A gradient cladding layer for super / heater tubes and its preparation method and application

Abstract

The application belongs to the field of metal welding materials, and discloses a gradient cladding layer for a superheater / reheater tube and a preparation method and application thereof. The gradient cladding layer comprises: forming a primer laser cladding layer on the superheater / reheater tube, forming an electric arc surfacing cladding layer on the primer laser cladding layer, and the electric arc surfacing cladding layer being directly contacted with a furnace atmosphere; the primer laser cladding layer comprises the following components: Ni: 30-40, Cr: 22-25, Mo: 5-8, Nb: 0.8-2, Si: 0.5-1, Mn: 0.5-1, and the balance of Fe, and the total is 100%; the electric arc surfacing cladding layer comprises the following components: Cr: 40-45, Fe: 16-19, Nb: 12-15, Mo: 8-10, Ti: 3-4, Al: 1.5-2.5, and the balance of Ni, and the total is 100%. The gradient cladding layer has excellent corrosion resistance and low stress, and meets the safe service of the superheater / reheater.

Description

Technical Field

[0001] This invention belongs to the field of metal welding materials technology, specifically relating to a gradient cladding layer for superheater / reheater tubes, its preparation method, and its application. Background Technology

[0002] The superheater / reheater is a key component of a supercritical boiler unit, responsible for recovering energy from coal-fired flue gas, heating steam, and achieving energy conversion. It is the part of the boiler that withstands the highest pressure, temperature, and harshest operating environment. Currently, the preferred materials for superheaters / reheaters mainly include ferritic heat-resistant steel and austenitic heat-resistant steel. Among them, austenitic steel is one of the preferred materials for the final stage superheater / reheater of the unit, as it can meet the mechanical performance requirements for operation under 650℃ steam conditions.

[0003] Compared to water-cooled wall tubes, the increased chromium content in superheater / reheater tube alloys significantly improves their resistance to flue gas corrosion. While chromium content is crucial for enhancing the alloy's resistance, during boiler operation, coal ash containing large amounts of alkali metal oxides / chlorides deposits on the surface of the superheater / reheater tube alloys, forming salt deposits. These salt deposits react with chromium in the oxide film to form spinel phases such as sodium chromate and potassium chromate, leading to oxide film rupture and accelerating the corrosion rate of the superheater / reheater tubes. Furthermore, the corrosion rate increases with increasing salt deposit content. Chloride ions significantly accelerate the corrosion rate of the superheater / reheater tube alloys; the increase in corrosion weight is particularly pronounced when 300 mg / kg NaCl is added to the atmosphere.

[0004] Currently, research on the corrosion of superheater / reheater tube alloys mainly focuses on improving their corrosion resistance by increasing the chromium content. However, due to the complex operating environment of superheater / reheater tubes inside boilers, the oxides generated by chromium alone are insufficient to meet increasingly stringent operating conditions. Furthermore, excessively high chromium content can lead to the formation of brittle phases, resulting in poorer machinability of the alloy. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a gradient cladding layer for superheater / reheater tubes, its preparation method, and its application. Since the superheater / reheater tubes are made of iron-based alloys, this invention employs an Fe-Ni-Cr alloy system. A base laser cladding layer formed from iron-based alloy powder is prepared on the superheater / reheater tube using laser cladding to ensure excellent metallurgical bonding between the base laser cladding layer and the superheater / reheater tube substrate. Simultaneously, based on the base laser cladding layer, an arc-welded cladding layer is formed by arc welding of welding wire, directly contacting the furnace atmosphere. Because the arc-welded cladding layer is in direct contact with the furnace atmosphere, it is necessary to ensure that the arc-welded cladding layer possesses high-temperature corrosion resistance under long-term service. This invention designs and prepares alloy powder in raw materials with a gradient distribution of chromium and nickel content in the superheater / reheater tube, the initial laser cladding layer, and the arc welding cladding layer, where the chromium content increases gradientally and the nickel content decreases gradientally. This serves two purposes: first, it allows chromium and nickel to act as a phase transition in the gradient cladding layer, effectively reducing the stress distribution within the layer; second, it increases the upper limit of Cr content in the arc welding cladding layer, thereby improving the high-temperature corrosion resistance of the gradient cladding layer.

[0006] The gradient cladding layer for superheater / reheater tubes of the present invention is achieved through the following technical solution:

[0007] The first objective of this invention is to provide a gradient cladding layer for superheater / reheater tubes, comprising: a base laser cladding layer formed on the superheater / reheater tube, wherein an arc weld cladding layer is formed on the base laser cladding layer, wherein the arc weld cladding layer is in direct contact with the furnace atmosphere.

[0008] The base laser cladding layer, by mass percentage, consists of the following components:

[0009] Ni: 30%–40%, Cr: 22%–25%, Mo: 5%–8%, Nb: 0.8%–2%, Si: 0.5%–1%, Mn: 0.5%–1%, balance Fe, total 100%.

[0010] The arc welding cladding layer is obtained by arc welding with flux-cored welding wire.

[0011] The flux-cored wire comprises a wire sheath and flux powder, with the flux powder filling the wire sheath. The flux powder, by mass percentage, consists of the following components:

[0012] Cr: 40%–45%, Fe: 16%–19%, Nb: 12%–15%, Mo: 8%–10%, Ti: 3%–4%, Al: 1.5%–2.5%, balance Ni, total 100%.

[0013] Preferably, the weld skin is a Cr50Ni50 strip with a thickness of 0.4 mm and a width of 7 mm.

[0014] It should be noted that since the superheater / reheater tube operates in a high-temperature and corrosive atmosphere, ensuring corrosion resistance is a prerequisite, and Cr is the most critical element determining corrosion resistance. This invention adds Cr to both the initial laser cladding layer and the arc weld cladding layer. Because the arc weld cladding layer is in direct contact with the high-temperature corrosive fumes in the furnace, the Cr content in its raw materials is higher than that in the initial laser cladding layer. In other words, the Cr content in the raw materials exhibits a gradually increasing gradient across the superheater / reheater tube, the initial laser cladding layer, and the arc weld cladding layer. This serves two purposes: firstly, to mitigate stress concentration caused by abrupt changes in composition, and secondly, to increase the upper limit of Cr content in the arc weld cladding layer. According to the Cr-Fe binary phase diagram, both have the risk of forming a brittle FeCr phase. Therefore, the gradient Cr content is beneficial for forming a high-performance gradient cladding layer.

[0015] This invention incorporates Cr and Ni elements into both the initial laser cladding layer and the arc welding cladding layer to optimize their performance and forming quality. According to the Ni-Cr binary phase diagram, Ni and Cr are infinitely miscible and exhibit excellent weldability. Therefore, introducing Ni into the initial laser cladding powder and the arc welding wire helps improve the metallurgical bonding ability of the gradient cladding layer and ensures interlayer bonding strength. Simultaneously, the Ni content exhibits a gradually decreasing gradient distribution across the superheater / reheater tube, the initial laser cladding layer, and the arc welding layer. This design not only matches the increasing trend of Cr, further mitigating stress concentration caused by abrupt compositional changes, but also optimizes the thermal expansion matching of the cladding layer, thereby reducing interfacial residual stress and improving the overall service performance of the cladding layer.

[0016] This invention enhances mechanical properties and high-temperature stability by introducing Mo into both the initial laser cladding layer and the arc welded cladding layer. Mo is incorporated into the cladding layer design to improve these properties. In the initial laser cladding layer, Mo strengthens through solid solution: due to its large atomic radius, Mo dissolves in the ferrite matrix, causing significant lattice distortion and thus increasing the strength of the initial laser cladding layer. For the arc welded layer, which directly contacts a high-temperature corrosive environment, adding Mo to the welding wire offers dual advantages: firstly, based on the infinite mutual solubility shown in the Cr-Mo binary phase diagram, Mo can form a stable solid solution with Cr; secondly, this alloying synergy not only improves the strength of the welded layer but also enhances its high-temperature creep resistance, ensuring the long-term reliability of the cladding layer under harsh conditions. This multi-scale strengthening mechanism design results in a gradient cladding layer that possesses both excellent mechanical properties and high-temperature durability.

[0017] This invention adds Nb to both the initial laser cladding layer and the arc-welded cladding layer. In the initial laser cladding layer, Nb enhances the mechanical properties of the nickel matrix through a solid solution strengthening mechanism. In the arc-welded layer, the high-temperature precipitation of Nb forms an Nb-rich phase, which effectively pins grain boundaries, inhibits grain boundary migration at high temperatures, and improves creep resistance. Furthermore, since the substrate itself contains Nb, the Nb content in the gradient cladding layer can be designed to form a composition buffer zone, effectively inhibiting the diffusion and loss of Nb from the substrate to the cladding layer, and maintaining the original properties of the substrate material.

[0018] This invention addresses the issue by adding Fe to both the initial laser cladding layer and the arc welding cladding layer. The primary role of Fe in these layers is to enhance the fluidity of the molten pool, thereby ensuring the formation of the gradient cladding layer. Furthermore, the superheater / reheater tube is an iron-based alloy, with Fe as its main element. The gradient distribution of Fe in both the superheater / reheater tube and the gradient cladding layer reduces the diffusion rate of other alloying elements within the superheater / reheater pipe.

[0019] The laser cladding layer of the present invention also contains Si and Mn elements. Firstly, Si and Mn have a combined deoxidizing effect; secondly, the laser cladding layer is an Fe-Ni-Cr alloy system, and Si and Mn are dissolved in the Fe-based lattice, which has a certain solid solution strengthening effect.

[0020] The arc-welded cladding layer of this invention also contains Ti and Al elements. The arc-welded cladding layer is an Fe-Ni-Cr alloy system. Ti and Al are common strengthening elements in nickel-based alloys. By generating the Ni3(Al,Ti) precipitate phase, the high-temperature creep strength and creep resistance of the nickel-based cladding layer are significantly improved. However, since Ti and Al are detrimental to arc stability, their addition amount should be strictly controlled.

[0021] Preferably, the thickness of the laser cladding layer is 0.6mm to 1.2mm, and the thickness of the arc welding layer is 2.0mm to 3.0mm.

[0022] Preferably, the particle size of the raw materials used to prepare the base laser cladding layer is 100 mesh to 200 mesh.

[0023] Preferably, the purity of the raw materials used to prepare the base laser cladding layer is higher than 99.99%.

[0024] A second objective of this invention is to provide a method for preparing the aforementioned gradient cladding layer, comprising the following steps:

[0025] S1. Mix the raw materials for preparing the base laser cladding layer to obtain laser cladding powder; use the laser cladding powder as raw material to form a base laser cladding layer on the superheater / reheater tube by laser cladding.

[0026] It should be noted that the specific preparation process of the base laser cladding layer of the present invention is as follows: according to the proportion of the raw materials for the preparation of the base laser cladding layer, the raw materials are mixed and vacuum melted, and atomized powder is obtained by gas atomization; the atomized powder is sieved to a particle size of 270-500 mesh to obtain laser cladding powder; the laser cladding powder is clad onto the superheater / reheater tube by laser cladding to form the base laser cladding layer.

[0027] Preferably, the conditions for gas atomization are: the atomizing gas is N2, the atomization pressure is 6 MPa, and the superheat of the melt is maintained at 100℃~150℃ during the gas atomization process.

[0028] Preferably, the flowability of the laser cladding powder is required to be 25s / 100g to 40s / 100g.

[0029] Preferably, the laser power of the laser cladding is 2.5kW to 3kW, and the overlap rate is 40% to 50%.

[0030] S2. Using flux-cored welding wire as raw material, an arc-welded cladding layer is formed on the laser cladding layer to obtain a gradient cladding layer.

[0031] It should be noted that the specific preparation process of the arc-welded cladding layer of the present invention is as follows: According to the proportion of the raw materials for preparing the arc-welded cladding layer, each raw material is heated at 220℃~260℃ for 1h~2h; then mixed in a powder mixer for 1h~2h to obtain flux powder; the flux powder is wrapped inside the weld bead, and a first drawing is performed, followed by gradually decreasing the drawing die aperture to obtain a flux-cored wire with a diameter of 1.0mm~1.2mm; the flux-cored wire is then used to perform arc welding on the laser cladding layer to form the arc-welded cladding layer. The filling rate of the flux powder is 22%~25%.

[0032] It should also be noted that, before arc welding, the flux-cored wire needs to be wrapped with a welding layer. To ensure the flux is tightly wrapped and to prevent leakage after wrapping, the wrapped welding layer needs to pass through a die with a circular aperture. That is, the flux is wrapped inside the welding layer, and a first drawing process is performed. After the first drawing process is completed, the die aperture is gradually reduced to obtain a flux-cored wire with a diameter of 1.0 mm to 1.2 mm. Preferably, the first drawing die aperture is 2.6 mm.

[0033] To prevent porosity and cracks from forming during welding, which could affect the arc welding process, this invention heats the raw materials before mixing to remove crystal water and gas from the powder, and ensures the stability of the powder and the welding quality during the welding process.

[0034] Preferably, the welding current of the arc welding is 160A to 200A, and the interpass temperature is controlled below 100℃.

[0035] A third objective of this invention is to provide the application of the gradient cladding layer described above for superheater / reheater tubes in the preparation of protective coating materials for superheaters / reheaters.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] The laser cladding layer of this invention uses an Fe-Ni-Cr alloy system, with an Fe content of 23% to 41.6%. The superheater / reheater tube is an iron-based alloy, with Fe as the main matrix element, which ensures excellent metallurgical bonding between the laser cladding layer and the superheater / reheater tube matrix. Simultaneously, the Ni content in the laser cladding layer is 30% to 40%, and the Cr content is 22% to 25%, higher than the Ni and Cr content in the superheater / reheater tube. The arc welding cladding layer contains 4.5% to 19.5% Ni and 40% to 45% Cr. The 5% Cr content results in a gradual increase in Cr content across the superheater / reheater tube, the initial laser cladding layer, and the arc welded cladding layer; while the Ni content gradually decreases across these layers. This serves two purposes: firstly, it allows Cr and Nickel to act as a phase transition in the gradient cladding layer, mitigating stress concentration caused by abrupt compositional changes and effectively reducing stress distribution within the gradient cladding layer; secondly, it raises the upper limit of Cr content in the arc welded cladding layer, thereby enhancing the excellent high-temperature corrosion resistance of the gradient cladding layer on the superheater / reheater tube.

[0038] The laser cladding layer of this invention incorporates 5%–8% Mo, 0.8%–2% Nb, 0.5%–1% Si, and 0.5%–1% Mn. Mo dissolves into the ferrite matrix, causing lattice distortion and thus increasing the strength of the laser cladding layer. Nb dissolves into the Ni matrix within the laser cladding layer, strengthening the Ni matrix. Simultaneously, Si and Mn dissolve in the Fe-based lattice of the laser cladding layer, providing solid solution strengthening and further enhancing the strength of the laser cladding layer.

[0039] The arc-welded cladding layer of this invention is in direct contact with the flue gas in the furnace. Based on Ni and Cr elements, it is supplemented with 8%–10% Mo, 16%–19% Fe, and 12%–15% Nb, with a small amount of 3%–4% Ti and 1.5%–2.5% Al. The infinite interfusibility of Mo and Cr elements improves the strength of the arc-welded cladding layer; Fe element improves the fluidity of the molten pool, thus ensuring the formation of the gradient cladding layer; the Nb-containing arc-welded cladding layer precipitates a Nb-rich phase at high temperatures, inhibiting grain boundary migration; Ti and Al elements significantly improve the high-temperature creep strength and anti-creep properties of the nickel-based cladding layer by forming the Ni3(Al,Ti) precipitate phase, thereby enhancing the high-temperature corrosion resistance of the arc-welded cladding layer.

[0040] This invention employs laser cladding to prepare the initial laser cladding layer. The laser cladding powder of this invention can be used for both coaxial powder feeding laser cladding and powder spreading laser cladding, offering a wide range of applications. The arc welding cladding layer of this invention can be used for both TIG welding and MIG welding, demonstrating a wide range of applications and good welding process performance. The arc welding layer of this invention is suitable for welding the surface of superheater / reheater tubes in coal-fired power plant boilers, thereby ensuring the safe operation of superheater / reheater tubes under harsh conditions such as deep peak shaving and multi-coal blending. Attached Figure Description

[0041] Figure 1 Figure 1 is a schematic diagram of the gradient cladding layer prepared according to the present invention; wherein, Figure (a) is a base laser cladding layer formed on the superheater tube, and Figure (b) is an arc welding cladding layer formed on the base laser cladding layer.

[0042] Figure 2 The metallographic structure of the laser cladding layer prepared for Comparative Example 1 is shown.

[0043] Figure 3 The metallographic structure of the arc-welded cladding layer prepared in Example 1.

[0044] Figure 4 Scanning electron microscope (SEM) image of the high-temperature corrosion morphology of the gradient cladding layer prepared in Example 1.

[0045] Figure 5 Scanning electron microscope (SEM) image of the high-temperature corrosion morphology of the gradient cladding layer prepared for Comparative Example 3.

[0046] Explanation of reference numerals in the attached figures:

[0047] 1-TP347H tube; 2-Laser cladding layer for the base coat; 3-Arc welding cladding layer. Detailed Implementation

[0048] To enable those skilled in the art to better understand and implement the technical solutions of this invention, the invention will be further described below with reference to specific embodiments and accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0049] Example 1

[0050] A method for preparing a gradient cladding layer includes the following steps:

[0051] S1. Preparation of the initial laser cladding layer:

[0052] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0053] Ni: 40%, Cr: 25%, Mo: 8%, Nb: 2%, Si: 1%, Mn: 1%, balance Fe, total 100%.

[0054] The raw materials are mixed and vacuum melted. Using a gas atomization method, N2 is used as the atomizing gas, the atomization pressure is 6 MPa, and the superheat of the melt is maintained at 150°C during the atomization process to obtain atomized powder.

[0055] The atomized powder was sieved through a particle size distribution of 270-500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder was 25μm-53μm, and the flowability of the laser cladding powder was 25s / 100g-40s / 100g.

[0056] Laser cladding powder was clad onto the TP347H tube using coaxial powder feeding laser cladding to form a base laser cladding layer; the laser power was 3kW, the overlap rate was 50%, and the thickness of the base laser cladding layer was 1.2mm.

[0057] S2. Preparation of gradient cladding layer:

[0058] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0059] Cr: 45%, Fe: 19%, Nb: 15%, Mo: 10%, Ti: 4%, Al: 2.5%, balance Ni, total 100%.

[0060] Each raw material was placed in a vacuum furnace and heated to 260°C for 2 hours. Then it was placed in a powder mixer for thorough mixing for 2 hours to obtain the powder.

[0061] Alcohol was used to remove the grease from the surface of the Cr50Ni50 strip. The flux-cored wire was then drawn by wrapping the flux powder inside the Cr50Ni50 strip using a flux-cored wire drawing machine. The first drawing die had a diameter of 2.6 mm. The drawing die diameter was then gradually reduced to obtain a flux-cored wire with a diameter of 1.2 mm. The flux powder filling rate in the flux-cored wire was 22%.

[0062] Arc welding was performed on the laser cladding layer prepared by S1 using flux-cored welding wire to form an arc cladding layer, resulting in a gradient cladding layer. The welding current was 160A to 200A, the interpass temperature was controlled at 70℃, and the thickness of the arc cladding was 3.0mm.

[0063] Example 2

[0064] A method for preparing a gradient cladding layer includes the following steps:

[0065] S1. Preparation of the initial laser cladding layer:

[0066] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0067] Ni: 30%, Cr: 22%, Mo: 5%, Nb: 0.8%, Si: 0.5%, Mn: 0.5%, balance Fe, total 100%.

[0068] The raw materials are mixed and vacuum melted. Using a gas atomization method, N2 is used as the atomizing gas, the atomization pressure is 6 MPa, and the superheat of the melt is maintained at 100°C during the atomization process to obtain atomized powder.

[0069] The atomized powder was sieved through a particle size distribution of 270-500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder was 25μm-53μm, and the flowability of the laser cladding powder was 25s / 100g-40s / 100g.

[0070] Laser cladding powder was clad onto the TP347H tube using coaxial powder feeding laser cladding to form a base laser cladding layer; the laser power was 2.5kW, the overlap rate was 40%, and the thickness of the base laser cladding layer was 0.6mm.

[0071] S2. Preparation of gradient cladding layer:

[0072] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0073] Cr: 40%, Fe: 16%, Nb: 12%, Mo: 8%, Ti: 3%, Al: 1.5%, balance Ni, total 100%.

[0074] Each raw material was heated in a vacuum furnace at 220°C for 1 hour, and then mixed thoroughly in a powder mixer for 1 hour to obtain the powder.

[0075] Alcohol was used to remove the grease from the surface of the Cr50Ni50 strip. The flux-cored wire was then drawn by wrapping the flux powder inside the Cr50Ni50 strip using a flux-cored wire drawing machine. The first drawing die had a diameter of 2.6 mm. The drawing die diameter was then gradually reduced to obtain a flux-cored wire with a diameter of 1.2 mm. The flux powder filling rate in the flux-cored wire was 25%.

[0076] Arc welding was performed on the laser cladding layer prepared by S1 using flux-cored welding wire to form an arc cladding layer, resulting in a gradient cladding layer. The welding current was 160A to 200A, the interpass temperature was controlled at 80℃, and the thickness of the arc cladding was 2.0mm.

[0077] Example 3

[0078] This embodiment provides a method for preparing a gradient cladding layer, including the following steps:

[0079] S1. Preparation of the initial laser cladding layer:

[0080] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0081] Ni: 35%, Cr: 23%, Mo: 7%, Nb: 1.4%, Si: 0.7%, Mn: 0.7%, balance Fe, total 100%.

[0082] The raw materials are mixed and vacuum melted. Using a gas atomization method, N2 is used as the atomizing gas, the atomization pressure is 6 MPa, and the superheat of the melt is maintained at 120°C during the atomization process to obtain atomized powder.

[0083] The atomized powder was sieved through a particle size distribution of 270-500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder was 25μm-53μm, and the flowability of the laser cladding powder was 25s / 100g-40s / 100g.

[0084] Laser cladding powder was clad onto the TP347H tube to form a base laser cladding layer; the laser power was 2.7kW, the overlap rate was 45%, and the thickness of the base laser cladding layer was 0.9mm.

[0085] S2. Preparation of gradient cladding layer:

[0086] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0087] Cr: 43%, Fe: 17%, Nb: 13%, Mo: 9%, Ti: 3.5%, Al: 2%, balance Ni, total 100%.

[0088] Each raw material was heated in a vacuum furnace at 240°C for 1.5 hours and then mixed thoroughly in a powder mixer for 1.5 hours to obtain the powder.

[0089] The grease on the surface of the Cr50Ni50 strip is removed with alcohol. The flux-cored wire is then drawn by wrapping the flux powder inside the Cr50Ni50 strip with a flux-cored wire drawing machine. The first drawing die has a diameter of 2.6 mm. The drawing die diameter is then gradually reduced to obtain a flux-cored wire with a diameter of 1.2 mm. The flux powder filling rate in the flux-cored wire is 24%.

[0090] Arc welding was performed on the laser cladding layer prepared by S1 using flux-cored welding wire to form an arc cladding layer, resulting in a gradient cladding layer. The welding current was 160A to 200A, the interpass temperature was controlled at 60℃, and the arc cladding thickness was 2.5mm.

[0091] Example 4

[0092] This embodiment provides a method for preparing a gradient cladding layer, including the following steps:

[0093] S1. Preparation of the initial laser cladding layer:

[0094] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0095] Ni: 37%, Cr: 24%, Mo: 6%, Nb: 1.5%, Si: 0.6%, Mn: 0.6%, balance Fe, total 100%.

[0096] The raw materials are mixed and vacuum melted. The mixture is then atomized using N2 as the atomizing gas at a pressure of 6 MPa. The superheat of the melt is maintained at 125°C during the atomization process to obtain atomized powder.

[0097] The atomized powder was sieved through a particle size distribution of 270-500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder was 25μm-53μm, and the flowability of the laser cladding powder was 25s / 100g-40s / 100g.

[0098] Laser cladding powder was clad onto the TP347H tube to form a base laser cladding layer; the laser power was 2.7kW, the overlap rate was 42%, and the thickness of the base laser cladding layer was 0.7mm.

[0099] S2. Preparation of gradient cladding layer:

[0100] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0101] Cr: 44%, Fe: 18%, Nb: 14%, Mo: 9.5%, Ti: 3.2%, Al: 2.4%, balance Ni, total 100%.

[0102] Each raw material was heated in a vacuum furnace at 255°C for 1.2 hours and then mixed thoroughly in a powder mixer for 1.2 hours to obtain the powder.

[0103] The grease on the surface of the Cr50Ni50 strip is removed with alcohol. The flux-cored wire is then drawn by wrapping the flux powder inside the Cr50Ni50 strip with a flux-cored wire drawing machine. The first drawing die has a diameter of 2.6 mm. The drawing die diameter is then gradually reduced to obtain a flux-cored wire with a diameter of 1.2 mm. The flux powder filling rate in the flux-cored wire is 23%.

[0104] Arc welding was performed on the laser cladding layer prepared by S1 using flux-cored welding wire to form an arc cladding layer, resulting in a gradient cladding layer. The welding current was 160A to 200A, the interpass temperature was controlled at 50℃, and the arc cladding thickness was 2.8mm.

[0105] Example 5

[0106] A method for preparing a gradient cladding layer includes the following steps:

[0107] S1. Preparation of the initial laser cladding layer:

[0108] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0109] Ni: 31%, Cr: 22.5%, Mo: 5.8%, Nb: 0.82%, Si: 0.55%, Mn: 0.95%, balance Fe, total 100%.

[0110] The raw materials are mixed and vacuum melted. Using a gas atomization method, N2 is used as the atomizing gas, the atomization pressure is 6 MPa, and the superheat of the melt is maintained at 130°C during the atomization process to obtain atomized powder.

[0111] The atomized powder was sieved through a particle size distribution of 270-500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder was 25μm-53μm, and the flowability of the laser cladding powder was 25s / 100g-40s / 100g.

[0112] Laser cladding was used to clad laser powder onto the TP347H tube to form a base laser cladding layer; the laser power was 2.8kW, the overlap rate was 48%, and the thickness of the base laser cladding layer was 1.1mm.

[0113] S2. Preparation of gradient cladding layer:

[0114] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0115] Cr: 41%, Fe: 16.5%, Nb: 12.5%, Mo: 8.5%, Ti: 3.8%, Al: 2.1%, balance Ni, total 100%.

[0116] Each raw material was heated in a vacuum furnace at 235°C for 1.3 hours and then mixed thoroughly in a powder mixer for 1.9 hours to obtain the powder.

[0117] The grease on the surface of the Cr50Ni50 strip is removed with alcohol. The flux-cored wire is then drawn by wrapping the flux powder inside the Cr50Ni50 strip with a flux-cored wire drawing machine. The first drawing die has a diameter of 2.6 mm. The drawing die diameter is then gradually reduced to obtain a flux-cored wire with a diameter of 1.0 mm. The flux powder filling rate in the flux-cored wire is 22%.

[0118] Arc welding was performed on the laser cladding layer prepared by S1 using flux-cored welding wire to form an arc cladding layer, resulting in a gradient cladding layer. The welding current was 160A to 200A, the thickness of the arc cladding layer was 2.1mm, and the interpass temperature was controlled at 40℃.

[0119] Comparative Example 1

[0120] A method for preparing a gradient laser cladding layer includes the following steps:

[0121] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0122] Ni: 40%, Cr: 25%, Mo: 8%, Nb: 2%, Si: 1%, Mn: 1%, balance Fe, total 100%.

[0123] The raw materials are mixed and vacuum melted. Using a gas atomization method, N2 is used as the atomizing gas, the atomization pressure is 6 MPa, and the superheat of the melt is maintained at 150°C during the atomization process to obtain atomized powder.

[0124] The atomized powder was sieved through a particle size distribution of 270-500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder was 25μm-53μm, and the flowability of the laser cladding powder was 25s / 100g-40s / 100g.

[0125] Laser cladding powder is clad onto the superheater / reheater tube using coaxial powder feeding laser cladding to form a base laser cladding layer; wherein, the laser power is 3kW, the thickness of the base laser cladding layer is 1.2mm, and the overlap rate is 50%.

[0126] Comparative Example 2

[0127] A method for preparing a gradient laser cladding layer includes the following steps:

[0128] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0129] Ni: 40%, Cr: 25%, Mo: 8%, Nb: 2%, Si: 1%, Mn: 1%, balance Fe, total 100%.

[0130] The raw materials are mixed and vacuum melted. Using a gas atomization method, N2 is used as the atomizing gas, the atomization pressure is 6 MPa, and the superheat of the melt is maintained at 150°C during the atomization process to obtain atomized powder.

[0131] The atomized powder was sieved through a particle size distribution of 270-500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder was 25μm-53μm, and the flowability of the laser cladding powder was 25s / 100g-40s / 100g.

[0132] The laser cladding powder was applied to the TP347H tube by coaxial powder feeding for the first cladding, forming a base laser cladding layer; the laser power was 3kW, the overlap rate was 50%, and the thickness of the base laser cladding layer was 1.2mm.

[0133] Under the same conditions, based on the prepared base laser cladding layer, the laser cladding powder is clad a second time using coaxial powder feeding laser cladding to form a second laser cladding layer. The thickness of the second base laser cladding layer is 1.2 mm, resulting in a gradient laser cladding layer.

[0134] Comparative Example 3

[0135] A method for preparing a gradient laser cladding layer.

[0136] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0137] Cr: 45%, Fe: 19%, Nb: 15%, Mo: 10%, Ti: 4%, Al: 2.5%, balance Ni, total 100%.

[0138] Each raw material was placed in a vacuum furnace and heated to 260°C for 2 hours. Then it was placed in a powder mixer for thorough mixing for 2 hours to obtain the powder.

[0139] Alcohol was used to remove the grease from the surface of the Cr50Ni50 strip. The flux-cored wire was then drawn by wrapping the flux powder inside the Cr50Ni50 strip using a flux-cored wire drawing machine. The first drawing die had a diameter of 2.6 mm. The drawing die diameter was then gradually reduced to obtain a flux-cored wire with a diameter of 1.2 mm. The flux powder filling rate in the flux-cored wire was 22%.

[0140] The first arc welding was performed on the TP347H pipe using flux-cored welding wire to form the first arc welding cladding layer; the welding current was 160A to 200A, the interpass temperature was controlled at 70℃, and the thickness of the arc welding cladding layer was 3.0mm.

[0141] Under the same conditions, a second arc welding layer is formed on top of the first arc welding cladding layer using flux-cored welding wire, resulting in a second arc welding cladding layer with a thickness of 3.0 mm, thus obtaining a gradient arc welding cladding layer.

[0142] Comparative Example 4

[0143] A method for preparing a gradient cladding layer includes the following steps:

[0144] S1. Preparation of the initial arc welding cladding layer:

[0145] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0146] Cr: 45%, Fe: 19%, Nb: 15%, Mo: 10%, Ti: 4%, Al: 2.5%, balance Ni, total 100%.

[0147] Each raw material was placed in a vacuum furnace and heated to 260°C for 2 hours. Then it was placed in a powder mixer for thorough mixing for 2 hours to obtain the powder.

[0148] Alcohol was used to remove the grease from the surface of the Cr50Ni50 strip. The flux-cored wire was then drawn by wrapping the flux powder inside the Cr50Ni50 strip using a flux-cored wire drawing machine. The first drawing die had a diameter of 2.6 mm. The drawing die diameter was then gradually reduced to obtain a flux-cored wire with a diameter of 1.2 mm. The flux powder filling rate in the flux-cored wire was 22%.

[0149] Arc welding was performed on TP347H pipe using flux-cored welding wire to form a root pass arc weld cladding layer; the welding current was 160A to 200A, the interpass temperature was controlled at 70℃, and the thickness of the root pass arc weld cladding layer was 3.0mm.

[0150] S2. Preparation of gradient cladding layer:

[0151] Weigh out the corresponding mass of the raw materials according to the following mass percentage ratios, and set aside:

[0152] Ni: 40%, Cr: 25%, Mo: 8%, Nb: 2%, Si: 1%, Mn: 1%, balance Fe, total 100%.

[0153] The raw materials are mixed and vacuum melted. Using a gas atomization method, N2 is used as the atomizing gas, the atomization pressure is 6 MPa, and the superheat of the melt is maintained at 150°C during the atomization process to obtain atomized powder.

[0154] The atomized powder was sieved through a particle size distribution of 270-500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder was 25μm-53μm, and the flowability of the laser cladding powder was 25s / 100g-40s / 100g.

[0155] Laser cladding is performed by applying laser cladding powder onto the underlying arc welding cladding layer using coaxial powder feeding. The laser power is 3kW, the overlap rate is 50%, and the thickness of the laser cladding layer is 1.2mm, resulting in a gradient cladding layer.

[0156] Figure 1 This is a schematic diagram of the gradient cladding layer prepared according to an embodiment of the present invention; wherein, (a) is a base laser cladding layer formed on the superheater / reheater tube, and (b) is an arc-welded cladding layer formed on the base laser cladding layer. Figure 1 As can be seen in (a), a base laser cladding layer 2 is formed on the TP347H tube 1; as Figure 1 As shown in (b), an arc welding cladding layer 3 is formed on the base laser cladding layer 2 to obtain a gradient cladding layer.

[0157] Experimental Test

[0158] 1. Surface morphology test

[0159] The present invention analyzes the surface morphology of the base laser cladding layer prepared in Comparative Example 1, and the gradient cladding layers prepared in Example 1 and Comparative Example 3.

[0160] Figure 2 The metallographic structure of the laser cladding layer prepared for Comparative Example 1 is shown. Figure 2As can be seen, the base laser cladding layer is mainly composed of fine γ-Ni austenite, exhibiting a columnar dendritic morphology, and no cracks or pore defects were observed.

[0161] Figure 3 The metallographic structure of the arc-welded cladding layer prepared in Example 1. From... Figure 3 As can be seen, the arc-welded cladding layer is mainly composed of γ-Ni austenite, exhibiting a columnar dendritic morphology, which is consistent with... Figure 2 Compared with the metallographic structure of the laser cladding layer prepared in Comparative Example 1, the dendritic morphology is coarser, and no cracks or pore defects were observed.

[0162] Figure 4 The image shows the high-temperature corrosion morphology of the gradient cladding layer prepared in Example 1. From... Figure 4 As can be seen, the surface of the gradient cladding layer is covered by a dense Cr2O3 oxide layer, indicating that the gradient cladding layer prepared in this invention has excellent corrosion resistance.

[0163] like Figure 5 As shown, in Comparative Example 3, the microstructure of both the first and second layers of the gradient cladding layer is mainly nickel-based austenitic, but crack defects were observed in the microstructure. This is because the Cr content in the arc cladding layer is high. Although it has good corrosion resistance, the high Cr content easily reacts with Fe in the matrix to form a brittle FeCr phase. Therefore, arc cladding wire cannot be used to form an arc cladding layer as the underlayer on the matrix.

[0164] 2. Hardness test

[0165] The microhardness of the gradient cladding layers prepared in Examples 1 to 5 was tested, and the microhardness data are shown in Table 1.

[0166] Table 1. Microhardness data of the gradient cladding layers prepared in Examples 1 to 5

[0167] Example Example 1 Example 2 Example 3 Example 4 Example 5 Microhardness 230HV0.2 220HV0.2 225HV0.2 225HV0.2 270HV0.2

[0168] As can be seen from Table 1, the microhardness values ​​of the gradient cladding layers prepared in Examples 1 to 5 are 230HV0.2, 220HV0.2, 225HV0.2, 225HV0.2 and 270HV0.2, respectively, indicating that the gradient cladding layers prepared in the embodiments of the present invention have high hardness and excellent resistance to deformation.

[0169] 3. Thermogravimetric test

[0170] The gradient cladding layers prepared in Examples 1 to 5, as well as the cladding layers prepared in Comparative Examples 2 and 4, were subjected to thermogravimetric analysis at 700°C. The weight of the sample before treatment, i.e. the mass of the base material, was measured using a balance. The sample was then subjected to high-temperature corrosion at 700°C for 120 hours, and the weight of the sample after treatment was measured using a balance.

[0171] The weight loss of the gradient cladding layers prepared in Examples 1 to 5 at 700℃ was 0.5, 0.4, 0.6, 0.55 and 0.4 times that of the base material, respectively. The weight loss of the gradient cladding layers was lower than that of the base material, indicating that the corrosion resistance of the gradient cladding layers under corrosive media was better than that of the base material.

[0172] In Comparative Example 2, all gradient cladding layers were laser cladding layers. After high-temperature corrosion at 700℃, the weight loss of the gradient cladding layer was twice that of the base material TP347H. This is because the second laser cladding layer in the gradient cladding layer has a low Cr content and poor high-temperature corrosion resistance, and therefore cannot directly contact the furnace atmosphere. Thus, laser cladding layers cannot be used for cladding layers that directly contact the furnace atmosphere.

[0173] In the gradient cladding layer prepared in Comparative Example 4, the bottom layer was an arc-welded cladding layer with a high Cr content, which was prone to cracking; the cladding layer that was in direct contact with the furnace atmosphere was a laser cladding layer. Due to its low Cr content, the weight loss of the cladding layer after high-temperature corrosion at 700℃ was 2.2 times that of the base material TP347H.

[0174] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. If these modifications and variations fall within the scope of equivalents of this invention, then this invention also intends to include these modifications and variations.

Claims

1. A gradient cladding layer for superheater / reheater tubes, characterized in that, include: A laser cladding layer is formed on the superheater tube, and an arc welding cladding layer is formed on the laser cladding layer, wherein the arc welding cladding layer is in direct contact with the furnace atmosphere. The base laser cladding layer, by mass percentage, consists of the following components: Ni: 30%–40%, Cr: 22%–25%, Mo: 5%–8%, Nb: 0.8%–2%, Si: 0.5%–1%, Mn: 0.5%–1%, balance Fe, total 100%; The arc welding cladding layer is obtained by arc welding with flux-cored welding wire. The flux-cored wire comprises a wire sheath and flux powder, with the flux powder filling the wire sheath. The flux powder, by mass percentage, consists of the following components: Cr: 40%–45%, Fe: 16%–19%, Nb: 12%–15%, Mo: 8%–10%, Ti: 3%–4%, Al: 1.5%–2.5%, balance Ni, total 100%; The weld bead is a Cr50Ni50 strip; The Cr and Ni contents in the laser cladding layer and the arc welding cladding layer of the superheater tube exhibit a gradient distribution, with the Cr content increasing and the Ni content decreasing.

2. The gradient cladding layer for superheater / reheater tubes according to claim 1, wherein the thickness of the initial laser cladding layer is 0.6 mm to 1.2 mm, and the thickness of the arc welding layer is 2.0 mm to 3.0 mm.

3. The gradient cladding layer for superheater / reheater tubes according to claim 1, characterized in that, The filling rate of the medicinal powder is 22% to 25%.

4. A method for preparing a gradient cladding layer for superheater / reheater tubes according to any one of claims 1 to 3, characterized in that, Includes the following steps: The raw materials for preparing the base laser cladding layer are mixed to obtain laser cladding powder; using the laser cladding powder as raw material, a base laser cladding layer is formed on the superheater / reheater tube by laser cladding. Using flux-cored welding wire as raw material, an arc-welded cladding layer is formed on the underlying laser cladding layer through arc welding, resulting in a gradient cladding layer.

5. The method for preparing a gradient cladding layer for superheater / reheater tubes according to claim 4, characterized in that, The underlying laser cladding layer is obtained through the following steps: According to the proportion of raw materials for preparing the base laser cladding layer, the raw materials are mixed and vacuum melted, and then atomized powder is obtained by gas atomization. The atomized powder was sieved through a particle size range of 270 to 500 mesh to obtain laser cladding powder; Laser cladding is used to clad laser cladding powder onto the superheater / reheater tube to form a base laser cladding layer.

6. The method for preparing a gradient cladding layer for superheater / reheater tubes according to claim 5, characterized in that, The gas atomization process maintains the superheat of the melt at 100°C to 150°C.

7. The method for preparing a gradient cladding layer for superheater / reheater tubes according to claim 5, characterized in that, The laser power of the laser cladding is 2.5kW to 3kW, and the overlap rate is 40% to 50%.

8. The method for preparing a gradient cladding layer for superheater / reheater tubes according to claim 4, characterized in that, The arc welding cladding layer is obtained through the following steps: According to the proportion of raw materials for preparing the arc welding cladding layer, the raw materials are mixed to obtain the powder. The flux powder is wrapped inside the welding wire and then drawn to obtain a flux-cored welding wire with a diameter of 1.0 mm to 1.2 mm. Arc welding is performed on the laser cladding layer using flux-cored welding wire to form an arc welding cladding layer.

9. The method for preparing a gradient cladding layer for superheater / reheater tubes according to claim 8, characterized in that, The interpass temperature of the arc welding is controlled below 100°C.

10. The use of the gradient cladding layer for superheater tubes as described in any one of claims 1 to 3 in the preparation of protective coating materials for superheaters / reheaters.

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