JPS6217169A - Surface coating method for metallic material - Google Patents

Abstract

PURPOSE:To improve the erosion resistance of the surface of a heat transfer pipe of a boiler to fly ash, etc. by forming an Al film on the surface of said pipe, forming a mixed layer composed of the oxide of a specific metal and Al metal thereon and subjecting the film and the mixed layer to a sealing treatment than oxidizing the remaining Al. CONSTITUTION:The film 2 of Al or Al alloy is formed by a thermal spraying, vapor deposition or plating method on the surface of the in-boiler heat transfer pipe 1 made of an alloy steel contg. at least one kind among Fe, Cr, Ni, Co, Mn, Ti and Cu. The mixed layer 3 composed of the Al or Al alloy and the metallic oxide such as ZrO2, TiO2 or Cr2O3 is formed thereon and is heated in a reducing or inert gaseous atmosphere to form an counter diffusion layer 4 between the Al of the film 2 and the Fe of the alloy steel; at the same time, the layer is subjected to the sealing treatment by penetrating the Al into the pores in the film 2 and the layer 3. Such pipe is heated at 500-650 deg.C in an oxidizing atmosphere to oxide the Al remaining in the film 2 and the layer 3, by which an Al2O3 layer 5 is formed.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention provides a coating that has good adhesion to the surface of a metal material, is dense with few pores, and has a high bonding force between metal oxide particles constituting the coating. The present invention relates to a method for forming a metal oxide film having excellent erosion resistance and heat resistance.

[Background of the invention]

In a boiler that burns coal or a mixture of heavy oil and coal as fuel, when the ash contained in the coal becomes fly ash and scatters in the flow of combustion gas, the It is well known that ash erosion occurs, which is a phenomenon in which ash collides with a group of heat transfer tubes and wears out and thins the surface of the heat transfer tubes. This ash erosion phenomenon is a particular problem in areas where the combustion gas flow is concentrated or unevenly flowing near the wall of the boiler combustion chamber, where the primary superheater and economizer are installed, and where the flow of combustion gas becomes faster. It becomes.

The rate of wear and thinning of heat transfer tubes due to ash erosion depends on the ash composition, shape, and size of the combustion gas produced by coal combustion, and the temperature of the boiler heat transfer tubes.

This varies depending on the material (usually carbon steel or low-alloy steel) and combustion gas flow rate, but in severe cases, the amount of wall thinning can reach several meters per year, which poses a serious problem for boiler operation. It has become. One method for preventing such thinning of heat exchanger tubes due to ash erosion is to provide a protector made of a material with excellent wear resistance on the outside of the heat exchanger tubes. According to this method, a gap is created between the protector and the heat exchanger tube, which significantly reduces the heat absorption of the heat exchanger tube.If the heat exchanger tube causes ash erosion over a wide area, this protector can be used. The heat exchange efficiency of the group deteriorates, causing both technical and economical problems. In addition, as another method to prevent wall thinning due to ash erosion of heat exchanger tubes,
There is a method of using a double tube in which the outer tube of the heat transfer tube is made of a metal material with good wear resistance and the inner tube is made of a normal heat transfer tube material, but the double tube is very expensive and cannot be used in large quantities. When used in a heat exchanger tube group, the material cost of the heat exchanger tube group increases significantly, resulting in a problem that the boiler equipment cost increases. and,
By adopting the above-mentioned protector method or double tube method, a certain degree of wall thinning can be achieved in heat transfer tubes used in boilers that use low-grade coal with a high ash content as fuel, or in locations where the combustion gas flow rate is extremely high. Although it is possible to reduce the speed, it has the disadvantage that it cannot be used as a permanent measure to prevent ash erosion. In addition, in the case of boiler heat exchanger tubes, the maximum material temperature of the heat exchanger tubes is 6.
To be used at temperatures as high as 00℃.

Heat resistance is required in addition to wear resistance.

Compared to the above-mentioned protector method or double tube method, a method that is relatively inexpensive and suitable for preventing structural erosion is to thermally spray a material with excellent wear resistance and heat resistance on the surface of the boiler heat transfer tubes. There is a way to form it. Heat-resistant and wear-resistant thermal spraying materials that can be used to prevent ash erosion include (1) Fe-Cr-based or Ni-C
Alloy materials such as r-based, (2) WC-Co-based, Cr5
G, cermet-based materials such as -NiCr-based, and (
3) Roughly divided into three types: metal oxide materials such as alumina and zirconia.

As an example of using the alloy-based thermal spray material of (1) above, chromium steel or nickel-chromium steel is thermally sprayed overlay on the boiler pipe wall, and Afl is further thermally sprayed thereon.

A method has been proposed in which M is heated above the melting point of M to infiltrate the air holes formed in the sprayed overlay to improve the adhesion between the sprayed overlay and the boiler pipe wall. Kosho 49
-32174). However, although this method relatively improves heat resistance and corrosion resistance, since the thermal sprayed overlay is an alloy, it is less effective than the cermet-based material (2) or metal oxide-based material (3) described above. It has the drawbacks of being soft and having poor lotion resistance, and inferior heat resistance compared to metal oxide materials.

There are flame spraying methods and plasma spraying methods as thermal spray coating methods for metal oxide materials, which are suitable thermal spray materials for improving the lotion resistance and heat resistance of boiler heat exchanger tubes. Since the metal oxide powder is a high melting point material, it does not adhere well to the base material to be coated using flame spraying, and the bonding strength between the metal oxide particles that make up the coating is weak. There was a problem that a strong film could not be formed. In addition, the plasma spraying method uses 10,000℃
This method is suitable for thermal spraying of high-melting point materials such as ceramics, as it is a method in which a plasma arc with an extremely high temperature exceeding However, since the temperature of the base material coated during plasma spraying is approximately 120 to 200°C, the sprayed powder instantly re-solidifies on the surface of the base material.
Voids (pores) are likely to be formed between the solidified oxide particles, and it is inevitable that several percent of pores will be formed in the sprayed coating.

Therefore, even if plasma spraying is applied to boiler heat transfer tubes in locations where the combustion gas flow rate is high and the collision force of ply ash is strong, considerable voids (pores) will occur between the metal oxide particles that make up the sprayed coating. However, since the binding force between the particles becomes weak, the film is peeled off when the fly ash collides with the fly ash, resulting in a drawback that sufficient wear resistance and heat resistance are not exhibited.

In order to improve the bonding strength between the metal oxide particles that make up this sprayed coating, it is necessary to increase the spraying speed extremely high, which makes the spraying equipment large and very expensive. Since it requires electric power, it poses a problem particularly in terms of economic efficiency when applied to the surface coating of a wide range of boiler heat exchanger tubes in actual equipment. In addition, if the difference in thermal expansion coefficient between the metal oxide to be sprayed and the base metal to be sprayed is large, even in the case of plasma sprayed coatings, during coating formation or during thermal cycles of starting and stopping thermal spraying, , the formed film is easily peeled off. In order to prevent this, before spraying the metal oxide, a material having approximately the same coefficient of thermal expansion as the base metal is sprayed as a binder, and then the metal oxide and the binder are mixed. Thermal spray coating must be carried out without changing the ratio, which makes the thermal spraying operation very complicated and at the same time has the disadvantage of extremely poor workability.

[Purpose of the invention]

The object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and to provide a coating to the surface of a metal material that has good adhesion, is dense with few pores, and has a high bonding force between metal oxide particles constituting the coating. An object of the present invention is to provide a method for easily forming a metal oxide film having excellent lotion resistance and heat resistance.

[Summary of the invention]

The present invention involves (1) first forming a film of aluminum or aluminum alloy on the surface of a metal material to which a lotion-resistant and heat-resistant coating is to be applied;
) on aluminum or aluminum alloy, aluminum oxide, which has wear resistance and heat resistance,
After forming a mixed layer of zirconium oxide, titanium oxide, chromium oxide, or metal oxides containing these as main components, (3) heating in a reducing atmosphere or inert atmosphere to coat. Metal materials and the above (
At the same time, by mutually diffusing the aluminum or aluminum alloy film of 1) to form a strong alloy layer by forming a solid solution or an intermetallic compound, etc.
Infiltrating aluminum or its alloy into the pores formed in the film and the mixed layer of (2) to form a dense film;
(4) The metal material to be coated is further heated in an oxidizing atmosphere at a temperature below the melting point of aluminum or aluminum alloy to convert the remaining aluminum or aluminum alloy components into oxides. The basic idea is to form a film that has good adhesion, is dense, has a high bonding force between the metal oxide particles constituting the film, and has excellent two-lotion resistance and heat resistance.

In the present invention, the metal material forming the lotion-resistant and heat-resistant film is an element that reacts with aluminum or an aluminum alloy to form a solid solution or an intermetallic compound, such as Fe, Cr, Ni, Go, Mn.
Any metal material containing elements such as , Ti, or Cu may be used.
Further, these elements may be included in the aluminum or aluminum alloy film formed on the surface of the metal material to be coated.

In the present invention, the metal oxide forming the mixed layer with aluminum or aluminum alloy is An20
3. Any oxide such as TiO2, ZrO2 or Cr, 03, or a metal oxide having wear resistance and heat resistance based on these oxides is sufficient, and there are no particular limitations on the composition thereof. .

[Embodiments of the invention]

An embodiment of the present invention will be described below in more detail with reference to the drawings.

FIGS. 1(a), (b), (c) and (d) are explanatory diagrams showing step-by-step an example of a method for forming a heat-resistant, lotion-resistant film on the surface of a metal material according to the present invention.

After shot blasting the surface of commercially available carbon steel 1,
Metal aluminum was sprayed to a thickness of 0.2 mm by flame spraying using acetylene gas to form an aluminum layer 2 [FIG. 1(a)]. Next, a mixture of aluminum oxide and metal aluminum with a mixing ratio of 10=1 was sprayed onto the aluminum layer 2 by flame spraying using acetylene gas, and the aluminum oxide and metal aluminum were sprayed to a thickness of about 0.3 nwn. A mixed layer 3 was formed with [first
Figure (b)].

Thereafter, the carbon steel 1 on which the aluminum layer 2 and the mixed layer 3 have been formed is inserted into a heat treatment furnace, and heat treated at about 900°C for 5 hours while passing hydrogen gas.
An alloy layer 4 of aluminum and iron was formed between the aluminum and carbon ml [FIG. 1(c)]. Next, the atmosphere in the heat treatment furnace is made oxidizing and heated at about 600° C. for 5 hours to oxidize the aluminum in the mixed layer 3 of aluminum oxide and metal aluminum and transform it into a single layer 5 of aluminum oxide. [Figure 1(d)].

Table 1 shows the test results of the porosity (%), adhesion, and abrasion resistance of the heat-resistant and erosion-resistant film of the present invention produced by the above process.

The porosity (%) of the film was determined by measuring the number of pores in the cross section of the film using an optical microscope. The adhesion of the film is determined by repeated rapid heating and cooling (heating to 500°C, cooling to 20°C, and
00°C and cooled to 20°C), and the number of times until the film peeled off was measured. In addition, the abrasion resistance of the film is
It was determined by adding SiO□ with a particle size of around 1100 t1 into air at a flow rate of 20 m/s, spraying it onto each film for 2 hours, and measuring the weight change before and after that.

The films produced as shown in Table 1 had extremely low porosity of 1 to 3%, indicating that a very dense film was formed. In addition, the adhesion between the coating and the base material carbon steel is 500°C and 700°C.
In both heating and quenching tests at ℃, it showed extremely good adhesion without peeling after 10 times, and the abrasion resistance of the film was an excellent value of 7 compared to 100% carbon steel. It can be seen that the lotion properties are significantly improved.

As shown in the examples above, the reason why the film formed by the method of the present invention exhibits excellent adhesion to the base metal is that the aluminum layer is coated by heat treatment in a reducing atmosphere of hydrogen gas. Interdiffusion occurs between 2 and carbon steel 1, and intermetallic compounds of FeAnz or F
A solid solution of e-An is formed at the boundary between the aluminum layer 2 and the carbon steel 1, forming a strong aluminum-iron alloy layer 4.
This is because it is formed. In addition, as the base metal material forming the heat-resistant and lotion-resistant film, Fe, Cr, which forms an intermetallic compound or solid solution with aluminum,
An alloy containing elements such as Ni, Co, Mn, Ti, and Cu can form a film with good adhesion.

In this example, aluminum was sprayed by flame spraying using acetylene gas, but if the coefficient of thermal expansion is significantly different from that of the metal material on which the film is to be formed, aluminum, which has a small difference in coefficient of thermal expansion, may be used as the main component. By thermal spraying the alloy, the adhesion of the film can be further improved. In addition, although acetylene gas flame spraying is used as the thermal spray coating method for aluminum in this example, a film with good adhesion can also be formed by vapor deposition, dip plating, electroplating, etc. can.

In this embodiment, hydrogen gas was used as the heat treatment atmosphere for forming the aluminum-iron alloy layer 4, but an atmosphere of an inert gas such as nitrogen or argon is also effective. Further, the heat treatment temperature may be at least the temperature at which aluminum or aluminum alloy and the metal material to be coated are alloyed, and this temperature varies depending on the type of metal material to be coated and is not particularly limited.

Next, a mixed layer 3 of aluminum oxide and metal aluminum
The reason why the porosity of aluminum oxide decreases and the bonding strength between aluminum oxide particles increases is due to the heat treatment (900°C x 5 hours) in hydrogen gas in this example.
This is because the aluminum oxide particles are remelted and penetrated into the pores in the mixture M3, significantly reducing the porosity, and at the same time, the aluminum oxide particles are firmly bonded via the remelted aluminum. The above treatment enhances the adhesion of the film and the bonding force between aluminum oxide particles, but since metallic aluminum remains in the mixed layer 3,
The two-lotion resistance and heat resistance of the film cannot be said to be sufficient. Therefore, we next performed heat treatment in an oxidizing atmosphere (6
00° C. for 5 hours) to oxidize the remaining aluminum and transform the mixed layer 3 into a single layer 5 of aluminum oxide, thereby forming a film having lotion resistance and heat resistance. This heat treatment in an oxidizing atmosphere is because the oxidation rate of aluminum is slow at low temperatures, making it difficult to form sufficient aluminum oxide.

It is desirable to heat the aluminum to 500°C or higher, at which point the oxidation rate of aluminum changes linearly with time.On the other hand, heating to 660°C or higher, which is the melting point of aluminum, will rapidly oxidize the aluminum and cause deterioration of the film. To prevent this, the upper limit of heating is 65°C, which is just below the melting point of aluminum.
It is desirable to set the temperature to 0°C.

In this example, the mixed layer 3 is a mixture of aluminum oxide and metal aluminum, but the aluminum in the mixed layer 3, which is remelted during heat treatment in a reducing or inert atmosphere, has other erosion-resistant and heat-resistant properties. It has good wettability even with oxides with good properties such as chromium oxide, titanium oxide, and zirconium oxide, and can provide strong bonding strength between metal oxide particles, so it can be used in the method of the present invention. I can do it. In addition, for alloy 1 of aluminum and metal oxide-forming elements, for example, chromium oxide,
Alloys of aluminum and chromium can also be used to achieve the objectives of the invention.

The mixing ratio of the metal oxide and aluminum or aluminum alloy in the present invention is such that when the size of the metal oxide particles is relatively large, it is necessary to increase the content of aluminum or its alloy. In this case, since the particle size of the metal oxide is around 10 mm, it is sufficient to add about several percent to the metal oxide. Further, when a mixture of metal oxide and aluminum or aluminum alloy is sprayed by flame spraying using acetylene gas, the porosity in the mixed layer 3 is about 10%, and when spraying using plasma, the porosity is about 10%.
Since pores with a porosity of about 5% are generated, as a sealing treatment, a solution containing aluminum or its alloy in the form of powder or ions is applied after thermal spraying to impregnate it, and then the above heat treatment is performed. It is also possible to further improve the properties of the film of the present invention.

〔Effect of the invention〕

As explained in detail above, the surface coating method for metal materials of the present invention has better adhesion to the metal material to be coated than conventional thermal spraying methods, and the metal oxide layer has fewer pores and is denser. , and the bonding strength between the metal oxide particles constituting the film makes it possible to easily form a film with excellent lotion resistance and heat resistance, so when this film is applied to boiler heat exchanger tubes, Its service life can be dramatically improved.

Furthermore, the method of the present invention can be applied to the surface coating of metal materials containing elements that form a solid solution or intermetallic compound with aluminum, so it is not limited to the boiler heat exchanger tubes mentioned above, but can also be applied to other materials that require wear resistance and heat resistance. It can be widely applied in various fields, and its industrial value is extremely large.

[Brief explanation of the drawing]

FIGS. 1(a), (b), (c) and (d) are explanatory diagrams showing step by step the process of forming a two-lotion resistant and heat resistant film in an example of the present invention. 1... Carbon steel 2... Aluminum layer 3
...Mixed layer of aluminum oxide and metallic aluminum

Claims (5)

[Claims] (1) After forming a film of aluminum or aluminum alloy on the surface of the metal material to be coated and further forming a mixed layer of aluminum or aluminum alloy and metal oxide on the film, A pore-sealing heat treatment is performed by heating in an atmosphere or an inert atmosphere to cause interdiffusion between the metal material and the coating and to infiltrate the aluminum or aluminum alloy into the pores of the coating and the mixed layer, and then the oxidizing A method for surface coating a metal material, comprising performing an oxidation heat treatment in an atmosphere at a temperature below the melting point of the aluminum or aluminum alloy to convert the aluminum or aluminum alloy remaining in the film and the mixed layer into an oxide. (2) The metal material to be coated is Fe, Cr, Ni, Co
2. The method of surface coating a metal material according to claim 1, wherein the alloy is an alloy containing at least one element selected from among , Mn, Ti, and Cu. (3) The metal oxide constituting the mixed layer with aluminum or aluminum alloy contains at least one oxide selected from aluminum oxide, zirconium oxide, titanium oxide, and chromium oxide, or mainly contains the above oxides. 2. The method for surface coating a metal material according to claim 1, wherein the metal oxide is a component. (4) Formation of the aluminum or aluminum alloy film and the mixed layer of aluminum or aluminum alloy and metal oxide is performed by at least one method selected from thermal spraying, vapor deposition, hot-dip plating, and electroplating. A method for surface coating a metal material according to claim 1, characterized in that: (5) The method for surface coating a metal material according to claim 1, wherein the temperature range of the oxidation heat treatment is 500 to 650°C.