Gradient cladding layer for superheater / reheater tube as well as preparation method and application of gradient cladding layer

Abstract

The invention 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 of the gradient cladding layer. The gradient cladding layer is characterized in that a bottoming laser cladding layer is formed on a superheater / reheater tube, an electric arc surfacing cladding layer is formed on the bottoming laser cladding layer, and the electric arc surfacing cladding layer is in direct contact with a hearth atmosphere; the bottoming laser cladding layer comprises the following components in percentage by weight: 30-40% of Ni, 22-25% of Cr, 5-8% of Mo, 0.8-2% of Nb, 0.5-1% of Si, 0.5-1% of Mn and the balance of Fe; the arc surfacing cladding layer comprises the following components: 40-45% of Cr, 16-19% of Fe, 12-15% of Nb, 8-10% of Mo, 3-4% of Ti, 1.5-2.5% of Al and the balance of Ni, and the total percentage is 100%. The gradient cladding layer disclosed by the invention is excellent in corrosion resistance and low in stress, and meets the safe service of the superheater / reheater.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal welding materials, and particularly relates to a gradient cladding layer for superheater / reheater tubes, a preparation method thereof, and an application thereof. Background Art

[0002] Superheater / reheater components are key components in a supercritical unit boiler responsible for recovering the energy of coal combustion flue gas, heating steam, and realizing energy conversion. They are the parts in the boiler that bear the greatest pressure, the highest temperature, and the harshest service environment. At present, the preferred materials for superheaters / reheaters mainly include ferritic heat-resistant steels and austenitic heat-resistant steels. Among them, austenitic steel is one of the preferred materials for the final-stage superheater / reheater of the unit, and it can meet the mechanical property requirements for service under steam conditions of 650°C.

[0003] Compared with water wall tubes, the chromium content in the superheater / reheater tube alloy is increased, and its resistance to flue gas corrosion is greatly improved. Although the chromium content is the key to improving the corrosion resistance of the alloy to flue gas, during the operation of the boiler, coal ash containing a large amount of alkali metal oxides / chlorides will deposit on the surface of the superheater / reheater tube alloy to form deposited salts. The deposited salts will react with chromium in the oxide film to generate spinel phases such as sodium chromate and potassium chromate, thereby causing the oxide film to rupture and accelerating the corrosion rate of the superheater / reheater tubes. And as the content of the deposited salts increases, the corrosion rate also increases. Chloride ions will significantly accelerate the corrosion rate of the superheater / reheater tube alloy. When 300 mg / kg of NaCl is added to the atmosphere, the corrosion weight gain is very significant.

[0004] At present, the corrosion research on superheater / reheater tube alloys mainly focuses on improving the corrosion resistance of superheater / reheater tube alloys by increasing the chromium content in the alloy. However, due to the very complex service environment of superheater / reheater tubes inside the boiler, the oxide formed solely by chromium is not sufficient to meet the increasingly stringent working conditions requirements. In addition, too high a chromium content will lead to the generation of brittle phases, thereby deteriorating the processing performance of the alloy. Summary of the Invention

[0005] To solve the above technical problems, the present invention provides a gradient cladding layer for a superheater / reheater tube, its preparation method and application. Since the superheater / reheater tube is a ferrous alloy, the present invention adopts the Fe-Ni-Cr alloy system, and a backing laser cladding layer formed by laser cladding of ferrous alloy powder is prepared on the superheater / reheater tube to ensure excellent metallurgical bonding between the backing laser cladding layer and the matrix of the superheater / reheater tube; at the same time, on the basis of the backing laser cladding layer, a submerged arc overlaying cladding layer is formed by means of submerged arc welding wire and is in direct contact with the furnace atmosphere; since the submerged arc overlaying cladding layer is in direct contact with the furnace atmosphere, it is necessary to ensure that the submerged arc overlaying cladding layer has high temperature corrosion resistance under long-term service. The present invention designs the chromium content and nickel content of the alloy powder in the preparation raw materials to show a gradient distribution in the superheater / reheater tube, the backing laser cladding layer and the submerged arc overlaying cladding layer in turn, wherein the chromium content shows a gradient increase and the nickel content shows a gradient decrease; one is to make the chromium element and nickel element play a role in phase transition in the gradient cladding layer, effectively reducing the stress distribution in the gradient cladding layer; the other is to increase the upper limit of the Cr content in the submerged arc overlaying cladding layer, thereby improving the high temperature corrosion resistance of the gradient cladding layer.

[0006] The gradient cladding layer for a superheater / reheater tube of the present invention is realized through the following technical solutions:

[0007] The first object of the present invention is to provide a gradient cladding layer for a superheater / reheater tube, including: a backing laser cladding layer formed on the superheater / reheater tube, and a submerged arc overlaying cladding layer is formed on the backing laser cladding layer, wherein the submerged arc overlaying cladding layer is in direct contact with the furnace atmosphere.

[0008] The backing laser cladding layer is composed of the following components by mass percentage:

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

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

[0011] The flux-cored wire includes a weld skin and welding powder, and the welding powder is filled in the weld skin. The welding powder is composed of the following components by mass percentage:

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

[0013] Preferably, the welding 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 service environment of the superheater / reheater tubes is high temperature and there is a corrosive atmosphere, ensuring corrosion resistance is a prerequisite. Cr is the most critical element determining corrosion resistance. In the present invention, Cr elements are added to both the backing laser cladding layer and the arc surfacing layer. Since the arc surfacing cladding layer is directly in contact with the high-temperature corrosive flue gas in the furnace, the Cr content in its preparation raw materials is higher than that in the backing laser cladding layer. That is to say, the Cr element content in the preparation raw materials shows a gradually increasing gradient change in the superheater / reheater tubes, the backing laser cladding layer, and the arc surfacing cladding layer. On the one hand, it is to relieve the stress concentration caused by the sudden change in composition, and on the other hand, it is to increase the upper limit of the Cr content in the arc surfacing cladding layer. According to the Cr-Fe binary phase diagram, there is a risk of forming a brittle FeCr phase between the two. Therefore, the gradient change of Cr elements is beneficial to the formation of a high-performance gradient cladding layer.

[0015] In the present invention, Cr and Ni elements are added to both the backing laser cladding layer and the arc surfacing cladding layer to optimize the performance and forming quality of the cladding layer. According to the Ni-Cr binary phase diagram, Ni and Cr are infinitely soluble and have excellent weldability. Therefore, introducing Ni elements into the backing laser cladding powder and the arc surfacing wire helps to improve the metallurgical bonding ability of the gradient cladding layer and ensure the interlayer bonding strength. At the same time, the content of Ni elements shows a gradually decreasing gradient distribution in the superheater / reheater tubes, the backing laser cladding layer, and the arc surfacing layer. This design not only matches the increasing trend of Cr elements, further relieves the stress concentration caused by the sudden change in composition, but also optimizes the thermal expansion matching of the cladding layer, thereby reducing the interfacial residual stress and improving the overall service performance of the cladding layer.

[0016] In the present invention, Mo elements are introduced into the backing laser cladding layer and the arc surfacing cladding layer to enhance the mechanical properties and high-temperature stability. Mo elements are introduced into the cladding layer design to enhance the mechanical properties and high-temperature stability. In the backing laser cladding layer, Mo improves the strength through solid solution strengthening: due to its large atomic radius, Mo dissolved in the ferrite matrix will cause significant lattice distortion, thereby increasing the strength of the backing laser cladding layer. For the arc surfacing layer directly in contact with the high-temperature corrosive environment, adding Mo elements to the wire has two advantages: on the one hand, based on the infinitely soluble characteristic shown by the Cr-Mo binary phase diagram, Mo can form a stable solid solution with Cr; on the other hand, this alloying synergistic effect not only increases the strength of the surfacing layer but also enhances its high-temperature creep resistance, ensuring the long-term reliability of the cladding layer under harsh working conditions. This multi-scale strengthening mechanism design enables the gradient cladding layer to have excellent mechanical properties and high-temperature durability.

[0017] In the present invention, by adding Nb element to the backing laser cladding layer and the arc surfacing cladding layer, in the backing laser cladding layer, Nb enhances the mechanical properties of the nickel matrix through the solid solution strengthening mechanism; in the arc surfacing layer, the high-temperature precipitation behavior of Nb forms an Nb-rich phase, which can effectively pin the grain boundaries and inhibit the grain boundary migration at high temperatures, thus improving the creep resistance. In addition, since the base material itself contains Nb element, the design of the Nb content in the gradient cladding layer can form a composition buffer zone, effectively inhibiting the diffusion loss of Nb element in the base material to the cladding layer and maintaining the original properties of the matrix material.

[0018] In the present invention, Fe element is added to the backing laser cladding layer and the arc surfacing cladding layer. The main function of Fe element in the backing laser cladding layer and the arc surfacing cladding layer is to improve the fluidity of the molten pool, thereby ensuring the formation of the gradient cladding layer. In addition, the superheater / reheater tube is a ferrous alloy, and the main element is Fe. The gradient distribution of Fe element in the superheater / reheater tube and the gradient cladding layer can reduce the diffusion rate of the remaining alloy elements in the superheater / reheater pipeline.

[0019] Si element and Mn element are also added to the backing laser cladding layer of the present invention. One is to utilize the combined deoxidation effect of Si and Mn; the other is that the backing laser cladding layer is a Fe-Ni-Cr alloy system, and Si and Mn are dissolved in the Fe-based lattice, having a certain solid solution strengthening effect.

[0020] Ti element and Al element are also added to the arc surfacing cladding layer of the present invention. The arc surfacing cladding layer is a Fe-Ni-Cr alloy system. Ti and Al are common strengthening elements of nickel-based alloys. By generating Ni3(Al,Ti) precipitation phase, the high-temperature creep rupture strength and creep resistance of the nickel-based cladding layer are significantly improved. However, since Ti and Al are harmful to the arc stability, their addition amounts should be strictly controlled.

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

[0022] Preferably, the particle sizes of the raw materials for preparing the backing laser cladding layer are all 100 mesh to 200 mesh.

[0023] Preferably, the purities of the raw materials for preparing the backing laser cladding layer are all higher than 99.99%.

[0024] The second object of the present invention is to provide a method for preparing the above-mentioned gradient cladding layer, including the following steps:

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

[0026] It should be noted that the specific preparation process of the backing laser cladding layer of the present invention is as follows: according to the ratio of the preparation raw materials of the backing laser cladding layer, mix and vacuum melt each preparation raw material, and use the gas atomization method to obtain atomized powder; perform particle size screening on the atomized powder with 270 meshes to 500 meshes to obtain laser cladding powder; use the laser cladding method to clad the laser cladding powder on the superheater / reheater tube to form a backing laser cladding layer.

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

[0028] Preferably, the fluidity requirement of the laser cladding powder is 25 s / 100 g to 40 s / 100 g.

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

[0030] S2. Using the flux-cored wire as the raw material, form an arc surfacing cladding layer on the backing laser cladding layer by arc surfacing to obtain a gradient cladding layer.

[0031] It should be noted that the specific preparation process of the arc surfacing cladding layer of the present invention is as follows: according to the ratio of the preparation raw materials of the arc surfacing cladding layer, heat each preparation raw material at 220 °C to 260 °C for 1 h to 2 h; then mix in a powder mixer for 1 h to 2 h to obtain powder; wrap the powder in the welding skin, perform the first drawing, and then gradually reduce the aperture of the drawing die for drawing to obtain a flux-cored wire with a diameter of 1.0 mm to 1.2 mm; use the flux-cored wire to perform arc surfacing on the backing laser cladding layer to form an arc surfacing cladding layer. The filling rate of the powder is 22% to 25%.

[0032] It also should be noted that before the arc surfacing of the present invention, the powder needs to be wrapped with the welding skin. In order to wrap the powder tightly and avoid powder leakage after the welding skin is wrapped, the wrapped welding skin needs to pass through a die with a circular aperture. That is, wrap the powder in the welding skin, perform the first drawing, and after the first drawing process is completed, gradually reduce the die aperture to obtain a flux-cored wire with a diameter of 1.0 mm to 1.2 mm. Preferably, the aperture of the first drawing die is 2.6 mm.

[0033] In order to prevent the generation of pores and cracks during the welding process, which may affect the arc surfacing process, the present invention heats each preparation raw material before mixing to remove the crystal water and gas in the powder and ensure the stability of the powder and the welding quality during the welding process.

[0034] Preferably, the welding current for the arc surfacing is 160 A to 200 A, and the interlayer temperature is controlled below 100 °C.

[0035] The third object of the present invention is to provide the application of the above-mentioned gradient cladding layer for the superheater / reheater tube in the preparation of the protective coating material for the superheater / reheater.

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

[0037] The backing laser cladding layer of the present invention adopts the Fe-Ni-Cr alloy system, with the Fe content being 23% to 41.6%. The superheater / reheater tube is a ferrous alloy, and its matrix element is mainly the Fe element, which can ensure excellent metallurgical bonding between the backing cladding layer and the matrix of the superheater / reheater tube. At the same time, the Ni element content in the backing laser cladding layer is 30% to 40% and the Cr element content is 22% to 25%, which is higher than the Ni element and Cr element content in the superheater / reheater tube; the Ni element content in the arc surfacing cladding layer is 4.5% - 19.5% and the Cr element content is 40% - 45%; this makes the Cr element content show a gradually increasing gradient change in the superheater / reheater tube, the backing laser cladding layer, and the arc surfacing cladding layer; the Ni element content shows a gradually decreasing gradient change in the superheater / reheater tube, the backing laser cladding layer, and the arc surfacing cladding layer; firstly, the chromium element and nickel element play a role in phase transition in the gradient cladding layer, alleviating the stress concentration caused by the sudden change in composition, and effectively reducing the stress distribution of the gradient cladding layer; secondly, it increases the upper limit of the Cr content in the arc surfacing cladding layer, thereby enhancing the excellent high-temperature corrosion resistance of the gradient cladding layer on the superheater / reheater tube.

[0038] In the backing laser cladding layer of the present invention, 5% to 8% of Mo element, 0.8% to 2% of Nb element, 0.5% to 1% of Si element, and 0.5% to 1% of Mn element are added. The Mo element is dissolved into the ferrite matrix to cause lattice distortion, thereby improving the strength of the backing laser cladding layer; the Nb element is dissolved into the Ni matrix in the backing laser cladding layer to strengthen the Ni matrix in the backing laser cladding layer; at the same time, Si and Mn are dissolved in the Fe-based lattice in the backing laser cladding layer, having a certain solid solution strengthening effect, thereby enhancing the strength of the backing laser cladding layer.

[0039] The arc surfacing cladding layer of the present invention is directly in contact with the flue gas in the furnace. On the basis of Ni and Cr elements, 8% - 10% of Mo element, 16% - 19% of Fe element and 12% - 15% of Nb element are added, and 3% - 4% of Ti element and 1.5% - 2.5% of Al element are added in small amounts. The infinite mutual solubility of Mo element and Cr element improves the strength of the arc surfacing cladding layer; Fe element improves the fluidity of the molten pool, thus ensuring the formation of the gradient cladding layer; the arc surfacing cladding layer containing Nb will precipitate a Nb-rich phase at high temperature, which plays a role in inhibiting grain boundary migration; Ti element and Al element significantly improve the high-temperature creep rupture strength and creep resistance of the nickel-based cladding layer by generating Ni3(Al,Ti) precipitate phase, thereby enhancing the high-temperature corrosion resistance of the arc surfacing cladding layer.

[0040] The present invention prepares a bottom laser cladding layer by laser cladding. The laser cladding powder of the present invention can be used for both coaxial powder feeding laser cladding and powder spreading laser cladding, with a wide range of applications. The arc surfacing cladding layer of the present invention can be used for both TIG welding and MIG welding, with a wide range of applications and good welding process performance. The arc surfacing layer of the present invention is applicable to the surfacing of the surface of the superheater / reheater tubes in coal-fired power plant boilers, thus ensuring the safe service of the superheater / reheater tubes under the harsh working conditions of deep peak shaving and co-firing of multiple coal types. Brief Description of the Drawings

[0041] Figure 1 It is a schematic structural diagram of the gradient cladding layer prepared by the present invention; among them, Fig. (a) is the bottom laser cladding layer formed on the superheater / reheater tube, and Fig. (b) is the arc surfacing cladding layer formed on the bottom laser cladding layer.

[0042] Figure 2 It is the metallographic structure of the bottom laser cladding layer prepared in Comparative Example 1.

[0043] Figure 3 It is the metallographic structure of the arc surfacing cladding layer prepared in Example 1.

[0044] Figure 4 It is a scanning electron microscope picture of the high-temperature corrosion morphology of the gradient cladding layer prepared in Example 1.

[0045] Figure 5 It is a scanning electron microscope picture of the high-temperature corrosion morphology of the gradient cladding layer prepared in Comparative Example 3.

[0046] Description of the Reference Numerals:

[0047] 1 - TP347H tube; 2 - bottom laser cladding layer; 3 - arc surfacing cladding layer. Detailed Description of the Invention

[0048] To enable those skilled in the art to better understand and implement the technical solution of the present invention, the present invention will be further described below in conjunction with specific embodiments and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0049] Example 1

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

[0051] S1. Prepare a bottom laser cladding layer:

[0052] Weigh the corresponding masses of the preparation 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%, the balance is Fe, totaling 100%.

[0054] Mix and vacuum melt the various preparation raw materials, and use the gas atomization method with N2 as the atomization gas, the atomization pressure is 6 MPa, and the superheat degree of the melt is maintained at 150 °C during the atomization process to obtain atomized powder.

[0055] Screen the atomized powder with a mesh size of 270 to 500 meshes to obtain laser cladding powder. The particle size range of the laser cladding powder is 25 μm to 53 μm, and the fluidity of the laser cladding powder is 25 s / 100 g to 40 s / 100 g.

[0056] Use coaxial powder feeding laser cladding to clad the laser cladding powder on the TP347H tube to form a bottom laser cladding layer; the laser power is 3 kW, the overlap rate is 50%, and the thickness of the bottom laser cladding layer is 1.2 mm.

[0057] S2. Prepare a gradient cladding layer:

[0058] Weigh the corresponding masses of the preparation 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%, the balance is Ni, totaling 100%.

[0060] Place the various preparation raw materials in a vacuum furnace and heat at 260 °C for 2 h, then place them in a powder mixer for sufficient mixing for 2 h to obtain the powder.

[0061] Remove the grease on the surface of the Cr50Ni50 strip with alcohol, wrap the powder inside the Cr50Ni50 strip through a flux-cored wire drawing device, and perform the first drawing. The aperture of the first drawing die is 2.6 mm; then gradually reduce the aperture of the drawing die for drawing to obtain a flux-cored wire with a diameter of 1.2 mm, and the powder filling rate in the flux-cored wire is 22%.

[0062] Use the flux-cored wire to perform arc surfacing on the backing laser cladding layer prepared in S1 to form an arc surfacing cladding layer and obtain a gradient cladding layer; among them, the welding current is 160 A to 200 A, the interlayer temperature is controlled at 70 °C, and the thickness of the arc surfacing cladding is 3.0 mm.

[0063] Example 2

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

[0065] S1. Prepare a backing laser cladding layer:

[0066] Weigh the corresponding masses of the preparation 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%, the balance is Fe, totaling 100%.

[0068] Mix and vacuum melt the preparation raw materials, and use the gas atomization method. Take N2 as the atomization gas, the atomization pressure is 6 MPa, and keep the superheat degree of the melt at 100 °C during the atomization process to obtain atomized powder.

[0069] Perform particle size screening on the atomized powder with 270 mesh to 500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder is 25 μm to 53 μm, and the fluidity of the laser cladding powder is 25 s / 100 g to 40 s / 100 g.

[0070] Use coaxial powder feeding laser cladding to clad the laser cladding powder on the TP347H tube to form a backing laser cladding layer; the laser power is 2.5 kW, the overlapping rate is 40%, and the thickness of the backing laser cladding layer is 0.6 mm.

[0071] S2. Prepare a gradient cladding layer:

[0072] Weigh the corresponding masses of the preparation 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%, the balance is Ni, totaling 100%.

[0074] Place each preparation raw material in a vacuum furnace and heat it at 220°C for 1 hour of heat preservation. Then place it in a powder mixer for sufficient mixing for 1 hour to obtain the medicinal powder.

[0075] Use alcohol to remove the grease on the surface of the Cr50Ni50 strip. Wrap the medicinal powder inside the Cr50Ni50 strip through a flux-cored wire drawing device for the first drawing. The aperture of the first drawing die is 2.6 mm; then gradually reduce the aperture of the drawing die for drawing to obtain a flux-cored wire with a diameter of 1.2 mm. The filling rate of the medicinal powder in the flux-cored wire is 25%.

[0076] Use the flux-cored wire to perform arc surfacing on the bottom laser cladding layer prepared in S1 to form an arc surfacing cladding layer and obtain a gradient cladding layer; among them, the welding current is 160 A - 200 A, the interlayer temperature is controlled at 80°C, and the thickness of the arc surfacing cladding is 2.0 mm.

[0077] Example 3

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

[0079] S1. Prepare the bottom laser cladding layer:

[0080] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

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

[0082] Mix and vacuum melt each preparation raw material. Use the gas atomization method with N2 as the atomizing gas and an atomizing pressure of 6 MPa. Keep the superheat of the melt at 120°C during the atomization process to obtain atomized powder.

[0083] Perform particle size screening on the atomized powder with 270 - 500 meshes to obtain laser cladding powder. The particle size range of the laser cladding powder is 25 μm - 53 μm, and the fluidity of the laser cladding powder is 25 s / 100 g - 40 s / 100 g.

[0084] Use laser cladding to clad the laser cladding powder on the TP347H pipe to form a bottom laser cladding layer; the laser power is 2.7 kW, the overlapping rate is 45%, and the thickness of the bottom laser cladding layer is 0.9 mm.

[0085] S2. Prepare the gradient cladding layer:

[0086] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

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

[0088] Place each preparation raw material in a vacuum furnace and heat it at 240 °C for 1.5 h of heat preservation, then place it in a powder mixer for sufficient mixing, and the mixing time is 1.5 h to obtain powder.

[0089] Use alcohol to remove the grease on the surface of the Cr50Ni50 strip, wrap the powder in the Cr50Ni50 strip through a flux-cored wire drawing device, and perform the first drawing. The aperture of the first drawing die is 2.6 mm; then gradually reduce the aperture of the drawing die for drawing to obtain a flux-cored wire with a diameter of 1.2 mm, and the filling rate of the powder in the flux-cored wire is 24%.

[0090] Use the flux-cored wire to perform arc surfacing on the backing laser cladding layer prepared in S1 to form an arc surfacing cladding layer and obtain a gradient cladding layer; among them, the welding current is 160 A - 200 A, the interlayer temperature is controlled at 60 °C, and the arc surfacing cladding thickness is 2.5 mm.

[0091] Example 4

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

[0093] S1. Prepare a backing laser cladding layer:

[0094] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

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

[0096] Mix and vacuum melt each preparation raw material, and use the gas atomization method. Take N2 as the atomization gas, the atomization pressure is 6 MPa, and keep the superheat degree of the melt at 125 °C during the atomization process to obtain atomized powder.

[0097] Perform particle size screening on the atomized powder from 270 mesh to 500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder is 25 μm - 53 μm, and the fluidity of the laser cladding powder is 25 s / 100 g - 40 s / 100 g.

[0098] Use powder laying laser cladding to clad the laser cladding powder on the TP347H pipe to form a backing laser cladding layer; the laser power is 2.7 kW, the overlapping rate is 42%, and the thickness of the backing laser cladding layer is 0.7 mm.

[0099] S2. Prepare a gradient cladding layer:

[0100] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

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

[0102] Place each of the preparation raw materials in a vacuum furnace and heat at 255 °C for 1.2 h, then place them in a powder mixer for sufficient mixing for 1.2 h to obtain powder.

[0103] Use alcohol to remove the grease on the surface of the Cr50Ni50 strip, wrap the powder inside the Cr50Ni50 strip through a flux-cored wire drawing device, and perform the first drawing. The aperture of the first drawing die is 2.6 mm; then gradually reduce the aperture of the drawing die for drawing to obtain a flux-cored wire with a diameter of 1.2 mm, and the powder filling rate in the flux-cored wire is 23%.

[0104] Use the flux-cored wire to perform arc surfacing on the backing laser cladding layer prepared in S1 to form an arc surfacing cladding layer and obtain a gradient cladding layer; among them, the welding current is 160 A - 200 A, the interlayer temperature is controlled at 50 °C, and the arc surfacing cladding thickness is 2.8 mm.

[0105] Example 5

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

[0107] S1. Prepare a backing laser cladding layer:

[0108] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

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

[0110] Mix and vacuum melt each of the preparation raw materials, and use the gas atomization method with N2 as the atomizing gas, the atomizing pressure is 6 MPa, and keep the superheat degree of the melt at 130 °C during the atomization process to obtain atomized powder.

[0111] Perform particle size screening on the atomized powder from 270 mesh to 500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder is 25 μm - 53 μm, and the fluidity of the laser cladding powder is 25 s / 100 g - 40 s / 100 g.

[0112] The laser cladding powder is cladded on the TP347H tube by powder bed laser cladding to form a bottom laser cladding layer; the laser power is 2.8 kW, the overlapping rate is 48%, and the thickness of the bottom laser cladding layer is 1.1 mm.

[0113] S2. Prepare the gradient cladding layer:

[0114] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

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

[0116] Place the preparation raw materials in a vacuum furnace and heat them at 235 °C for 1.3 h, then place them in a powder mixer for sufficient mixing for 1.9 h to obtain the powder.

[0117] Use alcohol to remove the grease on the surface of the Cr50Ni50 strip, wrap the powder inside the Cr50Ni50 strip through a flux-cored wire drawing device, and perform the first drawing. The aperture of the first drawing die is 2.6 mm; then gradually reduce the aperture of the drawing die for drawing to obtain a flux-cored wire with a diameter of 1.0 mm, and the powder filling rate in the flux-cored wire is 22%.

[0118] Use the flux-cored wire to perform arc surfacing on the bottom laser cladding layer prepared in S1 to form an arc surfacing cladding layer and obtain the gradient cladding layer; among them, the welding current is 160 A - 200 A, the thickness of the arc surfacing cladding layer is 2.1 mm, and the interlayer temperature is controlled at 40 °C.

[0119] Comparative Example 1

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

[0121] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

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

[0123] Mix and vacuum melt the preparation raw materials, and use the gas atomization method with N2 as the atomizing gas and an atomizing pressure of 6 MPa. Keep the superheat of the melt at 150 °C during the atomization process to obtain the atomized powder.

[0124] The atomized powder is screened with a mesh size of 270 to 500 meshes to obtain laser cladding powder. The particle size range of the laser cladding powder is 25 μm to 53 μm, and the fluidity of the laser cladding powder is 25 s / 100 g to 40 s / 100 g.

[0125] The laser cladding powder is cladded on the superheater / reheater tube by coaxial powder feeding laser cladding to form a bottom laser cladding layer. Among them, the laser power is 3 kW, the thickness of the bottom laser cladding layer is 1.2 mm, and the overlapping rate is 50%.

[0126] Comparative Example 2

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

[0128] Weigh the corresponding masses of the preparation 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%, and the balance is Fe, totaling 100%.

[0130] Mix and vacuum melt the various preparation raw materials, and use the gas atomization method with N2 as the atomization gas, an atomization pressure of 6 MPa, and keep the superheat degree of the melt at 150 °C during the atomization process to obtain atomized powder.

[0131] The atomized powder is screened with a mesh size of 270 to 500 meshes to obtain laser cladding powder. The particle size range of the laser cladding powder is 25 μm to 53 μm, and the fluidity of the laser cladding powder is 25 s / 100 g to 40 s / 100 g.

[0132] The laser cladding powder is first cladded on the TP347H tube by coaxial powder feeding laser cladding to form a bottom laser cladding layer. The laser power is 3 kW, the overlapping rate is 50%, and the thickness of the bottom laser cladding layer is 1.2 mm.

[0133] Under the same conditions, on the basis of the prepared bottom laser cladding layer, the laser cladding powder is secondarily cladded by coaxial powder feeding laser cladding to form a second laser cladding layer. The thickness of the second bottom laser cladding layer is 1.2 mm to obtain a gradient laser cladding layer.

[0134] Comparative Example 3

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

[0136] Weigh the corresponding masses of the preparation 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%, the balance is Ni, totaling 100%.

[0138] Place each preparation raw material in a vacuum furnace and heat it at 260 °C for 2 hours of heat preservation, then place it in a powder mixer for sufficient mixing, with the mixing time being 2 hours, to obtain the powder.

[0139] Use alcohol to remove the grease on the surface of the Cr50Ni50 strip, wrap the powder inside the Cr50Ni50 strip through a flux-cored wire drawing device, and perform the first drawing. The aperture of the first drawing die is 2.6 mm; then gradually reduce the aperture of the drawing die for drawing to obtain a flux-cored wire with a diameter of 1.2 mm, and the filling rate of the powder in the flux-cored wire is 22%.

[0140] Use the flux-cored wire to perform the first arc surfacing on the TP347H pipe to form the first layer of arc surfacing cladding layer; among them, the welding current is 160 A - 200 A, the interlayer temperature is controlled at 70 °C, and the thickness of the arc surfacing cladding layer is 3.0 mm.

[0141] Under the same conditions, use the flux-cored wire to perform the second arc surfacing on the first layer of arc surfacing cladding layer to form the second layer of arc surfacing cladding layer, with the thickness of the second layer of arc surfacing cladding layer being 3.0 mm, to obtain the gradient arc surfacing cladding layer.

[0142] Comparative Example 4

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

[0144] S1. Prepare the backing arc surfacing cladding layer:

[0145] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

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

[0147] Place each preparation raw material in a vacuum furnace and heat it at 260 °C for 2 hours of heat preservation, then place it in a powder mixer for sufficient mixing, with the mixing time being 2 hours, to obtain the powder.

[0148] Use alcohol to remove the grease on the surface of the Cr50Ni50 strip, wrap the powder inside the Cr50Ni50 strip through a flux-cored wire drawing device, and perform the first drawing. The aperture of the first drawing die is 2.6 mm; then gradually reduce the aperture of the drawing die for drawing to obtain a flux-cored wire with a diameter of 1.2 mm, and the filling rate of the powder in the flux-cored wire is 22%.

[0149] The flux-cored wire is used for arc surfacing on the TP347H pipe to form a backing arc surfacing clad layer; among them, the welding current is 160 A to 200 A, the interlayer temperature is controlled at 70 °C, and the thickness of the backing arc surfacing clad layer is 3.0 mm.

[0150] S2. Prepare the gradient clad layer:

[0151] Weigh the corresponding masses of the preparation raw materials according to the following mass percentage ratios for standby:

[0152] Ni: 40%, Cr: 25%, Mo: 8%, Nb: 2%, Si: 1%, Mn: 1%, and the balance is Fe, with a total of 100%.

[0153] Mix and vacuum melt the various preparation raw materials, and use the gas atomization method. Using N2 as the atomization gas, the atomization pressure is 6 MPa, and the superheat degree of the melt is maintained at 150 °C during the atomization process to obtain atomized powder.

[0154] Screen the atomized powder to a particle size of 270 mesh to 500 mesh to obtain laser cladding powder. The particle size range of the laser cladding powder is 25 μm to 53 μm, and the fluidity of the laser cladding powder is 25 s / 100 g to 40 s / 100 g.

[0155] Use coaxial powder feeding laser cladding to laser clad the laser cladding powder on the backing arc surfacing clad layer to form a laser clad layer; the laser power is 3 kW, the overlapping rate is 50%, and the thickness of the laser clad layer is 1.2 mm to obtain the gradient clad layer.

[0156] Figure 1 It is a schematic structural diagram of the gradient clad layer prepared in the embodiment of the present invention; among them, (a) is the backing laser clad layer formed on the superheater / reheater pipe, and (b) is the arc surfacing clad layer formed on the backing laser clad layer. From Figure 1 As can be seen from (a) in, the backing laser clad layer 2 is formed on the TP347H pipe 1; as Figure 1 shown in (b) in, the arc surfacing clad layer 3 is formed on the backing laser clad layer 2 to obtain the gradient clad layer.

[0157] Experimental testing

[0158] 1. Surface morphology testing

[0159] The present invention analyzes the surface morphologies of the backing laser clad layer prepared in Comparative Example 1, and the gradient clad layers prepared in Example 1 and Comparative Example 3.

[0160] Figure 2 It is the metallographic structure of the backing laser clad layer prepared in Comparative Example 1. From Figure 2It can be seen that the underlying laser cladding layer is mainly composed of fine γ-Ni austenite structure, presenting a columnar dendritic morphology, and no crack and pore defects are observed.

[0161] Figure 3 is the metallographic structure of the arc surfacing cladding layer prepared in Example 1. From Figure 3 it can be seen that the arc surfacing cladding layer is mainly composed of γ-Ni austenite structure, presenting a columnar dendritic morphology. Compared with Figure 2 the metallographic structure of the underlying laser cladding layer prepared in Comparative Example 1, the dendritic morphology is coarser, and no crack and pore defects are observed.

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

[0163] As Figure 5 shown, the structures of the first surfacing cladding layer and the second surfacing cladding layer in the gradient cladding layer prepared in Comparative Example 3 are both mainly composed of nickel-based austenite structure, but crack defects are observed in the structure. This is because the Cr content in the arc surfacing layer is relatively high. Although the corrosion resistance is good, the relatively high Cr content is prone to form brittle FeCr phase with Fe in the substrate. Therefore, an arc surfacing cladding layer formed by an arc surfacing welding wire cannot be used as the underlying layer on the substrate.

[0164] 2. Hardness test

[0165] The present invention conducts microhardness tests on the gradient cladding layers prepared in Examples 1 to 5, 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] It can be seen from Table 1 that 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 examples of the present invention have high hardness and excellent anti-deformation ability.

[0169] 3. Thermogravimetric test

[0170] The gradient cladding layers prepared in Examples 1 to 5 and the cladding layers prepared in Comparative Example 2 and Comparative Example 4 were subjected to thermogravimetric tests at 700°C. The weight of the sample before treatment, i.e., the mass of the base material, was weighed with a balance, and high-temperature corrosion was carried out at 700°C for 120 h, and then the weight of the sample after treatment was weighed with a balance.

[0171] The weight losses of the gradient cladding layers prepared in Examples 1 to 5 at 700°C due to high-temperature corrosion were 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 layer was lower than that of the base material, indicating that its corrosion resistance in corrosive media was better than that of the base material.

[0172] The gradient cladding layers prepared in Comparative Example 2 were all laser cladding layers. After high-temperature corrosion at 700°C, the weight loss of the gradient cladding layer was 2 times that of the base material TP347H. This was because the Cr content in the second-layer laser cladding layer of the gradient cladding layer was low, and its high-temperature corrosion resistance was poor, so it could not be directly in contact with the furnace atmosphere. Therefore, the laser cladding layer could not be used for the cladding layer directly in contact with the furnace atmosphere.

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

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

Claims

1. A gradient cladding layer for superheater / reheater tubes, characterized in that, Comprising: A backing laser cladding layer formed on the superheater / reheater tube, and an arc surfacing cladding layer is formed on the backing laser cladding layer, wherein the arc surfacing cladding layer is in direct contact with the furnace atmosphere; The backing laser cladding layer is composed of the following components by mass percentage: Ni: 30% - 40%, Cr: 22% - 25%, Mo: 5% - 8%, Nb: 0.8% - 2%, Si: 0.5% - 1%, Mn: 0.5% - 1%, the balance is Fe, totaling 100%; The arc surfacing cladding layer is obtained by arc surfacing with a flux-cored wire; The flux-cored wire includes a weld covering and a powder, and the powder is filled in the weld covering. The powder is composed of the following components by mass percentage: Cr: 40% - 45%, Fe: 16% - 19%, Nb: 12% - 15%, Mo: 8% - 10%, Ti: 3% - 4%, Al: 1.5% - 2.5%, the balance is Ni, totaling 100%; The weld covering is a Cr50Ni50 strip; The Cr content and Ni content in the backing laser cladding layer and the arc surfacing cladding layer show a gradient distribution in the superheater / reheater tube, the backing laser cladding layer and the arc surfacing cladding layer. Among them, the Cr content shows a gradient increase, and the Ni content shows a gradient decrease.

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

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

4. A method for preparing a gradient cladding layer for a superheater / reheater tube according to any one of claims 1 to 3, characterized in that, Including the following steps: Mix the preparation raw materials of the backing laser cladding layer to obtain laser cladding powder; use the laser cladding powder as the raw material and adopt the laser cladding method to form a backing laser cladding layer on the superheater / reheater tube; Use the flux-cored wire as the raw material and form an arc surfacing cladding layer on the backing laser cladding layer through arc surfacing to obtain a gradient cladding layer.

5. The method for preparing the gradient cladding layer for the superheater / reheater tube according to claim 4, characterized in that, The backing laser cladding layer is obtained through the following steps: According to the ratio of the preparation raw materials of the backing laser cladding layer, mix and vacuum melt the various preparation raw materials, and use the gas atomization method to obtain atomized powder; Perform particle size screening on the atomized powder with 270 - 500 meshes to obtain laser cladding powder; Adopt the laser cladding method to melt the laser cladding powder on the superheater / reheater tube to form a backing laser cladding layer.

6. The method for preparing the gradient cladding layer for the superheater / reheater tube according to claim 5, wherein During the gas atomization process, the superheat degree of the melt is maintained at 100°C - 150°C.

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

8. The preparation method of the gradient cladding layer for the superheater / reheater tube according to claim 4, characterized in that, The arc surfacing cladding layer is obtained through the following steps: According to the ratio of the preparation raw materials of the arc surfacing cladding layer, mix the various preparation raw materials to obtain powder; Wrap the powder in the weld covering and perform drawing to obtain a flux-cored wire with a diameter of 1.0 mm - 1.2 mm; Use the flux-cored wire to perform arc surfacing on the backing laser cladding layer to form an arc surfacing cladding layer.

9. The method for preparing a gradient cladding layer for an overheater / reheater tube according to claim 8, characterized in that, The interlayer temperature of the arc surfacing is controlled below 100°C.

10. Use of the gradient cladding layer for the superheater / reheater tubes according to any one of claims 1 to 3 in preparing a protective coating material for a superheater / reheater.

Images