Machining method of high-strength butt welding flange

Abstract

The invention discloses a processing method of a high-strength butt welding flange, which belongs to the field of flange processing, and comprises the following steps: S1, adopting a low-carbon medium alloy aluminum plate as a raw material, and carrying out directional deformation treatment on the raw material through a temperature control roll forging process to obtain a forging stock with a grain refinement structure; s2, plasma preheating treatment is conducted on the forging stock; s3, the preheated forging stock is placed in a multi-zone temperature difference control forging die, and a high-frequency precise hot forging technology is adopted for one-time forming; s4, solution treatment and controlled cooling treatment are conducted on the flange blank, and then staged temperature control cooling is achieved through a multi-stage slow cooling furnace; s5, a low-residual-stress precision turning device is used for conducting double-face high-precision numerical control machining, and a finished flange with the inner hole deviation smaller than + / -0.03 mm and the end face perpendicularity deviation smaller than 0.02 mm is obtained; and S6, low-temperature gas nitriding treatment is conducted on the finished flange. The method has the beneficial effects that the flange blank compactness is improved; the yield strength and the ductility are improved. The welding reliability is improved; repeated machining correction in a traditional process is avoided; the device is especially suitable for high-voltage occasions.

Description

Technical Field

[0001] The present invention relates to the field of flange processing, and more specifically, to a processing method for high-strength butt-welding flanges. Background Art

[0002] As a key component in aerospace, the processing method of high-strength butt-welding flanges has gradually evolved from traditional forging processing to precision forming and automation control technology. In the early days, butt-welding flanges mainly relied on free forging or die forging for forming. The process was simple, but the dimensional accuracy and mechanical properties fluctuated greatly. Subsequently, the introduction of hot die forging, isothermal forging, and heat treatment processes significantly improved the tissue uniformity and strength stability of the flanges. Entering the 21st century, advanced manufacturing technologies such as numerical control machining, laser cutting, and automatic welding have been widely used, greatly improving the processing efficiency, accuracy, and consistency of the flanges, meeting the usage requirements under harsh working conditions such as high pressure and high temperature. In addition, with the development of materials science, the application of new materials such as corrosion-resistant high-strength alloy aluminum has also promoted the wide application of high-performance flanges.

[0003] However, there is still a prominent problem in the current processing technology of high-strength butt-welding flanges: it is difficult to balance low cost and green manufacturing while ensuring high strength and high precision. Complex processes, high-temperature treatment, and the use of high-end materials result in high processing energy consumption, low material utilization rate, and the easy generation of a large amount of waste and heat treatment pollution problems during the processing, restricting its popularization in the mid- and low-end markets and the realization of the green manufacturing goal. Summary of the Invention

[0004] The purpose of the present invention is to provide a processing method for high-strength butt-welding flanges to solve the problems raised in the above background art: it is difficult to balance low cost and green manufacturing while ensuring high strength and high precision. Complex processes, high-temperature treatment, and the use of high-end materials result in high processing energy consumption, low material utilization rate, and the easy generation of a large amount of waste and heat treatment pollution problems during the processing, restricting its popularization in the mid- and low-end markets and the realization of the green manufacturing goal.

[0005] Technical Solution: The processing method for a high-strength butt-welding flange includes the following steps: S1. Select low-carbon medium-alloy aluminum sheets with a yield strength of not less than 350 MPa. After surface rust removal, cutting, and stress relief annealing pretreatment, perform multi-pass directional deformation treatment by temperature-controlled roll forging. The roll forging temperature is controlled at 1000 - 1100 °C. Apply radial and axial stresses through alternating longitudinal and transverse rolling to make the material generate a uniform fiber structure along the flange circumference, and at the same time refine the grains to 5 - 10 μm, finally obtaining a pre-forged blank with an isotropic strength distribution.

[0006] S2. Rapidly preheat the pre-forged blank by plasma beam in an oxygen-free atmosphere environment. The heating area is maintained between 850 and 950 °C under the protection of high-purity argon gas for 10 to 20 minutes, so that a small thermal gradient is formed between the surface layer and the interior of the forging blank, reducing the internal stress during the subsequent hot forging process and ensuring that no oxide film or metal decarburized layer is generated.

[0007] S3. Apply high-frequency precision hot forging forming to the preheated forging blank. Use a multi-cavity combined die and an intelligent zoning heating system to control the temperature distribution of the outer edge, middle, and core areas of the die cavity respectively, control the temperature difference not exceeding 100 °C, synchronously control the deformation rate within the range of 0.5 to 2 mm / s, and the pressing time is 20 to 45 seconds, so as to realize the simultaneous dense forming of the flange neck and the annular flange area, and obtain a flange blank with an average dimensional tolerance less than ±0.05 mm and a density not less than 98%.

[0008] S4. Transfer the hot-forged flange blank to a temperature-controlled resistance furnace for solution heat treatment. The temperature is set at 1000 to 1080 °C, and the holding time is 1.5 to 3 hours. Then, use a segmented slow cooling process to slowly cool the blank to room temperature. The cooling rate in the first stage is controlled at 20 °C / min, the second stage is 10 °C / min, and the final stage does not exceed 5 °C / min; S5. Machine the heat-treated flange blank, successively complete double-end face turning, inner hole boring, outer circle precision turning, and end face precision milling. The reserved machining allowance for each process is set not to exceed 0.1 mm. The tool is made of cemented carbide or CBN material, and the spindle runout is controlled within 0.005 mm. Finally, the internal and external dimensional tolerances are controlled within ±0.03 mm, the end face perpendicularity error is less than 0.02 mm, and the surface roughness Ra is lower than 0.4 μm; Specifically, the flange blank is subjected to solution heat treatment at a temperature of 1000 to 1080 °C for 1.5 to 3 hours; a three-stage slow cooling method is adopted: In the first stage, it is cooled from 1080 °C to 800 °C at a uniform speed in a temperature-controlled furnace; In the second stage, it is cooled from 800 °C to 500 °C by high-purity argon gas, and the temperature difference ≤ 15 °C / min; In the third stage, it is cooled from 500 °C to room temperature by intermittent water mist with a particle size of 50 to 80 μm, the spraying frequency is once every 10 seconds, and the total time does not exceed 15 minutes. The above method controls the tissue evolution, reduces the internal stress and deformation.

[0009] S6. The machined flange is subjected to low-temperature gas nitriding treatment. The treatment temperature range is 480 - 520 °C, the ratio of ammonia to hydrogen in the atmosphere is 3:1, the flow rate is kept stable at 100 - 150 mL / min, and the nitriding time is set to 4 - 6 hours. The surface nitrogen content gradient distribution is achieved through a three-stage treatment method of layer-by-layer heating, isothermal diffusion, and isothermal holding. Finally, a nitrided layer with a thickness of 0.3 - 0.6 mm and a hardness between 800 - 950 HV is formed, which has high wear resistance and high fatigue strength.

[0010] Preferably, in the S1 temperature-controlled roll forging, a combined multi-pass roll forging path planning process is adopted. In the first pass, the reduction amount along the axial direction is 12% - 15% of the material thickness, and then it gradually decreases to 5% - 8% in subsequent passes. After each pass, by adjusting the roll spacing and roll temperature, the thermal field uniformity in the workpiece deformation zone is maintained, preventing plastic non-uniformity or micro-crack initiation caused by central stress concentration. The material structure gradually extends along the processing direction to a fibrous shape, effectively suppressing the formation of columnar crystals and coarse grains, and significantly improving the mechanical property matching degree along the radial and axial directions, forming continuous and consistent metal flow lines and balanced deformation characteristics.

[0011] Preferably, after each pass is completed, the forging billet is sent into the induction heating zone for secondary heating. The temperature rise is maintained in the range of 95% - 98% of the initial roll forging temperature. The induction frequency is set to 8 - 12 kHz, and the heating time is controlled within 45 - 90 seconds. Through this thermal compensation process, the cooling lag in the core area of the billet is avoided, and at the same time, the internal and external temperature difference caused by premature surface cooling is prevented, thereby maintaining the overall deformation coordination, suppressing the occurrence of tissue non-uniformity, thermal fatigue, and stress rebound, and improving the subsequent forging quality and grain size consistency.

[0012] Preferably, in the plasma beam preheating treatment, the preheating scan adopts a spiral concentric circle wiring method. The scan step is set to 2 mm, the scan rate is 50 mm / s, high-purity argon gas is output from the nozzle annular distribution structure at a constant flow rate of 150 mL / min, the nozzle is kept 10 - 12 mm away from the workpiece surface, and the plasma arc beam width is 4 - 6 mm. By controlling the above process parameters, a heating zone with a thermal affected zone depth not greater than 0.5 mm is formed on the surface layer of the billet, and at the same time, the metal surface oxide layer is controlled below 10 nm, suppressing the growth of the oxide film and the migration of carbon elements, and avoiding cracks or local peeling caused by surface embrittlement in subsequent forging.

[0013] Preferably, during the high-frequency precision hot forging process, the electromagnetic induction heating method is adopted to set the heating power ratios of the outer edge, middle section, and center of the mold to 3:2:1 respectively. The temperatures of each cavity area are stabilized at 880°C, 920°C, and 950°C respectively. At the same time, a high-temperature heat-conducting ceramic coating with a thickness of not less than 1.5 mm is coated on the mold surface to improve the thermal field distribution efficiency. The forging forming rate is controlled at 0.8 mm / s, the pressing duration is 40 seconds, and the pressure is maintained above 120 MPa when the mold is closed. Thus, a neck structure with high density and no defects is formed. The thickness error of the ring part is controlled within the range of ±0.05 mm, and the compression ratio reaches more than 1.8, effectively enhancing the overall mechanical consistency and service life of the butt-welding flange.

[0014] Preferably, after the hot forging pressing is completed, the mold is kept closed for 10 seconds. By applying a constant-pressure load, the metal undergoes secondary micro-plastic deformation at high temperature, promoting grain boundary migration and tissue compaction, and further reducing micro-cracks or residual pores generated due to rapid unloading. Subsequently, it is cooled at a rate of 10°C / min under an inert atmosphere of 0.3 MPa until the temperature drops below 700°C, and then transferred to a slow-cooling furnace to continue cooling, finally obtaining a high-density flange forging with a uniform structure, no stress concentration at the edge, and a micro-porosity lower than 0.1%.

[0015] Preferably, the segmented slow cooling includes three stages. In the first stage, it slowly cools from 1080°C to 800°C, and the temperature is uniformly controlled in a heat preservation furnace to ensure sufficient transformation of austenite in the metal into pearlite or bainite. In the second stage, it cools from 800°C to 500°C, and is switched to high-purity argon gas cooling. The gas temperature is maintained in the range of normal temperature to 300°C, and the cooling rate is reduced to 15°C / min by continuous ventilation. In the third stage, it cools from 500°C to normal temperature, and is switched to intermittent water mist-assisted cooling. The water mist particle size is controlled within the range of 50 - 80 μm, the spray frequency interval is set to once every 10 seconds, and the total spraying duration does not exceed 15 minutes. Thus, a stable tissue evolution path is established, the formation of internal stress is reduced, and finally a heat treatment structure with a gentle hardness gradient and no precipitation phase agglomeration is obtained.

[0016] Preferably, the mechanical processing step adopts a five-axis linkage processing technology. The cutting tool selects a cemented carbide arc cutting tool with a diameter of 4 - 8 mm, the spindle speed is set at 1500 - 1800 rpm, the feed rate is 0.2 - 0.4 mm / rev, the machining path generates the minimum interference trajectory through the CAD / CAM system, and the trajectory overlap is set to not less than 30%. The end face machining adopts an alternating two-way milling mode to improve the flatness consistency. After each process, in-situ dimensional inspection is carried out, and the parts with an error exceeding ±0.03 mm are re-machined precisely once. Finally, the inner hole size deviation of the flange is controlled within ±0.02 mm, and the end face flatness is less than 0.01 mm, ensuring sufficient contact and minimal deformation of the flange welding surface.

[0017] Preferably, in the nitriding treatment step, a dynamic process of alternating ammonia and hydrogen atmospheres is used. In the initial stage, a mixed gas with a volume ratio of 3:1 is introduced and maintained for 1 hour. After that, every 60 minutes, it is switched to a pure hydrogen atmosphere for heat preservation for 10 minutes, and then restored to the original ratio of the mixed gas. This cycle is carried out 6 times. The nitriding temperature is kept constant at 500 °C, and the total treatment time does not exceed 6 hours. The thickness of the nitrided layer is controlled in the range of 0.35 - 0.5 mm by metallographic microscopy analysis. The nitrogen concentration gradient is stable, and a fine diffusion layer and a composite phase layer structure are formed on the surface. The Vickers hardness gradually transitions to the level of the core base material, while avoiding grain boundary embrittlement and the formation of low toughness regions, significantly enhancing the heat shock resistance and fatigue resistance of the flange.

[0018] Compared with the prior art, the advantages of the present invention are as follows: (1) By preheating with a plasma beam and combining it with temperature-controlled roll forging, internal shrinkage cavities and inclusions are effectively eliminated, and the density of the flange blank is improved.

[0019] (2) The heat treatment process precisely controls the tissue transformation to obtain fine and uniform tempered sorbite, improving the yield strength and ductility.

[0020] (3) The microstructure of the welded joint has a continuous transition, avoiding problems such as weld softening or hard brittleness, and improving the welding reliability.

[0021] (4) The whole process is temperature-controlled + a deformation amount feedback mechanism is used to control the forming size, avoiding repeated machining corrections in the traditional process. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the overall process flow of a processing method for a high-strength butt-welding flange of the present invention; DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] Embodiment, please refer to Figure 1 The processing method for a high-strength butt-welding flange includes the following steps: A processing method for a high-strength butt-welding flange includes the following steps: S1. Select a low-carbon medium-alloy aluminum sheet with a yield strength not less than 350 MPa. After surface rust removal, cutting, and stress relief annealing pretreatment, multi-pass directional deformation treatment is carried out by temperature-controlled roll forging. Among them, the roll forging temperature is controlled at 1000 - 1100 °C. Radial and axial stresses are applied by alternating longitudinal and transverse roll pressing, so that the material generates a uniform fiber structure along the circumference of the flange. At the same time, the grain size is refined to 5 - 10 μm, and finally a pre-forged blank with an isotropic strength distribution is obtained. Specifically, a low-carbon medium-alloy aluminum sheet with a yield strength of not less than 350 MPa is selected. The medium alloying elements are selected from one or more of Cr, Mo, Ni, and V, and the alloy content is controlled at 0.5% - 1.5%. The thickness of the aluminum sheet is not less than 30 mm. The aluminum sheet is mechanically shot-peened to remove rust, cut into the target shape using a plasma cutting device, and then stress-relieved annealed at 650 - 700 °C for 1 - 2 hours.

[0024] S2. The pre-forged blank is subjected to plasma beam rapid preheating treatment in a non-oxidizing atmosphere environment. The heating area is maintained between 850 - 950 °C under the protection of high-purity argon for 10 - 20 minutes, so that a small thermal gradient is formed between the surface and the interior of the forging blank, reducing the internal stress in the subsequent hot forging process and ensuring that no oxide film or metal decarburized layer is generated. Specifically, a spiral concentric circle scanning path is adopted, the step distance is set at 2 mm, the scanning speed is 50 mm / s, the nozzle is 10 - 12 mm from the surface, the argon gas flow rate is 150 mL / min, and the depth of the heat-affected zone is controlled ≤ 0.5 mm, and the surface oxide layer ≤ 10 nm to prevent metal decarburization and surface embrittlement.

[0025] S3. High-frequency precision hot forging forming is applied to the preheated forging blank. A multi-cavity combined die and an intelligent zoning heating system are used to control the temperature distribution of the outer edge, middle, and core areas of the die cavity respectively, and the temperature difference is controlled not to exceed 100 °C. The deformation rate is synchronously controlled within the range of 0.5 - 2 mm / s, and the pressing time is 20 - 45 seconds, so as to realize the simultaneous dense forming of the flange neck and the annular flange area, and obtain a flange blank with an average dimensional tolerance of less than ±0.05 mm and a density of not less than 98%. Specifically, an electromagnetic induction heating die is used. The die cavity is divided into an outer edge, a middle section, and a central area, and the set heating power ratio is 3:2:1. The temperatures of each area are 880 °C, 920 °C, and 950 °C respectively. The surface of the die is coated with a high-temperature heat-conducting ceramic layer with a thickness ≥ 1.5 mm to improve the thermal uniformity. The pressure when the die is closed is ≥ 120 MPa.

[0026] S4. The hot-forged flange blank is transferred to a temperature-controlled resistance furnace for solution heat treatment. The temperature is set at 1000 - 1080 °C, and the holding time is 1.5 - 3 hours. Then, the blank is slowly cooled to room temperature using a segmented slow-cooling process, where the cooling rate in the first stage is controlled at 20 °C / min, the second stage is 10 °C / min, and the final stage does not exceed 5 °C / min. S5. Machine process the heat-treated flange blank, successively completing double-end turning, internal hole boring, external circle finish turning and end face finish milling. The reserved machining allowance for each process is set to not exceed 0.1 mm. The cutting tool is made of cemented carbide or CBN material. The spindle runout is controlled within 0.005 mm. Finally, the internal and external dimensional tolerances are controlled within ±0.03 mm, the end face perpendicularity error is less than 0.02 mm, and the surface roughness Ra is lower than 0.4 μm. Specifically, the flange blank is subjected to solution heat treatment at a temperature of 1000 - 1080 °C for 1.5 - 3 hours, and a three-stage slow cooling method is adopted: In the first stage, it is cooled from 1080 °C to 800 °C at a uniform speed in a temperature-controlled furnace. In the second stage, it is cooled from 800 °C to 500 °C by high-purity argon gas, with a temperature difference ≤ 15 °C / min. In the third stage, it is cooled from 500 °C to room temperature by intermittent water mist with a particle size of 50 - 80 μm, with a spraying frequency of once every 10 seconds and a total time not exceeding 15 minutes. The above method controls the tissue evolution, reduces internal stress and deformation.

[0027] S6. Perform low-temperature gas nitriding treatment on the machined flange. The treatment temperature range is 480 - 520 °C. The ratio of ammonia to hydrogen in the atmosphere is 3:1. The flow rate is kept stable at 100 - 150 mL / min. The nitriding time is set to 4 - 6 hours. The surface nitrogen content gradient distribution is achieved through a three-stage treatment method of layer-by-layer heating, isothermal diffusion and constant-temperature heat preservation. Finally, a nitrided layer with a thickness of 0.3 - 0.6 mm and a hardness between 800 - 950 HV is formed, which has high wear resistance and high fatigue strength.

[0028] S1 The temperature-controlled roll forging adopts a combined multi-pass roll forging path planning process. In the first pass, the reduction amount along the axial direction is 12% - 15% of the material thickness, and in subsequent passes, it is successively reduced to 5% - 8%. After each pass, by adjusting the roll spacing and roll temperature, the thermal field uniformity in the deformation zone of the workpiece is maintained, preventing plastic non-uniformity or micro-crack initiation caused by central stress concentration. The material structure gradually extends along the processing direction into a fibrous shape, effectively suppressing the formation of columnar crystals and coarse grains, and significantly improving the mechanical property matching degree along the radial and axial directions, forming a continuous and consistent metal streamline and an anisotropic balanced deformation characteristic.

[0029] After each pass is completed, the forging blank is fed into the induction heating zone for secondary temperature rise. The temperature rise is maintained in the range of 95% - 98% of the initial roll forging temperature. The induction frequency is set to 8 - 12 kHz, and the heating time is controlled within 45 - 90 seconds. Through this thermal compensation process, the cooling lag in the core area of the blank is avoided, and at the same time, the internal and external temperature difference caused by premature cooling of the surface layer is prevented, thereby maintaining the overall deformation coordination, suppressing the occurrence of non-uniform organization, thermal fatigue, and stress rebound, and improving the subsequent forging quality and grain size consistency.

[0030] In the plasma beam preheating treatment, the preheating scan adopts a spiral concentric circle wiring method. The scan step is set to 2 mm, and the scan rate is 50 mm / s. High-purity argon gas is output from the nozzle annular distribution structure at a constant flow rate of 150 mL / min. The nozzle is kept at a distance of 10 - 12 mm from the workpiece surface, and the plasma arc beam width is 4 - 6 mm. By controlling the above process parameters, a heating zone with a heat-affected zone depth of no more than 0.5 mm is formed on the surface layer of the blank, and at the same time, the metal surface oxide layer is controlled below 10 nm, suppressing the growth of the oxide film and the migration of carbon elements, and avoiding cracks or local peeling caused by surface embrittlement in subsequent forging.

[0031] During the high-frequency precision hot forging process, the electromagnetic induction heating method is used to set the heating power ratios of the outer edge, middle section, and center of the die to 3:2:1 respectively. The temperatures of each die cavity area are stabilized at 880°C, 920°C, and 950°C respectively. At the same time, a high-temperature heat-conducting ceramic coating with a thickness of not less than 1.5 mm is coated on the die surface to improve the heat field distribution efficiency. The forging forming rate is controlled at 0.8 mm / s, the pressing duration is 40 seconds, and the pressure during die closing is maintained above 120 MPa. Thus, a neck structure with high density and no defects is formed, the thickness error of the ring part is controlled within the range of ±0.05 mm, and the compression ratio reaches more than 1.8, effectively enhancing the overall mechanical consistency and service life of the butt welding flange.

[0032] After the hot forging pressing is completed, the die is kept closed for 10 seconds. Through the application of a constant pressure load, the metal undergoes secondary micro-plastic deformation at high temperature, promoting grain boundary migration and tissue compaction, and further reducing micro-cracks or residual pores generated by rapid unloading. Subsequently, it is cooled at a rate of 10°C / min under an inert atmosphere of 0.3 MPa until the temperature drops below 700°C, and then transferred to a slow-cooling furnace for further cooling. Finally, a high-density flange forging with a uniform structure, no stress concentration at the edge, and a micro-porosity below 0.1% is obtained.

[0033] The stepped slow cooling includes three stages. In the first stage, it is slowly cooled from 1080°C to 800°C, and uniform temperature control is adopted in a holding furnace to ensure sufficient transformation of metal austenite into pearlite or bainite. In the second stage, it is cooled from 800°C to 500°C, and switched to high-purity argon gas cooling. The gas temperature is maintained in the range of normal temperature to 300°C, and the cooling rate is reduced to 15°C / min by continuous ventilation. In the third stage, it is cooled from 500°C to normal temperature, and switched to intermittent water mist-assisted cooling. The water mist particle size is controlled within 50 - 80μm, the spray frequency interval is set to once every 10 seconds, and the total spraying duration does not exceed 15 minutes. Thus, a stable tissue evolution path is established, the formation of internal stress is reduced, and finally a heat treatment structure with a gentle hardness gradient and no precipitation phase agglomeration is obtained.

[0034] The machining steps adopt a five-axis linkage machining process. The cutting tool selects a cemented carbide circular arc tool with a diameter of 4 - 8mm. The spindle speed is set at 1500 - 1800rpm, and the feed rate is 0.2 - 0.4mm / rev. The machining path generates the minimum interference trajectory through a CAD / CAM system, and the trajectory overlap is set to not less than 30%. The end face machining adopts an alternating two-way milling mode to improve the flatness consistency. In-situ dimensional inspection is carried out after each process. The parts with an error exceeding ±0.03mm are re-machined once. Finally, the inner hole size deviation of the flange is controlled within ±0.02mm, and the end face flatness is less than 0.01mm, ensuring sufficient contact and minimal deformation of the flange welding surface.

[0035] In the nitriding treatment step, a dynamic process of alternating ammonia gas and hydrogen gas atmosphere is used. In the initial stage, a mixed gas with a volume ratio of 3:1 is introduced and maintained for 1 hour. Then, every 60 minutes, it is switched to a pure hydrogen atmosphere for heat preservation for 10 minutes, and then restored to the original ratio of the mixed gas. This cycle is carried out 6 times. The nitriding temperature is kept constant at 500°C, and the total treatment time does not exceed 6 hours. The nitrided layer thickness is controlled in the range of 0.35 - 0.5mm by metallographic microscopy analysis. The nitrogen concentration gradient is stable, and a fine diffusion layer and a composite phase layer structure are formed on the surface. The Vickers hardness gradually transitions to the level of the core base material, while avoiding grain boundary embrittlement and the formation of low toughness zones, significantly enhancing the heat shock resistance and fatigue resistance of the flange.

[0036] The above shows and describes the basic principles, main features and advantages of the present invention; those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions in the specification are only preferred examples of the present invention and do not limit the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed; the scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A processing method for a high-strength butt-welding flange, characterized in that, The processing method of the high-strength butt-welding flange includes the following steps: S1. Select low-carbon medium-alloy aluminum sheets with a yield strength of not less than 350 MPa. After surface rust removal, cutting, and stress-relieving annealing pretreatment, multi-pass directional deformation treatment is carried out by means of temperature-controlled roll forging. The roll-forging temperature is controlled at 1000-1100 °C, and radial and axial stresses are applied by alternating longitudinal and transverse rolling to generate a uniform fiber structure along the flange circumference. At the same time, the grain size is refined to 5-10 μm, and finally a pre-forged blank with an isotropic strength distribution is obtained; S2. The pre-forged blank is subjected to plasma beam rapid preheating treatment in a non-oxidizing atmosphere environment. The heating area is maintained between 850-950 °C under the protection of high-purity argon for 10-20 minutes, so that a small thermal gradient is formed between the surface layer and the interior of the forging blank, reducing the internal stress in the subsequent hot forging process and ensuring that no oxide film or metal decarburized layer is generated; S3. High-frequency precision hot forging forming is applied to the preheated forging blank. A multi-cavity combined die and an intelligent zoning heating system are used to control the temperature distribution of the outer edge, middle, and core areas of the die cavity respectively, with the temperature difference controlled within 100 °C. The deformation rate is synchronously controlled within the range of 0.5-2 mm / s, and the pressing time is 20-45 seconds, so as to realize the simultaneous dense forming of the flange neck and the annular flange area, and obtain a flange blank with an average dimensional tolerance of less than ±0.05 mm and a density of not less than 98%; S4. The hot-forged flange blank is transferred to a temperature-controlled resistance furnace for solution heat treatment. The temperature is set at 1000-1080 °C, and the holding time is 1.5-3 hours. Then, the blank is slowly cooled to room temperature by a segmented slow-cooling process. The cooling rate in the first stage is controlled at 20 °C / min, the second stage is 10 °C / min, and the final stage does not exceed 5 °C / min; S5. The heat-treated flange blank is machined mechanically, and double-end face turning, inner hole boring, outer circle precision turning, and end face precision milling are completed in sequence. The reserved machining allowance for each process is set not to exceed 0.1 mm. The cutting tool is made of cemented carbide or CBN material, and the spindle runout is controlled within 0.005 mm. Finally, the internal and external dimensional tolerances are controlled within ±0.03 mm, the end face perpendicularity error is less than 0.02 mm, and the surface roughness Ra is lower than 0.4 μm; S6. The machined flange is subjected to low-temperature gas nitriding treatment. The treatment temperature range is 480-520 °C, the ratio of ammonia to hydrogen in the atmosphere is 3:1, the flow rate is kept stable at 100-150 mL / min, and the nitriding time is set at 4-6 hours. The surface nitrogen content gradient distribution is realized through a three-stage treatment method of layer-by-layer heating, isothermal diffusion, and constant-temperature holding. Finally, a nitrided layer with a thickness of 0.3-0.6 mm and a hardness between 800-950 HV is formed, with high wear resistance and high fatigue strength.

2. The processing method of a high-strength butt-welding flange according to claim 1, wherein in the S1 temperature-controlled roll forging, a combined multi-pass roll forging path planning process is adopted. In the first pass, the reduction amount along the axial direction is 12% - 15% of the material thickness, and then the reduction amount in subsequent passes is sequentially reduced to 5% - 8%. After each pass, the roll spacing and the roll temperature are adjusted.

3. The processing method of a high-strength butt-welding flange according to claim 2, wherein in the multi-pass roll forging, after each pass is completed, the forging blank is sent into the induction heating zone for secondary temperature rise. The temperature rise is maintained in the range of 95% - 98% of the initial roll forging temperature. The induction frequency is set to 8 - 12 kHz, and the heating time is controlled within 45 - 90 seconds.

4. The processing method of a high-strength butt-welding flange according to claim 1, wherein in the S2 plasma beam preheating treatment, the preheating scan adopts a spiral concentric circle wiring method. The scan step is set to 2 mm, the scan rate is 50 mm / s, high-purity argon gas is output from the nozzle annular distribution structure at a constant flow rate of 150 mL / min. The nozzle is kept 10 - 12 mm away from the workpiece surface, and the plasma arc beam width is 4 - 6 mm. By controlling the above process parameters, a heating zone with a heat-affected zone depth of no more than 0.5 mm is formed on the surface layer of the blank, and at the same time, the metal surface oxide layer is controlled below 10 nm to inhibit the growth of the oxide film and the migration of carbon elements.

5. The processing method of a high-strength butt-welding flange according to claim 1, wherein in the S3 high-frequency precision hot forging forming, the electromagnetic induction heating method is used to set the heating power ratios of the outer edge, the middle section, and the center of the mold to 3:2:1 respectively. The temperatures of each cavity area are stabilized at 880 °C, 920 °C, and 950 °C respectively. At the same time, a high-temperature heat-conducting ceramic coating with a thickness of not less than 1.5 mm is coated on the mold surface. The forging forming rate is controlled at 0.8 mm / s, the pressing duration is 40 seconds, and the pressure during mold closing is maintained above 120 MPa. Thus, a neck structure that is dense and defect-free is formed, the thickness error of the ring part is controlled within the range of ±0.05 mm, and the compression ratio reaches more than 1.

8.

6. The processing method of a high-strength butt-welding flange according to claim 5, wherein after the hot forging pressing is completed, the mold is kept in the closed state for 10 seconds. By applying a constant pressure load, the metal undergoes secondary micro-plastic deformation at a high temperature state. Subsequently, it is cooled at a rate of 10 °C / min in an inert atmosphere of 0.3 MPa until the temperature drops below 700 °C, and then transferred to a slow-cooling furnace for further cooling. Finally, a high-density flange forging with a uniform structure, no stress concentration at the edge, and a micro-porosity lower than 0.1% is obtained.

7. The processing method of a high-strength butt-welding flange according to claim 1, wherein the S4 segmented slow cooling includes three stages. In the first stage, it slowly cools from 1080°C to 800°C, and uniform temperature control is adopted in an insulation furnace to fully transform metal austenite into pearlite or bainite. In the second stage, it cools from 800°C to 500°C, and is switched to high-purity argon gas cooling. The gas temperature is maintained in the range of normal temperature to 300°C, and the cooling rate is reduced to 15°C / min by continuous ventilation. In the third stage, it cools from 500°C to normal temperature, and is switched to intermittent water mist-assisted cooling. The water mist particle size is controlled within 50 - 80 μm, the spray frequency interval is set to once every 10 seconds, and the total spraying duration does not exceed 15 minutes. Thus, a stable tissue evolution path is established, the formation of internal stress is reduced, and finally a heat treatment structure with a gentle hardness gradient and no precipitation phase agglomeration is obtained.

8. The processing method of a high-strength butt-welding flange according to claim 1, wherein the S5 machining adopts a five-axis linkage machining process. The cutting tool selects a cemented carbide circular arc cutting tool with a diameter of 4 - 8 mm, the spindle speed is set at 1500 - 1800 rpm, the feed rate is 0.2 - 0.4 mm / rev, the machining path generates the minimum interference trajectory through a CAD / CAM system, and the trajectory overlap is set to not less than 30%. The end face machining adopts an alternating two-way milling mode. In-situ dimensional inspection is carried out after each process, and the parts with an error exceeding ±0.03 mm are re-machined precisely once. Finally, the inner hole size deviation of the flange is controlled within ±0.02 mm, and the end face flatness is less than 0.01 mm.

9. The processing method of a high-strength butt-welding flange according to claim 1, wherein the S5 nitriding treatment uses a process of alternating dynamic ammonia gas and hydrogen gas atmospheres. In the initial stage, a mixed gas with a volume ratio of 3:1 is introduced and maintained for 1 hour, and then every 60 minutes, it is switched to a pure hydrogen atmosphere for heat preservation for 10 minutes, and then restored to the original ratio of the mixed gas. This cycle is carried out 6 times. The nitriding temperature is kept constant at 500°C, and the total treatment time does not exceed 6 hours. The nitrided layer thickness is controlled within the range of 0.35 - 0.5 mm by metallographic microscopy analysis.

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