Method for repairing an article having a self-fluxing alloy coating

Abstract

To provide a repairing method that can form a repair film which enables large-area repairs with high work efficiency, includes neither a cavity nor a crack inside to have superior denseness, and also has superior adhesion to a substrate surface.SOLUTION: The present invention relates to a repairing method for an article having a self-fluxing film on a substrate surface, and the repairing method includes: a patch preparation process of preparing a patch for repair of less than 30% in porosity which is made of self-fluxing alloy shaped adaptively to a repaired part including a defective part of the self-fluxing film; a surface treatment process of performing a surface treatment on the repaired part; a repaired part preheating process of preheating the repaired part after the surface treatment; a patch arranging process of arranging the patch in contact with the preheated repaired part; and a joining process of heating and joining the patch to the repaired part to form the repair film.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for repairing an article having a self-fluxing alloy coating on a substrate surface. [Background technology]

[0002] As a method for repairing defects in a corrosion-protected body having a corrosion-prevention thermal spray coating formed on the surface of a substrate such as a steel structure, a method of flame spraying metal powder using a handy type thermal spraying device has been introduced (see Patent Document 1 below). However, this repair method using a thermal spraying device is limited in that the area that can be repaired is small and the work efficiency is poor, so it is not suitable as a repair method for repairing large areas on-site.

[0003] On the other hand, as a method for coating a self-fluxing alloy layer on the surface of a substrate such as a steel material, the present inventors have proposed a method in which a slurry-like composition containing a self-fluxing alloy powder, a binder resin, and a solvent is applied to the substrate surface, the coating is dried, and then the dried coating is heated to a sintering temperature under specified heating conditions in the atmosphere, and the self-fluxing alloy powder is sintered at the sintering temperature, thereby forming a self-fluxing alloy layer made of a sintered body on the substrate surface (see Patent Document 2 below). It is conceivable that such a method could be used to repair a repaired portion (a missing portion of the self-fluxing alloy coating) of an article having a self-fluxing alloy coating on the substrate surface. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2006-213988 A [Patent Document 2] JP 2015-157971 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, when repairing an article using the method described in Patent Document 2, the following problems arise. (1) Temperature control up to the sintering temperature is difficult, and depending on the heating conditions, the powder surface may oxidize, making it impossible to form the desired coating on the sintered body. (2) The film formed may have voids or cracks due to residual combustion gases from the binder resin, or the flow of the alloy powder or sintered body. (3) The sintered body formed may have a high porosity and lack density. Repaired parts made with self-fluxing alloys that lack density cannot fully fulfill the performance required of a self-fluxing alloy layer (corrosion resistance, wear resistance, etc.). (4) Because bonding to the substrate surface and sintering are performed simultaneously, bonding may be insufficient, and partial peeling may occur between the self-fluxing alloy layer (repair coating) and the substrate surface. This means that the adhesion of the repair coating to the substrate surface is not sufficient. (5) When attempting to form a thick self-fluxing alloy layer for repair because the defective portion of the self-fluxing alloy film in the repaired portion is deep, it is necessary to apply the slurry composition to the repaired portion in layers to form a thick coating film, which is troublesome, and it takes a long time to dry the coating film, making it impossible to carry out repair efficiently. This is a significant problem especially when the area of ​​the repaired portion is large.

[0006] As described above, in the repair method in which a self-fluxing alloy layer (repair film) is formed from a dried coating containing a self-fluxing alloy powder and a binder resin, a sintering process is carried out after the coating is formed on the substrate surface, which may result in poor performance of the repair film due to changes in the properties of the coating (film) (due to the presence of voids, cracks, or high porosity) or poor adhesion of the film to the substrate surface. For these reasons, no suitable method has been proposed for repairing an article having a self-fluxing alloy coating on the substrate surface.

[0007] The present invention has been made based on the above circumstances. The object of the present invention is to provide a repair method which is highly efficient, enables repair of large areas, and is capable of forming a repair coating which is free of voids or cracks and has excellent density and adhesion to the surface of a substrate. [Means for solving the problem]

[0008] (1) The repair method of the present invention is a method for repairing an article having a self-fluxing alloy coating on a surface of a substrate, comprising the steps of: a patch preparation process for preparing a repair patch having a porosity of less than 30% and made of a self-fluxing alloy shaped to a shape that fits the part to be repaired, including the defective part of the self-fluxing alloy coating; A surface preparation process for preparing the repaired portion; a repair portion preheating step of preheating the repair portion that has been subjected to a surface treatment; a patch placement step of placing the patch in contact with the preheated repaired portion; and a joining step of heat-joining the patch to the repaired portion to form a repair coating. The present invention is characterized by comprising: Here, the porosity is defined as the ratio of the area of ​​the pores in the cross section of a patch or film, and is a value obtained by dividing the area of ​​the pores by the area of ​​the cross section, expressed as a percentage. For example, an optical microscope photograph of the cross section of the film in the thickness direction is taken, and the photograph is binarized into white and black parts by image processing, and the area ratio of the parts corresponding to the pores in the obtained binarized image is calculated, and this value is regarded as the porosity.

[0009] According to this repair method, an article can be repaired by a simple operation of placing a patch on the part to be repaired and heating and bonding it. Furthermore, by producing a large-sized patch, it is possible to accommodate repair areas having a large area. A patch made of a self-fluxing alloy with a porosity of less than 30% can be firmly adhered to the part to be repaired by heating the contact surface with the part to be repaired to a certain temperature or higher during the joining process. In addition, the porosity is reduced by remelting through heat treatment, further improving the performance of the final repair coating.

[0010] (2) In the patch preparation step, it is preferable to prepare a repair patch having a porosity of less than 20%.

[0011] (3) In the patch fabrication step, the patch is preferably fabricated by thermal spraying. This ensures that a patch with low porosity and no voids is produced.

[0012] (4) In the patch production step of the repair method (3) above, it is preferable to form a thermal sprayed coating on a mold surface having the same or similar shape as the part to be repaired, and to produce the patch by peeling off the thermal sprayed coating from the mold surface. As a result, in cases where the shape of the part to be repaired is somewhat predetermined due to the structure of the article, more efficient repairs can be achieved by preparing a patch that fits that shape in advance.

[0013] (5) In the repair method of (4) above, it is preferable that the mold having the mold surface is prepared by molding the part to be repaired. This allows the creation of a patch that perfectly matches the shape of the part to be repaired.

[0014] (6) In the repair method of the present invention, it is preferable that the patch is made of an alloy having the same composition as the self-fluxing alloy coating formed on the substrate surface of the article. This further improves adhesion to the remaining self-fluxing alloy layer in the repaired portion, and allows the self-fluxing alloy layer to exhibit the same performance (corrosion resistance, wear resistance, etc.) as before it was damaged. Effect of the Invention

[0015] The repair method of the present invention is highly efficient and allows repair of large areas. The repair method of the present invention makes it possible to form a repair coating that is free of voids or cracks, has excellent density, and exhibits excellent adhesion to the substrate surface. [Brief description of the drawings]

[0016] [Figure 1] 1 is a photograph showing the appearance of a repaired portion according to an embodiment of the present invention. [Diagram 2]1 is a photomicrograph showing a cross section of a repaired portion according to an embodiment of the present invention. [Diagram 3] 4 is a photograph showing the appearance of a repaired part in a comparative example (sintering of a coating film containing self-fluxing alloy powder). [Figure 4] 1 is a photomicrograph showing a cross section of a repaired portion according to a comparative example (sintering of a coating film containing self-fluxing alloy powder). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The repair method of the present invention is a method for forming a repair coating on a portion to be repaired, including a defective portion of a self-fluxing alloy coating, in an article having a self-fluxing alloy coating on the surface of the substrate. Examples of "articles" having a self-fluxing alloy coating on the surface of a substrate include boiler parts such as pipe components (boiler tubes) that make up the heat transfer piping of various boilers, and plate-tube composite panels (boiler furnace panels) that make up a furnace housing with cooling water channels.

[0018] The repair method of the present invention includes a patch preparation step, a surface preparation step, a preheating step of the portion to be repaired, a patch placement step, and a joining step.

[0019] <Patch manufacturing process> This process involves producing a repair patch made of a self-fluxing alloy shaped to fit the part to be repaired. Here, the "part to be repaired" includes the substrate surface exposed due to the loss of the self-fluxing alloy coating and the remaining self-fluxing alloy coating. Therefore, the repair coating is formed on the substrate surface and / or the surface of the self-fluxing alloy coating.

[0020] The shape of the part to be repaired may vary depending on the shape of the base material (three-dimensional or flat), and may be, for example, the outer circumferential shape (cylindrical) of a pipe part. A shape that fits the part to be repaired means a shape that, when the patch is placed on the part to be repaired, can come into contact with the part to be repaired over the entire joint surface of the patch (for example, the gap between the part to be repaired is within 5 mm). For example, a patch that fits around the outer circumference of a pipe component may have a partially circular tubular (trough) shape with an inside diameter that is approximately the same as the outside diameter of the pipe component.

[0021] The repair patch is made of a self-fluxing alloy. Examples of the self-fluxing alloy include known self-fluxing alloys that can impart wear resistance and corrosion resistance to a metal material (base material), but a preferred self-fluxing alloy is a Ni-based alloy with 1 to 5 mass% B, 1 to 5 mass% Si, 10 to 40 mass% Cr, 4 mass% or less Mo, and 1 mass% or less C. The self-fluxing alloy constituting the patch preferably has the same composition as the self-fluxing alloy film formed on the surface of the substrate.

[0022] The repair patch typically has a porosity of less than 30%, preferably less than 20%, and more preferably less than 15%. If the porosity is 30% or more, the repair coating formed through the joining process may have poor bonding to the part to be repaired, or voids may remain in the repair coating, preventing it from achieving the performance required of a self-fluxing alloy coating.

[0023] The thickness of the patch is usually 300 μm or more, and preferably 500 to 3000 μm. If the thickness of the patch is too small, a repair coating of sufficient thickness cannot be formed, and the protective coating cannot exhibit its performance such as abrasion resistance and corrosion resistance. On the other hand, if the patch is too thick, it may be difficult to make it conform to the shape of the substrate in the area to be repaired. The size (area) of the patch is not particularly limited, and a patch of an appropriate size can be prepared depending on the size of the part to be repaired, etc.

[0024] One method for making a patch involves preparing a mold having a mold surface that is the same or similar in shape to the part to be repaired, spraying a self-fluxing alloy powder onto the mold surface to form a coating, and then peeling off the sprayed coating from the mold surface.

[0025] The thermal spraying method may be a conventionally known method. The surface of the mold may be treated with a release agent such as a boron nitride release agent so that the sprayed coating (patch) can be easily peeled off from the surface of the mold. Fabricating the patch by thermal spraying ensures that the patch is uniform in thickness, void-free, and has less than 30% porosity.

[0026] The patches may be pre-fabricated, such as in a factory. For example, the patch to be placed on the outer periphery of the pipe part as described above can be formed in large numbers using the outer periphery of a pipe part of the same diameter as the mold surface. However, it is necessary to prevent the bonding surface of the patch that comes into contact with the part to be repaired from being oxidized. Furthermore, if the part to be repaired has a special shape, a mold may be made by taking a mold from the part to be repaired, thereby making it possible to produce a patch that perfectly matches the shape of the part to be repaired.

[0027] <Surface preparation process> This step is a step of removing oxide films, corroded areas, etc. on the surface of the part to be repaired by subjecting the part to a surface treatment. As the surface treatment method, a conventionally known treatment method such as grinding or shot blasting can be used.

[0028] <Preheating process for the part to be repaired> This step involves preheating the area to be repaired that has been pre-treated. The preheating method is not particularly limited, and examples of the method include heating (preheating) the surface of the part to be repaired by roasting with a gas burner or the like. The preheating temperature is usually 600°C or higher, preferably 700 to 900°C, and a suitable example is 800°C. By preheating the area to be repaired, melting of the joining surface of the patch can be promoted, enabling suitable thermal joining to be achieved.

[0029] <Patch placement process> This process involves placing a patch in contact with the part to be repaired. For example, a partially circular tubular (gutter-shaped) patch is placed on the outer periphery of a preheated pipe part. This step is carried out immediately after the completion of the repair part preheating step. The patch to be placed may be preheated.

[0030] <Joining process> This process involves heat-bonding a patch to the part to be repaired to form a repair coating. In one example of the bonding process, a partially circular tubular patch is placed on the outer periphery of a pipe part (the part to be repaired) and heated from its outer periphery by a gas burner. Here, the heating temperature is usually 1000°C or higher, preferably 1020 to 1080°C, and a suitable example is 1050°C. As a result, the patch in contact with the outer circumferential surface of the pipe component is thermally bonded to the pipe component, and a repair coating is formed on the outer circumferential surface of the pipe component by the bonded patch. At this time, the patch made of the thermal spray coating is remelted, so that its porosity is reduced, resulting in a denser repair coating. Here, the porosity of the repair coating after remelting is generally less than 15%, preferably less than 10%, and more preferably less than 5%. A repair coating with a porosity of less than 15% is highly dense and can fully exhibit the wear resistance, corrosion resistance, and other properties required of a self-fluxing alloy coating. EXAMPLES

[0031] <Example> (1) Preparation of the item to be repaired: A boiler tube was manufactured by spraying a self-fluxing alloy (equivalent to SFNi4) specified in JIS H 8303 onto the outer surface of a carbon steel tube for boilers and heat exchangers (equivalent to JIS G 3461, STB410) with an outer diameter of 63.5 mm and a wall thickness of 6.5 mm, forming a self-fluxing alloy coating with a thickness of 2000 μm and a porosity of 0.1 to 1%, as measured by image analysis of the cross section observed with an optical microscope. A part of the self-fluxing alloy coating formed on the obtained boiler tube was removed using a grinder to expose the substrate surface (the outer peripheral surface of the steel tube) in a substantially rectangular shape. The size of the exposed substrate surface (the circumferential length of the steel tube × the axial length of the steel tube) was about 60 mm × about 20 mm.

[0032] (2) Patch production process: A repair patch was prepared as follows, using the outer peripheral surface of a carbon steel pipe for boilers and heat exchangers having the same shape as that used in (1) above as a mold surface. Masking tape was attached to the outer circumferential surface (mold surface) of the steel pipe in a frame shape. The size of the mold surface surrounded by the masking tape (circumferential length of the steel pipe × axial length of the steel pipe) was 80 mm × 30 mm. A self-fluxing alloy (equivalent to SFNi4) with the same composition as the self-fluxing alloy coating was sprayed onto the mold surface surrounded by masking tape, and then the masking tape was removed and the sprayed coating was peeled off from the mold surface to obtain a repair patch (80 mm x 30 mm x 2.5 mm). The cross section of the obtained patch was observed by an optical microscope, and the porosity measured by image analysis was 5 to 15%.

[0033] (3) Surface preparation process: The surface of the substrate exposed in (1) above and its surroundings (the portion to be repaired in which at least a part of the self-fluxing alloy coating is missing) were subjected to surface preparation by grinding with a grinder.

[0034] (4) Preheating process for the part to be repaired: The area to be repaired, which had been subjected to the surface preparation in (3) above, was preheated to approximately 800°C by roasting with a gas burner.

[0035] (5) Patch placement process: The patch prepared in (2) above was placed in contact with the preheated area to be repaired such that the exposed substrate surface was completely covered.

[0036] (6)Joining process: The outer peripheral surface of the patch placed on the part to be repaired was heated to approximately 1050° C. by roasting with a gas burner, thereby bonding (welding) the patch to the part to be repaired and forming a repair coating.

[0037] (7) Observation of repaired area: Figure 1 is a photograph showing the appearance of the repaired portion (repair film formed on the surface of the substrate) according to this example. As shown in Figure 1, no irregularities or cracks were observed on the outer surface of the repair film, and the surface was excellent in smoothness. In addition, the repair film was integrated with the boiler tube.

[0038] FIG. 2 is a photomicrograph showing the cross-sectional state of the repaired portion (repair coating formed on the surface of the substrate) according to this embodiment. The cross-section of the repaired portion, cut in a direction perpendicular to the axial direction of the boiler tube, was etched using the nitric acid-ethanol method described in JIS G 0553, and the photo was taken with an optical microscope. As shown in Figure 2, the interface between the repair coating and the substrate is smooth, with no partial peeling observed between the two, indicating excellent adhesion of the repair coating to the substrate surface. In addition, no large voids (voids) or cracks are observed within the repair coating. In addition, the cross section of this repair coating was observed under an optical microscope, and the porosity measured by image analysis was 0.3 to 3%, indicating that the coating was dense.

[0039] <Comparative Example> (1) Preparation of the item to be repaired: An article to be repaired (a boiler tube having a portion to be repaired in which the surface of the substrate was exposed) was prepared in the same manner as in Example (1).

[0040] (2) Preparation of a slurry composition: A slurry-like composition was prepared by mixing 62.50 mass% of self-fluxing alloy powder (corresponding to SFNi4), 0.60 mass% of binder resin (PVB), 0.75 mass% of plasticizer (DBP), 31.23 mass% of solvent (ethanol), and 4.92 mass% of other components.

[0041] (3) Surface preparation process: The surface of the substrate exposed in (1) above and its surroundings (the portion to be repaired in which at least a part of the self-fluxing alloy coating is missing) were subjected to surface preparation by grinding with a grinder.

[0042] (4) Coating and drying process of the slurry composition: The slurry composition prepared in (2) above was applied by airbrush to the part to be repaired that had been subjected to the surface preparation in (3) above to form a coating film, and then the boiler tube, which was the item to be repaired, was left to stand for 6 hours in an environment of 20°C and 60% relative humidity to remove the solvent and form a dry coating film 2.5 mm thick.

[0043] (5) Repair film formation process: The dried coating film formed in (4) above was heated to about 800°C by roasting with a gas burner, and then the heating temperature was increased to about 1050°C to sinter the dried coating film and form a repair coating (self-fluxing alloy layer).

[0044] (6) Observation of repaired area: Fig. 3 is a photograph showing the appearance of the repaired portion (repair film formed on the surface of the substrate) according to this comparative example. As shown in Fig. 3, the outer surface of the repair film had noticeable irregularities and partial cracks.

[0045] FIG. 4 is a photomicrograph showing the cross-sectional state of the repaired portion (repair coating formed on the surface of the substrate) according to this comparative example. As shown in Figure 4, the interface between the repair coating and the substrate is not smooth, with partial peeling observed between the two, indicating poor adhesion of the repair coating to the substrate surface. In addition, large voids (voids) and cracks were observed within the repair coating. In addition, the cross section of this repair coating was observed under an optical microscope, and the porosity measured by image analysis was 20-40%, indicating that the coating lacked density.

Claims

1. A method for repairing an article having a self-fluxing alloy coating on a substrate surface, comprising the steps of: a patch preparation step of preparing a repair patch having a porosity of less than 30% and a thickness of 300 μm or more, the repair patch being made of a self-fluxing alloy shaped into a shape that fits the portion to be repaired, including the defective portion of the self-fluxing alloy coating; A surface preparation process for preparing the repaired portion; a repair portion preheating step of preheating the surface-treated repair portion to a temperature of 600° C. or higher; a patch placement step of placing the patch in contact with the preheated repaired portion; and a joining step of heat-joining the patch to the repaired portion at a temperature of 1000° C. or higher to form a repair coating. A repair method including:

2. The repair method according to claim 1 , wherein in the patch preparation step, a repair patch having a porosity of less than 20% is prepared.

3. 3. The repair method according to claim 1, wherein in the patch preparation step, the patch is prepared by thermal spraying.

4. 4. The method for repairing an article according to claim 3, wherein in the patch preparation step, a thermal sprayed coating is formed on a mold surface having the same or similar shape as the part to be repaired, and the patch is prepared by peeling off the thermal sprayed coating from the mold surface.

5. 5. The method for repairing an article according to claim 4, wherein the mold having the mold surface is prepared by molding the part to be repaired.

6. 6. The repair method according to claim 1, wherein the patch is made of an alloy having the same composition as the self-fluxing alloy coating formed on the substrate surface of the article.

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