JPH05195161A - Double layer heat resistant steel pipe excellent in carburization resistance - Google Patents

Abstract

PURPOSE:To improve the carburization resistance of a heat resistant steel pipe reactor for petrochemical industry or the like. CONSTITUTION:A tube wall consists of a double layer structure and the 1st wall layer is formed from a Fe-Cr-Ni base heat resistant steel having a proper chemical composition and the 2nd wall layer in need of carburization resistance is formed from a heat resistant steel containing 0.01-0.5% C, >4% and <7% Si, <=3% Mn, 20-35% Cr, 20-60% Ni, 2-15% Mo and balance substantially Fe. The heat resistant steel of the 2nd wall layer is made by adding, if necessary, one or two or more kinds of elements of (I) 1-10% Ni, (II)>=0.5% Co (where 20-60% Ni+Co), (III) 0.02-1% Al, 0.02-0.5% Ti, <=5% W, 0.001-0.5% CaO, <=0.05% B, <=0.5% Y, <=0.5% Hf.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant steel pipe having excellent carburization resistance, which is suitable as a pyrolysis / reforming reaction pipe for hydrocarbons in the petrochemical industry.

[0002]

2. Description of the Related Art Pyrolysis / reforming reaction tubes for hydrocarbons, such as cracking tubes for ethylene production, use naphtha or the like passing through the tubes at high temperature and pressure (temperature: about 750 to 1050 ° C., pressure: about 10 kgf / cm ^2). It is a reactor that thermally decomposes under the conditions of (below), and recently, due to demands for efficiency of reaction, improvement of productivity, etc., high temperature operation exceeding a temperature of about 1100 ° C is becoming common.

The above reaction is accompanied by the precipitation phenomenon of solid carbon from the reaction system. The deposited carbon adheres to the inner surface of the pipe wall and diffuses (carburizes) inside the pipe wall under the influence of high temperature heat. The generation and progress of carburization causes deterioration of the material of the reaction tube. In particular, the ductility is remarkably deteriorated, which causes the tube to crack due to embrittlement during high temperature operation.

Therefore, carburization resistance is a very important characteristic in practical use of the reaction tube. Needless to say, it is necessary to have not only carburization resistance but also strength to withstand high temperature and high pressure, oxidation resistance, and the like, and weldability for pipe construction by welding. Conventionally, as the reaction tube, HP40 material of ASTM standard (0.4 C-25Cr-35N
i-Fe) and its improved material (0.4 C-25Cr-35Ni-
Mo, Nb, W-Fe) have been used.

[0005]

The above HP40 material is 900
It is a material used in the temperature range of ℃ to 1050 ℃, and it is unavoidable that the carburization resistance and creep rupture strength are insufficient in the higher temperature range. The HP-improved material shows good carburization resistance even at 1000 ° C or higher, but if it exceeds 1100 ° C, the carburization resistance and creep rupture strength are greatly reduced, and especially the carburization resistance is significantly deteriorated at a temperature of around 1150 ° C. It is hard to say that it can sufficiently cope with the recent high temperature operation. The present invention has been made in view of the above circumstances, and provides a heat-resistant steel pipe excellent in carburizing resistance and the like that enables stable use in recent high-temperature operations exceeding 1100 ° C.

[0006]

The heat-resistant steel pipe of the present invention comprises a first wall layer and a second wall layer, and the first wall layer which does not particularly require carburization resistance is Fe-Cr. -The second wall layer formed of Ni-based heat-resistant steel and requiring carburization resistance is
C: 0.01 to 0.5%, Si: more than 4%, 7% or less, Mn: 3
% Or less, P: 0.03% or less, S: 0.03% or less, Cr: 20 to
It is characterized in that it is a two-layer laminated tube made of heat-resistant steel having a chemical composition of 35%, Ni: 20-60%, Mo: 2-15%, and the balance being substantially Fe.

The heat-resisting steel forming the second wall layer of the heat-resisting steel pipe of the present invention, if desired, may be combined with any of the above elements in any one of the following items (i) to (iii) and at least two items. A chemical composition containing the listed elements is provided. (I) Nb: 1-10% (ii) Co: 0.5% or more (however, Co + Ni: 20-60
%) (Iii) Al: 0.02 to 1%, Ti: 0.02 to 0.5%,
One or more elements selected from the group consisting of W: 5% or less, Ca: 0.001 to 0.5%, B: 0.05% or less, Y: 0.5% or less, and Hf: 0.5% or less. Regarding the heat-resistant steel pipe of the present invention, the reasons for limiting the components of the heat-resistant steel having the improved carburization resistance forming the second wall layer will be described below.

C: 0.01 to 0.5% C improves the castability of the alloy and forms carbides such as Cr and Mo, which enhances the high temperature creep rupture strength by its precipitation dispersion strengthening action. At least 0.01% is required to obtain this effect. The effect is increased by increasing the amount of addition, but if a large amount is added, excessive precipitation of secondary carbides is caused, which causes deterioration of aging ductility and weldability, so the upper limit is 0.5%.

Si: more than 4% and less than 7% Si is an element that has the effect of deoxidizing the molten alloy and improving the fluidity. However, in the present invention, it does not stop there, but it also plays an important role as a carburization resistance improving element. It is an element to fulfill. That is, S
When i is added in a large amount, thick and dense SiO ₂ is generated in the oxide film formed on the alloy surface by high temperature heating, and the effect of suppressing C diffusion in a high temperature carburizing environment is obtained.
The effect of preventing carburization by suppressing the diffusion is remarkable. The lower limit of the amount added exceeds 4% in order to make the carburizing prevention effect sufficient in the high temperature region around 1150 ° C. Although the effect is increased by increasing the addition amount, the upper limit is 7% because the ductility decreases and the weldability tends to decrease.

Mn: 3% or less Mn has a deoxidizing effect and fixes S, which is a harmful impurity, as MnS, and is effective in improving weldability and the like. This effect is 3
%, It is possible to obtain a sufficient amount, and no further addition is required, so the content is made 3% or less.

P: 0.03% or less, S: 0.03% or less P and S are harmful impurities that cause reduction in high temperature strength and increase in hot cracking susceptibility during welding. If it does, there is no substantial impact.

Cr: 20-35% Cr is an element that enhances high temperature strength, oxidation resistance and the like and also improves carburization resistance. In particular, in order to make these effects sufficient for use in a high temperature range around 1150 ° C, it is necessary to add at least 20% or more. The effect increases as the amount of addition increases, but if it is too large, it causes excessive precipitation of Cr carbide in the process of use at high temperature and deteriorates the aging ductility. Therefore, the upper limit is 35%.

Ni: 20-60% Ni is an element that forms a stable austenite matrix together with Cr, Fe and the like, and enhances high temperature strength, oxidation resistance and the like.
Further, it has the effect of reducing the amount of C solid solution in the matrix and suppressing the diffusion and penetration of C. At least 20% addition is required to ensure these effects at high temperatures around 1150 ° C. These effects increase with an increase in the amount of Ni, but addition up to 60% is sufficient, and addition exceeding this impairs economic efficiency.

Mo: 2% to 15% Mo is an element effective in improving carburization resistance, and particularly has an action of suppressing diffusion of the alloy from the surface layer to the inside. This effect is exhibited by the addition of 2% or more. The effect increases as the addition amount increases, and the high temperature strength is also improved. However, if it exceeds 15%, the effect is almost saturated, and if the amount is further increased, the oxidation resistance is lowered. Therefore, the upper limit is 15%.

Nb: 1-10% Nb forms eutectic carbides at grain boundaries, strengthens grain boundary fracture resistance, and improves high temperature creep rupture strength. In addition, it improves resistance to weld cracking and is effective in stabilizing the quality of welded joints. This effect can be obtained by adding 1% or more, but if added in a large amount, ductility is deteriorated and oxidation resistance is deteriorated, so the upper limit is 10%.

Co: 0.5% or More Co is an element that contributes to the stabilization of an austenite matrix like Ni and has the effect of improving high temperature strength and oxidation resistance. The effect is enhanced by the synergistic action with Ni. Therefore, when it is desired to further increase the high temperature strength, it is preferable that a part of Ni is replaced with 0.5% or more of Co so that it coexists with Ni. However, since Co and Ni are both austenite-forming elements, the total content of both elements is 20 to 60%, which is the same as the Ni content when Co is not added, so as not to impair the balance with other elements. Range.

Al: 0.02-1% Al has the effect of improving carburization resistance. That is, the addition of Al produces alumina on the surface of the alloy in the high temperature region, and this oxide suppresses the diffusion and penetration of C. Also, the oxidation resistance is improved. These effects are obtained by adding 0.02% or more, and the effects are increased by increasing the addition amount. However, addition of a large amount causes a decrease in room temperature elongation, a decrease in weldability, etc., so the upper limit is 1%.

Ti: 0.02 to 0.5% Ti suppresses and delays agglomeration and coarsening of Cr carbides in a high temperature region to improve creep rupture strength, and strengthens carburization resistance by a synergistic effect with Al. This effect can be obtained by adding 0.02% or more. However, if added in a large amount, the amount of oxide-based inclusions increases and the precipitates become coarse, resulting in a decrease in strength. Therefore, the upper limit is 0.5%.

W: 5% or less W has the effect of forming a solid solution in the austenite matrix to strengthen the matrix, and suppresses the diffusion of C to enhance the carburization resistance. However, addition of a large amount impairs the ductility of the alloy.
% Or less.

Ca: 0.001 to 0.5% Ca forms an oxide on the alloy surface in the high temperature range, and C
Control the spread and invasion of. This carburization resistance improvement effect is 0.001
It is obtained by adding more than 100%. However, if added in a large amount, the weldability is impaired, so 0.5% is made the upper limit.

B: 0.05% or less B strengthens the grain boundaries and enhances the high temperature creep rupture strength. However, if added in a large amount, the weldability and the like deteriorate, so it is made 0.05% or less.

Y: 0.5% or less Y has an effect of improving carburization resistance. This effect is sufficiently obtained with addition of 0.5% or less, and there is no benefit of adding a large amount beyond that, so it is set to 0.5% or less.

Hf: 0.5% or less Hf is an element effective in improving the carburization resistance as in the case of Y. This effect can be sufficiently secured by adding up to 0.5%, and there is no benefit of adding more than that.

In the heat-resistant steel pipe of the present invention, the first wall layer made of Fe—Cr—Ni heat-resistant steel is laminated with the second wall layer made of heat-resistant steel having the above chemical composition and having improved carburization resistance. It has a two-layer structure formed. The heat-resistant steel pipe of the present invention is a two-layer laminate, which is improved by forming the pipe surface on the side in contact with the pyrolysis / reforming reaction system of hydrocarbons with the second pipe wall material. In addition to ensuring carburization resistance and oxidation resistance, the first pipe wall made of Fe-Cr-Ni heat-resistant steel laminated on the first pipe wall is intended to compensate for the high temperature strength required as a reaction pipe. Is.

The grade of the Fe-Cr-Ni heat-resisting steel forming the first wall layer may be appropriately selected depending on the specific application of the pipe and the use conditions. When used as an ethylene cracking tube, HP40 material (0.35 to 0.75C-
33 to 37Ni-24 to 28Cr-Fe), and 1 to 2 of Nb of 2% or less, Mo of 2% or less, W of 2% or less.
It is sufficient to apply an HP improving material or the like to which one or more elements are added. Further, the material is not limited to this, and a material having required high temperature creep rupture strength, oxidation resistance, weldability, etc. can be arbitrarily used.

FIG. 1 schematically shows a two-layer laminated structure of the heat resistant steel pipe of the present invention. (A) is an outer side wall layer, (b) is an inner side wall layer. First made of the above Fe-Cr-Ni heat resistant steel
The positional relationship between the inner and outer layers of the pipe wall formed by the above wall layer and the second wall layer made of the above heat-resistant steel having improved carburization resistance varies depending on the specific use mode of the pipe. In a normal ethylene cracking tube, since the inner space of the tube serves as a reaction system, the inner wall layer (b) is formed of heat-resistant steel having carburization resistance as the second wall layer, and the outer wall layer (a) is formed as the second wall layer. As the wall layer of No. 1, a two-layer structure formed of Fe-Cr-Ni heat resistant steel is provided. Needless to say, in the case of a usage mode in which the outside of the pipe is a carburizing environment, the application of the above two kinds of heat resistant steels to the inner and outer wall layers is the reverse of the above. The layer thickness of the second wall layer made of the carburization resistance improving material is not particularly limited, but when it is used as either the inner wall layer or the outer wall layer, it is about 0.5 to 3 mm from the viewpoint of the carburization suppression prevention function. Is sufficient, and it is not necessary to intentionally make the layer thicker than that.

The heat-resistant steel pipe of the present invention can be manufactured by a two-stage casting operation using the centrifugal force casting method. When the inner surface of the tube requires carburization resistance, as the first stage casting, a molten metal of Fe-Cr-Ni heat-resisting steel, which is the first wall layer material, is injected into the centrifugal casting mold. The outer wall layer (a) is formed, and then the inner wall layer (b) is formed by the second-stage casting in which a molten second-thickness material of heat-resistant steel having improved carburization resistance is injected. By forming, a desired double-layer tube is obtained. If the outer wall layer (a) is required to have carburization resistance, the injection of the two types of heat-resistant steel melt may be reversed. According to centrifugal casting, the outer wall layer (a)
It is easy to form a metallurgically fusion-bonded state at the interface between the inner wall layer (b) and the inner wall layer (b), and the obtained double-layer pipe has excellent laminated structure robustness and stability, and also in the casting process. The non-metallic inclusions and bubbles floating in the molten metal are centrifugally separated due to the difference in specific gravity from the molten metal due to the action of the centrifugal force, and thus the casting quality is excellent.

The production of the heat-resistant steel pipe having the double-layer structure of the present invention is not limited to the above-mentioned casting method, but it can be produced by, for example, a hot extrusion molding method. When a hot extrusion method is applied to produce, for example, a two-layer pipe in which the inner wall surface of the pipe is required to have carburization resistance, the Fe-Cr-Ni-based material that is the first wall layer material that is the outer layer is used. A heat-resistant steel ingot and a powder having an improved carburization-resistant heat-resistant steel composition, which is a second wall layer material serving as an inner layer, are prepared. After hollowing out the ingot to form a hollow cylindrical body to be the outer layer, the inner surface of the hollow hole is filled with heat-resistant steel powder to be the inner layer through the encapsulant, and hot isostatic pressing is performed. A sintered alloy is formed by applying a sintering method such as pressure sintering. Then, it is subjected to hot extrusion forming processing to obtain an intended two-layer laminated tube.

In addition to the straight pipes (straight pipes), bend pipes such as U-shaped pipes and elbow pipes are incorporated to construct the ethylene cracking tubes in the furnace. As these bend pipes, straight pipes obtained by centrifugal casting, hot extrusion molding, or the like, which have been subjected to bending (high-frequency bending, etc.) and formed into a desired shape can be used.

[0030]

[Examples] Production of test tubes In a high-frequency melting furnace, heat-resistant steel as the first wall layer material and second
Heat-resistant steel (carburizing resistance improving material), which is the material for the wall layer, is subjected to centrifugal casting and is a two-layer laminated pipe by two-stage casting (Invention Examples) No. 1 and No. I made 2. In both cases, the outer wall layer a was the first wall layer material, and the inner wall layer b was the second wall layer material (carburization resistance improving material). The tube size (after machining) is 140 mm in outer diameter, 10 mm in wall thickness (7 mm in outer wall layer, 3 mm in inner wall layer), and 520 mm in length. As a comparative example, Fe-Cr which is a conventional reaction tube material
A single-layer pipe (centrifugal casting) made of Ni-based heat-resistant steel was prepared. The tube size (outer diameter, wall thickness, length) is the same as above.

Table 1 shows the wall layer composition and chemical composition of each test tube. The grades of the outer wall layers of the two-layer pipes of the invention examples (No. 1 and 2) and the grades of the comparative examples No. 11 and No. 12 (single-layer pipes) were all equivalent to HP40 improved material. It is heat resistant steel.

High-temperature carburizing test From each test tube, a test piece of the shape shown in FIG. 3 (test surface is inner surface, finished surface roughness: 25 S. width w: 30 mm, length 1: 70 mm)
Is cut out and embedded in a solid carburizing agent (Degussa KG30). This is heated to 850 ℃, it takes 1 hour from the same temperature for 30 hours.
The temperature was raised to ℃, held at the same temperature for 18 hours, and then the heat pattern of decreasing temperature was repeated 17 times (test time: (30Hr + 18H
r) × 17 times = 816 Hr). After the test, test surface (inner surface)
To a depth of 5 mm with a pitch of 0.5 mm (total 10
Chips are collected from the location) and the amount of C is analyzed by chemical analysis, and the amount of carbon increase is determined by subtracting the amount of C before the test from the analytical value. The right column of Table 1 shows the carbon increase amount of each test tube (total value of carbon increase amount at each of 10 positions in the depth direction).
FIG. 2 shows the distribution of carbon increase in the depth direction from the test surface for the test tubes No. 1 and 2 (invention example). As is clear from these test results, the sample tube No.
Nos. 1 and 2 have remarkably excellent carburization resistance due to the second wall layer (inner wall layer in this example), and the amount of carbon increase is extremely small compared to the conventional material, and the first wall layer (outer side) It can be seen that the diffusion and penetration of carbon into the wall layer) is almost completely blocked.

Weldability test A U-shaped groove was formed on the end surface of the test pipe material, and GTA was used under the following conditions.
Butt welding by W welding was performed to form a pipe joint.
Each pipe joint was inspected by die check for cracks in the first layer and the final layer. Inventive sample tube (No. 1,
No. 2), like the conventional test tubes No. 11 and No. 12, has no cracks and has a good welding quality. Welding position Downward (welding under rotation by turning roller) Welding rod 0.4 C-1Si-1Mn-25Cr-45Ni-5
Mo Welding current: 85 to 130 A, welding speed 8 to 13 cm / min Number of built-up layers: 6 layers

[0034]

[Table 1]

[0035]

INDUSTRIAL APPLICABILITY The heat-resistant steel pipe of the present invention having a two-layer structure of the first wall layer and the second wall layer is significantly carburized as compared with the heat-resistant steel pipe conventionally used as an ethylene cracking tube. Has resistance. The heat-resistant steel pipe of the present invention can be used stably as a reaction pipe for the petrochemical industry in the recent high-temperature operation over the conventional heat-resistant steel reaction pipe due to improved carburization resistance and the like, and has improved durability. It also has the effects of reducing maintenance and improving the efficiency of reaction operations. In addition, the heat-resistant steel pipe of the present invention is also useful as, for example, a radiant tube of a heat treatment furnace for steel products.

[Brief description of drawings]

FIG. 1 is a radial cross-sectional view schematically showing an example of a two-layer structure of a two-layer heat resistant steel pipe of the present invention.

FIG. 2 is a graph showing the distribution of carbon increase in the depth direction of a carburized test piece.

FIG. 3 is a diagram illustrating the shape of a carburized test piece.

[Explanation of symbols]

 a outer wall layer, b inner wall layer.

Claims (4)

[Claims] 1. A first Fe-Cr-Ni heat-resisting steel.
Wall layer of C: 0.01 to 0.5%, Si: more than 4%, 7% or less, Mn: 3% or less, P: 0.03% or less, S: 0.03% or less, Cr: 20 to 35%, Ni: 20 ~ 60%, Mo: 2-15
%, The remainder being a two-layer laminated pipe with a second wall layer made of a heat-resistant steel having a chemical composition of substantially Fe. A heat-resistant steel pipe excellent in carburization resistance. 2. The heat resistant steel pipe according to claim 1, wherein the heat resistant steel forming the second wall layer contains 1 to 10% of Nb. 3. The heat resistant steel forming the second wall layer contains 0.5% or more of Co, and Co + Ni is 20 to 60%.
Alternatively, the heat-resistant steel pipe according to claim 2. 4. The heat resistant steel forming the second wall layer is Al: 0.02.
~ 1%, Ti: 0.02 to 0.5%, W: 5% or less, Ca: 0.
Any one of claims 1 to 3 containing one or more elements selected from the group consisting of 001 to 0.5%, B: 0.05% or less, Y: 0.5% or less, and Hf: 0.5% or less. Heat-resistant steel pipe described in one.