JPH1046315A - Corrosion resistant heat transfer tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion resistant heat transfer tube having long life and usable over a long period even in a waste incineration furnace. SOLUTION: The outside peripheral surface of a steel tube is coated, by thermal spraying, with an Ni-base self-fluxing alloy of 25-50wt.% Cr content to about 1-3mm thickness and the resultant sprayed coating is subjected to heating and remelting treatment, by which the heat resistance heat transfer tube, having the Ni-base self-fluxing alloy film adhering and joining to the steel tube as base material and free from pores, is produced. Because of high Cr content in the Ni-based self-luxing alloy film, this corrosion resisting heat transfer tube has excellent corrosion resistance even against the severe corrosive atmosphere in a waste incineration furnace and can be used over a long period with prolonged service life.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a corrosion-resistant heat transfer tube suitable for use in a steam power generation facility installed for effectively utilizing the waste heat of a refuse incinerator.

[0002]

2. Description of the Related Art One of the solutions aimed at solving the problem of refuse disposal, which is a recent concern, is a power generation system combined with an incinerator. A major challenge in the above method is to increase power generation efficiency.
Increase in heating temperature of steam by combustion gas (currently 300 ° C
To 500 ° C.), but this goal has not yet been achieved. This is because heat transfer tubes for steam generation or overheating are exposed to combustion gas containing various corrosion media generated in the refuse combustion furnace, and ash containing various corrosion media adheres to the heat transfer tubes. However, since it is placed in an extremely severe corrosive atmosphere, the outer surface of the heat transfer tube is greatly corroded. In particular, when the heating temperature of the steam is increased, this corrosion is further increased and the life is shortened. Therefore, in order to suppress corrosion when used in refuse incinerators, public and private researches are being actively conducted on measures to prevent corrosion of heat transfer tubes. It has been reported that a Ni-based high Cr material (for example, Cr of 25% or more) is good as a metal material.

[0003]

However, such N
Although the i-based high Cr material is present, it is not easy to form a pipe and has low toughness, so that it is still in a stage where improvement and development are still required for use as a solid pipe material. It is also expensive.

The present invention has been made in view of such conventional problems, and can exhibit excellent corrosion resistance, strength, and toughness even when used at a high temperature in a refuse incinerator, and can be manufactured easily and at low cost. An object of the present invention is to provide a corrosion-resistant heat transfer tube that can be manufactured.

[0005]

Means for Solving the Problems The present inventors have made the above Ni
As a result of studying to solve the problem when using the base high Cr material, that is, the problem of pipe forming or toughness, this problem can be solved by spray-coating the Ni base high Cr material on the outer surface of the steel tube having high toughness. That is, the present invention has been completed. That is, according to the present invention, the outer peripheral surface of the steel pipe is provided with Ni
A Ni-based self-fluxing alloy having a Cr weight content of 25 to 50%, which is a base high Cr material, is spray-coated to a thickness of about 1 to 3 mm,
Thereafter, the formed thermal spray coating is heated and re-melted. As described above, the configuration using thermal spraying can be implemented by adjusting the components of the thermal spray material in the current technology, solving the problem of pipe making, and the steel pipe as the base material exhibits strength and toughness. In addition, the sprayed coating of Ni-based high Cr material on the surface exhibits excellent corrosion resistance, and can be used for a long time even in a refuse incinerator.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a method for manufacturing a Ni-based self-fluxing alloy having a Cr content of 25 to 50% on an outer peripheral surface of a steel pipe by 1 to 3 mm.
This is a corrosion-resistant heat transfer tube characterized in that a thermal spray coating is formed to a thickness of about a degree and then the formed thermal spray coating is heated and re-melted.

The steel pipe used in the present invention has an anticorrosion film formed by thermal spraying on the outer peripheral surface, which exhibits corrosion protection against combustion gases. Therefore, the steel pipe itself is not required to have corrosion protection against combustion gases. Any strength, toughness, heat resistance, etc. required when used as a heat transfer tube for a boiler is arbitrary.For example, a carbon steel tube, an alloy steel tube, etc. used in a normal boiler can be used. Can be used.

The remarkable anticorrosion effect of the Ni-based self-fluxing alloy film formed on the surface of the steel pipe can be obtained at a Cr content by weight of 25% or more, but is reduced to 50% or less in view of preparation of a thermal spray material or thermal spray suitability. Must be fastened. Accordingly, in the present invention, a Ni-based self-fluxing alloy having a Cr content of 25 to 50% is used.

Conventionally, it has been considered difficult to spray such a Ni-based high Cr material, but it has been found that it can be carried out by ordinary flame spraying. That is, if thermal spraying of about 100 μm / 1 pass is repeated, thermal spraying can be performed without cracking. By the way, a thermal sprayed coating that has only been sprayed not only has a large number of pores, but also has an imperfect adhesion to a base steel pipe, and as such, it cannot exhibit sufficient corrosion resistance.
Therefore, after forming a coating by thermal spraying, the coating is heated to a temperature corresponding to the melting point of the thermal sprayed material to perform a re-melting process. When this remelting treatment is performed, in the process, boron and silicon in the film alloy convert the gas and metal oxide in the film into borosilicate glassy slag and float on the surface of the film, thereby forming pores in the film. And a complete bond between the thermal spray coating and the base material can be obtained by the creation of a thin alloy layer. In this way, it is possible to obtain a film having no pores and firmly adhered to the steel pipe as the base material, and by this film, it is possible to reliably prevent the corrosion medium from reaching the base steel pipe and to improve the adhesion with the base. To avoid thermal strain destruction at high temperatures.

[0010] In performing the re-melting treatment of the above-mentioned sprayed coating, as a method of heating the coating, heating by a gas flame, induction heating, heating by a furnace, and the like can be mentioned. Among them, induction heating is preferable. By using this induction heating, it is possible to heat quickly and easily to a desired temperature, and since the heating is generated from the surface of the base steel pipe, the coating is heated from the bottom to the surface, and the coating and the mother pipe are heated. Assured close contact with the steel pipe and elimination of pores can be obtained.

By the way, if the re-melting treatment of the thermal spray coating is performed in the air, oxidation in the molten state proceeds, and the surface of the thermal spray coating may be roughened. Therefore, in order to prevent this roughening, after forming a thermal spray coating, apply an antioxidant such as flux to the surface of the thermal spray coating and dry it.
It is preferable to perform the re-melting treatment by induction heating. Alternatively, place the thermal spray coating in an inert atmosphere,
In this state, it is preferable to perform the remelting treatment by induction heating. By employing these methods, it is possible to prevent rough surface due to oxidation of the surface at the time of re-melting, and to form a high quality metal spray coating.

Furthermore, as a result of confirmation by the present inventors, the quality of the coating after the re-melting treatment of the thermal sprayed coating depends on the length of time since the coating started melting, irrespective of the suitability of the processing temperature. It is greatly affected, and it is possible to obtain a film of almost constant quality by setting a temperature rising rate in a temperature range from a temperature at which the film starts to melt to a predetermined appropriate remelting temperature to a certain lower limit or more, It has been found that this specific lower limit is 2 ° C./sec for the Ni-based self-fluxing alloy film. Therefore, when the film is heated in the re-melting process, the temperature at which the film starts to melt (about 900
° C) to an appropriate remelting temperature (about 1050 to 1200 ° C depending on the Cr content)
It is preferable to heat at a temperature rising rate of 2 ° C./sec or more, whereby a film of good quality (having almost no pores and well adhered to the base material) can be obtained.

Further, when the base material steel pipe is a low alloy steel or an austenitic stainless steel, when the coating is induction-heated during the remelting treatment, the properties of the base material change before and after the treatment depending on the cooling rate at that time, and after the remelting treatment, There is a possibility that cracks may occur in the thermal spray coating. Therefore, it is desirable to adjust the cooling rate after heating so that the properties of the base material do not change. For example, when the base material steel pipe is a low alloy steel, in the re-melting treatment of the coating, after induction heating, it is allowed to cool to a temperature at which the re-melted coating becomes a stable state, and the base metal starts to transform, and the troostite Covering with a heat insulating material before the temperature does not result in formation of a bainite structure, and cooling gradually, the structure of the steel pipe becomes the same as the structure before treatment, the hardness does not change, and cracks occur in the coating after remelting treatment. Can be prevented. Also, when the base steel pipe is austenitic stainless steel, if cooling is performed normally in the cooling process after remelting and heating, sensitization will occur, and in order to prevent this, air for cooling is flowed through the inner surface of the steel pipe. If the heating speed is increased while moving and heating, the sensitization can be prevented, and a high-quality sprayed coating can be formed.

In the present invention, as described above, a thermal spray coating of a Ni-based high Cr material is formed on the outer peripheral surface of a steel pipe, and the thermal spray coating provides corrosion resistance. However, although the corrosion resistance was improved by providing the thermal spray coating, the corrosion of the thermal spray coating itself due to the molten salt of high-temperature chlorides and sulfates as the corrosive medium was not zero, so the thickness of the thermal spray coating was reduced. About 1m
m or more to secure corrosion allowance. In addition, the thickness of about 1 mm or more substantially eliminates the problem of pinholes. On the other hand, a thickness of about 3 mm or more is not practical because there is a risk of cracking due to accumulation of thermal strain. Therefore, in the present invention, the thickness of the film is set to about 1 to 3 mm.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific examples will be described below. [Example 1] Steel pipe (material: STPA22, dimensions: 48.
A Ni-based high Cr material (composition: 40.5% Cr, 3.5% B, 3.0% Si) is formed on an outer peripheral surface of 8 mmφ × 5.1 mmt).
%, Mo 2.3%, Cu 2.0%, C 0.7%, NiB
al) was sprayed to a thickness of 1 mm by flame spraying.
Thereafter, in a state where the flux material was applied to the surface of the sprayed coating, the coating was heated to about 1150 ° C. by high frequency induction heating to perform a re-melting treatment, and then allowed to cool. The heating rate during this heating is 5 between 900 ° C and 1150 ° C.
° C / sec. As described above, it was possible to obtain a good film having a smooth surface and no pores inside.

[Example 2] Steel pipe (material: STPA22,
Dimensions: 48.8mmφ × 5.1mmt)
i-based high Cr material (composition: Cr 26.0%, Fe1.0
%, Si 4.0%, B 3.3%, C 1.0%, NiBa
1) was sprayed to a thickness of 1 mm by flame spraying. Thereafter, in a state where the flux material was applied to the surface of the sprayed coating, the coating was heated to about 1080 ° C. by high frequency induction heating to perform a re-melting treatment, and then allowed to cool. The heating rate during this heating is 5 ° between 900 ° C. and 1080 ° C.
C / sec. As described above, it was possible to obtain a good film having a smooth surface and no pores inside.

[Embodiment 3] Round bar of steel (material: SS40)
0, dimensions: 15mmφ × 50mm length)
The same Ni as in Examples 1 and 2 was applied to all the surfaces.
The base self-fluxing alloy was flame-sprayed to a thickness of 1 mm, and then heated and re-melted in the same manner as in Examples 1 and 2 to obtain test pieces 1 and 2. For comparison, a Ni-based low Cr material (composition: Cr 10.0%, B 2.0%, Si 3.0%, C
0.5%, Fe 3.0%, NiBal) is sprayed to a thickness of 1 mm by flame spraying, and then heated to about 1050 ° C. by high frequency induction heating in a state where a flux material is applied to the surface of the sprayed film. Then, re-melting treatment was performed, and then the mixture was allowed to cool to obtain a test piece 3 having a film formed on the entire surface. The heating rate during this heating was 5 ° C / sec from 900 ° C to 1050 ° C.

In order to measure the corrosion resistance of the test pieces 1, 2, and 3 thus obtained, the test pieces 1, 2, and 3 coated with incinerator ash were inserted into the experimental furnace, and the atmosphere of the actual furnace was substantially reproduced. Corrosive gas (8% CO ₂ + 8% O ₂ + 18% H ₂ O +
0.1% HCl + N ₂ ) was supplied, and the furnace temperature was kept at 500 ° C. to perform a corrosion test for 72 hours.

As a result of observing the appearance of the test pieces 1, 2, and 3 after the test, no local corrosion was observed, and the test pieces seemed to be slightly corroded as a whole. The weight of each test piece was measured, and the amount of corrosion was measured from the difference from the weight before the test. The results were as follows. Test piece 1 (Example of the present invention) 0.05 mg / cm ² / h Test piece 2 (Example of the present invention) 0.06 mg / cm ² / h Test piece 3 (Comparative example) 0.10 mg / cm ² / h As can be seen from the results, the coating of the Ni-based high Cr material had better corrosion resistance than the coating of the Ni-based low Cr material.

[0020]

As described above, according to the present invention, a Ni-based self-fluxing alloy having a Cr content of 25 to 50% is spray-coated on the outer peripheral surface of a steel pipe to a thickness of about 1 to 3 mm, and then formed. Since the sprayed coating is heated and re-melted, the surface of the steel pipe is covered with a sprayed coating with excellent corrosion resistance, so that it is placed in a refuse incinerator exposed to high temperatures and in a complex salt environment. It can exhibit excellent corrosion resistance as a heat transfer tube to be arranged, and when used as a boiler heat transfer tube installed in a refuse incinerator, can greatly contribute to improving the life of the boiler heat transfer tube whose operating temperature has been increased, This has the effect of increasing power generation efficiency. Thus, the present invention has an extremely large industrial effect and economic effect, and greatly contributes to the development of industry.

Claims (1)

[Claims] 1. A steel pipe having an outer peripheral surface having a Cr content by weight of 25 to 25%.
A corrosion-resistant heat transfer tube characterized in that a 50% Ni-based self-fluxing alloy is spray-coated to a thickness of about 1 to 3 mm, and then the formed sprayed coating is heated and re-melted.