JPH07138727A - Method for coating of wear resistant film - Google Patents

Abstract

PURPOSE:To provide a coated film excellent in wear resistance by forming thermally sprayed film with a specified thickness on the surface of a projective objects by a thermal spraying material in which specified ratios of C, Cr, V, Mo and B are incorporated into Fe, and holding it under heating under specified conditions. CONSTITUTION:A thermal spraying material essentially contg., by weight, 4 to 6% C, >=20% Cr, 0 to 10% V, 0 to 10% Mo and 0 to 5% B, and the balance Fe is used, and thermally sprayed film with >=0.2mm thickness is formed on the surface of a protective object (such as the heat transfer pipe of a coal burning boiler) by a thermal spraying method. Next, this thermally sprayed film is held under heating in the temp. range of 600 to 800 deg.C for >=30min to largely crystallize out multiple carbides of M7C3 and to highly harden the thermally sprayed film. Thus, the ash erosion resistance of the protective object can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of coating a wear-resistant film most suitable for application to, for example, a heat transfer tube which constitutes a coal-fired boiler.

[0002]

2. Description of the Related Art For example, in a coal-fired boiler, heat transfer tubes such as a boiler evaporation tube, a economizer tube, a heater tube and a reheater tube are provided and arranged in a furnace. FIG. 5 shows a cross-sectional structure of a boiler evaporation tube which is one of conventional heat transfer tubes.
In the figure, reference numeral 21 denotes an evaporation pipe, which is a pipe made of carbon steel, CrMo (chromium molybdenum) steel, or stainless steel. Reference numeral 22 denotes a welded portion, which is made of carbon steel, CrMo steel, stainless steel, or the like, and is used to connect the evaporation pipes 21 to each other. Reference numeral 23 denotes a boiler furnace surface of the boiler evaporation pipe having such a configuration, and the boiler furnace surface 23 is exposed to the flame in the furnace.

By the way, in coal, which is a fuel for a coal-fired boiler, about 30% or less of an inorganic substance exists depending on the type of coal, and these are decomposed and evaporated by combustion. However, most of them are mainly Al ₂ O ₃ (aluminum oxide),
It becomes fly ash of SiO ₂ (silicon dioxide) and scatters on the combustion gas flow.

Such fly ash collides with heat transfer tubes such as a economizer tube, an evaporator tube, a heater tube and a reheater tube,
Causes wear and thinning of the pipe surface. The action of such wear and thinning caused by fly ash is called ash erosion. And because of the heat erosion of the heat transfer tube, maintenance of the equipment is performed regularly.
Therefore, the heat transfer tube, which is subject to severe wear and thinning, is to be replaced. Therefore, this maintenance requires a lot of time and money.

[0005]

In a coal-fired boiler, fly ash generated by burning coal, which is a fuel, scatters along with a combustion gas flow. And
This collides with various heat transfer tubes installed in the furnace and causes ash erosion. Therefore, due to this ash erosion, the heat transfer tube is regularly maintained, and the heat transfer tube, which is heavily worn and thinned, is to be replaced. Therefore, it takes a lot of time and cost.

By the way, as measures for preventing ash erosion, [1] optimization of combustion gas flow and prevention of uneven flow by improvement of boiler structure and combustion conditions [2] protection of heat transfer tube by mounting protector [3] high protection by spraying method It is actually used by forming a film of hardness on the tube surface to improve erosion resistance. However, the measure described in the above [1] is technically very difficult because it is related to the essence of the cause of ash erosion, and it has not been sufficiently effective. The above-mentioned measure [2] is a very easy means, but there is no material having high hardness at high temperature, and the current stainless steel and heat-resistant steel have the same ash erosion resistance as the heat transfer tube, Therefore, only the ash erosion resistance corresponding to the thickness can be improved. In addition, this method requires high construction cost and causes problems such as reduction of heat transfer efficiency and damage due to falling of the protector.

According to the above-mentioned measure [3], a high-hardness film can be formed relatively easily even on a large-area metal surface, and erosion resistance can be improved. The technical problems such as the construction method, coating characteristics, and application range cannot be balanced with the price of coating material and the construction cost, and they are not sufficiently effective.

In other words, in the commonly used Ni-based self-fluxing alloy, "spraying" + "melting treatment" is performed, so that the adhesion of the coating is good, but the melting treatment causes deformation of the heat transfer tube and cracking of the coating. There are drawbacks such as easy occurrence. Further, depending on the material of the heat transfer tube, it cannot be used because the material changes due to the melting process, and the construction cost is high.

Further, although the high hardness WC, Cr ₃ C ₂ cermet coating has excellent ash erosion resistance among the thermal spray coatings, the adhesion of the coating is insufficient and strict quality control during construction is required. is necessary. Also, WC
Since the Cr ₃ C ₂ cermet material and Cr ₃ C ₂ are expensive materials, the material cost is very high for a heat transfer tube such as a boiler having a large area, which is not practical.

Therefore, there is a strong demand for the development of a technique capable of inexpensively obtaining the ash erosion resistance of heat transfer tubes such as a economizer tube, an evaporator tube, a heater tube and a reheater tube.
The present invention has been made in view of the above circumstances, and its purpose is to prevent ash erosion due to collision of fly ash in a protection target such as a heat transfer tube of a coal-fired boiler. Another object of the present invention is to provide a method for coating an abrasion resistant coating that can be performed at low cost.

[0011]

In order to achieve the above object, the present invention is as follows. That is, mainly C is 4 to 6 wt%, Cr is 20 wt% or more, and V is 0 to 10 w.
t%, 0 to 10 wt% Mo, 0 to 5 wt% B,
After forming a thermal spray coating with a thickness of 0.2 mm or more on the surface of the object to be protected using a thermal spray material with the remainder being Fe, this thermal spray coating is further heated and held at a temperature of 600 to 800 ° C. for 30 minutes or more. To do.

[0012]

[Function] By the thermal spraying method, the content of C is mainly 4-6w.
t%, Cr content is 20 wt% or more, V content is 0
-10 wt%, Mo content 0-10 wt%, B content 0-5 wt% and the rest 0.2% formed on the surface of the object to be protected by using a thermal spray material having a composition of Fe. The above coating is 3 at a temperature of 600-800 ° C.
By heating for 0 minutes or more, a large amount of M ₇ C ₃ composite carbide crystallizes out, resulting in high hardness. In general, ash erosion resistance is proportional to the hardness of the material, so when the above-mentioned high-hardness thermal spray coating is formed on the surface of a protection target such as a heat transfer tube, the protection target is protected by the high-hardness coating, Ash erosion is improved. And since the thermal spray material is composed of inexpensively available components,
It can be implemented at low cost.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. The present invention improves the ash erosion resistance by forming a high hardness coating of an inexpensive material on the heat transfer tube by thermal spraying, and realizes cost reduction and maintenance-free.

FIG. 1 shows a sectional structure of a boiler evaporation tube which is one of the heat transfer tubes of the present invention. In FIG. 1, reference numeral 1 denotes an evaporation pipe, which is a pipe made of carbon steel, CrMo steel, stainless steel, or the like. Further, 2 is a joint portion for connecting the evaporation tubes 1 to each other, and is made of carbon steel, CrMo steel or stainless steel. Reference numeral 3 is a plasma spray coating formed on the boiler furnace surface of the evaporation tube 1. A plurality of evaporation pipes 1 are arranged close to each other, adjacent evaporation pipes 1 are joined to each other at a joining portion 2, and a plasma sprayed coating 3 is formed on one surface of the evaporation pipe assembly thus formed. Is used for the boiler furnace surface.

In the heat transfer tube having such a structure, the plasma spray coating 3 is formed. In order to form the plasma spray coating 3, in the present invention, the spray material mainly contains 4 to 6 wt% of C (carbon). % (Wt%), Cr (chromium) content of 20 wt% or more, V (vanadium) content of 0-10 wt%, Mo (molybdenum) content of 0-10 wt%, B (boron) content The amount is 0-5wt%
Then, a powder having the rest of Fe (iron) and having a particle size of −45 μm or less was used.

The conditions for plasma spraying are that the plasma gas is Ar 45 l / min + H ₂ 10 l /
min (namely, 45 liters / minute of Ar (argon gas) as plasma gas and 1 / minute of H ₂ (hydrogen gas))
0 liters), plasma arc current 600A, plasma voltage 65V, and spray distance 100-150m
m.

A thermal spray coating having a thickness of 0.2 mm or more formed by plasma spraying the thermal spray material of the above components under such conditions is heated to 600 ° C. in the atmosphere, and this temperature is set to 30.
Held minutes. Due to this heat treatment, the thermal spray coating has M
_A large amount of ₇ C ₃ composite carbide is crystallized, and the thermal spray coating is made harder.

Therefore, in the heat transfer tube of the present invention applied with the thermal spraying material of the above components by plasma spraying under the above plasma spraying conditions to form a sprayed coating, and further subjecting to heat treatment under the above conditions, a large amount of composite carbides are crystallized. Since the exposed high-hardness sprayed coating is formed on the furnace surface side, it has excellent ash erosion resistance and excellent wear resistance (wear resistance). Further, since the thermal spray material related to the thermal spray coating of the present invention is low in price and is easy to perform thermal spraying with a metal-based material, the construction cost is significantly higher than that of the conventional thermal spray coating using a high hardness material such as WC. Is reduced to.

FIG. 2 shows the Vickers hardness [Hv] of carbon steel, stainless steel, and thermal spray coating, which are the materials for the heat transfer tube.
FIG. 3 is a diagram showing Vickers hardness [Hv] of a self-fluxing alloy film, a Cr ₃ C ₂ cermet, and a WC, which are high hardness materials, as a reference example. The Vickers hardness of carbon steel and stainless steel is approximately 120 [MH
v], approximately 150 [MHv], but the Vickers hardness of the thermal spray coating according to the present invention is approximately 820 [MHv]. From FIG. 3, the hardness of the thermal spray coating according to the present invention is It turns out that it is much higher than that.

Further, the conventional high hardness material W
About 90 for C, Cr ₃ C ₂ cermet and self-fluxing alloy film
0 [MHv], about 700 [MHv], about 750 [MH
v], and it can be seen that it exhibits high hardness comparable to those.

FIG. 3 shows the maximum wear depth of each material shown in FIG. 2, by which the ash erosion resistance was evaluated. As conditions for the evaluation test, the fly ash collected from the operating boiler was used at a flow rate of 50 m /
s was sprayed on the test piece for 100 hours. Then, the maximum wear depth of the test piece after the test is measured.

The maximum wear depths of carbon steel and stainless steel are about 70 mm and about 80 mm, respectively, but the maximum wear depth of the thermal spray coating according to the present invention is about 36 mm.
It can be seen from FIG. 3 that the ash erosion resistance of the thermal spray coating according to the present invention is much better than that of the heat transfer tube material.

Further, W which is a conventional high hardness material
About 30 for C, Cr ₃ C ₂ cermet and self-fluxing alloy film
mm, about 45 mm, about 50 mm, which is comparable to those, or shows more ash erosion resistance.

As described above, the ash erosion resistance of the thermal spray coating of the present invention is much higher than that of the heat transfer tube material, and the conventional expensive and high hardness material WC,
It is possible to obtain ash erosion resistance equivalent to or higher than that of Cr ₃ C ₂ cermet and self-fluxing alloy film.

When the thickness of the sprayed coating according to the present invention is less than 0.2 mm, the effect of the ash erosion resistance decreases, so the thickness of the sprayed coating is 0.2 m.
m or more is required.

From the above results, mainly 4 to 6 wt% of C
%, Cr 20 wt% or more, V 0-10 wt%, Mo
0 to 10 wt%, B 0 to 5 wt% and the rest F
It was found that the thermal spray coating formed by using the thermal spray material constituted by e and subjected to a predetermined heat treatment has excellent ash erosion resistance. Therefore, the thermal spray coating formed by the thermal spray coating forming method according to the present invention is applied to It can be easily estimated that the heat tube has excellent ash erosion resistance.

(Reason for determining the content ratio of each component of the thermal spray material)
Here, the reason for determining the content ratio of each component of the thermal spray material used in the present invention will be described.

It is well known to form a thermal spray coating on a heat transfer tube. For example, the content of C as a thermal spray material is 4 to 5 wt.
%, Cr content is 5 to 15 wt%, content of the third element such as V, Mo and B is several wt%, and the rest is Fe to form a thermal spray coating using a high carbon iron-based material. There is an example. However, in this case, the hardness is not sufficient.

On the other hand, in the thermal spray material of the present invention, the content of C is 4 to 6 wt%, the content of Cr is 20 wt% or more, and the contents of the third elements of V, Mo and B are several numbers. %, Mo
Content of 0-10wt%, B content of 0-5w
It is a high-carbon, high-chromium iron-based material consisting of t% and the remainder being Fe.

In other words, the Cr content is completely different, and the thermal spray material of the present invention has a high Cr content. In the present invention, a thermal spray coating made of such a thermal spray material containing a large amount of Cr is subjected to heat treatment, and a large amount of chromium carbide is crystallized by this heat treatment to obtain a coating having high hardness. . In the present invention, the lower limit of chromium is set to 20 wt% in order to maximize the crystallization effect of the chromium carbide of the thermal spray material by the above heat treatment.

Further, in order to form a film of high hardness in which a large amount of chromium carbide is crystallized, the thermal spray material used in the present invention contains 4 wt% of C as a component.
% And Cr, and 20 wt% or more of Cr was added to obtain a high carbon, high chromium iron-based material. Then, elements V and Mo were selected as additive components in order to further increase the hardness of the carbide.

Additive components V and Mo further increase the hardness of the above-mentioned carbides, and partially form high-hardness carbides to increase the hardness of the film. However, if the addition amount is too large, it becomes brittle, so the upper limit of each is set to 10 wt%.
Thereby, the toughness of the coating is maintained and cracking and peeling of the thermal spray coating can be suppressed.

Further, in order to improve the wear resistance of the film, the element B is also used as an additive component. The additive component B increases the hardness of the matrix in the coating structure to improve the wear resistance of the coating, and maintains the toughness of the coating by setting its upper limit to 5 wt%.

However, V, Mo and B are not indispensable, and even if they are partially lacking or are not contained at all, the present invention contributes sufficiently to the improvement of ash erosion resistance. To do.

The components of the coating film of the present invention used in the experiments of FIGS. 2 and 3 and the content ratio thereof are 5.2 wt% for C and 2 for Cr.
4wt%, V 5wt%, Mo 4wt%, B 2wt%
% And the balance is Fe.

(Heat Treatment Conditions) Next, heat treatment conditions that play an important role in the present invention will be described. Considering the heating temperature in the heat treatment of the thermal spray coating from various experimental results, it was found that at 400 ° C., the precipitation of carbides was small and the hardness could not be increased. Also, 6
It was found that precipitation of fine carbides was clearly observed at a temperature of 00 ° C or higher, and accompanying this, an increase in hardness was also obtained.

However, the steel pipe for a boiler has a transformation temperature of 7 to several tens of degrees although it varies depending on the material, and changes the structure and characteristics. Further, at 800 ° C. or higher, the film undergoes high temperature oxidation. Therefore, considering these things, the optimum heating temperature range is 600 to 800 ° C.

On the other hand, considering the holding time for keeping the optimum heating temperature, the heating temperature is 600 to 800 ° C.
When it is held for 30 minutes or more, the precipitation of fine carbide is clearly observed, and the hardness is increased accordingly. Also,
An increase in hardness can be obtained as the holding time becomes longer.

However, even if the holding time is lengthened, the increase in hardness commensurate with the length of time is not obtained, and it is inefficient to lengthen the length unnecessarily, which leads to cost increase. Therefore, the retention time is 30
It is sufficient to secure more than minutes, but about 30 minutes is optimal when comprehensively judged.

(Comparative Example to the Present Invention) In order to confirm the wear resistance of the thermal spray coating according to the present invention, a film was formed by using a thermal spray material having a composition close to the content ratio of each component in the thermal spray powder of the present invention. Confirmation of the limit thickness of the coating and hardness test were carried out. FIG. 4 shows the components of various comparative examples and the presence / absence of heat treatment, the limit thickness of the coating, and the hardness of the coating, and the components of various examples of the present invention and the presence / absence of heat treatment, the limiting thickness of the coating, and the hardness of the coating. It is the figure which put together.

As components of various examples of the present invention, C is 5 wt% and Cr is 25 wt%, but Mo and V are 5 wt% and 10 wt%, and B is 5 wt%.
Was tested for 4 cases using 3 wt% and 5 wt%. All are C4-6w which is the compounding range of this invention.
t%, Cr 20 wt% or more, V 0-10 wt%, M
o 0-10 wt%, B 0-5 wt% and balance Fe
Is a thermal spray material.

The comparative examples are 16 examples which were compounded in the vicinity of the compounding range of the present invention. Here, the film is formed by carbon steel pipe φ45 ×
The film formation was continued on the surface of 100 ^l × 7 ^t mm until the film started peeling. However, when a film having a film thickness of 2 mm or more was formed, the film formation was stopped at that point and the hardness test was performed.

The thermal spraying is plasma spraying, and the plasma spraying conditions are Ar 45 l / plasma gas.
min + H ₂ 10 1 / min, the plasma arc current was 600 A, the plasma voltage was 65 V, and the spraying distance was 150 mm. Heat treatment after film formation is performed by heating to 600 ° C. in an air atmosphere, and this temperature is set to 3
Hold for 0 minutes.

The results of the above test are shown in FIG. 4, and it can be seen that a high hardness thick film is formed by heat-treating the thermal spray coating according to the present invention to which chemical components are added. The feature of the present invention is that C is 4 to 6 wt% and Cr is 20 w.
t% or more, V 0 to 10 wt%, Mo 0 to 10 wt%
%, B of 0 to 5 wt% and the rest of Fe were used, and the thickness of the heat transfer tube was set to 0.
Form a sprayed coating of 2 mm or more and further 600-800
That is, the heat treatment was performed by heating and holding at a temperature of 30 ° C. for 30 minutes or more.

The wear life of the member depends not only on the hardness but also on the thickness of the coating. Therefore, the higher the hardness and the thickness of the coating, the longer the life. Therefore, comparing the effect of the present invention with the hardness and the limit thickness shown in FIG. 4, the high hardness coating of the comparative example has a small limit thickness and a low hardness, but a high hardness and a high limit thickness. It can be said that the thermal spray coating has excellent wear resistance.

The present invention is not limited to the embodiments described above and shown in the drawings, but can be appropriately modified and implemented within the scope of the invention. For example, the above embodiment has been described focusing on an example applied to a heat transfer tube of a coal-fired boiler, but abrasion resistance is required, and thermal spraying is also possible, protection of structures or materials that can be subjected to heat treatment, manufactured products, etc. As long as it is an object, it can be used as a surface protective film, and if it is a protected object such as a structure, material or manufactured product that satisfies these conditions, it can be applied regardless of its material. is there. Further, the thermal spraying method is not limited to plasma spraying.

[0047]

As described above in detail, according to the present invention,
A spray coating with excellent ash erosion resistance can be formed on a metal surface such as a heat transfer tube, and even if there is a collision of fly ash, the spray coating with excellent ash erosion resistance protects the metal surface. It is possible to improve the erosion resistance of the heat tube and obtain wear resistance, and in a coal-fired boiler, etc., the heat transfer tube installed in the furnace can be protected from ash erosion. In addition to being able to promote maintenance-free operation, the material for forming the sprayed coating is also inexpensive, so it can be implemented at low cost. Furthermore, the above-mentioned maintenance-free method adopts the heat transfer tube to which the present invention is applied. Then, it is possible to obtain great effects such as improvement of operation efficiency of the boiler and reduction of operation maintenance cost.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention,
FIG. 3 is a cross-sectional view of an evaporation tube which is one of the heat transfer tubes according to the first embodiment of the present invention.

FIG. 2 is an explanatory view showing an example of measurement of Vickers hardness of carbon steel, stainless steel, which is a material of a heat transfer tube, a thermal spray coating according to the present invention, and various conventional thermal spray coatings of high hardness.

FIG. 3 is an explanatory view showing the maximum wear depth for each material due to collision of fly ash.

FIG. 4 shows components of Examples and Comparative Examples according to various composition examples to which the present invention is applied, presence / absence of heat treatment, and limit thickness of a film,
The figure which showed the relationship of the hardness of a film.

FIG. 5 is a view for explaining a conventional example and is a cross-sectional view of an evaporation tube which is one of conventional heat transfer tubes.

[Explanation of symbols]

 1, 21 ... Evaporation pipe 2.22 ... Welded portion 3 ... Thermal spray coating 23 ... Furnace surface

Claims (1)

[Claims] 1. Mainly 4 to 6 wt% of C and 20 of Cr
wt% or more, V 0 to 10 wt%, Mo 0 to 10 wt
%, B in an amount of 0 to 5 wt%, and the remainder being Fe, and a thickness of 0.
After forming a thermal spray coating of 2 mm or more, this thermal spray coating is further heated and held at a temperature of 600 to 800 ° C. for 30 minutes or more, to provide a wear-resistant coating method.