Multi-layer composite protective coating for boiler pipeline and preparation method of multi-layer composite protective coating

Abstract

The invention provides a multi-layer composite protective coating for a boiler pipeline and a preparation method of the multi-layer composite protective coating, and the preparation method comprises the following steps: carrying out cleaning and sand blasting roughening treatment on the surface of a pipeline matrix to obtain a treated pipeline matrix; the surface of the treated pipeline base body is sequentially sprayed with a bottom layer, a middle layer and a surface layer, the pipeline base body sprayed with multiple layers of composite protective coatings is obtained, during spraying, spraying materials of the bottom layer, the middle layer and the surface layer are NiCrNbMoB alloy powder, thermal fatigue, oxidation corrosion and abrasion under the working condition of deep peak shaving can be effectively resisted, the service life of the pipeline base body is prolonged, and the service life of the pipeline base body is prolonged. The boiler pipeline explosion risk is obviously reduced, and the service life of the pipeline is prolonged.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature protective coating technology, specifically relating to a multi-layer composite protective coating for boiler pipes and its preparation method. Background Technology

[0002] With the continuous increase in the installed capacity of new energy sources, the frequency and magnitude of thermal power generating units participating in deep peak shaving have significantly increased. During deep peak shaving, the frequent start-up and shutdown of the units and the rapid changes in load cause boiler pipes (such as TP347H stainless steel pipes) to be subjected to severe temperature fluctuations and thermal stress shocks, which can easily lead to thermal fatigue, oxidation corrosion and wear, and in severe cases, tube rupture accidents, affecting the safe and stable operation of the units.

[0003] Traditional single-component protective coatings are difficult to maintain stable service under combined conditions of high temperature, corrosion, and wear. Existing coatings generally suffer from problems such as insufficient bonding strength, mismatched coefficients of thermal expansion, low high-temperature hardness, and poor corrosion resistance, and cannot meet the stringent requirements for pipeline protection in deep peak-shaving environments. Summary of the Invention

[0004] The purpose of this invention is to provide a multi-layer composite protective coating for boiler pipes and its preparation method, which solves the above-mentioned deficiencies in the prior art. The multi-layer composite protective coating described in this application has excellent bonding strength, thermal shock resistance, high temperature hardness and corrosion resistance, and can effectively suppress the risk of pipe rupture.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a method for preparing a multilayer composite protective coating for boiler pipes, comprising the following steps: The surface of the pipe substrate is cleaned and roughened by sandblasting to obtain the treated pipe substrate. A primer, intermediate layer, and top coat are sequentially sprayed onto the surface of the treated pipe substrate to obtain a pipe substrate coated with a multi-layer composite protective coating, wherein: During spraying, the coating materials for the bottom layer, middle layer, and top layer are all NiCrNbMoB alloy powder.

[0006] Preferably, by mass percentage, the NiCrNbMoB alloy powder corresponding to the bottom layer comprises 18-20% Cr, 3-5% Nb, 2-3% Mo and 0.5-1% B, with the remainder being Ni.

[0007] Preferably, by mass percentage, the NiCrNbMoB alloy powder corresponding to the middle layer comprises 15-18% Cr, 8-12% Nb, 6-8% Mo and 1-2% B, with the remainder being Ni.

[0008] Preferably, by mass percentage, the NiCrNbMoB alloy powder corresponding to the surface layer comprises 20-22% Cr, 15-18% Nb, 10-12% Mo and 2-3% B, with the remainder being Ni.

[0009] Preferably, the surface roughness of the treated pipe substrate is 5.0 μm - 7.0 μm.

[0010] Preferably, plasma is used to sequentially spray a base layer, a middle layer, and a top layer onto the surface of the treated pipe substrate to obtain a multi-layer composite protective coating for boiler pipes.

[0011] Preferably, a multi-layer composite protective coating for boiler pipes is obtained by sequentially spraying a base layer, a middle layer, and a top layer onto the surface of the treated pipe substrate using a supersonic flame.

[0012] Preferably, the preparation method further includes: heat-treating the obtained pipe substrate coated with a multi-layer composite protective coating.

[0013] Secondly, the present invention provides a multi-layer composite protective coating for boiler pipes, which is prepared using the aforementioned preparation method.

[0014] Preferably, the thickness of the multilayer composite protective coating is 300-400 μm. Compared with the prior art, the beneficial effects of the present invention are: This invention provides a method for preparing a multi-layer composite protective coating for boiler pipes. Through compositional gradient design and functional layering, the overall performance of the coating is significantly improved, effectively addressing the severe temperature fluctuations and thermal stress impacts experienced by boiler pipes during deep peak shaving due to frequent start-ups and shutdowns and rapid load changes. The coating uses NiCrNbMoB alloy powder. Through optimized composition ratios of the bottom, middle, and top layers, the bottom layer, with its high nickel content, ensures strong bonding with the substrate and matches the coefficient of thermal expansion, alleviating thermal stress. The middle layer achieves dispersion strengthening by adding elements such as Nb and Mo to form hard phases (e.g., NbC, MoB), improving overall hardness and support. The top layer, with its high Cr, Nb, Mo, and B content, imparts excellent high-temperature hardness, wear resistance, and corrosion resistance. The preparation process employs plasma spraying or supersonic flame spraying, combined with vacuum heat treatment, to eliminate internal stress in the coating, promote inter-layer element interdiffusion, and further enhance bonding strength and density. Actual tests show that the coating has a bonding strength of over 65 MPa, a surface microhardness of 650-800 HV0.3, and can withstand more than 50 cycles of water quenching from 800℃ to room temperature without peeling. The microstructure shows that the layers are well bonded and free of cracks and pores. These characteristics enable the coating to operate stably for a long time under combined high-temperature, corrosive, and abrasion conditions, fundamentally suppressing the risk of pipe rupture, extending the service life of pipelines, and ensuring the safe and stable operation of thermal power generating units under deep peak shaving. Detailed Implementation

[0015] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0016] It should be understood that, when used in this application specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or a collection thereof.

[0017] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0018] As used in this application specification and the appended claims, the term "if" may be interpreted, depending on the context, as "when," "once," "in response to determination," or "in response to detection." Similarly, the phrase "if determined" or "if detected [the described condition or event]" may be interpreted, depending on the context, as meaning "once determined," "in response to determination," "once detected [the described condition or event]," or "in response to detection [the described condition or event]."

[0019] Furthermore, in the description of this application and the appended claims, the terms "first," "second," "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0020] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0021] Example 1 This embodiment provides a method for preparing a multi-layer composite protective coating for boiler pipes, comprising the following steps: The surface of the pipe substrate is cleaned and roughened by sandblasting to obtain the treated pipe substrate. A primer, intermediate layer, and top coat are sequentially sprayed onto the surface of the treated pipe substrate to obtain a pipe substrate coated with a multi-layer composite protective coating, wherein: During spraying, the coating materials for the bottom layer, middle layer, and top layer are all NiCrNbMoB alloy powder.

[0022] Example 2 Based on Example 1, this example provides a method for preparing a multi-layer composite protective coating for boiler pipes. By mass percentage, the NiCrNbMoB alloy powder corresponding to the bottom layer includes 18-20% Cr, 3-5% Nb, 2-3% Mo and 0.5-1% B, with the remainder being Ni.

[0023] By mass percentage, the NiCrNbMoB alloy powder corresponding to the middle layer includes 15-18% Cr, 8-12% Nb, 6-8% Mo and 1-2% B, with the remainder being Ni. By mass percentage, the NiCrNbMoB alloy powder corresponding to the surface layer comprises 20-22% Cr, 15-18% Nb, 10-12% Mo and 2-3% B, with the remainder being Ni.

[0024] The bottom layer is used to improve the bonding strength between the coating and the substrate, alleviate the mismatch of thermal expansion coefficients, and prevent the excessive formation of hard and brittle phases.

[0025] The middle layer serves as a performance transition zone, enhancing overall hardness through the dispersion strengthening effect of hard phases such as NbC and MoB.

[0026] The surface layer has high hardness, wear resistance and corrosion resistance, making it suitable for high temperature, corrosion and wear-prone working conditions.

[0027] Example 3 This embodiment uses TP347H stainless steel pipes commonly used in power plant boilers as the substrate to illustrate a method for preparing a multi-layer composite protective coating to prevent pipe rupture under deep peak-shaving conditions in thermal power generating units, including the following steps: Step 1, Pretreatment of substrate surface Cleaning and degreasing: Use acetone or anhydrous ethanol to thoroughly clean the surface of the TP347H stainless steel substrate to remove oil and impurities until the surface reaches a hydrophilic state.

[0028] Sandblasting roughening: Abrasive type: Brown corundum (Al2O3) angular sand with a particle size range of 16 mesh.

[0029] Process parameters: compressed air pressure is 0.6MPa, spray gun angle is 75°, and spray gun distance is 100mm.

[0030] Objective: To roughen the substrate surface to achieve a surface roughness (Ra) of 5.0 μm, significantly increasing the bonding area between the coating and the substrate and providing a good mechanical bonding foundation. After sandblasting, the surface is cleaned with compressed air to remove residual abrasive dust.

[0031] Step 2, spray the base coat (adhesive transition layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, the NiCrNbMoB alloy powder comprises 18% Cr, 5% Nb, 2% Mo, and 1% B, with the remainder being Ni. The particle size of the NiCrNbMoB alloy powder is 45 μm. The role of the NiCrNbMoB alloy powder in mitigating the risk of pipe rupture is as follows: Ni forms the matrix to ensure coating toughness and adhesion; Cr enhances high-temperature oxidation / corrosion resistance; Nb and Mo form the NbC and MoB hard phases to strengthen hardness and wear resistance; and B lowers the alloy melting point, promotes element diffusion, and alleviates thermal stress. Structural level: Gradient composition design allows the coating to gradually match the thermal expansion coefficient of the pipe substrate, reducing thermal stress caused by temperature fluctuations and preventing crack initiation; Key process parameters: Plasma gas and flow rate: The main gas is argon (Ar), with a flow rate of 40 Standard Liters Per Minute (SLPM); the auxiliary gas is hydrogen (H2), with a flow rate of 8 SLPM. The addition of hydrogen helps to increase the enthalpy and voltage of the plasma arc, thereby improving the melting effect of the powder.

[0032] Arc current and voltage: The current is set to 550A and the voltage is approximately 70V (determined by the gas flow rate).

[0033] Spraying distance: 100mm. This distance ensures that the powder melts completely while preventing excessive oxidation or cooling of the molten droplets.

[0034] Powder feeding gas and powder feeding rate: Argon is used as the powder feeding gas, and the powder feeding rate is controlled at 30g / min. The powder feeding rate needs to be stable to ensure uniform coating deposition.

[0035] Spray gun movement speed: 800mm / s.

[0036] Coating thickness: The thickness of the coating applied to the base layer is approximately 80 μm.

[0037] Step 3, apply the intermediate layer (reinforcing support layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, the NiCrNbMoB alloy powder comprises 18% Cr, 8% Nb, 6% Mo, and 2% B, with the remainder being Ni. The particle size of the NiCrNbMoB alloy powder is 60 μm.

[0038] Key process parameters: Plasma gas and flow rate: To provide higher heat to melt the added high-melting-point elements such as Nb and Mo, the hydrogen (H2) flow rate was slightly increased to 10 SLPM, while the argon (Ar) flow rate remained unchanged.

[0039] Arc current and voltage: The current is set to 600A to obtain a higher energy plasma jet.

[0040] Spraying distance: 120mm. Increase the distance appropriately to prevent overheating of the substrate and undercoat.

[0041] Powder delivery rate: 35g / min.

[0042] Spray gun movement speed: 800mm / s.

[0043] Coating thickness: The thickness of the intermediate layer is approximately 100μm.

[0044] Step 4, apply the surface coating (functional protective layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, the NiCrNbMoB alloy powder comprises 21% Cr, 16% Nb, 11% Mo, and 2.5% B, with the remainder being Ni. The powder particle size ranges from 30 μm. Using finer powder helps to form a denser, harder surface layer.

[0045] Key process parameters: Plasma gas and flow rate: Due to the highest content and melting point of the powder alloy elements in this layer, a higher energy input is required. The hydrogen (H2) flow rate is further increased to 12 SLPM, while the argon (Ar) flow rate remains unchanged.

[0046] Arc current and voltage: Current is set to 650A.

[0047] Spraying distance: 120mm.

[0048] Powder delivery rate: 30g / min.

[0049] Spray gun movement speed: 800mm / s.

[0050] Coating thickness: The thickness of the surface layer after spraying is approximately 100 μm. The total coating thickness is controlled at 300 μm.

[0051] Step 5, subsequent heat treatment To eliminate internal stress in the coating, promote inter-layer element interdiffusion, and improve interlayer bonding strength and overall coating density, vacuum heat treatment is performed.

[0052] Equipment: Vacuum sintering furnace.

[0053] Heat treatment process: Vacuum degree: ≤1.0×10 - ²Pa.

[0054] Heating program: Heat to 950-1050℃ at a heating rate of 5-8℃ / min.

[0055] Insulation: Keep warm at the target temperature for 1.5-2.5 hours.

[0056] Cooling: After cooling in the furnace to below 300℃, remove from the furnace and air cool.

[0057] Implementation effect The NiCrNbMoB gradient coating prepared by the above plasma spraying process and parameters was tested and found to be: Coating bond strength: ≥65MPa (according to ASTM C633 standard).

[0058] Coating microhardness: It exhibits a gradient increase from the bottom layer to the surface layer, with the surface hardness reaching 650-800 HV0.3.

[0059] Thermal shock resistance: It can withstand more than 50 cycles of water quenching thermal shock test from 800℃ to room temperature without peeling.

[0060] Microstructure: Metallographic microscopy and SEM observation showed that the layers were well bonded together, with no obvious cracks or pores, and the surface layer had the densest structure.

[0061] This embodiment demonstrates that the plasma spraying process can successfully prepare NiCrNbMoB gradient coatings that are suitable for harsh working conditions with deep peak sizing, have high bonding strength, and excellent comprehensive protective performance.

[0062] Example 4 This embodiment uses TP347H stainless steel pipes commonly used in power plant boilers as the substrate to illustrate a method for preparing a multi-layer composite protective coating to prevent pipe rupture under deep peak-shaving conditions in thermal power generating units, including the following steps: Step 1, Pretreatment of substrate surface Cleaning and degreasing: Use acetone or anhydrous ethanol to thoroughly clean the surface of the TP347H stainless steel substrate to remove oil and impurities until the surface reaches a hydrophilic state.

[0063] Sandblasting roughening: Abrasive type: Brown corundum (Al2O3) angular sand with a particle size range of 25 mesh.

[0064] Process parameters: compressed air pressure is 0.6MPa, spray gun angle is 80°, and spray gun distance is 130mm.

[0065] Objective: To roughen the substrate surface to achieve a surface roughness (Ra) of 6.0 μm, significantly increasing the bonding area between the coating and the substrate and providing a good mechanical bonding foundation. After sandblasting, the surface is cleaned with compressed air to remove residual abrasive dust.

[0066] Step 2, spray the base coat (adhesive transition layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, the NiCrNbMoB alloy powder comprises 19% Cr, 3% Nb, 2.5% Mo, and 0.5% B, with the remainder being Ni. The particle size of the NiCrNbMoB alloy powder is 60 μm. The role of the NiCrNbMoB alloy powder in mitigating the risk of pipe rupture is as follows: Ni forms the matrix to ensure coating toughness and adhesion; Cr enhances high-temperature oxidation / corrosion resistance; Nb and Mo form the NbC and MoB hard phases to strengthen hardness and wear resistance; and B lowers the alloy melting point, promotes element diffusion, and alleviates thermal stress. Structural level: Gradient composition design allows the coating to gradually match the thermal expansion coefficient of the pipe substrate, reducing thermal stress caused by temperature fluctuations and preventing crack initiation; Key process parameters: Plasma gas and flow rate: The main gas is argon (Ar), with a flow rate of 43 Standard Liters Per Minute (SLPM); the auxiliary gas is hydrogen (H2), with a flow rate of 10 SLPM. The addition of hydrogen helps to increase the enthalpy and voltage of the plasma arc, thereby improving the melting effect of the powder.

[0067] Arc current and voltage: The current is set to 600A and the voltage is approximately 73V (determined by the gas flow rate).

[0068] Spraying distance: 110mm. This distance ensures that the powder melts completely while preventing excessive oxidation or cooling of the molten droplets.

[0069] Powder feeding gas and powder feeding rate: Argon is used as the powder feeding gas, and the powder feeding rate is controlled at 35g / min. The powder feeding rate needs to be stable to ensure uniform coating deposition.

[0070] Spray gun movement speed: 900mm / s.

[0071] Coating thickness: The thickness of the coating applied to the base layer is approximately 90 μm.

[0072] Step 3, apply the intermediate layer (reinforcing support layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, the NiCrNbMoB alloy powder comprises 15% Cr, 10% Nb, 7% Mo, and 1% B, with the remainder being Ni. The particle size of the NiCrNbMoB alloy powder is 45 μm.

[0073] Key process parameters: Plasma gas and flow rate: To provide higher heat to melt the added high-melting-point elements such as Nb and Mo, the hydrogen (H2) flow rate was slightly increased to 8 SLPM, while the argon (Ar) flow rate remained unchanged.

[0074] Arc current and voltage: The current is set to 650A to obtain a higher energy plasma jet.

[0075] Spraying distance: 130 mm. Increase the distance appropriately to prevent overheating of the substrate and the undercoat.

[0076] Powder delivery rate: 40g / min.

[0077] Spray gun movement speed: 900mm / s.

[0078] Coating thickness: The thickness of the intermediate layer is approximately 130 μm.

[0079] Step 4, apply the surface coating (functional protective layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, the NiCrNbMoB alloy powder comprises 21% Cr, 16% Nb, 11% Mo, and 2.5% B, with the remainder being Ni. The powder particle size ranges from 55 μm. Using finer powder helps to form a denser, harder surface layer.

[0080] Key process parameters: Plasma gas and flow rate: Due to the highest content and melting point of the powder alloy elements in this layer, a higher energy input is required. The hydrogen (H2) flow rate is further increased to 13 SLPM, while the argon (Ar) flow rate remains unchanged.

[0081] Arc current and voltage: Current is set to 650-750A.

[0082] Spraying distance: 130mm.

[0083] Powder delivery rate: 35g / min.

[0084] Spray gun movement speed: 900 mm / s.

[0085] Coating thickness: The thickness of the surface layer after spraying is approximately 130 μm. The total coating thickness is controlled at 350 μm.

[0086] Step 5, subsequent heat treatment To eliminate internal stress in the coating, promote inter-layer element interdiffusion, and improve interlayer bonding strength and overall coating density, vacuum heat treatment is performed.

[0087] Equipment: Vacuum sintering furnace.

[0088] Heat treatment process: Vacuum degree: ≤1.0×10 - ²Pa.

[0089] Heating program: Heat to 950-1050℃ at a heating rate of 5-8℃ / min.

[0090] Insulation: Keep warm at the target temperature for 1.5-2.5 hours.

[0091] Cooling: After cooling in the furnace to below 300℃, remove from the furnace and air cool.

[0092] Example 5 This embodiment uses TP347H stainless steel pipes commonly used in power plant boilers as the substrate to illustrate a method for preparing a multi-layer composite protective coating to prevent pipe rupture under deep peak-shaving conditions in thermal power generating units, including the following steps: Step 1, Pretreatment of substrate surface Cleaning and degreasing: Use acetone or anhydrous ethanol to thoroughly clean the surface of the TP347H stainless steel substrate to remove oil and impurities until the surface reaches a hydrophilic state.

[0093] Sandblasting roughening: Abrasive type: Brown corundum (Al2O3) angular sand with a particle size range of 40 mesh.

[0094] Process parameters: compressed air pressure is 0.7MPa, spray gun angle is 90°, and spray gun distance is 150mm.

[0095] Objective: To roughen the substrate surface to achieve a surface roughness (Ra) of 7.0 μm, significantly increasing the bonding area between the coating and the substrate and providing a good mechanical bonding foundation. After sandblasting, the surface is cleaned with compressed air to remove residual abrasive dust.

[0096] Step 2, spray the base coat (adhesive transition layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, the NiCrNbMoB alloy powder comprises 20% Cr, 4% Nb, 3% Mo, and 1% B, with the remainder being Ni. The particle size of the NiCrNbMoB alloy powder is 90 μm. The role of the NiCrNbMoB alloy powder in mitigating the risk of pipe rupture is as follows: Ni forms the matrix to ensure coating toughness and adhesion; Cr enhances high-temperature oxidation / corrosion resistance; Nb and Mo form the NbC and MoB hard phases to strengthen hardness and wear resistance; and B lowers the alloy melting point, promotes element diffusion, and alleviates thermal stress. Structural level: Gradient composition design allows the coating to gradually match the thermal expansion coefficient of the pipe substrate, reducing thermal stress caused by temperature fluctuations and preventing crack initiation; Key process parameters: Plasma gas and flow rate: The main gas is argon (Ar), with a flow rate of 45 Standard Liters Per Minute (SLPM); the auxiliary gas is hydrogen (H2), with a flow rate of 12 SLPM. The addition of hydrogen helps to increase the enthalpy and voltage of the plasma arc, thereby improving the melting effect of the powder.

[0097] Arc current and voltage: The current is set to 650A and the voltage is approximately 75V (determined by the gas flow rate).

[0098] Spraying distance: 120mm. This distance ensures that the powder melts completely while preventing excessive oxidation or cooling of the molten droplets.

[0099] Powder feeding gas and powder feeding rate: The powder feeding gas is argon, and the powder feeding rate is controlled at 40 g / min. The powder feeding rate needs to be stable to ensure uniform coating deposition.

[0100] Spray gun movement speed: 1000mm / s.

[0101] Coating thickness: The thickness of the coating applied to the base layer is approximately 100 μm.

[0102] Step 3, apply the intermediate layer (reinforcing support layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, NiCrNbMoB alloy powder comprises 16% Cr, 12% Nb, 8% Mo, and 1.5% B, with the remainder being Ni. The particle size of NiCrNbMoB alloy powder is 45μm-90μm.

[0103] Key process parameters: Plasma gas and flow rate: To provide higher heat to melt the added high-melting-point elements such as Nb and Mo, the hydrogen (H2) flow rate was slightly increased to 14 SLPM, while the argon (Ar) flow rate remained unchanged.

[0104] Arc current and voltage: The current is set to 700A to obtain a higher energy plasma jet.

[0105] Spraying distance: 140 mm. Increase the distance appropriately to prevent overheating of the substrate and the undercoat.

[0106] Powder delivery rate: 45g / min.

[0107] Spray gun movement speed: 1000mm / s.

[0108] Coating thickness: The thickness of the intermediate layer is approximately 150 μm.

[0109] Step 4, apply the surface coating (functional protective layer). Spraying process: Atmospheric plasma spraying Coating material: By mass percentage, the NiCrNbMoB alloy powder comprises 22% Cr, 18% Nb, 12% Mo, and 3% B, with the remainder being Ni. The powder particle size ranges from 75 μm. Using finer powder contributes to the formation of a denser, harder surface layer.

[0110] Key process parameters: Plasma gas and flow rate: Due to the highest content and melting point of the powder alloy elements in this layer, a higher energy input is required. The hydrogen (H2) flow rate is further increased to 15 SLPM, while the argon (Ar) flow rate remains unchanged.

[0111] Arc current and voltage: Current is set to 750A.

[0112] Spraying distance: 140mm.

[0113] Powder delivery rate: 40g / min.

[0114] Spray gun movement speed: 1000 mm / s.

[0115] Coating thickness: The thickness of the surface layer after spraying is approximately 150 μm. The total coating thickness is controlled at 400 μm.

[0116] Step 5, subsequent heat treatment To eliminate internal stress in the coating, promote inter-layer element interdiffusion, and improve interlayer bonding strength and overall coating density, vacuum heat treatment is performed.

[0117] Equipment: Vacuum sintering furnace.

[0118] Heat treatment process: Vacuum degree: ≤1.0×10 - ²Pa.

[0119] Heating program: Heat to 950-1050℃ at a heating rate of 5-8℃ / min.

[0120] Insulation: Keep warm at the target temperature for 1.5-2.5 hours.

[0121] Cooling: After cooling in the furnace to below 300℃, remove from the furnace and air cool.

[0122] Example 6 This embodiment provides a multi-layer composite protective coating for boiler pipes, which is prepared based on the preparation method described in Example 1.

[0123] The thickness of the bottom layer is 80-120μm; the thickness of the middle layer is 100-150μm; and the thickness of the top layer is 100-150μm.

[0124] The thickness of the multilayer composite protective coating is 300-400μm.

[0125] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A method for preparing a multilayer composite protective coating for boiler pipes, characterized in that, Includes the following steps: The surface of the pipe substrate is cleaned and roughened by sandblasting to obtain the treated pipe substrate. A primer, intermediate layer, and top coat are sequentially sprayed onto the surface of the treated pipe substrate to obtain a pipe substrate coated with a multi-layer composite protective coating, wherein: During spraying, the coating materials for the bottom layer, middle layer, and top layer are all NiCrNbMoB alloy powder.

2. The method for preparing a multi-layer composite protective coating for boiler pipes according to claim 1, characterized in that, By mass percentage, the NiCrNbMoB alloy powder corresponding to the bottom layer comprises 18-20% Cr, 3-5% Nb, 2-3% Mo and 0.5-1% B, with the remainder being Ni.

3. The method for preparing a multi-layer composite protective coating for boiler pipes according to claim 1, characterized in that, By mass percentage, the NiCrNbMoB alloy powder corresponding to the middle layer comprises 15-18% Cr, 8-12% Nb, 6-8% Mo and 1-2% B, with the remainder being Ni.

4. The method for preparing a multi-layer composite protective coating for boiler pipes according to claim 1, characterized in that, By mass percentage, the NiCrNbMoB alloy powder corresponding to the surface layer comprises 20-22% Cr, 15-18% Nb, 10-12% Mo and 2-3% B, with the remainder being Ni.

5. The method for preparing a multi-layer composite protective coating for boiler pipes according to claim 1, characterized in that, The surface roughness of the treated pipe substrate is 5.0 μm - 7.0 μm.

6. The method for preparing a multi-layer composite protective coating for boiler pipes according to claim 1, characterized in that, By using plasma to sequentially spray the base layer, middle layer, and top layer onto the surface of the treated pipe substrate, a multi-layer composite protective coating for boiler pipes is obtained.

7. The method for preparing a multi-layer composite protective coating for boiler pipes according to claim 1, characterized in that, By using a supersonic flame to sequentially spray a base layer, a middle layer, and a top layer onto the surface of the treated pipe substrate, a multi-layer composite protective coating for boiler pipes is obtained.

8. The method for preparing a multi-layer composite protective coating for boiler pipes according to claim 1, characterized in that, The preparation method further includes: heat treatment of the obtained pipe substrate coated with a multi-layer composite protective coating.

9. A multi-layer composite protective coating for boiler pipes, characterized in that, It was prepared using the preparation method described in claim 1.

10. A multi-layer composite protective coating for boiler pipes according to claim 9, characterized in that, The thickness of the multilayer composite protective coating is 300-400μm.