JP2817587B2 - Wear resistant multi-layer steel pipe for boiler and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hard bed having an outer surface layer required for a coal-fired fluidized-bed boiler, the outer layer of which has excellent abrasion resistance against collisions of unburned coal fine powder and coal-burning ash. And a method for producing the same.

[0002]

2. Description of the Related Art In recent years, with the diversification of energy sources, there has been a tendency to use coal again as an energy source instead of oil. In particular, coal-fired fluidized bed boilers and pressurized fluidized bed boilers have high power generation efficiency, can suppress the generation of nitrogen oxides, and can also use raw materials such as coal and coal ash that contain sulfur and have a high ash content. So it is getting more attention.

[0003] However, in these power generation methods, hard particles such as unburned coal fine powder and coal-burning ash are scattered in the boiler, causing high-temperature solids generated by high-speed collision with boiler members such as heater tubes and evaporator tubes. Particle erosion is becoming increasingly recognized as a serious form of damage to boiler components. In particular, recently, since it is desired that the power generation capacity and power generation efficiency per unit are comparable to those of oil-fired power generation, a boiler member that is less likely to undergo erosion damage even under increasingly severe conditions is required.

[0004] Hard materials such as Co-based alloys such as stellite can be overlaid and sprayed on the surface layer of steel pipes for boilers and heat exchangers to improve erosion resistance. Is inferior, so that bending cannot be performed, and there is a trouble that on-site thermal spraying is required.
Further, the Co-based alloy has a disadvantage that the production cost is increased because it is an expensive material.

JP-A-57-16149 and JP-A-57-16151
In the publication, C: 0.3 to 0.9%, Si: 1.5% or less, Mn: 1.5%
%, Cr: 3.5 to 12% as the basic composition, Ni,
A precipitation hardening wear resistant nitriding steel containing Al, Cu, Mo, W, V, etc. is shown. These steels contain a large amount of S in order to improve machinability, and even if such steels are applied to the outer layer of wear-resistant multi-layer steel pipes for boilers, low melting point compounds are precipitated and heat is generated. The brittleness becomes remarkable, and hot extrusion is difficult.

Japanese Unexamined Patent Publication No. Sho 60-196502 discloses a coal-fired boiler comprising a steel pipe for a boiler / heat exchanger and an outer layer steel having excellent high-temperature particle erosion resistance laminated on the outer surface thereof. Double-walled steel pipes have been proposed. However, this outer layer steel is an extremely common steel based on high Si and high Cr, cannot be said to have sufficient erosion resistance, and is insufficient as a coal-fired boiler member.

Japanese Patent Application Laid-Open No. 61-110714 discloses a steel pipe for a boiler / heat exchanger and an outer layer steel pipe of a precipitation hardening alloy laminated on the outer surface thereof. A heat transfer tube characterized by hardening the outer layer steel pipe by performing aging treatment after adding the above process has been proposed. However, in this method, although the hardness is increased by the aging treatment, the hardness is up to about Hv350, and a heat transfer tube having sufficient performance for high-temperature particle erosion resistance in a severe environment cannot be obtained.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a wear-resistant multi-layer steel pipe for a boiler having the following characteristics and a method for producing the same.

It is necessary to have sufficient abrasion resistance on the outer layer tube surface against wear due to collision of high-temperature and high-hardness particles, as in a coal-fired pressurized fluidized-bed boiler, and to have strength as a boiler steel tube.

[0010] The heat treatment of the steel tube for the boiler / heat exchanger as the inner tube also significantly increases the hardness of the outer tube.

[0011] The steel of the outer tube is easy to obtain a high hardness nitrided layer by gas nitriding.

[0012] The outer tube having a high hardness layer and the steel tube for the boiler / heat exchanger of the inner tube are completely adhered to each other, so that there is no hindrance to heat conduction and no hot spot due to poor local adhesion is generated.

In the fluidized-bed boiler, a bent pipe portion is also worn, so that a bent pipe member of a multilayer steel pipe can be manufactured.

[0014]

The gist of the present invention is as follows.
(1) The wear-resistant multi-layer steel pipe for a boiler according to (2) and the production method thereof according to (3) to (5).

(1) The inner layer is a steel tube for a boiler / heat exchanger, and the outer layer is by weight, C: 0.1-1.2%, Si: 0.1-1.5.
%, Mn: 0.1-1.5%, Cr: 6-20%, Mo: 0.1-10%,
V: 0.1 to 4%, Nb: 0.1 to 3% and Al: 0.1 to 3%, the balance being Fe and inevitable impurities, P in the impurities is 0.01% or less, and S is 0.01% or less in the impurities. A wear-resistant multi-layer steel pipe for a boiler, which is a steel pipe obtained by nitriding hardening type steel for nitriding.

(2) The weight percentage of the outer layer is, in addition to the chemical components described in (1) above, Cu: 2% or less, Ni: 4% or less,
W: 4% or less, Ti: 3% or less, Zr: 1% or less, and Ta:
The above-mentioned (1), which is a steel pipe obtained by nitriding a precipitation hardening type nitriding steel containing at least one of 1% or less.
The wear-resistant multi-layer steel pipe for a boiler according to the above.

(3) A steel pipe for a boiler / heat exchanger and a steel pipe manufactured from a precipitation hardening type nitriding steel having the chemical composition described in the above (1) or (2) are arranged outside the steel pipe. The billet is assembled, the composite billet is hot-extruded to produce a clad steel pipe, then the steel pipe for a boiler / heat exchanger is subjected to a normalizing treatment, a tempering treatment is then performed, and a nitriding treatment is further performed to precipitate outside. A method for producing a wear-resistant multi-layer steel pipe for a boiler, comprising forming a surface hardened layer on a hardening type steel pipe for nitriding.

(4) A thin-walled metal cylinder is concentrically arranged outside the steel pipe for a boiler / heat exchanger, and one side of the steel pipe and the cylinder is fixed with a disk member to form a capsule pipe formed of a steel pipe and a cylinder.
After filling the precipitation-hardening type nitriding steel powder having the chemical composition described in the above (1) or (2) into the annular gap of the capsule tube, the open end of the capsule tube is placed in the second disk member. To form a hollow composite billet, hot extruding this composite billet to produce a clad steel pipe, then normalizing the steel pipe for boilers and heat exchangers, then tempering it, and then nitriding And forming a hardened surface layer on the outer precipitation hardening type steel pipe for nitriding by using a method for producing a wear-resistant multilayer steel pipe for a coal boiler.

(5) In the manufacturing method described in the above (3) or (4), the hollow composite billet is hot-extruded to produce a clad steel pipe, and then subjected to a bending process.
A heat-resistant steel pipe for a heat exchanger, which is subjected to a normalizing treatment, then to a tempering treatment, and further to a nitriding treatment to form a hardened layer on the outer precipitation hardening type steel pipe for nitriding. A method for manufacturing a layered steel pipe.

[0020]

[Action] As a coal-fired fluidized-bed boiler, a steel pipe that has both sufficient abrasion resistance on the outer surface and the strength of a boiler steel pipe against abrasion due to the collision of high-temperature and high-hardness particles, the operating conditions of the equipment Steel pipes applicable to steel pipes for boilers and heat exchangers conforming to (for example, carbon steel pipes for boilers and heat exchangers specified in JIS G 3461, alloy steel pipes for boilers and heat exchangers specified in JIS G 3462, Further, a clad steel pipe in which a high-temperature, high-hardness material is multi-layered and adhered to the outer layer of the inner layer pipe, which is a stainless steel pipe for boilers and heat exchangers specified in JIS G 3463, is most suitable.

A precipitation hardening type nitriding steel is used as a high hardness material to be adhered to the outer layer of the tube. The hardness of the precipitation hardening type nitriding steel is increased by the precipitation hardening treatment, and the surface hardness is increased by performing the nitriding treatment, and the attained value satisfies the object of the present invention. Stellite, or high speed steel or cemented carbide as a tool material.

The advantages of such a hardness-improving effect and cost advantages over cemented carbides can be attributed to precipitation hardening-type nitriding steel using an ingot produced by the ingot method, or powder produced by the N ₂ or Ar gas atomizing method. The same is true in any case of precipitation hardening type nitriding steel.

The reason why the alloying elements of the precipitation hardening type nitriding steel and the appropriate contents thereof are determined as described above and the characteristics of this steel will be described in detail.

C: 0.1-1.2% If less than 0.1%, the nitride layer becomes brittle. On the other hand, if it exceeds 1.2%, nitriding becomes difficult. Therefore, the range of the C content is
0.1 to 1.2%.

Si: 0.1-1.5% Deoxidizing agent, a content of 0.1% or more is required to obtain this effect. On the other hand, if it exceeds 1.5%, it becomes brittle. Therefore, the range of the Si content is set to 0.1 to 1.5%.

Mn: 0.1-1.5% Mn is an element necessary for improving hardenability. To obtain this effect, a content of 0.1% or more is required. Meanwhile, 1.
If it exceeds 5%, it becomes brittle. Therefore, the range of Mn content is
0.1 to 1.5%.

Cr: 6 to 20% At less than 6%, erosion wear becomes significant. But,
Forms over 20% of giant carbides and deteriorates toughness.
Therefore, the range of the Cr content is set to 6 to 20%.

Mo: 0.1-10% Mo is contained to prevent temper brittleness and strengthen solid solution. 0.
If less than 1%, this effect cannot be obtained. However, if it exceeds 10%, the effect will be saturated, and the economic efficiency will deteriorate. Therefore, the range of the Mo content is set to 0.1 to 10%.

V: 0.1-4%, for tempering the steel pipe for the boiler / heat exchanger, which is the inner pipe, so that the outer pipe is also precipitated and hardened.
If less than 0.1%, this effect cannot be obtained. On the other hand, if it exceeds 4%, the effect will be saturated and the economic efficiency will deteriorate. Therefore, the range of the V content is set to 0.1 to 4%.

Nb: 0.1-3% Nb is a carbide and nitride forming element and is contained in order to improve wear resistance and heat resistance. If less than 0.1%, this effect cannot be obtained. On the other hand, if it exceeds 3%, the effect becomes saturated, so that the economic efficiency deteriorates. Therefore, the range of the Nb content is set to 0.1 to 3%.

Al: 0.1 to 3% Al is a nitride-forming element and is contained to increase the surface hardness by nitriding. If less than 0.1%, this effect cannot be obtained. On the other hand, if it exceeds 3%, the melting point of the steel decreases and hot workability deteriorates. Therefore, the range of Al content is 0.1
To 3%.

In order to further improve the properties of the precipitation hardening type nitriding steel, one or more of the following Cu, Ni, W, Ti, Zr and Ta are selected and contained as required.

Cu: 2% or less Cu is contained in order to form an intermetallic compound and strengthen precipitation. However, if it exceeds 2%, the hot workability deteriorates, so the upper limit was made 2%.

Ni: 4% or less Ni is added to improve ductility and to form an intermetallic compound to strengthen precipitation. However, when the content exceeds 4%, the effect of suppressing the formation of the nitride layer is obtained.
And

W: 4% or less W is contained in order to increase the tempering softening resistance. However, if it exceeds 4%, a large carbide is formed to deteriorate the toughness, so the upper limit was made 4%.

Ti: 3% or less Ti is an element forming nitride and carbide, and is contained in order to improve wear resistance and heat resistance. However, above 3% the effect saturates. The upper limit of the Ti content is set to 3% in consideration of economy.

Zr: 1% or less Zr is an element for forming nitride and carbide, and is contained in order to improve wear resistance and heat resistance. However, if it exceeds 1%, the effect is saturated. The upper limit of the Zr content is set to 1% in consideration of economy.

Ta: 1% or less Nitride and carbide Nitride forming element contained in order to improve wear resistance and heat resistance. However, if it exceeds 1%, the effect is saturated. In consideration of economy, the upper limit of the Ta content is set to 1.0%.

P and S: P and S are impurities, but if both exceed 0.01%, a low-melting-point compound is precipitated, hot brittleness becomes remarkable, and extrusion becomes difficult. There must be.

The steel pipe manufactured from the precipitation hardening type nitriding steel as described above is subjected to a normalizing treatment (JIS G3461, JIS G3462 and JIS G3462) for the boiler / heat exchanger steel pipe as the inner layer pipe.
(Specified in SG3463)
Carbide precipitates, and the hardness of the entire steel pipe greatly increases.

Since the nitriding steel can be cured at a considerably low temperature (about 500 ° C.) by the nitriding treatment, even if the nitriding treatment is performed after the tempering treatment on the steel pipe for the boiler / heat exchanger, its properties are adversely affected. Never. Therefore, the wear-resistant multi-layer steel pipe for a boiler of a straight pipe can be manufactured by a method of processing, heat-treating, and then nitriding using a conventional manufacturing process of a steel pipe for a boiler.

Further, after the hot extruded clad steel pipe is subjected to hot, warm or cold, preferably hot or warm curved pipe processing, and then subjected to the above-mentioned heat treatment, followed by nitriding treatment. By performing this method, it is also possible to manufacture a multi-layer curved pipe portion having an outer layer pipe having a high hardness.

The hardness of the steel pipe for nitriding after heat treatment of the steel pipe for boiler / heat exchanger is Hv at high temperature (350 ° C.).
350 or more, the highest surface hardness after nitriding is Hv
It is desirable to be 600 or more. Usually, the thickness of the outer layer of the precipitation hardening type nitrided steel pipe is sufficient to be about 2 to 4 mm. For example, this thickness is 4 mm, and the whole high temperature hardness is Hv400 by heat treatment.
When the outermost surface high-temperature hardness is set to Hv 1000 by nitriding treatment, the high-temperature hardness distribution in the depth (thickness) direction of the precipitation hardening type nitrided steel tube of the outer layer is about Hv 800, about 0.5 mm
Hv 400 from the depth of 4mm to 4mm.

The features and advantages of the above precipitation hardening type nitrided steel cannot be obtained with stellite or tool steel. That is, since stellite does not soften even by heat treatment or the like, when manufacturing a boiler steel tube of an intimate two-layer tube, 5% is used in cold rolling.
Cracks occur at the following rolling reduction, and cold working for dimensional correction is practically impossible. In the case of tool steel, the heat treatment of the boiler steel pipe causes a decrease in hardness.

Next, a method of forming a hollow composite billet to be subjected to hot extrusion in the production method of the present invention is shown in FIG.
This will be described with reference to FIG.

FIG. 1 is a longitudinal sectional view of a composite billet showing an example in which a precipitation hardening type steel powder for nitriding is used. boiler·
A thin-walled low-carbon steel cylinder 2 is concentrically arranged outside an inner-layer steel pipe 1 which is a steel pipe for a heat exchanger, and one side (the lower side in FIG. 1) is fixed with a carbon steel disk member 4 to form a capsule pipe. This gap is filled with precipitation hardening type steel powder 3 having a particle size of 500 μm or less, and the open end (upper side in FIG. 1) is sealed with a second carbon steel disk member 4 ′, preferably about 400
A layer of the powder 3 is compression-molded by applying a cold isostatic pressure at a pressure of MPa to obtain a hollow composite billet in which the powder 3 is densified.
In this case, it is necessary to form a clad steel pipe by heating and hot extruding in the presence of the low carbon steel cylinder 2, and then to remove a portion corresponding to the clad steel pipe by cutting or the like.

The reason for applying the cold isostatic pressing method when using the steel powder for nitriding is as follows. First, in the upsetting state at the time of hot extrusion, since the precipitation hardening type steel powder for nitriding is higher in hardness than the steel pipe for the boiler, which is the inner layer pipe, the powder layer enters the steel pipe surface,
Destroys the oxide film on the steel pipe surface. Since the interface is extended with the extrusion, the oxide film is divided. Under such circumstances, diffusion of elements occurs at the interface between the powder layer and the steel pipe, and a strong bond is achieved. Also, since the powder 3 is densified by compression molding using cold isostatic pressure,
When the hollow composite billet is heated, its temperature distribution is stabilized.

FIG. 2 is a longitudinal sectional view of a composite billet showing an example in which a steel pipe made of a precipitation hardening type nitriding steel manufactured by a normal melting method is formed by a cold press-fitting method. A steel pipe 5 made of precipitation hardening type steel for nitriding is cold-pressed and closely attached to the outer surface of the inner steel pipe 1, preferably at both ends (up and down in the figure).
Is sealed with a carbon steel disk member 4 to form a hollow composite billet.

When sealed with the carbon steel disk member 4, both the steel pipes 1 and 5 undergo thermal expansion during heating before hot extrusion, but the effect is to prevent a gap from being formed at the interface and prevent the interface from being oxidized. effective. Cold injection is used because the outer surface of the inner steel pipe and the inner surface of the outer steel pipe are in close contact with each other, causing the diffusion of elements at the interface during heating before hot extrusion, and forming a clad steel pipe with good interface adhesion after processing. It is for manufacturing.

The relationship between the outer diameter of the inner steel pipe and the inner diameter of the outer steel pipe at the time of cold press fitting is equal or the inner diameter is larger than the outer diameter.
It is desirable to reduce the thickness by 0.00 to 1.00 mm so that close contact at the time of press-fitting is sufficiently performed.

FIG. 3 is a longitudinal sectional view of a hollow composite billet showing an example of a case where a steel pipe made of a precipitation hardening type steel for nitriding is formed by an insertion method. A steel pipe 5 of precipitation hardening type nitriding steel is inserted into the outer surface of the inner steel pipe 1, and both ends (upper and lower in the figure) are sealed with a carbon steel disk member 4.

In the case of the insertion method, the relation between the outer diameter of the inner steel pipe 1 and the inner diameter of the precipitation hardening type steel pipe 5 of the outer layer is such that the inner diameter is 0.1 to 2.0 mm larger than the outer diameter. This is because if the gap between the two steel pipes is too large, there will be places where the interface between the clad steel pipes after hot extrusion becomes insufficiently adhered. In this case, it is merely insertion, and since air is present in the gap between the two steel pipes, the deaeration pipe 6 is attached to one place serving as a clad surface as shown in the figure to perform vacuum deaeration. Thereafter, the deaeration tube 6 is sealed by welding to form a hollow composite billet.

By using these three kinds of methods, an oxide film which is easily formed on the clad surface when the hollow composite billet is heated is suppressed. In the method using the precipitation hardening type nitriding steel powder shown in FIG. 1, the degassing pipe 6 shown in FIG. 3 can be used, but this is not always necessary. The total amount of oxygen and nitrogen to be picked up by the air remaining in the layer of powder 3 being dissolved in the precipitation hardening type steel powder for nitriding is about 0.01.
This is because it is about weight%, and hardly affects the mechanical properties of the joining or the multi-layer steel pipe for boilers.

In the above-mentioned hollow composite billet, the interface between the powder and the steel pipe or the interface between the both steel pipes is obtained by the cold isostatic pressing method or the cold press-fitting method from the time of heating, and the insertion method by the hot extrusion method. Thus, diffusion of elements occurs, and good bonding is formed, and a clad steel pipe having excellent interface adhesion can be obtained.

[0055]

【Example】

[Invention Example 1] A precipitation hardening type nitriding steel powder having a chemical composition of type 1 to type 9 and type 15 to type 18 shown in Table 1 was produced by the N ₂ gas atomizing method, and the method shown in FIG. 1 was used. JIS STBA22 inner steel pipe 1 (outer diameter 130m
m, an inner diameter of 31 mm, a length of 780 mm) and an outer thin-walled low carbon steel cylinder 2 filled with a nitriding steel powder 3 having a particle diameter of 500 μm or less and sealed with carbon steel disk members 4 and 4 ′, Cold isostatic pressing at a pressure of 400 MPa, outer diameter 174mm, inner diameter 31mm, length 80
A hollow composite billet of 0 mm was produced. 12 this billet
After heating to 00 ° C, hot extrusion at an extrusion ratio of 12
A clad steel pipe having a size of 57 mm × inner diameter 28 mm × length 7 m was manufactured, and a portion corresponding to the outer thin low-carbon steel cylinder 2 was removed by cutting.

These clad steel pipes were heated at 950 ° C. for 10 minutes, subjected to hot bending at a bending radius of 2D (twice the diameter D of the steel pipe), and then annealed at 960 ° C. × 10 minutes. Process,
After tempering at 680 ° C for 60 minutes (the steel pipe for nitriding is equivalent to precipitation hardening), the temperature is 530 ° C in decomposed ammonia gas.
The surface was nitrided by nitriding treatment for × 40 hours to produce a multilayer steel pipe. Thereafter, samples were cut out from the surface layer portion of the curved pipe portion, and the following evaluation tests were performed.

(1) High-temperature hardness test: Measured at a load of 50 g and a test temperature of 350 ° C. using a high-temperature Vickers hardness tester.
The position is 0.1mm from the outermost layer.

(2) Warm erosion test: The test was carried out for 5 hours under the conditions shown in Table 2 using a blast erosion test apparatus, and the thinning rate at that time was measured.

[Invention Example 2] Particle diameters of the chemical compositions of types 10 to 12 shown in Table 1 by Ar gas atomizing method.
A precipitation hardening type steel powder for nitriding of 250 μm or less was produced, and a hollow composite billet having the same dimensions as in Example 1 of the present invention was produced. After heating this billet to 1150 ° C., it was hot-extruded at an extrusion ratio of 15 to produce a clad steel pipe having an outer diameter of 53 mm × inner diameter of 28 mm × length of 9 m, and a portion corresponding to an outer thin-walled low carbon steel cylinder 2. Was removed by cutting.

These clad steel pipes were subjected to a normalizing treatment at 930 ° C. for 20 minutes and a tempering treatment at 680 ° C. for 30 minutes (the steel pipe portion for nitriding corresponds to a precipitation hardening treatment). The multi-layered steel pipe was manufactured by holding at 500 ° C. × 20 hours and further nitriding the surface at 530 ° C. × 40 hours. After that, samples were cut out from the respective surface layers, and the same evaluation test as in Example 1 of the present invention was performed.

[Invention Example 3] An outer layer steel pipe (outer diameter of 17) was prepared from an ingot of precipitation hardening type nitriding steel having a chemical composition of type 13 shown in Table 1.
2mm, inner diameter 139.9mm, length 680mm), and using the method shown in Fig. 2, the inner layer steel pipe 1 (outer diameter) of JIS standard STBA22
140mm, 31mm inside diameter, 680mm length)
After cold-pressing with a press, both ends are sealed with a carbon steel disk member 4, and the outer diameter is 172mm, the inner diameter is 31mm, and the length is 700mm.
Was prepared. This billet is 1200 ℃
After being heated to an extrusion ratio of 20, the product was hot-extruded to an outside diameter of 48 mm.
A clad steel pipe having an inner diameter of 28 mm and a length of 12 m was manufactured.

The clad steel pipe is bent in a cold at a bending radius of 2D.
After bending at 1000 ° C x 10 minutes, normalize at 1000 ° C x 10 minutes, and then temper at 660 ° C x 60 minutes (the steel pipe for nitriding is equivalent to precipitation hardening). By nitriding at 530 ° C. for 40 hours, the surface was nitrided to produce a multi-layer steel pipe. Thereafter, a sample was cut out from the surface layer portion of the curved tube portion, and the following evaluation test was performed.

(1) High-temperature hardness test: Measured under the conditions of a load of 100 g and a test temperature of 350 ° C. using a high-temperature Vickers hardness tester.
The position is 0.1mm from the outermost layer.

(2) Warm erosion test: The test was carried out for 2 hours under the conditions shown in Table 2 using a blast type erosion test apparatus, and the thinning rate at that time was measured.

[Invention Example 4] An outer layer steel pipe (outer diameter 172 mm, inner diameter 140.1 mm, length 680 mm) was manufactured from a precipitation hardening type ingot for nitriding steel having a chemical composition of type 14 shown in Table 1, and Using a method shown in FIG. 3, a nitriding steel pipe 5 is inserted outside the inner steel pipe 1 (outer diameter 140 mm, inner diameter 31 mm, length 680 mm) of STBA22 of JIS standard. Seal and attach a deaeration pipe 6 and use a rotary pump to
After degassing for a time, the degassing pipe 6 is sealed by welding, and the outer diameter is 172 mm.
A hollow composite billet having an inner diameter of 31 mm and a length of 700 mm was produced.
The billet was heated to 1200 ° C. and then hot-extruded at an extrusion ratio of 12 to produce a clad steel pipe having an outer diameter of 57 mm, an inner diameter of 28 mm and a length of 8 m.

The clad steel pipe was subjected to a normalizing treatment at 960 ° C. for 20 minutes and a tempering treatment at 700 ° C. for 30 minutes (the steel pipe portion for nitriding corresponds to a precipitation hardening treatment). The surface was nitrided by nitriding at 40 ° C. for 40 hours to produce a multi-layer steel pipe.

Thereafter, a sample was cut out from the surface layer and subjected to the same evaluation test as that of Example 3 of the present invention.

Comparative Example A normal hollow billet shown in FIG. 4 was produced from ingots of steel (STBA22 of JIS standard) having a chemical composition of type 19 shown in Table 1.

FIG. 4 is a longitudinal sectional view of a billet used for manufacturing a conventional alloy steel pipe for a boiler / heat exchanger. In the drawing, reference numeral 7 denotes an outer diameter of 174 mm manufactured from the above STBA22 steel.
It is a hollow billet with an inner diameter of 31 mm and a length of 700 mm.

After heating this billet to 1200 ° C., it was hot-extruded at an extrusion ratio of 20 to obtain an outer diameter of 48 mm × inner diameter of 28 mm × length of 12 mm.
m was produced.

Next, normalizing treatment was performed at 930 ° C. for 20 minutes,
After tempering at 0 ° C. for 30 minutes, the steel pipe is bent in a cold at a bending radius of 2D.
The surface was nitrided by a nitriding treatment at 0 ° C. × 40 hours. Thereafter, a sample was cut out from the surface layer portion of the curved tube portion, and the same evaluation test as in Example 1 of the present invention was performed.

The results of these tests are also shown in Table 1.

[0073]

[Table 1 (1)]

[0074]

[Table 1 (2)]

[0075]

[Table 2]

As is clear from Table 1, the multilayer steel pipe manufactured under the conditions specified in the present invention has excellent erosion resistance.

[0077]

The wear-resistant multi-layer steel pipe for a boiler according to the present invention is excellent in erosion resistance, heat transfer efficiency and workability required for a steel pipe for a coal-fired power generation boiler. It can be implemented in the process.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view illustrating a hollow composite billet when a precipitation hardening type steel powder for nitriding is used.

FIG. 2 is a longitudinal sectional view illustrating a hollow composite billet manufactured by a cold press-sealing method when using a steel pipe manufactured from an ingot of precipitation hardening type steel for nitriding.

FIG. 3 is a longitudinal sectional view illustrating a hollow composite billet manufactured by an insertion-sealing-degassing method when using a steel pipe manufactured from an ingot of precipitation hardening type steel for nitriding.

FIG. 4 is a vertical cross-sectional view illustrating a billet used for manufacturing an alloy steel pipe for a normal boiler / heat exchanger.

[Explanation of Signs] 1; inner layer steel pipe, 2: low carbon steel cylinder, 3: precipitation hardening type steel powder for nitriding, 4, 4 ': carbon steel disk member, 5: precipitation hardening type steel pipe for nitriding steel, 6 : Deaeration tube, 7: Normal hollow billet

Claims (5)

(57) [Claims] 1. The inner layer is a steel tube for a boiler / heat exchanger, and the outer layer is in weight%, C: 0.1-1.2%, Si: 0.1-1.5%, Mn:
0.1 to 1.5%, Cr: 6 to 20%, Mo: 0.1 to 10%, V: 0.1 to
4%, Nb: 0.1-3% and Al: 0.1-3%, the balance being Fe and unavoidable impurities.
A wear-resistant multi-layer steel pipe for boilers, characterized in that the pipe is a steel pipe obtained by nitriding a precipitation hardening type steel for nitriding of 0.01% or less and S of 0.01% or less. 2. The inner layer is a steel tube for a boiler / heat exchanger, and the outer layer is in weight%, C: 0.1-1.2%, Si: 0.1-1.5%, Mn:
0.1 to 1.5%, Cr: 6 to 20%, Mo: 0.1 to 10%, V: 0.1 to
4%, Nb: 0.1-3% and Al: 0.1-3%, Cu: 2% or less, Ni: 4% or less, W: 4% or less, Ti: 3
% Or less, Zr: 1% or less and Ta: 1% or less, with the balance being Fe and unavoidable impurities, where P is 0.01% or less and S is 0.01% or less. A wear-resistant multi-layer steel pipe for a boiler, which is a steel pipe obtained by nitriding hardening type steel for nitriding. 3. A hollow composite billet comprising a steel pipe for boiler / heat exchanger and a steel pipe made of precipitation hardening type nitriding steel having the chemical composition according to claim 1 or 2 disposed outside said steel pipe. Assembling, hot extruding this composite billet to produce clad steel pipes, then normalizing the steel pipes for boilers and heat exchangers, then performing tempering, and then nitriding to form the outer precipitation hardening type A method for producing a wear-resistant multi-layer steel pipe for a boiler, comprising forming a surface hardened layer on a steel pipe for nitriding. 4. A thin-walled metal cylinder is arranged concentrically outside a steel pipe for a boiler / heat exchanger, and one side of the steel pipe and the cylinder is fixed with a disk member to form a capsule pipe formed of a steel pipe and a cylinder. After filling the annular gap with the precipitation hardening type steel powder for nitriding having the chemical composition according to claim 1 or 2, the open end of the capsule tube is sealed with a second disk member and hollow. Assembling the composite billet, hot extruding the composite billet to produce a clad steel pipe, then performing a normalizing process on the steel tube for a boiler / heat exchanger, performing a tempering process, further performing a nitriding process, and performing an outer nitriding process. A method for producing a wear-resistant multi-layer steel pipe for a boiler, comprising forming a surface hardened layer on a precipitation hardening type steel pipe for nitriding. 5. The method according to claim 3, wherein the hollow composite billet is hot-extruded to produce a clad steel pipe, and then subjected to a bending process.
A heat-resistant steel pipe for a heat exchanger, which is subjected to a normalizing treatment, then to a tempering treatment, and further to a nitriding treatment to form a hardened layer on the outer precipitation hardening type steel pipe for nitriding. A method for manufacturing a layered steel pipe.