia coating. Can be prevented.

  Further, even when a chromia coating is formed on the as-cast part 22 and the bead 33 on the heat-resistant tube 20, the alumina coating is formed on the periphery of the weld sandwiched between these chromia coatings, thereby peeling off the alumina coating. Can be effectively blocked.

  The heat-resistant tube is formed of an austenitic high Ni alloy containing at least Cr: 15% to 50%, Ni: 18% to 70%, and Al: 1 to 6%. In the present specification, “%” is “% by mass” unless otherwise indicated.

  Further, the heat-resistant tube is C: 0.05% to 0.7%, Si: more than 0% to 2.5% or less, Mn: more than 0% to 5% or less, rare earth element: 0.005% to Desirably, the heat-resistant alloy further includes 0.4% and W: 0.5% to 10% and / or Mo: 0.1% to 5%, and the balance is Fe and inevitable impurities.

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

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

  Furthermore, it is desirable that the heat-resistant tube 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. Moreover, when there is little content of Ni, as a result of the content of Fe becoming relatively large, it becomes easy to produce | generate a Cr-Fe-Mn oxide on the pipe inner surface and the pipe outer surface of a heat-resistant pipe. Thereby, the production | generation of the alumina barrier layer of a pipe inner surface is inhibited. Moreover, the production | generation of the alumina film to the welding peripheral part of a pipe outer surface is also inhibited. 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%
Al is added to form an alumina barrier layer excellent in carburization resistance and coking resistance on the inner surface of the tube and to form an alumina coating on the weld peripheral portion of the outer surface of the tube. 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.

  Al is contained in an amount of at least 1% in order to satisfactorily form an alumina barrier layer on the inner surface of the tube and an alumina coating on the weld peripheral portion of the outer surface of the tube. However, when the Al content exceeds 6%, the formation effect of the alumina barrier layer and the alumina coating 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%.

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 and the alumina coating is suppressed. Insufficient supply of Al to the part occurs, and local cuts of the alumina barrier layer and the alumina coating occur, and the continuity of the alumina barrier layer and the alumina coating 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% and 2.5% or less Si is included as a deoxidizer for molten alloy and to increase the fluidity of the molten alloy. However, if the content is too high, the high temperature creep rupture strength is reduced. Therefore, the upper limit is 2.5%. The Si content is more preferably 2.0% 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 an alumina barrier layer and an alumina coating.

When the formation of the alumina barrier layer or the alumina coating is performed by heat treatment in a high-temperature oxidizing atmosphere, the rare earth element is contained in an amount of 0.005% or more, which effectively contributes to the formation of the alumina barrier layer or the alumina coating.
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 the movement of Al forming the alumina barrier layer and the alumina coating is suppressed. Insufficient supply of Al to the surface portion of the body occurs, the local breakage of the alumina barrier layer and the alumina coating occurs, and the continuity of the alumina barrier layer and the alumina coating 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 and Cr and preventing the formation of an alumina barrier layer and an alumina coating. 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, It is an element that tends to form carbides, and it does not dissolve as much in the matrix as W and Mo. Therefore, no special action is observed in the formation of an alumina barrier layer or an alumina coating, but it has the effect 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 and the alumina coating. 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.

<Heat resistant tube>
The heat-resistant tube is made of a molten metal and cast to the above composition by centrifugal casting. Note that it may be stationary casting or the like.

  The obtained heat-resistant pipe is processed so that the inner diameter of the pipe inner surface 23 becomes uniform by machining or polishing, and the groove outer surface 21 is subjected to groove processing as shown in FIG. . In the illustrated embodiment, the groove processing portion 25 has a U-shaped groove portion 26 at the tip, and a cast-out portion 22 of the heat-resistant tube 20 from the parallel portion 27 and the parallel portion 27 continuous to the groove portion 26. And a tapered portion 28 that is continuous with the tape.

  The groove processed portion is preferably processed so that the surface roughness (Ra) of the parallel portion and the tapered portion is 6.3 μm or less, and the surface roughness (Ra) is 2.5 μm to 4.0 μm. It is more desirable to do. By adjusting the surface roughness (Ra) as described above, formation of a chromia coating can be suppressed, and an alumina coating can be more suitably formed by subsequent welding treatment and heat treatment.

  In addition, you may heat-process to the heat-resistant pipe which gave groove processing beforehand before the welding process demonstrated below. By performing heat treatment on the heat-resistant pipe subjected to groove processing, Al in the groove processed portion is combined with oxygen, and the alumina coating is preliminarily formed into the groove processed portion, particularly a parallel portion and a tapered portion that are thermally affected by welding. Can be formed. This heat treatment may be performed on the entire heat-resistant tube, or locally on the groove processing portion.

<Welding process>
While the obtained heat-resistant pipes 20 and 20 are rotated in the same direction in a state in which the end portions are butted, by performing TIG welding or the like, as shown in FIG. 2 and FIG. The base material of the groove portion 26 is melted, and the beads 33 are sequentially stacked on the groove portion 26. In the case of TIG welding, examples of the welding conditions include welding current 70A to 180A, welding voltage 7V to 21V, welding speed 70cm / min to 180cm / min, but the welding temperature applied to the heat-resistant tube is within the range of 700 ° C to 900 ° C. It is necessary to manage so that it fits in. If the welding temperature is lower than the above, it is difficult to form an alumina coating, and if the welding temperature is higher than the above, a chromia coating is formed instead of the alumina coating.

By performing this welding process in an oxidizing atmosphere, for example, in an oxidizing environment containing 20% by volume or more of oxygen, such as air, steam or CO ₂ , the groove processing section 25 is more detailed. In addition, the weld peripheral portion 35 of the pipe outer surface 21 is heated under the influence of welding and heated, and Al of the heated weld peripheral portion 35 is oxidized, and a thin alumina coating 40 is formed on the weld peripheral portion 35. Can be formed.

  FIG. 5 is a photograph after welding of Invention Example 6 in an example described later. When the periphery of the weld was observed with an SEM (scanning electron microscope), it was found that a thin alumina film having a thickness of 0.1 μm was formed. The thickness of the alumina film formed after welding may be 0.001 μm or more, but more preferably 0.01 μm or more.

<Heat treatment>
Then, by applying heat treatment to the heat-resistant pipes 20 and 20 after welding, a thin alumina film formed at the time of welding can be grown on the weld peripheral portion 35, and a thicker film 40 can be formed. At this time, the alumina film is formed to 0.05 or more, and more preferably 0.5 μm or more. 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 heat-resistant pipe | tube in a heating furnace.

The heat treatment is performed in an oxidizing atmosphere. As described above, 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. Examples of the heat treatment include 900 ° C. to 1100 ° C. The heat treatment is desirably performed for 5 hours or more.

  Since a thin alumina film is formed in advance on the weld peripheral part by welding, chromium oxide or the like is not formed, and oxygen passing through the thin alumina film and Al in the vicinity of the surface of the weld peripheral part 35 are preferentially combined. The alumina coating 40 grows thick.

  FIG. 6 is a photograph after the heat treatment of FIG. 5 described above. When the periphery of the weld was observed by SEM, it was found that the alumina coating had grown to a thickness of 0.1 μm to 2 μm.

  The thick alumina coating formed on the periphery of the weld is highly dense and acts as a barrier to prevent oxygen, carbon, and nitrogen from entering the base metal from the outside, so the weld periphery is exposed to a high-temperature oxidizing atmosphere during operation. Even if it is done, the embrittlement and intergranular corrosion of the structure are suppressed, and the deterioration of the oxidation resistance is also prevented, so that the strength can be prevented from being lowered.

  In the present invention, as shown in FIG. 3, the alumina coating 40 is sandwiched between chromia coatings 50 and 50 formed on the as-cast portion 22 and the bead 33. Thereby, peeling of the alumina coating 40 can be suitably suppressed.

  The molten metal was melted by melting in the air in a high-frequency induction melting furnace, and two heat-resistant tubes each having an alloy composition listed in Table 1 below were produced and machined by die centrifugal casting. 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.

  As shown in FIG. 4, the obtained heat-resistant pipes were subjected to groove processing as shown in FIG. 4, and pipe bodies having the same composition as a pair were joined to each other by butt TIG welding. In addition, the surface roughness (Ra) of the parallel part 27 and the taper part 28 is 3.2 micrometers. Moreover, TIG welding conditions are welding current 90A-150A, welding voltage 9V-17V, welding speed 80-160 cm / min, and about the welding temperature added to a heat-resistant pipe, invention examples are 700 to 900 degreeC, and a comparative example is 900. The temperature was adjusted to over ° C.

  FIG. 5 is a photograph of Example 6 after welding. Observation of the weld peripheral part by SEM revealed that a thin alumina film having a thickness of 0.005 μm or more was formed. Also, in other invention examples, a thin alumina film having a thickness of 0.1 μm to 1.5 μm having a thickness of 0.1 μm or more was formed on the periphery of the weld.

  On the other hand, when the comparative example was similarly observed by SEM, an alumina coating was not formed and only a chromia coating was formed. This is because the chromia coating was formed before the alumina coating was formed because the welding temperature applied to the heat-resistant tube exceeded 900 ° C.

  Subsequently, the heat-resistant pipes joined by welding were each heated in the atmosphere (oxygen of about 21%) at 1000 ° C. for 8 hours, followed by a furnace cooling treatment.

  About the invention example and the comparative example, the welding peripheral part was observed by SEM, and the presence or absence of formation of an alumina film was investigated. The results are shown in Table 2. In Table 2, “◯” indicates that the alumina coating is formed on the periphery of the weld, and “X” indicates that the chromia coating is formed.

  Referring to Table 2, it can be seen that in all of the inventive examples, the alumina coating is well formed on the periphery of the weld. FIG. 6 is a photograph after heat treatment of Invention Example 6. Observation of the welded peripheral portion of Invention Example 6 by SEM revealed that the alumina coating had grown to a thickness of about 0.1 μm to 2 μm. In the other invention examples as well, a thick alumina coating having a thickness of 0.1 μm to 2 μm was formed on the periphery of the weld.

  On the other hand, when the comparative example was similarly observed by SEM, a chromia coating was formed on the periphery of the weld. FIG. 7 is a photograph after heat treatment of Comparative Example 2. Referring to FIG. 7, in Comparative Example 2, it was observed that a chromia coating was formed not only on the as-cast part and the bead but also on the weld peripheral part.

  As for the invention example, the thick alumina film was formed as described above because the welded peripheral part was previously formed with a thin alumina film by welding, and chrome oxide or the like was not formed, but passed through the thin alumina film. This is because oxygen and Al in the vicinity of the surface of the periphery of the weld were preferentially bonded and an alumina coating was grown.

  In the comparative example, as a result of forming a chromia film on the weld peripheral part by welding, it was considered that even when heat treatment was performed, the thickness of the chromia film only increased on the weld peripheral part, and no alumina film was formed. .

  In addition, when the as-cast part and the bead were observed in the inventive examples, a chromia coating was formed in each case, and the alumina coating at the weld peripheral portion was sandwiched between these chromia coatings. Thereby, it is thought that peeling of the alumina film formed in the welding peripheral part can be suitably suppressed.

  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.

20 heat resistant tube 22 as cast part 25 groove processing part 26 groove part 27 parallel part 28 taper part 31 weld part 33 bead 40 alumina coating 50 chromia coating

Claims (11)

A welded structure of a heat-resistant pipe formed by welding austenitic high Ni heat-resistant pipes containing at least Cr: 15% to 50%, Ni: 18% to 70%, and Al: 1% to 6%. There,
An alumina coating containing Al oxide was formed on the periphery of the welded outer surface of the heat-resistant tubes,
This is a heat-resistant pipe welded structure. The weld peripheral part is a pipe outer surface that is affected by heat when welding heat-resistant pipes,
The welding structure of the heat-resistant pipe according to claim 1. The alumina coating has a thickness of 0.1 μm to 2 μm.
The welded structure of the heat-resistant pipe according to claim 1 or 2. The heat-resistant tube 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, rare earth elements: 0.005% to 0.4%, and ,
W: 0.5% to 10% and / or Mo: 0.1% to 5%,
Consisting of the balance Fe and inevitable impurities,
The welded structure of a heat-resistant pipe according to any one of claims 1 to 3. The heat-resistant tube 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%,
The welded structure of the heat-resistant pipe according to claim 4. The rare earth element is at least one of La, Y, and Ce.
The heat-resistant pipe welded structure according to claim 4 or 5. The heat-resistant tube is in mass%,
B: 0.001% to 0.5% is contained,
The heat-resistant pipe welded structure according to any one of claims 4 to 6. The heat-resistant tube is in mass%,
N: 0.005% to 0.2% is contained,
The heat-resistant pipe welded structure according to any one of claims 4 to 7. The heat-resistant tube is in mass%,
Ca: 0.001% to 0.5% is contained,
The heat-resistant pipe welded structure according to any one of claims 4 to 8. The weld peripheral portion has a surface roughness (Ra) of 6.3 μm or less.
The heat-resistant pipe welded structure according to any one of claims 1 to 9.   A heating furnace having a heat-resistant pipe welded structure according to any one of claims 1 to 10.