Manufacturing method of high temperature and high pressure resistant boiler flue tubes

Abstract

This invention discloses a method for manufacturing high-temperature and high-pressure resistant boiler flue tubes, comprising the following steps: feeding and melting raw materials, followed by cooling to obtain a blank material; forging and sandblasting the blank material to obtain a steel bar; forming the steel bar to obtain a seamless tube; cold working the seamless tube; rolling the seamless tube to obtain a spiral seamless tube; electron beam irradiation of the spiral seamless tube; instantaneous pulse current heating of the spiral seamless tube; salt bath treatment of the spiral seamless tube; magnetic field quenching of the inner and outer surfaces of the spiral seamless tube; gradient deep cryogenic treatment of the spiral seamless tube; tempering of the spiral seamless tube; shaping and straightening of the spiral seamless tube; ultrasonic impact treatment of the spiral seamless tube; shot peening of the spiral seamless tube; water flow strengthening treatment of the inner wall of the spiral seamless tube; and pickling of the spiral seamless tube. Boiler flue tubes prepared by this method can maintain excellent high-temperature strength and thermal fatigue resistance in high-temperature flue gas environments, possessing high strength and toughness, and preventing safety hazards such as leakage and bursting.

Description

Technical Field

[0001] This invention relates to a method for manufacturing boiler flue tubes, specifically a method for manufacturing high-temperature and high-pressure resistant boiler flue tubes. Background Technology

[0002] Boiler flue tubes are one of the core pressure-bearing components of hot water boilers, playing a crucial role in transferring heat from high-temperature flue gas to the water medium inside the boiler. In actual operation, flue tubes operate under extremely harsh conditions: the inside of the tubes carries flue gas at temperatures reaching hundreds or even thousands of degrees Celsius, often containing corrosive components such as sulfur, carbon oxides, and water vapor; the outside of the tubes is in direct contact with the water inside the boiler. During boiler operation, the water is heated from room temperature to boiling, and the tube walls endure thermal stress caused by significant temperature changes, while simultaneously being exposed to the corrosive environment of high-temperature water and water vapor. Under the combined effects of high temperature, temperature difference cycles, and corrosive media, boiler flue tubes are highly susceptible to failure modes such as wall thinning, pitting, cracking, and even perforation. Once a flue tube leaks, it not only reduces the boiler's thermal efficiency but also forces the boiler to shut down for maintenance. Because the flue tubes are usually located inside the boiler shell, the maintenance space is limited, and the replacement process is time-consuming and labor-intensive, severely impacting the continuity of industrial production and economic efficiency.

[0003] Currently, conventional boiler flue tubes are mostly made of ordinary carbon steel or low-alloy steel, such as No. 20 steel or 12CrMoG. In terms of high-temperature resistance, these materials have a certain strength under normal or medium temperature conditions, but when exposed to high-temperature flue gas environments of several hundred degrees Celsius for a long time, the material structure will degrade, the high-temperature strength will decrease significantly, the resistance to thermal fatigue will be insufficient, and cracks will easily be generated due to thermal stress, leading to premature failure of the flue tube.

[0004] Regarding pressure resistance, as boilers develop towards higher efficiency and larger capacity, boiler operating pressures are continuously increasing, especially for hot water boilers with pressures above 1.4 MPa, which places higher demands on the pressure resistance of flue tubes. However, existing flue tube materials and manufacturing processes cannot achieve a sufficiently high balance of strength and toughness while ensuring good processing performance, resulting in insufficient safety margins. When boilers operate under pressure fluctuations or localized overpressure conditions, flue tubes are at risk of deformation or even rupture, posing significant safety hazards. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for manufacturing a high-temperature and high-pressure resistant boiler flue tube. The boiler flue tube prepared by the method can maintain excellent high-temperature strength and thermal fatigue resistance in a high-temperature flue gas environment, and has high strength and toughness, preventing safety hazards such as leakage and bursting.

[0006] To solve this technical problem, the present invention adopts the following technical solution:

[0007] A method for manufacturing high-temperature and high-pressure resistant boiler flue tubes, comprising the following steps in sequence:

[0008] (1) After feeding and melting, the blank material is cooled to obtain a blank material; (2) The blank material is forged and sandblasted to obtain a steel bar; (3) The steel bar is formed to obtain a seamless tube; (4) The seamless tube is cold-worked; (5) The seamless tube is rolled to obtain a spiral seamless tube; (6) The spiral seamless tube is irradiated with an electron beam; (7) The spiral seamless tube is instantaneously heated by a pulse current; (8) The spiral seamless tube is subjected to a salt bath; (9) The inner and outer surfaces of the spiral seamless tube are magnetically quenched; (10) The spiral seamless tube is subjected to gradient deep cooling; (11) The spiral seamless tube is tempered; (12) The spiral seamless tube is shaped and straightened; (13) The spiral seamless tube is subjected to ultrasonic impact; (14) The spiral seamless tube is shot-peened; (15) The inner wall of the spiral seamless tube is strengthened by water flow; (16) The spiral seamless tube is pickled.

[0009] The feeding in step (1) refers to feeding No. 20 steel, 12Cr2Mo1VR alloy steel, 40Cr alloy steel, fluorite and quicklime in a weight ratio of (47~53):(72~79):(12.5~17.5):(2.25~2.75):1; the melting temperature is 1610℃~1650℃ and the melting time is 6~8min.

[0010] Preferably, step (2) includes the following sub-steps performed sequentially:

[0011] (2.1) Surface pretreatment of blank: First, remove the surface oxide scale and rust, then rinse with clean water and dry with compressed air;

[0012] (2.2) Coating the surface of the blank with slurry: First, brush the slurry onto the surface of the blank, coat it evenly, and cover it with a thickness of 0.6~1.5mm. Then, air dry it at a constant temperature.

[0013] (2.3) Stepwise heating: The heating rate from room temperature to 280℃~300℃ is 60~80℃ / h, and the holding time is 11~12min; the heating rate from 280℃~300℃ to 640℃~665℃ is 70~90℃ / h, and the holding time is 15~16min; the heating rate from 640℃~665℃ to 1175℃~1210℃ is 75~85℃ / h, and the holding time is 30min±1min.

[0014] (2.4) Forging: initial forging temperature 1175℃~1210℃, final forging temperature ≥820℃;

[0015] (2.5) Step cooling after forging: The cooling rate is 80~125℃ / h when the final forging temperature reaches 640℃~665℃; the cooling rate is 45~72℃ / h when the temperature reaches 280℃~300℃ from 640℃~665℃; and the temperature is naturally air-cooled to room temperature when the temperature is below 280℃~300℃.

[0016] (2.6) Sandblasting removes the oxide layer on the surface of the steel bar.

[0017] More preferably, the slurry in step (2.2) is composed of 79% to 81% by mass of mixed powder and 19% to 21% by mass of binder; wherein the mixed powder contains 16.5% to 18.5% Y2O3, 1.6% to 2.2% Sc2O3, 4.7% to 5.5% Tb4O7, 2.4% to 3.3% HfO2 and 3.8% to 4.7% ZrO2 by mass.

[0018] Preferably, in step (9), a magnetic field quenching furnace is used to quench the inner and outer surfaces of the spiral seamless tube with a strong magnetic field strength of 0.8T±0.15T. The spiral seamless tube is heated to 780℃~810℃, alkaline water is sprayed onto the inner and outer surfaces of the spiral seamless tube, and then cooled to room temperature.

[0019] More preferably, the alkaline solution contains 0.19%~0.24% Na3PO4 and 0.23%~0.26% C7H5NaO2 by mass.

[0020] Preferably, in step (10), the spiral seamless tube is subjected to liquid nitrogen gradient deep cooling, with a cooling rate of 2.5~2.9℃ / min from room temperature to -75℃~-85℃; a cooling rate of 1.5~2.2℃ / min from -75℃~-85℃ to -162℃~-178℃; and after holding at the temperature for 50min±2min, it is returned to room temperature with a heating rate of 0.7~0.9℃ / min.

[0021] Preferably, in step (15), the inner wall of the spiral seamless tube is first pretreated with ultrasound, and after drying, the high-pressure water pump is started to impact the tube wall with water at 2.0℃±1.9℃, water pressure 70MPa±8MPa, pulse frequency 50Hz±5Hz, and nozzle moving speed 25~30mm / min; after impact, it is dried.

[0022] Preferably, in step (16), the seamless tube is pickled using pickling solution at a temperature of 38℃±3.5℃ for 10.5min±2min; after pickling, it is rinsed with water and dried with hot air.

[0023] More preferably, the pickling solution contains 0.10-0.14% La(NO3)3, 0.07-0.10% Ce(NO3)3 and 0.02-0.04% Y(NO3)3 by mass.

[0024] Preferably, the manufacturing method further includes the following steps:

[0025] (17) Liquid nitriding: Place the spiral seamless tube into a nitriding furnace and heat it to 545℃~568℃ for 2.5~3.0 hours; (18) Nickel plating: Use nickel plating solution to perform chemical nickel plating on the spiral seamless tube, with a plating thickness of 0.015~0.025mm; (19) Grinding and polishing: Grind and polish the spiral seamless tube, and the roughness Ra after polishing is 0.63.

[0026] The positive effects of this invention are as follows:

[0027] First, this invention uses No. 20 steel, 12Cr2Mo1VR alloy steel, and 40Cr alloy steel as main raw materials, supplemented with fluorite and quicklime, and melts them at a temperature of 1610℃~1650℃ for 6~8 minutes to obtain the blank material. Fluorite and quicklime, as slag-forming agents, can effectively remove impurities in the molten steel, improve the purity of the material, and lay the foundation for obtaining high-strength and high-toughness flue pipes in the future.

[0028] Secondly, this invention involves forging the blank material and coating its surface with a slurry containing rare earth oxides before forging. Y₂O₃ reduces cracks, refines grains, and improves strength, toughness, and fatigue resistance; Sc₂O₃ refines grains and inhibits cracking; Tb₄O₇ refines grains and improves thermal shock resistance; HfO₂ effectively inhibits crack propagation; and ZrO₂ prevents corner cracks. Among these components, Y₂O₃, Sc₂O₃, and Tb₄O₇ work synergistically to refine grains, while Y₂O₃, Sc₂O₃, and HfO₂ work synergistically to inhibit overall and internal cracks. After forging, stepped cooling further refines the grains, eliminates internal stress, and significantly improves the overall mechanical properties and fatigue life of the blank material. Sandblasting removes the surface oxide layer after forging, providing a clean surface for subsequent processing.

[0029] Third, this invention forms a seamless tube from a steel bar and then performs cold working. Cold working can introduce a work hardening effect, improving the strength and hardness of the seamless tube, while also providing good dimensional accuracy and surface finish for subsequent roll forming.

[0030] Fourth, this invention processes seamless tubes into spiral seamless tubes through a roll forming process. The spiral grooves formed not only enhance the structural strength of the flue tube, but also serve as a spiral guide for flue gas, increasing the contact area between the flue gas and the tube wall and significantly improving the heat exchange efficiency.

[0031] Fifth, this invention sequentially applies electron beam irradiation, pulsed current instantaneous heating, and salt bath treatment to the spiral seamless tube. Electron beam irradiation introduces high-density dislocations and lattice distortion into the material surface, pulsed current instantaneous heating utilizes the Joule heating effect to achieve rapid temperature rise, and salt bath treatment provides a uniform and stable heating medium. Through electron beam irradiation, pulsed current instantaneous heating, and salt bath treatment, the surface grains of the material are refined, and the microstructure is improved, laying a good foundation for subsequent quenching and strengthening treatment.

[0032] Sixth, this invention employs a strong magnetic field internal and external surface quenching process. The spiral seamless tube is heated to 780℃~810℃ and then cooled by spraying with an alkaline solution containing Na3PO4 and C7H5Na2. The strong magnetic field improves the uniformity of the quenched structure; Na3PO4 has rust-preventing and corrosion-inhibiting effects, enhancing hardening ability and giving the tube surface higher hardness; C7H5NaO2 ensures a more uniform hardness distribution in all parts of the tube after quenching, avoiding localized soft spots or excessive hardness differences. Through this step, the inner and outer surfaces of the tube obtain a martensitic structure with high hardness and high wear resistance.

[0033] Seventh, this invention applies liquid nitrogen gradient cryogenic treatment to quenched spiral seamless tubes. By controlling the staged cooling rate from room temperature to -75℃~-85℃, and then to -162℃~-178℃, especially the specific low temperature and holding time, as well as the reheating rate, the residual austenite can be fully transformed into martensite, effectively eliminating residual stress inside the material, improving the dimensional stability and fatigue resistance of the tube, and further enhancing its hardness and toughness.

[0034] Eighth, this invention performs tempering treatment on the spiral seamless tube after deep cooling. Tempering can eliminate the internal stress generated during quenching and deep cooling, regulate the decomposition of martensite and the precipitation of carbides, and improve the toughness and plasticity of the material while maintaining high hardness, so that the tube achieves a good balance between strength and toughness.

[0035] Ninth, this invention straightens and shapes the tempered spiral seamless tube to eliminate bending deformation that may occur during heat treatment, ensuring the straightness and dimensional accuracy of the flue tube and meeting the requirements for boiler installation and use.

[0036] Tenth, this invention applies ultrasonic impact treatment to spiral seamless tubes. Ultrasonic impact can introduce a deep compressive stress layer on the material surface, effectively offsetting the tensile stress generated during service, significantly inhibiting the initiation and propagation of fatigue cracks, and improving the fatigue life of the tube.

[0037] Eleventh, this invention applies shot peening to spiral seamless tubes. High-speed shot impacts the tube wall surface, further introducing surface compressive stress and refining surface grains, while simultaneously eliminating minor processing defects and improving surface finish and resistance to stress corrosion.

[0038] Twelfth, this invention applies a water flow strengthening treatment to the inner wall of a spiral seamless pipe, utilizing high-pressure pulsed water flow to impact the pipe wall. This treatment introduces a compressive stress layer on the inner wall surface, improving the microstructure of the inner wall surface, enhancing the corrosion resistance and erosion wear resistance of the inner wall, while simultaneously eliminating residual stress from the inner wall processing.

[0039] Thirteenth, this invention employs a composite pickling solution containing rare earth elements to pickle spiral seamless pipes. Specifically, La(NO3)3 improves surface smoothness and reduces over-corrosion and pitting; Ce(NO3)3 shortens pickling time and reduces over-corrosion and pitting; Y(NO3)3 inhibits grain boundary corrosion and purifies the surface; and Na2MoO4 reduces over-corrosion and pitting. The synergistic effect of these components effectively removes the surface oxide layer while simultaneously achieving rare earth surface modification, significantly improving the corrosion resistance and surface quality of the inner and outer surfaces of the pipe.

[0040] Fourteenth, the present invention further applies liquid nitriding treatment to the spiral seamless tube. At a temperature of 545℃~568℃, nitrogen penetrates into the material surface to form a nitride layer, which significantly improves the surface hardness, wear resistance, and anti-galling performance, while also enhancing corrosion resistance and fatigue resistance.

[0041] Fifteenth, this invention applies chemical nickel plating to the nitrided spiral seamless tube, with a plating thickness of 0.015~0.025mm. The nickel plating has good corrosion resistance and uniformity, effectively isolating the tube substrate from external corrosive media, further improving the tube's resistance to high-temperature flue gas corrosion and water-based media corrosion.

[0042] Sixteenth, this invention involves grinding and polishing the nickel-plated spiral seamless tube, achieving a surface roughness of Ra0.63 after polishing. The smooth surface reduces the adhesion and accumulation of corrosive media, lowering the risk of localized corrosion, while simultaneously improving the appearance quality and dimensional accuracy of the tube. Detailed Implementation

[0043] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] Example 1

[0045] The specific steps of this embodiment are as follows:

[0046] (1) After feeding and melting, the raw material is cooled to obtain the blank material:

[0047] The feeding refers to the feeding of No. 20 steel, 12Cr2Mo1VR alloy steel, 40Cr alloy steel, fluorite and quicklime in a weight ratio of 49.5:73.4:13.8:2.5:1; the melting temperature is 1626℃~1639℃, the melting time is 7min, and the blank is obtained by cooling.

[0048] (2) The blank material is forged and sandblasted to obtain a steel bar, including the following sub-steps performed in sequence:

[0049] (2.1) Surface pretreatment of blank:

[0050] First, use an angle grinder with a 120-grit grinding wheel to remove surface oxide scale and rust, exposing the metallic luster. Then rinse with clean water and dry with compressed air at a pressure of 0.27~0.33MPa.

[0051] (2.2) Coating the surface of the blank with slurry:

[0052] First, brush the slurry onto the surface of the blank, evenly coat it, and cover it with a thickness of 0.7~1.4mm; then air dry it at a constant temperature of 15℃~16℃ and humidity of 59%~60% until the surface is completely dry and no longer sticky.

[0053] The prepared coating slurry consists of mixed powder and binder, typically containing 79%–81% mixed powder and 19%–21% binder by mass percentage. The mixed powder contains, by mass percentage: 21.2%–22.5% SiO2 (silicon dioxide), 7.6%–8.5% Al2O3 (alumina), 16.5%–18.5% Y2O3 (yttrium oxide), 1.6%–2.2% Sc2O3 (scandium oxide), 4.7%–5.5% Tb4O7 (terbium oxide), 2.4%–3.3% HfO2 (hafnium oxide), 4.6%–5.8% B (amorphous boron), and 3... It contains 0.8%~4.7% ZrO2 (zirconia), 19.2%~20.8% C (graphite), 4.8%~5.7% Al (aluminum), 4.6%~5.3% FeSi (ferrosilicon), with the remainder being MgO (magnesium oxide) and / or CaO (calcium oxide); the binder contains 44%~46% Na2SiO3 and 5.3%~6.7% bentonite by mass, with the remainder being water; the modulus of Na2SiO3 is 2.8~3.2.

[0054] Specifically, in this embodiment, the prepared coating slurry contains 79.6% mixed powder and 20.4% binder. The chemical components in the mixed powder contain, by mass percentage, 21.9% SiO2 (silicon dioxide), 8.1% Al2O3 (alumina), 17.7% Y2O3 (yttrium oxide), 2.1% Sc2O3 (scandium oxide), 5.1% Tb4O7 (terbium oxide), 2.9% HfO2 (hafnium oxide), 5.1% B (amorphous boron), 4.2% ZrO2 (zirconia), 19.8% C (graphite), 5.2% Al (aluminum), and 4.9% FeSi (ferrosilicon). The remainder consists of trace components such as MgO (magnesium oxide) and CaO (calcium oxide). The mixture is 250-350 mesh, and after mixing and stirring, it is uniform and free of lumps. The binder contains the following chemical components in the following mass ratio: 44.9% water glass (Na2SiO3, modulus 2.8~3.2), 49.1% water, and 6% bentonite. The mixture is stirred until there is no separation.

[0055] (2.3) Stepped heating:

[0056] When heating from room temperature to 287℃, the heating rate is 65.5℃ / h, and the holding time is 11.5min; when heating from 287℃ to 655℃, the heating rate is 78.2℃ / h, and the holding time is 15.25min; when heating from 655℃ to 1197℃, the heating rate is 77.4℃ / h, and the holding time is 29.75min. The product is removed from the furnace immediately after the holding time is completed.

[0057] (2.4) Forging:

[0058] The initial forging temperature is 1191℃, and the final forging temperature is 828℃. The deformation is controlled, and after every 3-5 hammer blows, 4-6g of powder is added, evenly and thinly sprinkled, to forge into steel bars with a diameter of 38.1-42.0mm.

[0059] The forging powder typically contains, by mass percentage, 55.0%~59.0% C (graphite), 7.3%~8.7% BN (boron nitride), 4.0%~6.0% Y2O3 (yttrium oxide) micro powder, 1.0%~1.6% Sc2O3 (scandium oxide) micro powder, 1.5%~1.8% HfO2 (hafnium oxide) micro powder, 8.0%~9.0% B (boron) micro powder, 7.0%~7.6% V (vanadium) micro powder, and 11.2%~11.6% Ti (titanium) micro powder.

[0060] Specifically, in this embodiment, it contains 57.0% C (graphite), 8.0% BN (boron nitride), 4.7% Y2O3 (yttrium oxide) micro powder, 1.4% Sc2O3 (scandium oxide) micro powder, 1.6% HfO2 (hafnium oxide) micro powder, 8.6% B (boron) micro powder, 7.4% V (vanadium) micro powder, and 11.3% Ti (titanium) micro powder, with a particle size of 150~300 mesh.

[0061] (2.5) Step cooling after forging:

[0062] Final forging temperature up to 653℃: slow cooling in an insulated box at a rate of 89.4℃ / h; 653℃ to 290℃: slow cooling in an insulated box at a rate of 55.2℃ / h; below 290℃: natural air cooling to room temperature.

[0063] (2.6) Sandblasting removes the oxide layer on the surface of the steel bar, and the surface of the steel bar reaches Sa2.5 grade.

[0064] (3) Steel bars are formed to obtain seamless tubes:

[0065] The steel bar is heated to 1167℃ and extruded through a forming die to control the outer diameter to 39.5~40.5mm. It is then pierced on a piercing mill, and under the combined action of the rolls and the mandrel, it becomes a tube. The tube is heated to 866℃ and rolled on a tube rolling mill to form a seamless tube with a uniform wall thickness, an outer diameter of 57.5~58.5mm, and a wall thickness of 5.2~5.7mm. It is then cooled to room temperature in a furnace at a cooling rate of 67.3℃ / h.

[0066] (4) Cold working of seamless tubes:

[0067] (4.1) Cold drawing at room temperature: Cold drawing seamless tubes to an outer diameter of 56.5~57.5mm and a wall thickness of 4.7~5.3mm;

[0068] (4.2) Low temperature freezing: temperature -81℃~-83℃, freezing time 15.5min, temperature controlled by liquid nitrogen;

[0069] (4.3) Reheat treatment: Temperature 181℃~185℃, hold for 21.25min, furnace cool to 36℃~42℃ and remove from the furnace. Repeat steps (4.2) and (4.3) for 2 cycles of hot and cold treatment.

[0070] (5) Seamless tubes are rolled to obtain spiral seamless tubes:

[0071] At room temperature, a roller press is used to continuously roll spiral grooves into seamless tubes multiple times. The pitch is 49.50~50.50mm, the groove cross-section is arc-shaped, the arc radius is 4.85~5.25mm, the arc pressing depth is 2.77~3.22mm, the pressing depth of each roll is 0.22~0.29mm, and the total number of roll presses is 13.

[0072] (6) Electron beam irradiation of spiral seamless tubes:

[0073] The spiral seamless tube was irradiated with an electron beam using a high-energy electron beam irradiation device (model IS1010, manufactured by Jiangsu Tongwei Xinda Technology Co., Ltd.). The energy was 2.8MeV~2.9MeV, the beam current intensity was 0.60~0.64mA, and the time was 3.5min. After irradiation, the tube was cooled by air at a wind speed of 3.9~4.4m / s.

[0074] (7) Instantaneous heating of spiral seamless tube by pulsed current:

[0075] A CNC pulse device (specifically a 485 programmable pulse type CNC pulse device manufactured by Jinan Nenghua Electromechanical Equipment Co., Ltd.) is used to perform instantaneous pulse current heating on the spiral seamless tube, with a current density of 1.3A / mm²~1.4A / mm². 2 The frequency is 60Hz and the pulse width is 5.6ms. The two ends of the spiral seamless tube are clamped with chromium-zirconium-copper electrodes, heated by electricity, and then immediately transferred to a salt bath furnace after heating.

[0076] (8) Spiral seamless tube salt bath:

[0077] The suspended spiral seamless tube is fed into a salt bath furnace at a constant speed using a crane. The temperature is 844℃~867℃. It is slowly immersed in a specially prepared salt bath melt, avoiding violent shaking and ensuring no parts are suspended or exposed. The immersion time is 3.25 minutes. After the holding time is reached, the spiral seamless tube is lifted out of the salt bath furnace at a constant speed using a crane. It is held at the furnace opening for 6 seconds, and then transferred to a special cooling rack. After cooling to room temperature, it is washed in a hot water pool at 84~87℃ and dried with hot air.

[0078] The prepared salt bath formula typically contains, by mass percentage: 37.8%–39.3% KCl (potassium chloride), 42.6%–43.8% NaCl (sodium chloride), 8.5%–9.8% CaCl2 (calcium chloride), 2.5%–3.1% NaF (sodium fluoride), 2.7%–3.3% Na2B4O7 (borax), 1.8%–2.2% Na2CO3 (sodium carbonate), 1.0%–1.3% Na2SiO3 (sodium silicate), 0.08%–0.18% TiO2 (ultrafine titanium dioxide), 0.07%–0.13% La2O3 (ultrafine lanthanum oxide), 0.06%–0.11% CeO2 (ultrafine cerium oxide), and 0.03%–0.05% Y2O3 (ultrafine yttrium oxide).

[0079] Specifically, this embodiment contains 38.49% KCl (potassium chloride), 43.5% NaCl (sodium chloride), 8.9% CaCl2 (calcium chloride), 2.7% NaF (sodium fluoride), 2.9% Na2B4O7 (borax), 2.0% Na2CO3 (sodium carbonate), 1.2% Na2SiO3 (sodium silicate), 0.11% TiO2 (ultrafine titanium dioxide), 0.09% La2O3 (ultrafine lanthanum oxide), 0.07% CeO2 (ultrafine cerium oxide), and 0.04% Y2O3 (ultrafine yttrium oxide).

[0080] (9) Magnetic field hardening of the inner and outer surfaces of spiral seamless tubes:

[0081] The spiral seamless tube was subjected to strong magnetic field quenching of its inner and outer surfaces using a high magnetic field quenching furnace (model L-400-B / RCWM200-9, manufacturer: Taixing Huaneng Industrial Furnace Co., Ltd.). The magnetic field strength was 0.74T. The spiral seamless tube was heated to 796℃, and alkaline water was sprayed onto the inner and outer surfaces of the spiral seamless tube before cooling to room temperature.

[0082] The prepared alkaline solution generally contains, by mass fraction, 2.20%~2.50% NaOH (sodium hydroxide), 1.16%~1.85% KOH (potassium hydroxide), 1.45%~1.53% NaCl (sodium chloride), 1.38%~1.51% Na2CO3 (sodium carbonate), 0.47%~0.54% KCl (potassium chloride), 0.22%~0.29% Na2B4O7 (borax), 0.19%~0.24% Na3PO4 (trisodium phosphate), and 0.23%~0.26% C7H5NaO2 (sodium benzoate), with the remainder being deionized water.

[0083] Specifically, this embodiment contains 2.33% NaOH (sodium hydroxide), 1.44% KOH (potassium hydroxide), 1.49% NaCl (sodium chloride), 1.46% Na2CO3 (sodium carbonate), 0.50% KCl (potassium chloride), 0.24% Na2B4O7 (borax), 0.21% Na3PO4 (trisodium phosphate), 0.24% C7H5NaO2 (sodium benzoate), and the remainder is deionized water.

[0084] (10) Gradient cryogenic treatment of spiral seamless tubes:

[0085] A horizontal liquid nitrogen cryogenic chamber (specifically a Cryometal-9000 horizontal liquid nitrogen cryogenic chamber manufactured by Wuxi Aisco Cryogenic Technology Co., Ltd.) was used to perform gradient cryogenic treatment on the spiral seamless tubes. The cooling rate was 2.6℃ / min from room temperature to -81℃, and 1.6℃ / min from -81℃ to -170℃. After holding at these temperatures for 49.5 minutes, the tubes were returned to room temperature at a heating rate of 0.8℃ / min.

[0086] (11) Tempering of spiral seamless tubes:

[0087] (11.1) Heating: The spiral seamless tube is preheated to 268°C by slow heating at a rate of 33.5°C / h;

[0088] (11.2) Insulation: Insulation 38.5min;

[0089] (11.3) Cooling time: Cool to 158℃ with the furnace at a rate of 38.2℃ / h, and then cool naturally after exiting the furnace. The ambient temperature after exiting the furnace is 19.3℃ and the air humidity is 51%.

[0090] (12) Straightening and straightening of spiral seamless tubes:

[0091] The spiral seamless tube was straightened and shaped on a straightening machine (specifically, a JG10-150 horizontal ten-roll precision tube straightening unit, manufactured by China Heavy Machinery Research Institute Co., Ltd.), with a straightness error of 2.6 mm / m.

[0092] (13) Ultrasonic impact testing of spiral seamless tubes:

[0093] An ultrasonic generator and an impact gun (specifically the HY-2050 ultrasonic impact device, manufactured by Shandong Huayun Electromechanical Technology Co., Ltd.) were used to impact the outer surface of the spiral seamless tube. The ultrasonic frequency was 20kHz, the power was 745W~757W, and the amplitude was 20.5μm~21.7μm, ensuring full coverage and no missed impacts.

[0094] (14) Shot peening of spiral seamless tubes:

[0095] For blasting ceramic shot with a diameter of 0.13~0.24mm, the blasting time is 4.5min and the blasting speed is 121~146m / s.

[0096] Ceramic pellets typically contain 64.60%~65.50% ZrO2 (zirconia), 30.10%~31.00% SiO2 (silicon dioxide), 1.10%~1.33% CeO2 (cerium oxide), and 2.87%~3.10% Y2O3 (yttrium oxide) by mass percentage.

[0097] Specifically, this embodiment contains 65.20% ZrO2 (zirconia), 30.66% SiO2 (silicon dioxide), 1.16% CeO2 (cerium oxide), and 2.98% Y2O3 (yttrium oxide).

[0098] (15) Water flow enhancement treatment on the inner wall of spiral seamless pipe:

[0099] First, the inner wall of the spiral seamless pipe is pretreated with ultrasound. Then, it is dried with compressed air at 0.3~0.4MPa. A high-pressure water pump is then started, and low-temperature pure water (1.6℃) is used to impact the pipe wall at a water pressure of 69MPa~72MPa, a pulse frequency of 50Hz, and a nozzle movement speed of 25.5~29.6mm / min. After impaction, it is dried again with compressed air at 0.3~0.4MPa.

[0100] (16) Pickling of spiral seamless tubes:

[0101] Seamless tubes are pickled using a special pickling solution at a temperature of 38.9℃~40.1℃ for 11 minutes. After pickling, they are rinsed with deionized water and dried with hot air.

[0102] The specially prepared pickling solution generally contains, by mass percentage, 5%~7% HCl (hydrochloric acid), 1.4%~1.8% HF (hydrofluoric acid), 0.9%~1.4% NaHF2 (sodium hydrogen fluoride), 0.5%~0.8% Na2SiF6 (sodium fluorosilicate), 0.10~0.14% La(NO3)3 (lanthanum nitrate), 0.07~0.10% Ce(NO3)3 (cerium nitrate), 0.02%~0.04% Y(NO3)3 (yttrium nitrate), and 0.05%~0.10% Na2MoO4 (sodium molybdate), with the remainder being water; wherein the mass concentration of HCl (hydrochloric acid) is 31%, and the mass concentration of HF (hydrofluoric acid) is 40%.

[0103] Specifically, this embodiment contains 5.9% HCl (hydrochloric acid, 31% industrial grade), 1.5% HF (hydrofluoric acid, 40% industrial grade), 1.1% NaHF2 (sodium hydrogen fluoride), 0.6% Na2SiF6 (sodium fluorosilicate), 0.11% La(NO3)3 (lanthanum nitrate), 0.08% Ce(NO3)3 (cerium nitrate), 0.02% Y(NO3)3 (yttrium nitrate), 0.06% Na2MoO4 (sodium molybdate), and 90.63% deionized water.

[0104] (17) Liquid nitriding:

[0105] The spiral seamless tube is liquid nitrided using a nitriding furnace and nitriding liquid. The spiral seamless tube is placed in the nitriding furnace and heated to 554℃~560℃ for 2.8 hours.

[0106] The prepared nitriding solution typically contains, by mass percentage: 43%–45% K₂CO₃ (potassium carbonate), 21.1%–22.7% Na₂CO₃ (sodium carbonate), 3.6%–4.4% Li₂CO₃ (lithium carbonate), 9.3%–10.8% KCl (potassium chloride), 1.7%–2.4% K₂SO₄ (potassium sulfate), 1.6%–2.3% Na₂B₄O₇ (borax), 13.8%–16.1% CO(NH₂)₂ (urea), 0.11%–0.14% La₂O₃ (lanthanum oxide), 0.17%–0.19% CeO₂ (cerium oxide), 0.10%–0.13% Nd₂O₃ (neodymium oxide), and 0.06%–0.09% Y₂O₃ (yttrium oxide).

[0107] Specifically, this embodiment contains 44.8% K2CO3 (potassium carbonate), 22.1% Na2CO3 (sodium carbonate), 4.1% Li2CO3 (lithium carbonate), 10.2% KCl (potassium chloride), 1.9% K2SO4 (potassium sulfate), 1.9% Na2B4O7 (borax), 14.51% CO(NH2)2 (urea), 0.12% La2O3 (lanthanum oxide), 0.18% CeO2 (cerium oxide), 0.12% Nd2O3 (neodymium oxide), and 0.07% Y2O3 (yttrium oxide).

[0108] (18) Nickel plating:

[0109] The spiral seamless tube is electroless nickel plated using a nickel plating solution, with a coating thickness of 0.015~0.025mm.

[0110] Nickel plating solutions typically contain 24.5-26.5 g / L NiSO4 (nickel sulfate), 23.5-25.5 g / L NaH2PO2 (sodium hypophosphite), 14-16 g / L C4H6O4 (succinic acid), 20.5-23.5 g / L C4H6O5 (malic acid), 0.0025-0.0035 g / L (CH3COO)2Pb (lead acetate), 0.1-0.3 g / L Ce(NO3)3 (cerium nitrate), 1.8-2.1 g / L CoSO4 (cobalt sulfate), and 0.07-0.11 g / L Y(NO3)3 (yttrium nitrate), with the remainder being deionized water.

[0111] Specifically, this embodiment contains 25.7 g / L NiSO4 (nickel sulfate), 24.6 g / L NaH2PO2 (sodium hypophosphite), 14.9 g / L C4H6O4 (succinic acid), 23.1 g / L C4H6O5 (malic acid), 0.0034 g / L (CH3COO)2Pb (lead acetate), 0.14 g / L Ce(NO3)3 (cerium nitrate), 1.91 g / L CoSO4 (cobalt sulfate), and 0.08 g / L Y(NO3)3 (yttrium nitrate), with the remainder being deionized water. The temperature of the plating bath is between 91.9℃ and 92.8℃.

[0112] (19) Grinding and polishing:

[0113] The spiral seamless tube was polished with 600-mesh diamond abrasive, and the roughness Ra after polishing was 0.63.

[0114] The ambient temperature and room temperature mentioned refer to 15~25℃.

[0115] The product of Example 1 was tested: the high-temperature and high-pressure boiler flue tube prepared in the above example has an outer diameter of 57 mm and a length of 3.2 m. The mechanical parameters of Example 1 compared with existing high-pressure boiler flue tubes on the market are as follows:

[0116] Table 1

[0117] Tensile strength (MPa) 687.0 629.2 Lower yield strength (MPa) 475.1 447.2 Elongation after fracture (%) 20.4 19.2 <![CDATA[Impact toughness (J / cm 2 )]]> 55.2 44.4 Non-proportional elongation strength at 400℃ (MPa) 386.4 343.5

[0118] As shown in Table 1, the tensile strength, lower yield strength, elongation after fracture, impact toughness, and non-proportional elongation strength at 400℃ of Example 1 are significantly improved compared to commercially available high-pressure flue tubes. This indicates that the present invention significantly improves the high-temperature resistance and pressure-bearing capacity of the product, effectively solving the problems of fatigue cracking, susceptibility to high-temperature flue gas corrosion, and insufficient pressure bearing capacity of existing boiler flue tubes, and fully meeting the requirements for long-term stable operation of boiler flue tubes under harsh working conditions.

[0119] Example 2

[0120] The only difference between this embodiment and embodiment 1 is that the proportion of raw materials fed into the furnace in step (1) is different: the weight ratio of No. 20 steel, 12Cr2Mo1VR alloy steel, 40Cr alloy steel, fluorite and quicklime in this embodiment is 53:73.4:13.8:2.5:1, only the weight ratio of No. 20 steel is increased, and the other steps are the same as in embodiment 1.

[0121] The mechanical parameters of the product from Example 2 were tested and compared with those of Example 1 as follows:

[0122] Table 2

[0123] Tensile strength (MPa) 687.0 686.4 Lower yield strength (MPa) 475.1 474.6 Elongation after fracture (%) 20.4 20.1 <![CDATA[Impact toughness (J / cm 2 )]]> 55.2 54.6 Non-proportional elongation strength at 400℃ (MPa) 386.4 385.9

[0124] As shown in Table 2, the tensile strength, lower yield strength, elongation after fracture, impact toughness, and non-proportional elongation strength at 400℃ in Example 2 are slightly lower than those in Example 1. This indicates that increasing the weight proportion of No. 20 steel in the raw materials for furnace feeding, i.e., decreasing the weight proportion of alloying elements such as Cr, Mo, and V, will lead to a decrease in the high-temperature resistance and pressure resistance of the product.

[0125] Example 3

[0126] The only difference between this embodiment and Example 1 is the proportion of mixed powders in the coating slurry in step (2.2): The coating slurry of Example 3 contains 21.9% SiO2 (silicon dioxide), 8.1% Al2O3 (alumina), 16.5% Y2O3 (yttrium oxide), 1.6% Sc2O3 (scandium oxide), 4.7% Tb4O7 (terbium oxide), 2.4% HfO2 (hafnium oxide), 5.1% B (amorphous boron), 4.2% ZrO2 (zirconia), 19.8% C (graphite), 5.2% Al (aluminum), 4.9% FeSi (ferrosilicon), and the remainder are trace components such as MgO (magnesium oxide) and CaO (calcium oxide). The mass ratios of Y2O3 (yttrium oxide), Sc2O3 (scandium oxide), Tb4O7 (terbium oxide) and HfO2 (hafnium oxide) were slightly reduced, while the other steps were the same as in Example 1.

[0127] The mechanical parameters of the product from Example 3 were tested and compared with those of Example 1 as follows:

[0128] Table 3

[0129] Tensile strength (MPa) 687.0 686.9 Lower yield strength (MPa) 475.1 475.0 Elongation after fracture (%) 20.4 20.3 <![CDATA[Impact toughness (J / cm 2 )]]> 55.2 55.0 Non-proportional elongation strength at 400℃ (MPa) 386.4 386.2

[0130] As shown in Table 3, the tensile strength, lower yield strength, elongation after fracture, impact toughness, and non-proportional elongation strength at 400℃ in Example 3 are slightly lower than those in Example 1. This indicates that reducing the mass proportions of Y2O3 (yttrium oxide), Sc2O3 (scandium oxide), Tb4O7 (terbium oxide), and HfO2 (hafnium oxide) in the coating slurry on the outer surface of the blank will lead to a slight decrease in the product's high-temperature resistance and pressure resistance.

[0131] Comparative Example 1

[0132] The only difference between this comparative example and Example 1 is the proportion of the pickling solution in step (16): Comparative example 1 contains 5.9% HCl (hydrochloric acid, 31% industrial grade), 1.5% HF (hydrofluoric acid, 40% industrial grade), 1.1% NaHF2 (sodium hydrogen fluoride), 0.6% Na2SiF6 (sodium fluorosilicate), 0.06% Na2MoO4 (sodium molybdate), and 90.84% ​​deionized water. Only La(NO3)3 (lanthanum nitrate), Ce(NO3)3 (cerium nitrate), and Y(NO3)3 (yttrium nitrate) were not added; the other steps are the same as in Example 1.

[0133] The mechanical parameters of the product in Comparative Example 1 were tested and are as follows:

[0134] Table 4

[0135] Tensile strength (MPa) 687.0 686.9 Lower yield strength (MPa) 475.1 475.0 Elongation after fracture (%) 20.4 20.3 <![CDATA[Impact toughness (J / cm 2 )]]> 55.2 55.1 Non-proportional elongation strength at 400℃ (MPa) 386.4 386.3

[0136] As shown in Table 4, the tensile strength, lower yield strength, elongation after fracture, impact toughness, and non-proportional elongation strength at 400℃ in Comparative Example 1 are slightly lower than those in Example 1. This indicates that the present invention significantly improves the high-temperature resistance and pressure resistance of the product by adding La(NO3)3 (lanthanum nitrate), Ce(NO3)3 (cerium nitrate), and Y(NO3)3 (yttrium nitrate) to the pickling solution.

[0137] Comparative Example 2

[0138] The only difference between this comparative example and Example 1 is that steps (6) electron beam irradiation of the spiral seamless tube, (7) instantaneous pulse current heating of the spiral seamless tube, (8) salt bath of the spiral seamless tube, (9) magnetic field quenching of the inner and outer surfaces of the spiral seamless tube, (10) gradient deep cooling of the spiral seamless tube, and (13) ultrasonic impact of the spiral seamless tube were not performed. The other steps are the same as in Example 1.

[0139] The mechanical parameters of the product from Comparative Example 2, compared to those of Comparative Example 1, were tested as follows:

[0140] Table 5

[0141] Tensile strength (MPa) 687.0 650.84 Lower yield strength (MPa) 475.1 450.78 Elongation after fracture (%) 20.4 19.6 <![CDATA[Impact toughness (J / cm 2 )]]> 55.2 53.0 Non-proportional elongation strength at 400℃ (MPa) 386.4 366.1

[0142] As shown in Table 5, the tensile strength, lower yield strength, elongation after fracture, impact toughness, and non-proportional elongation strength at 400℃ in Comparative Example 2 are significantly lower than those in Example 1. This indicates that the synergistic combination of steps (6) electron beam irradiation of the spiral seamless tube, (7) instantaneous pulsed current heating of the spiral seamless tube, (8) salt bath of the spiral seamless tube, (9) magnetic field quenching of the inner and outer surfaces of the spiral seamless tube, (10) gradient deep cooling of the spiral seamless tube, and (13) ultrasonic impact of the spiral seamless tube significantly improves the high-temperature resistance and pressure resistance of the product.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for manufacturing a high-temperature, high-pressure resistant boiler flue tube, characterized in that... Manufactured according to the following steps in sequence: (1) After feeding and melting, the blank material is cooled to obtain a blank material; (2) The blank material is forged and sandblasted to obtain a steel bar; (3) The steel bar is formed to obtain a seamless tube; (4) The seamless tube is cold-worked; (5) The seamless tube is rolled to obtain a spiral seamless tube; (6) The spiral seamless tube is irradiated with an electron beam; (7) The spiral seamless tube is instantaneously heated by a pulse current; (8) The spiral seamless tube is subjected to a salt bath; (9) The inner and outer surfaces of the spiral seamless tube are magnetically quenched; (10) The spiral seamless tube is subjected to gradient deep cooling; (11) The spiral seamless tube is tempered; (12) The spiral seamless tube is shaped and straightened; (13) The spiral seamless tube is subjected to ultrasonic impact; (14) The spiral seamless tube is shot-peened; (15) The inner wall of the spiral seamless tube is strengthened by water flow; (16) The spiral seamless tube is pickled. The feeding in step (1) refers to feeding No. 20 steel, 12Cr2Mo1VR alloy steel, 40Cr alloy steel, fluorite and quicklime in a weight ratio of (47~53):(72~79):(12.5~17.5):(2.25~2.75):1; the melting temperature is 1610℃~1650℃ and the melting time is 6~8min.

2. The method for manufacturing high-temperature and high-pressure resistant boiler flue tubes according to claim 1, characterized in that... Step (2) includes the following sub-steps performed sequentially: (2.1) Surface pretreatment of blank: First, remove the surface oxide scale and rust, then rinse with clean water and dry with compressed air; (2.2) Coating the surface of the blank with slurry: First, brush the slurry onto the surface of the blank, coat it evenly, and cover it with a thickness of 0.6~1.5mm. Then, air dry it at a constant temperature. (2.3) Stepwise heating: The heating rate from room temperature to 280℃~300℃ is 60~80℃ / h, and the holding time is 11~12min; the heating rate from 280℃~300℃ to 640℃~665℃ is 70~90℃ / h, and the holding time is 15~16min; the heating rate from 640℃~665℃ to 1175℃~1210℃ is 75~85℃ / h, and the holding time is 30min±1min. (2.4) Forging: initial forging temperature 1175℃~1210℃, final forging temperature ≥820℃; (2.5) Step cooling after forging: The cooling rate is 80~125℃ / h when the final forging temperature reaches 640℃~665℃; the cooling rate is 45~72℃ / h when the temperature reaches 280℃~300℃ from 640℃~665℃; and the temperature is naturally air-cooled to room temperature when the temperature is below 280℃~300℃. (2.6) Sandblasting removes the oxide layer on the surface of the steel bar.

3. The method for manufacturing high-temperature and high-pressure resistant boiler flue tubes according to claim 2, characterized in that: The slurry in step (2.2) consists of 79% to 81% by mass of mixed powder and 19% to 21% by mass of binder; wherein the mixed powder contains 16.5% to 18.5% Y2O3, 1.6% to 2.2% Sc2O3, 4.7% to 5.5% Tb4O7, 2.4% to 3.3% HfO2 and 3.8% to 4.7% ZrO2 by mass.

4. The method for manufacturing high-temperature and high-pressure resistant boiler flue tubes according to claim 1, characterized in that: In step (9), the spiral seamless tube is subjected to strong magnetic field quenching on the inner and outer surfaces using a magnetic field quenching furnace with a magnetic field strength of 0.8T±0.15T. The spiral seamless tube is heated to 780℃~810℃, and alkaline water is sprayed onto the inner and outer surfaces of the spiral seamless tube before cooling to room temperature.

5. The method for manufacturing high-temperature and high-pressure resistant boiler flue tubes according to claim 4, characterized in that: The alkaline solution contains 0.19%~0.24% Na3PO4 and 0.23%~0.26% C7H5NaO2 by mass.

6. The method for manufacturing high-temperature and high-pressure resistant boiler flue tubes according to claim 1, characterized in that: In step (10), the spiral seamless tube is subjected to liquid nitrogen gradient deep cooling. The cooling rate is 2.5~2.9℃ / min from room temperature to -75℃~-85℃; the cooling rate is 1.5~2.2℃ / min from -75℃~-85℃ to -162℃~-178℃; after holding at the temperature for 50min±2min, the temperature is restored to room temperature at a heating rate of 0.7~0.9℃ / min.

7. The method for manufacturing high-temperature and high-pressure resistant boiler flue tubes according to claim 1, characterized in that: In step (15), the inner wall of the spiral seamless tube is first pretreated with ultrasound. After drying, the high-pressure water pump is started, and water at 2.0℃±1.9℃ is used to impact the tube wall. The water pressure is 70MPa±8MPa, the pulse frequency is 50Hz±5Hz, and the nozzle moving speed is 25~30mm / min. After the impact is completed, the tube is dried.

8. The method for manufacturing high-temperature and high-pressure resistant boiler flue tubes according to claim 1, characterized in that: In step (16), the seamless tube is pickled with pickling solution at a temperature of 38℃±3.5℃ for 10.5min±2min. After pickling, it is rinsed with water and dried with hot air.

9. The method for manufacturing high-temperature and high-pressure resistant boiler flue tubes according to claim 8, characterized in that: The pickling solution contains 0.10-0.14% La(NO3)3, 0.07-0.10% Ce(NO3)3 and 0.02-0.04% Y(NO3)3 by mass.

10. The method for manufacturing high-temperature and high-pressure boiler flue tubes according to any one of claims 1 to 9, characterized in that... The manufacturing method further includes the following steps: (17) Liquid nitriding: Place the spiral seamless tube into a nitriding furnace and heat it to 545℃~568℃ for 2.5~3.0 hours; (18) Nickel plating: Use nickel plating solution to perform chemical nickel plating on the spiral seamless tube, with a plating thickness of 0.015~0.025mm; (19) Grinding and polishing: Grind and polish the spiral seamless tube, and the roughness Ra after polishing is 0.63.