JPS61169174A - Member for boiler provided with co alloy build-up layer - Google Patents

Abstract

PURPOSE:To improve the resistance to erosion and corrosion by forming a Co alloy build-up layer into which specific % or above of Cr is incorporated and which has prescribed Vickers hardness. CONSTITUTION:The Co alloy build-up layer contg. >=20% Cr and having 400-550kgf/mm<2> Vickers hardness at an ordinary temp. is formed to a boiler pipe material consisting of a stainless steel or Cr-Mo steel, etc. The hardness after the building up is controlled by adjusting the content of C in this stage. The erosion resistance is remarkably improved if the hardness of the build-up layer is >=400kgf/mm<2> but the resistance to deformation decreases when the hardness exceeds 550kgf/mm<2>. Since the content of Cr is >=20%, the resistance to corrosion at a high temp. and sulfide corrosion is improved. The resistance to erosion and corrosion of the member for a boiler is thus improved by the above-mentioned method.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a method suitable for preventing erosion and corrosion occurring in boiler tubes of various boilers, particularly fluidized bed boilers.
The present invention relates to a boiler member having an alloy overlay layer.

(Prior Art) Recently, fluidized bed boilers have come into widespread use in order to save fuel costs and increase energy efficiency. This fluidized bed boiler is a type of boiler in which combustion is performed inside a fluidized bed formed by fluidizing solid particles such as limestone grains and silica sand grains, and is a type of coal-fired boiler. Heat transfer efficiency is extremely good due to the use of a fluidized bed, and at the same time it is possible to desulfurize in the furnace using limestone etc. It has advantages and is rapidly becoming popular. However, from a materials perspective, new and unique problems have emerged.

That is, in a fluidized bed boiler, the inner bed tube inserted into the fluidized bed, that is, the boiler tube, is placed in a special high temperature erosion and corrosion environment that has not been encountered in conventional boilers. For example, the corrosion environment is caused by an atmosphere with low oxygen partial pressure and high sulfur partial pressure, and the erosion environment is caused by an atmosphere of low oxygen partial pressure and high sulfur partial pressure.
O1 is based on a fluidized medium such as coal ash or sand.

When conventional boiler tubes are used in the environment found in fluidized bed boilers, they often suffer significant wear and tear due to erosion and corrosion, so designers are struggling to find countermeasures. There is. For example, measures have been taken from the perspective of pipe structure, such as covering the boiler pipes with protectors or adding studs, and operational measures, such as reducing the superficial velocity of the tower.

However, using a protector reduces thermal efficiency,
When installing studs, expensive construction costs are required, and lowering the superficial velocity causes problems such as a reduction in output. Measures to improve the quality of the material are strongly desired. In this respect, the same is true for pulverized coal-fired boiler tubes.

Furthermore, in a fluidized bed boiler, the boiler atmosphere is low in oxygen and high in sulfur partial pressure, and therefore, depending on the material of the boiler tubes, severe sulfide corrosion occurs.

(Problems to be Solved by the Invention) Thus, a first object of the present invention is to provide a boiler member that can withstand erosion and corrosion caused by high-temperature solid particles.

A second object of the present invention is to provide a boiler member that exhibits no sulfide corrosion resistance under the low oxygen, high sulfur atmosphere encountered in fluidized bed boilers or pulverized coal fired boilers.

A third object of the present invention is to provide a boiler member suitable for fluidized bed boilers and pulverized coal-fired boilers.

(Means for Solving the Problem) The inventors of the present invention did not achieve the objective stated in -1-, but after conducting repeated research focusing on the advantages of the overlay method, they obtained the following knowledge. .

■Boiler tube number for heat exchange in fluidized bed Because of the t, the temperature gradient within the tube wall thickness is large, and the thermal stress in the radial direction becomes large. Furthermore, since it is placed in the flow of a fluid medium, it is subject to bending stress and vibration. Therefore, if the surface is treated to have a low adhesion to the boiler tube mother +1 surface, it will be subject to mechanical failure due to thermal stress or bending stress before erosion or corrosion. From this point of view, overlay is more advantageous than thermal spraying.

■As a result of an erosion test assuming high-temperature solid particle erosion in a boiler, the Vickers hardness of the built-up layer at room temperature was 400 kgf/. A remarkable effect on erosion resistance was observed in cases of A and above.

■There are various overlay alloys with such hardness, but F
e-Cr alloys do not cause cracks during welding, and Co-based alloys are preferred. However, when applied to boiler tubes, even Co-based alloys need to be able to withstand deformation to a certain extent, and those with too high hardness have poor deformation resistance, with only a slight resistance to deformation in the flat test described below. In order to withstand severe deformation, it is necessary that the Vickers hardness is 550 kgf/mm or less.

(2) In order to maintain high-temperature corrosion resistance up to a maximum of 800°C, which is the operating temperature range of boiler tubes, it is desirable to have 20% or more of Cr.

By integrating these findings, the present invention was completed by discovering that a Co-based alloy is an overlay alloy suitable for application to boiler tubes.

Here, the gist of the present invention is that 20% or more of C
A high-temperature solid particle erosion and corrosion material made of a Co-based alloy containing This is a boiler member that has a sagging resistance to John.

In addition, examples of the base material on which the Co-based alloy build-up layer is provided include the outer surface of a boiler tube, a protector material, a stud hole, and the like. Further, the material is not particularly limited as long as it is conventionally used as a boiler member.

Generally, stainless steel, Cr-MO steel, etc. are mentioned.

Furthermore, the overlaying means itself (4) is extremely well known, and although any known means may be used, conventional G welding or shielded arc is generally preferred.

The preferred composition of the Co-based alloy used as the overlay alloy in the present invention is: C: o, 2 to 2.5%, Cr: 20 to 35%, Ill
: 3 to 20%, and in this case, by adjusting the amount of C, the hardness after overlaying can be rolled to Con 1.

Next, the present invention will be explained in further detail by way of examples.

Welding electrodes of various alloy compositions shown in Table 1 were formed, and a build-up layer of approximately 5.0 mm thickness was formed on the surface of a stainless steel (SUS 304) plate using the old G welding method Q. .

Test specimens with a thickness of 21 mm were cut from near the surface of the overlay layer from the plate obtained in this way, and exposed to an atmosphere with the following gas composition at 750°C for 500 hours to prevent high-temperature gas corrosion in a boiler atmosphere. The resistance of each build-up layer was evaluated.

Gas atmosphere = (A): N 2 Ba1. -15%
CO2-2%02 0.3%S02 Gas atmosphere-(B): N2Ba1. 1%H2-6%H
2O3%CO- 15%CO20, 3%H2S Note that gas atmosphere - (A) has a gas composition similar to a normal boiler combustion atmosphere, while gas atmosphere - (B) has a gas composition that is assumed in a fluidized bed boiler. This is an approximate gas composition in the low air ratio combustion (90% theoretical air) section.

The components of the test materials and the amount of corrosion are summarized in Table 1. Here, the amount of corrosion erosion is the amount of thinning due to surface scale formation (μm
) and the amount of internal erosion (μm). In addition,
The Vickers hardness of each build-up layer was 430 for positive No. 1, 420 for No. 2, and 350 for No. 3.

It can be seen that Alloy No. 1.2 in Table 1 shows better corrosion resistance than Alloy No. 3 and SO5304.

Actual Example 11 (In this example, high-temperature solid particle erosion was investigated using a high-temperature blast type 1I caution tester on a test piece obtained in the same manner as in Example 1.The overlay alloy test piece was
A 5 mm thick build-up was applied on the SUS 304 slope, and the surface of the build-up layer was polished and used.

Erosion in coal-fired boilers is caused by coal ash. In addition, fluidized bed boilers use various fluid media, and sand may be used depending on the boiler, and the hardness of these solid particles also varies, but in the case of this example, silica sand (
Erosion resistance was evaluated using Ilv #I 000 ).

Coal ash is also mainly composed of 5i02, and its hardness is said to be around 1000. Regardless of the absolute value, it can be said that the erosion resistance of the material can be judged relatively from the results of silica sand.

The experimental conditions were summarized as follows.

Silica sand 0.7-0.9mm 5 kg/h 2
0°1 Yaw Man's End L Input ■ Sword Flow Continuous Ar Gas 20 m/s Sample material components, hardness, and test results (maximum thinning depth of test piece) are summarized in Table 2.

For the erosion part, the trace thickness reduction that occurred in the center of a test piece with a width of 20M and a length of 30mm was measured using a surface roughness meter, and the maximum thickness reduction value was recorded.

As is clear from the results shown in Table 2, even when a Co-based alloy containing Cr2O% or more is used, Hv > 4
A particularly remarkable improvement in erosion resistance is observed in the case of 00 kgf/nA.

Although the alloy ll1o, 1 has poor erosion resistance, it has too high llv = 659 kgf/m, so the ductility of the built-up layer cannot be improved at all, as will be described in Example 3 below.

Table 2 Prisoner J canned side Next, the deformation resistance due to flattening was evaluated.

The test piece was 30mm with a diameter of 50.8x6t (mm).
Using a 411 steel pipe, a Co-based gold overlay layer of approximately 2 mm was applied to the surface of the steel pipe in the same manner as in Example 1, and then the entire steel pipe was flattened to reduce the height at which cracks would occur. Recorded. The results are summarized in Table 3 together with the alloy composition.

As is clear from the results in Table 3, all of them had low flatness resistance, and U-shaped bending of the tubes was completely impossible. Alloy 1.2 has no ductility at all and is expected to crack even with the slightest impact. On the other hand, I
Alloy Nos. 3 to 6 with lv≦550 can be slightly deformed, and can be used for practical purposes without causing LSI cracks due to slight deformation.

Claims (1)

[Claims] Made of a Co-based alloy containing 20% or more of Cr, the hardness at room temperature is Vickers hardness of 400 to 550 kgf/m.
A boiler member having excellent resistance to high-temperature solid particle erosion and corrosion, characterized by comprising a Co-based alloy build-up layer of m^2.