JPH08334205A - Waste-heat boiler tube excellent in anticorrosion - Google Patents

Abstract

PURPOSE: To achieve excellent anticorrosion under a corrosive environment of a garbage incinerator by arranging an inner layer made of carbon steel, low alloy steel or stainless steel and an outer layer made of the stainless steel while the outer layer is formed by spray coating or deposit welding. CONSTITUTION: A boiler tube 4 used for a combustion chamber 2 of a garbage incinerator comprises an inner layer made of carbon steel, low alloy steel or stainless steel and an outer layer made of the stainless steel. The outer layer of the stainless steel is formed by spray coating or deposit welding. As a result, a melted part is solidified immediately after the stray coating or deposit welding. But as the cooling speed at this point is by far large as compared with that during the casting period, the growth of gain particles is blocked to obtain a very fine crystal texture. In the very fine crystal texture, a smaller particle size of Cr carbide deposited allows a reduction in difference of the concentration of Cr between the Cr carbide and a base near the Cr carbide thereby achieving excellent anticorrosion of the Cr even under a severely corrosive environment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waste heat recovery boiler tube used in a domestic refuse incinerator.

[0002]

2. Description of the Related Art In recent years, attention has been paid to waste power generation from the viewpoint of effective use of resources. Garbage power generation uses the combustion heat generated when combusting domestic waste (3) put into the combustion chamber (2) from the crane bucket (1) in the waste incinerator as shown in Fig. 1. The water supplied to the boiler tube (4) is converted into high-temperature and high-pressure steam, and the steam is supplied to the generator (5) for power generation.

Conventionally, as a boiler tube material,
Carbon steel pipe (STB; JIS G 3461), low alloy steel pipe (STBA; JIS G 3462), stainless steel pipe
(SUS310TB etc .; JIS G 3463) is known.

By the way, since municipal waste contains plastics, various foods, etc., when these are burned in an incinerator, the combustion waste gas contains not only water vapor but also sulfate or chloride salt. A large amount of dust is contained. HC
When a corrosive gas such as 1, SO ₂ or the like adheres to the surface of the boiler tube, the corrosion tends to be significantly promoted by the combined action with water vapor.

For this reason, stainless steel having corrosion resistance is generally used for the waste heat recovery boiler tube of the refuse incinerator. However, in the case of stainless steel, Cr carbide precipitates at the grain boundaries during the casting of the pipe, so a portion with a low Cr concentration is formed in the vicinity of the precipitated Cr carbide, and it may not be possible to ensure the desired corrosion resistance. . Along with the severer corrosive environment in the refuse incinerator, further improvement in corrosion resistance is desired.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a boiler tube which exhibits excellent corrosion resistance in the corrosive environment of a refuse incinerator.

[0007]

In order to achieve the above object, the boiler tube of the present invention comprises an inner layer of carbon steel, low alloy steel or stainless steel, and an outer layer of stainless steel, and the outer layer of stainless steel is It is formed by thermal spraying or overlay welding.

The corrosion resistance required for boiler tubes is
The material of the inner layer that does not come into contact with corrosive gas may be carbon steel or low alloy steel because it is provided by the stainless steel of the outer layer. In fact, unless it is used particularly in a place exposed to high temperatures, it is not necessary to use expensive stainless steel for the inner layer material, and it is economically advantageous to use carbon steel or low alloy steel.

In order to secure desired corrosion resistance and high temperature strength, the stainless steel used for the outer layer is Cr: 20-40%, N:
High alloy stainless steel containing i: 20-40% is desirable. Further, C not only has the effect of improving the fluidity during melting and suppressing the inclusion of oxides, but also has a large quenching effect after melting. Therefore, C of stainless steel used for the outer layer
The amount is preferably about 0.2 to 0.6%. It is desirable that the thickness of the outer layer is set to be about 1/20 to 1/5 of the inner layer thickness.
This is because with this wall thickness ratio, cooling is performed from a state where the heat accumulation amount is high, so that a rapidly solidified structure is obtained in the outer layer. Although the thickness of the outer layer depends on the tube thickness of the boiler tube, it is generally desired to be about 0.3 to 2 mm.

[0010]

After the thermal spraying or the overlay welding, the molten portion immediately solidifies, but the cooling rate at this time is much higher than the cooling rate at the time of casting, so that the growth of crystal grains is hindered and very fine particles are obtained. A crystal structure is obtained. In the fine crystal structure, since the grain size of the precipitated Cr carbide is small, the difference in the Cr concentration between the Cr carbide and the matrix in the vicinity of the Cr carbide is small, and the excellent corrosion resistance of Cr can be exhibited even in a severe corrosive environment. .

[0011]

EXAMPLE A test piece (10 × 10 × 4 mm) was placed in an electric furnace, and a corrosion test (550 ° C. × 96 hours) simulating the corrosive environment of a municipal solid waste incinerator was conducted. . For the test piece taken out of the electric furnace, the surface deposits were scraped off thoroughly with a nylon scrubber, the dimensions were measured, and the difference from the dimensions before charging into the electric furnace was calculated to determine the erosion depth. Sought as. Table 1 shows the components, content, and supply amount of the simulated gas having substantially the same composition as the corrosive gas generated in the combustion chamber of the refuse incinerator. On the surface of each test piece, combustion ash actually collected from the combustion chamber of the refuse incinerator was applied in advance at a rate of 40 mg / cm ² before charging into the electric furnace. Table 2 shows the components of the combustion ash.

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[Table 1]

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[Table 2]

Table 3 shows the chemical composition of the test piece, the thickness of the built-up portion, and the average erosion depth due to corrosion. The sample No.
Nos. 1, No. 2 and No. 3 are comparative examples formed only from stainless steel, and Nos. 1A, No. 2A and No. 3A are examples of the present invention in which a build-up portion of stainless steel is formed by welding. Is. The weld overlay was formed by a TIG method under an argon stream, and a co-gold electrode rod having a diameter of 1.6 mm was used.

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[Table 3]

As is clear from the results shown in Table 3, the test pieces No. 1A, 2A and 3 of the present invention provided with the weld overlay were provided.
Samples A are test pieces No. 1, No. 2 and No.
It shows that the corrosion amount is smaller than that of No. 3, and the corrosion resistance is excellent.

FIG. 2 shows the metal structure of the sample No. 1A near the boundary between the base metal and the overlay. In FIG. 2, the left side is the structure of the base material, and the right side is the structure of the overlay portion. As is clear from FIG. 2, it is understood that the build-up portion has a very fine crystal structure as compared with the base material.

[0018]

As described above, the two-layer structure boiler tube of the present invention is formed by quenching and solidifying the outer layer of stainless steel by thermal spraying or overlay welding. In comparison, it exhibits excellent corrosion resistance against corrosive gases such as HCl and SO ₂ and is suitable as a boiler tube for waste power generation.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an outline of a waste combustion chamber of a waste incinerator.

FIG. 2 is a drawing-substitute photomicrograph (× 100) showing the structure of the welded joint of the boiler tube of the present invention.

[Explanation of symbols]

 (2) Combustion chamber (3) Garbage (4) Boiler tube

Claims (1)

[Claims] 1. Corrosion resistance, comprising an inner layer of carbon steel, low alloy steel or stainless steel and an outer layer of stainless steel, the outer layer of stainless steel being formed by thermal spraying or overlay welding. Boiler tube for excellent waste power generation.