Production process of high-strength corrosion-resistant welded bridge

Abstract

The application discloses a high-strength corrosion-resistant welded bridge production process and relates to the field of welded bridge production, which comprises the following steps: center pipe forming, flange assembly welding, accessory welding, end machining, shot blasting and aging treatment, surface spraying and the like. In the flange assembly welding step, the heated flange is sleeved into the center pipe end in an interference fit and is fixed by spot welding, the welding area is preheated, and then the outer welding seam is welded by synchronously feeding alloy powder into the molten pool by using the shielded metal arc welding, the inner welding seam is welded in the same way after cooling, heating and furnace cooling, and the whole workpiece is heated and stepwise cooled after welding; in the surface spraying step, the zinc-aluminum coating is sprayed on the surface of the workpiece after plasma cleaning by using the arc spraying process, the workpiece is sprayed with the penetration type epoxy sealing paint within a limited time while the workpiece is still warm, and then the epoxy primer and the polyester topcoat are coated in sequence. The application can significantly improve the comprehensive performance of the welding seam and the dimension precision of the component, and simultaneously improves the corrosion resistance of the welded bridge.

Description

Technical Field

[0001] This invention relates to the field of welded bridge manufacturing, and more particularly to a manufacturing process for high-strength, corrosion-resistant welded bridges. Background Technology

[0002] In the fields of engineering machinery, rail transportation, and heavy equipment manufacturing, welded bridge structural components, as key load-bearing and connecting parts, directly affect the service life and safety reliability of the entire machine due to their mechanical properties and corrosion resistance. These components are typically constructed by welding a central tube to flanges at both ends, with various connecting accessories welded onto the tube body, and finally achieving long-term corrosion protection through a surface protective coating system. However, because these components are subjected to alternating loads and exposed to harsh corrosive environments such as industrial atmospheres and marine salt spray for extended periods, extremely stringent requirements are placed on weld quality, structural dimensional accuracy, and the durability of the protective coating.

[0003] Currently, the welding of flanges and pipe fittings commonly employs gas metal arc welding (GMAW), with the conventional practice being the direct welding of ordinary low-alloy steel welding wire. While this welding method can meet basic connection strength requirements, it suffers from significant deficiencies in controlling the alloying of the weld metal. The lack of sufficient solid solution strengthening elements and carbide-forming elements during the solidification process results in low hardness and coarse microstructure in the weld zone, making it difficult to meet long-life design requirements in terms of fatigue resistance and electrochemical corrosion resistance. Especially when the weld is subjected to the superposition of welding residual stress and working stress, conventional welds are highly susceptible to microcracks. These microcracks propagate rapidly under the penetration of corrosive media, becoming the source of structural failure. Furthermore, if a conventional clearance fit is used when assembling the flange and center pipe, the lack of effective rigid constraints during welding leads to welding deformation caused by thermal stress. This can cause the flange end face runout and the coaxiality of the center hole to exceed design tolerances. Even with subsequent machining corrections, the deformation is often too large to fully compensate, severely affecting the assembly accuracy and transmission smoothness of the bridge body and adjacent components.

[0004] In terms of surface protection, existing processes often use arc-sprayed zinc-aluminum coatings as sacrificial anode underlayers, followed by coatings of epoxy primer and polyester topcoat to form a composite coating system. However, the inherent porosity of arc-sprayed zinc-aluminum coatings is relatively high. If a primer is directly applied to its surface, the high-viscosity primer cannot effectively penetrate into the pores to form a deep seal. Corrosive media can still penetrate to the substrate surface through the interconnected pore network, initiating galvanic corrosion at the coating-substrate interface, leading to blistering and peeling of the coating, and significantly reducing the protective life of the composite coating system. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention employs a series of synergistic methods, including simultaneous feeding of chromium-nickel-molybdenum alloy powder during flange welding, interference fit between the flange and the central tube, post-weld stepped cooling heat treatment, and applying a penetrating epoxy sealant within a limited time using the residual heat of the workpiece after zinc-aluminum coating spraying. These methods solve the technical problems of low hardness and easy cracking of conventional weld bridges, welding deformation leading to excessive flange dimensional and positional accuracy, and incomplete sealing of zinc-aluminum coating pores resulting in insufficient corrosion resistance of the composite coating system. Specifically, this is achieved through the following technical solutions.

[0006] A manufacturing process for high-strength, corrosion-resistant welded bridges includes the following steps: Step S1: Center tube forming, The central tube is placed in a bending die and bent into shape. After forming, the dimensions of the workpiece are inspected, and qualified products are transferred to the next stage. Step S2: Flange assembly and welding. After heating the flange, it is inserted into the end of the center tube. While it is still hot, the flange is adjusted to the target position. After the flange cools down, the installation dimensions are re-inspected. If the re-inspection is qualified, spot welding is performed to fix the center tube and flange at the joint. Preheat the flange welding area and use an automatic welding machine to weld the outer weld. The welding adopts gas metal arc welding. During the welding process, the powder feeding device is started simultaneously to send alloy powder containing chromium, nickel and molybdenum into the molten pool along with the shielding gas. When the arc is closed, the overlap exceeds the arc starting point by at least 10mm. After the outer weld is completed, the temperature is first lowered to 150-180℃ and held, then the temperature is raised to 280-320℃ and held, and then cooled to room temperature in the furnace. The welding method of the inner weld is the same as that of the outer weld. Repeat the entire process of flange assembly and welding described above to complete the assembly and welding of the flange on the other side of the workpiece. After both flanges are welded, heat the entire workpiece to 260°C and then cool it down to room temperature in stages. Step S3: Welding of accessories. The workpiece is transferred to the accessory welding station for spot welding and fixation. After spot welding is completed, additional welding is performed. After additional welding, the position of the accessory is inspected. After the inspection is passed, it is ground and cleaned. Step S4: End machining, The end faces of the flanges at both ends are machined, and the key dimensions are inspected after machining. Step S5: Shot blasting and aging treatment. The workpiece is shot blasted, and after shot blasting, it is subjected to low-temperature tempering. After tempering, the surface condition of the workpiece is checked. Step S6: Surface spraying, After plasma cleaning of the workpiece surface and installation of protective sleeves in non-sprayed areas, zinc-aluminum coating is sprayed onto the surface of the central tube and accessories using an electric arc spraying process. After the zinc-aluminum coating is applied, a penetrating epoxy sealant is sprayed within 30 minutes at a workpiece surface temperature of 40-80℃. After the sealant has dried completely, an epoxy primer and a polyester topcoat are sprayed in sequence. After the coating has dried completely, it is tested. If the test is qualified, it is packaged and put into storage.

[0007] Preferably, in step S2, the flange inner hole and the outer diameter of the central pipe are designed to be an interference fit, with an interference amount of 0.10 to 0.30 mm, and the flange heating temperature is 350 to 400°C.

[0008] Preferably, in step S2, when preheating the flange welding area, the preheating temperature is: 260-300℃ in summer and 300-340℃ in winter. An infrared thermometer is used for monitoring, and it is ensured that the temperature difference between the welding area and the adjacent area is not greater than 25℃.

[0009] Preferably, in step S2, the alloy powder is formulated as follows: 40-55 wt% chromium powder, 30-40 wt% nickel powder, and 5-15 wt% molybdenum powder, with a powder particle size of 50-100 μm, a powder feeding rate of 15-25 g / min, and the powder feeding nozzle is aligned with the center of the protective gas stream in front of the weld pool.

[0010] Preferably, during welding in step S2, the welding machine is NBC-500, the welding wire is ER70-G with a diameter of 1.2mm, the wire feed rate is 8-12m / min, 45° pull welding is used, the shielding gas is CO2 with a flow rate of 10-15L / min, the welding current is 220-240A, the voltage is 32-34V, and the welding speed is 330-370cm / min.

[0011] Preferably, in step S2, after the weld is completed, when the temperature is lowered to 150-180°C, it needs to be kept at that temperature for 2-5 minutes, and then when the temperature is raised to 280-320°C, it needs to be kept at that temperature for 5-10 minutes. The stepped cooling process is as follows: the first stage involves cooling to 80°C and holding for 4 hours; the second stage involves cooling to 60°C and holding for 4 hours; and the third stage involves cooling the furnace to room temperature.

[0012] Preferably, in step S5, the parameters for shot blasting are: steel shot size 0.6–1.0 mm, projection speed 80–120 m / s, shot blasting time 4–6 min, and coverage 200%–240%; The tempering temperature for low-temperature tempering is 180–220℃, the holding time is 1–2 hours, and the furnace is cooled to room temperature after the holding time is completed.

[0013] Preferably, in step S6, the zinc-aluminum coating has a zinc content of 85 wt%, an aluminum content of 15 wt%, and a coating thickness of 40–60 μm.

[0014] Preferably, in step S6, the penetrating epoxy sealing varnish is prepared by uniformly mixing component A and component B in a weight ratio of 4:1; Component A is prepared by mixing 25-35 parts by weight of bisphenol A type epoxy resin, 5-10 parts by weight of phenolic epoxy resin, 10-15 parts by weight of C12-C14 glycidyl ether reactive diluent, 28-36 parts by weight of xylene and 7-9 parts by weight of n-butanol. Component B is prepared by mixing 40-50 parts by weight of modified polyamide curing agent, 2-5 parts by weight of accelerator DMP-30, 36-44 parts by weight of xylene and 9-11 parts by weight of n-butanol; The modified polyamide curing agent is prepared by adding 10% to 20% of bisphenol A type epoxy resin by mass to polyamide resin and reacting at 60 to 80°C for 2 hours. The solids content of the penetrating epoxy sealing varnish is 18-22 wt%, and the thickness after drying is 10-20 μm.

[0015] Preferably, in step S6, the epoxy primer is sprayed in two coats, each coat being 25-35 μm thick, and each coat is surface dry for 25-35 minutes after spraying. The polyester topcoat is applied in two coats, each coat being 25–35 μm thick, and each coat takes 25–35 minutes to dry to dry.

[0016] After adopting the above technical solution, the beneficial effects of the present invention are: 1. By simultaneously feeding a specific ratio of chromium-nickel-molybdenum alloy powder into the molten pool during flange welding, the weld metal forms a uniform solid solution strengthening phase and a dispersed carbide strengthening phase during solidification, which significantly improves the hardness of the weld. At the same time, the introduction of alloying elements improves the oxidation resistance and fluidity of the molten pool, effectively eliminates internal microcracks and porosity defects, and endows the weld with excellent strength, toughness and resistance to electrochemical corrosion.

[0017] 2. The flange inner hole and the outer diameter of the central pipe are designed to be an interference fit. During assembly, the flange is heated to allow it to expand and fit into the pipe. After cooling, the pre-compression stress is generated to tighten the pipe end. This not only enhances the rigid constraint of the joint surface before welding and effectively suppresses the flange end face runout and the coaxiality deviation of the central hole caused by welding thermal stress, but also eliminates the root gap of the joint surface, effectively reducing unwelded defects. Structurally, this ensures the long-term reliability of welding accuracy and joint surface integrity.

[0018] 3. After the internal and external welds are completed, the workpiece is subjected to a heat treatment process of heating and then cooling in stages. By controlling the cooling rate and temperature steps, the residual stress caused by the concentrated heat of welding is fully released, and the uneven phase transformation and brittle martensite formation caused by rapid air cooling are avoided. This makes the hardness of the weld area uniform and stable, and inhibits the generation of discontinuous microcracks from the perspective of microstructure and stress.

[0019] 4. After arc spraying the zinc-aluminum sacrificial anode underlayer, a low-viscosity penetrating epoxy sealant is sprayed within a limited time using the residual heat window of the workpiece after spraying. The residual heat of the workpiece reduces the viscosity of the sealant and increases its fluidity. At the same time, the hot air in the pores of the zinc-aluminum layer is cooled by the sealant and contracts to generate local negative pressure. The two, together with the capillary effect of the coating pores, drive the sealant to penetrate deeply into the pores, forming a tightly sealed layer. This greatly reduces the interconnected porosity of the zinc-aluminum layer and provides a strong mechanical anchoring substrate for subsequent coatings, solving the problem of weak interlayer adhesion caused by insufficient penetration in traditional sealing processes.

[0020] 5. By constructing a multi-layer composite coating system consisting of a zinc-aluminum sacrificial anode underlayer, a dense transition layer of penetrating epoxy sealing paint, an epoxy primer, and a polyester topcoat, a dual protection mechanism of step-by-step shielding of corrosive media and cathodic protection is achieved. The sealing layer effectively blocks corrosion channels, and the multi-layer organic coating provides a strong physical barrier, enabling the welded bridge to exhibit a corrosion resistance life far exceeding that of conventional protection systems in a neutral salt spray environment. Detailed Implementation

[0021] To make the objectives, technical solutions, and technical effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0022] The manufacturing process for this high-strength, corrosion-resistant welded bridge includes the following steps: Step S1: Center tube forming, After the incoming material of the central tube passes inspection, it is placed into the bending mold. After being positioned and locked by tooling, it is bent into shape by hydraulic press. After the forming is completed, it is taken out for quality inspection. Workpieces with out-of-tolerance dimensions are isolated, and qualified products are transferred to the next stage.

[0023] The incoming central tube undergoes visual inspection, with the main items being defects such as cracks and deformation. The bending process uses a 315T hydraulic press with a system pressure set at 25MPa and a holding time of 4s. After forming, the quality inspection process mainly involves dimensional inspection of the workpiece. Specifically, a tape measure and height gauge are used to measure key dimensions such as the length and height of the central tube. The first piece is always inspected, and every 5 to 10 pieces thereafter are randomly inspected. Any piece with dimensions exceeding the tolerance is isolated.

[0024] Step S2: Flange assembly and welding. Before assembly, the flange is heated to 350-400℃. After heating, the flange is quickly inserted into the end of the bridge pipe. While it is still hot, the flange is adjusted to the target position. After the flange cools down, the installation dimensions are re-inspected. If the re-inspection is qualified, the center pipe and the flange are spot welded to fix them.

[0025] Preheat the flange welding area. After preheating to the required standard, use an automatic welding machine to weld the outer weld seam. The welding adopts gas metal arc welding. During the welding process, the powder feeding device is started simultaneously to send the alloy powder into the molten pool along with the shielding gas. After welding, the temperature is lowered to 150-180℃ and held for 2-5 minutes. Then the temperature is raised to 280-320℃ and held for 5-10 minutes before being cooled to room temperature with the furnace. Then the inner weld seam is welded. The welding method of the inner weld seam is the same as that of the outer weld seam.

[0026] Repeat the entire process of flange assembly and welding described above to complete the assembly and welding of the flange on the other side of the workpiece. After both flanges are welded, heat the entire workpiece to 260°C and then cool it down to room temperature in stages.

[0027] During the design phase, the flange inner hole and the outer diameter of the central tube are designed to be an interference fit with an interference amount of 0.10 to 0.30 mm. Before assembly, the flange is heated so that its inner hole expands due to thermal expansion. After the flange cools down, it forms an interference fit with the central tube, generating pre-compression stress at the root of the joint surface, which is beneficial to the tight connection between the central tube and the flange.

[0028] During the preheating process of the flange welding area, the preheating temperature is 260-300℃ in summer and 300-340℃ in winter. The temperature measurement process is monitored by an infrared thermometer, and the temperature difference between the welding area and the adjacent area is ensured to be ≤25℃.

[0029] The powder feeding device is installed on the automatic welding machine, and the powder feeding nozzle is aligned with the center of the protective gas stream in front of the weld pool. The powder feeding device contains uniformly mixed alloy powder with the following proportions: 40-55 wt% chromium powder, 30-40 wt% nickel powder, and 5-15 wt% molybdenum powder. The powder particle size is 50-100 μm, and the powder feeding rate is 15-25 g / min.

[0030] During the welding process described above, the welding machine model is NBC-500, the welding wire model is ER70-G, the welding wire diameter is 1.2mm, the welding wire feed rate is 8-12m / min, 45° pull welding is used, the shielding gas is CO2 with a flow rate of 10-15L / min, the welding current is 220-240A, the voltage is 32-34V, the welding speed is 330-370cm / min, and the overlap at the arc termination point must exceed the arc initiation point by at least 10mm.

[0031] The stepped cooling parameters are as follows: the first stage is to cool down to 80℃ and hold for 4 hours; the second stage is to cool down to 60℃ and hold for 4 hours; and the third stage is to cool down to room temperature with the furnace.

[0032] To ensure welding quality, the workpieces need to be randomly inspected after welding. The welding positions of the randomly inspected workpieces are cross-sectioned for inspection. 1 to 2 pieces are randomly selected from each production batch. The hardness of the weld is tested using a Vickers hardness tester. The weld hardness is required to be 190 to 380 HV. The internal defects of the weld are detected using flaw detection equipment. The weld penetration depth is required to be 2 to 3 mm, and there should be no cracks, porosity or other defects.

[0033] Step S3: Welding of accessories. The workpiece with flange welding completed in step S2 is transferred to the accessory welding station. The workpiece is positioned and installed using welding fixtures. The accessories to be welded are placed on the corresponding positioning points of the fixtures. After confirming that the position is accurate, they are tack welded and fixed. After the tack welding is completed, the workpiece is removed from the welding fixture for re-welding. After the re-welding, the position of the accessories is sampled and inspected using an inspection tool. After the inspection is qualified, the accessories are polished and cleaned using a grinder and then transferred to the next stage.

[0034] During the welding process of the above-mentioned accessories, an NBC-350 welding machine was used, the weld was set as a fillet weld, welded at 45°, the welding wire was THQ50-C with a diameter of 0.8mm, the gas protection was a mixture of argon and carbon dioxide, the gas flow rate was 10-15L / min, the current was 160-180A, the voltage was 27-29V, and the welding speed was 300-400mm / min.

[0035] In the above process, a polishing machine is used to remove all welding slag, spatter, weld beads and other residues, so that the welded surface is clean and free of residue.

[0036] Step S4: End machining, After welding, the end faces of the flanges at both ends of the axle are machined to ensure that they meet the dimensions and accuracy specified in the drawings. After machining, the iron filings are removed, and the key dimensions are inspected. Products that pass the inspection continue to be processed.

[0037] In the aforementioned machining process, the workpiece is fixed on a CNC machine tool for cutting. The machining parameters are set as follows: spindle speed 800-1200 r / min, cutting feed rate 0.05-0.15 mm / r, single cutting depth 0.2-0.5 mm. The deformation allowance caused by welding on the end faces of the flanges at both ends is removed, and the central shaft hole of the flange is bored. After machining, the dimensions of the flanges at both ends are inspected. It is required that the end faces of the flanges at both ends are perpendicular to the axis of the bridge pipe at both ends, the end face runout is within 0.05 mm, and the coaxiality error of the central holes of the flanges at both ends does not exceed 0.08 mm.

[0038] Step S5: Shot blasting and aging treatment. The workpiece is shot blasted using shot blasting equipment. After shot blasting, the workpiece is subjected to low-temperature tempering treatment at a temperature of 180-220℃ for 1-2 hours. After tempering, the workpiece is cooled to room temperature in the furnace. The surface condition of the workpiece is checked, and the surface hardness is randomly checked. Qualified workpieces are then transferred to the next stage.

[0039] The parameters for shot blasting are: steel shot size 0.6–1.0 mm, projection speed 80–120 m / s, shot blasting time 4–6 min, and coverage 200%–240%.

[0040] When inspecting the surface condition of the workpiece, visual inspection is used. It is required that there is no oxide scale or rust, no deformation of welded accessories, and the surface hardness reaches 220-260HV.

[0041] Step S6: Surface spraying, After plasma cleaning of the workpiece surface, a special protective sleeve is installed in the non-sprayed area. Using an arc spraying process, a zinc-aluminum coating with a thickness of 40-60 μm is sprayed onto the surface of the central tube and accessories to serve as the sacrificial anode underlayer and provide cathodic protection.

[0042] After the zinc-aluminum coating is applied, immediately apply a layer of low-viscosity sealing paint, followed by two coats of epoxy primer, each 25-35 μm thick. Each coat should be surface dry for 25-35 minutes, ensuring uniformity and no dripping. After this, apply two coats of polyester topcoat, each 25-35 μm thick, also requiring 25-35 minutes of surface dryness. Once the coating is fully dry, test its thickness, adhesion, and corrosion resistance. Workpieces meeting the testing standards are packaged and stored, while substandard products are isolated and disposed of.

[0043] In the aforementioned zinc-aluminum coating, the zinc content is 85wt% and the aluminum content is 15wt%.

[0044] In the above steps, the low-viscosity sealing paint is a penetrating epoxy sealing coating, which is prepared by uniformly mixing two components, A and B, in a weight ratio of 4:1. Component A is prepared by mixing 25-35 parts by weight of bisphenol A type epoxy resin, 5-10 parts by weight of phenolic epoxy resin, 10-15 parts by weight of C12-C14 glycidyl ether reactive diluent, 28-36 parts by weight of xylene, and 7-9 parts by weight of n-butanol. Component B is prepared by mixing 40-50 parts by weight of modified polyamide curing agent, 2-5 parts by weight of accelerator DMP-30, 36-44 parts by weight of xylene, and 9-11 parts by weight of n-butanol. The modified polyamide curing agent in component B is prepared by adding 10%-20% by weight of bisphenol A type epoxy resin to polyamide resin and reacting at 60-80°C for 2 hours.

[0045] The penetrating epoxy sealing paint has a solids content of 18-22 wt%. It is applied within 30 minutes after the zinc-aluminum coating is applied, and the surface temperature of the workpiece is 40-80℃ during the application. After the epoxy sealing paint dries, it has a thickness of 10-20 μm, forming a tightly sealed layer. This coating relies on the synergistic effect of capillary effect and temperature gradient to allow the sealing paint to penetrate into the pores of the zinc-aluminum layer, reducing the porosity of the underlying layer and providing a mechanically anchored substrate for subsequent coatings.

[0046] When inspecting the workpiece in the above process, the inspection standards are as follows: the coating thickness is not less than 100μm, and the coating thickness gauge is used for testing; the coating adhesion is grade 0 to 1, and the cross-cut test is used for testing; the corrosion resistance is tested using a salt spray test chamber, and the neutral salt spray test is required to be ≥2000h without rust.

[0047] To further understand the above technical solutions, several embodiments and comparative examples of the present invention are given below.

[0048] Example 1 Step S1: Center tube forming, The incoming center tubes are visually inspected and found to be free of cracks, deformations, and other defects. The qualified materials are then placed into the bending die, and after being positioned and locked using tooling, they are bent into shape using a 315T hydraulic press. The system pressure is set to 25MPa, and the pressure holding time is 4s. After the forming is completed, the tubes are removed for quality inspection. The first piece is always inspected, and one piece is randomly selected from every eight pieces thereafter. The length and height of the center tubes are measured using a tape measure and a height gauge. Workpieces with dimensions that are out of tolerance are isolated, and qualified products are transferred to the next batch.

[0049] Step S2: Flange assembly and welding. The flange inner diameter and the center pipe outer diameter are designed to be an interference fit with an interference of 0.20mm. Before assembly, heat the flange to 375℃ and quickly fit it onto the end of the bridge pipe. While it is hot, adjust the flange to the target position. After the flange cools down, re-inspect the installation dimensions. If the re-inspection is qualified, spot weld it at the joint between the center pipe and the flange.

[0050] Preheat the flange welding area to 280℃ in summer and 320℃ in winter. Use an infrared thermometer to monitor the temperature and ensure that the temperature difference between the welding area and the adjacent area is ≤25℃. After preheating, use an NBC-500 automatic welding machine to weld the outer seam. The welding wire is ER70-G with a diameter of 1.2mm. Use 45° pull welding. The shielding gas is CO2 with a flow rate of 12.5L / min. The welding current is 230A, the voltage is 33V, and the welding speed is 350cm / min. During the welding process, start the powder feeding device simultaneously to send the alloy powder into the molten pool along with the shielding gas. The powder feeding nozzle is aligned with the center of the shielding gas stream in front of the weld pool. The alloy powder ratio is 50wt% chromium powder, 38wt% nickel powder, and 12wt% molybdenum powder with a particle size of 75μm. The powder feeding rate is 20g / min, the welding wire feed rate is 10m / min, and the overlap at the arc termination point exceeds the arc initiation point by at least 10mm.

[0051] After the outer weld is completed, the temperature is lowered to 165℃ and held for 2 minutes. Then the temperature is raised to 300℃ and held for 5 minutes before being cooled to room temperature in the furnace. Then the inner weld is welded. The method and parameters for the inner weld are the same as those for the outer weld.

[0052] Repeat the entire process of flange assembly and welding described above to complete the assembly and welding of the flange on the other side of the workpiece. After both flanges are welded, heat the entire workpiece to 260°C and perform a stepped cooling process: the first stage is to keep it at 80°C for 4 hours, the second stage is to keep it at 60°C for 4 hours, and the third stage is to cool it to room temperature with the furnace.

[0053] After the workpiece is welded, one piece is randomly selected from each production batch for inspection. The weld position of the selected workpiece is cross-sectioned for inspection. The hardness of the weld is tested using a Vickers hardness tester, and internal defects of the weld are detected using flaw detection equipment. The penetration depth is then calculated.

[0054] Step S3: Welding of accessories. The workpiece with flange welding completed in step S2 is transferred to the accessory welding station. The workpiece is positioned and installed using the welding fixture. The accessory to be welded is placed on the corresponding positioning point of the fixture. After confirming that the position is accurate, it is tack welded and fixed. After the tack welding is completed, it is taken out of the welding fixture for further welding.

[0055] Welding was performed using an NBC-350 welding machine. The weld was set as a fillet weld, welded at 45°, using THQ50-C welding wire with a diameter of 0.8mm. Argon and carbon dioxide mixed gas protection was used, with a gas flow rate of 12.5L / min, a current of 170A, a voltage of 28V, and a welding speed of 350mm / min.

[0056] After the repair welding, the location of the accessory is sampled and inspected using a gauge. After the inspection is qualified, a grinder is used to grind and clean it to remove all welding slag, spatter, weld beads and other residues, so that the welded surface is clean and free of residue, and then it can continue to be processed.

[0057] Step S4: End machining, The workpiece is fixed on a CNC machine tool for cutting. The spindle speed is 1000 r / min, the cutting feed rate is 0.10 mm / r, and the single cutting depth is 0.35 mm. The deformation allowance caused by welding on the flange end faces at both ends is removed, and the flange center shaft hole is bored.

[0058] After processing, remove the iron filings and inspect the dimensions of the flanges at both ends. The end faces of both flanges are perpendicular to the axis of the bridge pipe, the end face runout is 0.03mm, and the coaxiality error of the center holes of the flanges at both ends is 0.05mm. Products that pass inspection continue to be processed.

[0059] Step S5: Shot blasting and aging treatment. The workpiece is shot blasted using a shot blasting equipment with a steel shot size of 0.8mm, a projection speed of 100m / s, a shot blasting time of 5min, and a coverage rate of 220%. After shot blasting, the workpiece is subjected to low-temperature tempering treatment at a tempering temperature of 200℃ and a holding time of 1.5h. After the holding time is completed, the workpiece is cooled to room temperature in the furnace.

[0060] Visually inspect the surface of the workpiece. There is no oxide scale or rust on the surface, and the welded accessories are not deformed. The surface hardness of the sampled surface is 245HV.

[0061] Step S6: Surface spraying, After plasma cleaning of the workpiece surface, a special protective sleeve is installed in the non-sprayed area. Then, an arc spraying process is used to spray a zinc-aluminum coating on the surface of the central tube and accessories. The zinc content is 85wt%, the aluminum content is 15wt%, and the coating thickness is 55μm.

[0062] After the zinc-aluminum coating is applied, maintain the workpiece surface temperature at 65°C and spray a low-viscosity sealing paint within 30 minutes. This sealing paint is a penetrating epoxy sealing coating, which is prepared by uniformly mixing components A and B in a weight ratio of 4:1.

[0063] Component A is prepared by mixing 30 parts by weight of bisphenol A type epoxy resin, 8 parts by weight of phenolic epoxy resin, 12 parts by weight of C12-C14 glycidyl ether reactive diluent, 32 parts by weight of xylene, and 8 parts by weight of n-butanol; Component B is prepared by mixing 45 parts by weight of modified polyamide curing agent, 3 parts by weight of accelerator DMP-30, 40 parts by weight of xylene, and 10 parts by weight of n-butanol; wherein the modified polyamide curing agent is prepared by adding 15% by weight of bisphenol A type epoxy resin to polyamide resin and reacting at 70°C for 2 hours.

[0064] This penetrating epoxy sealing varnish has a solids content of 20wt% and a thickness of 18μm after drying.

[0065] After the sealing paint has fully dried, apply two coats of epoxy primer evenly, each coat being 30μm thick. Each coat should be surface dry for 30 minutes after application, and should be uniform without sagging. After the epoxy primer is applied, apply two coats of polyester topcoat in sequence, each coat being 30μm thick. Each coat should be surface dry for 30 minutes after application, and should be uniform without sagging.

[0066] After the coating is completely dry, it is tested, and workpieces that meet the testing standards are packaged and put into storage.

[0067] Example 2 This embodiment is based on embodiment 1, with the same operating steps as embodiment 1, the only difference being the adjustment of the following parameters, specifically: Step S2: Flange interference fit 0.30mm, heating temperature 400℃; preheating temperature 300℃ in summer, 340℃ in winter; alloy powder ratio chromium 55wt%, nickel 40wt%, molybdenum 15wt%, powder particle size 100μm, powder feeding rate 25g / min, welding wire feed 12m / min; welding current 240A, voltage 34V, welding speed 370cm / min.

[0068] Step S5: Shot blasting with steel shot of 1.0mm particle size, 120m / s projection speed, 6min time, and 240% coverage; tempering temperature of 220℃ and holding for 2h, surface hardness of 255HV.

[0069] Step S6: Zinc-aluminum coating thickness 60μm, workpiece surface temperature 80℃, sealing paint dry thickness 20μm; epoxy primer and polyester topcoat each 35μm, total coating thickness 220μm.

[0070] The remaining steps, parameters, and detection methods are the same as in Example 1.

[0071] Example 3 This embodiment is based on embodiment 1, with the same operating steps as embodiment 1, the only difference being the adjustment of the following parameters, specifically: Step S2: Flange interference fit 0.10mm, heating temperature 350℃; preheating temperature 260℃ in summer, 300℃ in winter; alloy powder ratio 40wt% chromium, 30wt% nickel, 5wt% molybdenum, powder particle size 50μm, powder feeding rate 15g / min, welding wire feed 8m / min; welding current 220A, voltage 32V, welding speed 330cm / min.

[0072] Step S5: Shot blasting with steel shot of 0.6mm particle size, projection speed of 80m / s, time of 4min, coverage of 200%; tempering temperature of 180℃ and holding for 1h, surface hardness of 225HV.

[0073] Step S6: Zinc-aluminum coating thickness 40μm, workpiece surface temperature 40℃, sealing paint dry thickness 10μm; epoxy primer and polyester topcoat each 25μm, total coating thickness 150μm.

[0074] The remaining steps, parameters, and detection methods are the same as in Example 1.

[0075] Comparative Example 1 This comparative example is based on Example 1, in which the powder feeding device is turned off in step S2, and no alloy powder is fed in, while the other parameters remain unchanged. Specifically: Step S2: Flange assembly and welding. The flange inner diameter and the center pipe outer diameter are designed to be an interference fit with an interference of 0.20mm. Before assembly, heat the flange to 375℃ and quickly fit it onto the end of the bridge pipe. While it is hot, adjust the flange to the target position. After the flange cools down, re-inspect the installation dimensions. If the re-inspection is qualified, spot weld it at the joint between the center pipe and the flange.

[0076] Preheat the flange welding area to 280℃ in summer and 320℃ in winter. Use an infrared thermometer to monitor the temperature difference between the welding area and the adjacent area to ensure it is ≤25℃. After preheating, use an NBC-500 automatic welding machine to weld the outer seam. Use ER70-G welding wire with a diameter of 1.2mm. Use 45° pull welding. Use CO2 shielding gas with a flow rate of 12.5L / min. Use 230A welding current and 33V welding voltage. Use 350cm / min welding speed and 10m / min wire feed rate. When finishing the arc, the overlap should exceed the arc initiation point by at least 10mm.

[0077] After the outer weld is completed, the temperature is lowered to 165℃ and held for 2 minutes. Then the temperature is raised to 300℃ and held for 5 minutes before being cooled to room temperature in the furnace. Then the inner weld is welded. The method and parameters for the inner weld are the same as those for the outer weld.

[0078] Repeat the entire process of flange assembly and welding described above to complete the assembly and welding of the flange on the other side of the workpiece. After both flanges are welded, heat the entire workpiece to 260°C and perform a stepped cooling process: the first stage is to keep it at 80°C for 4 hours, the second stage is to keep it at 60°C for 4 hours, and the third stage is to cool it to room temperature with the furnace.

[0079] After the workpiece is welded, one piece is randomly selected from each production batch for inspection. The weld position of the selected workpiece is cross-sectioned for inspection. The hardness of the weld is tested using a Vickers hardness tester, and internal defects of the weld are detected using flaw detection equipment. The penetration depth is then calculated.

[0080] The remaining steps, parameters, and detection methods are the same as in Example 1.

[0081] Comparative Example 2 This comparative example is based on Example 1, with adjustments made to the proportion of alloy powder in step S2, specifically: Step S2: Flange assembly and welding. The flange inner diameter and the center pipe outer diameter are designed to be an interference fit with an interference of 0.20mm. Before assembly, heat the flange to 375℃ and quickly fit it onto the end of the bridge pipe. While it is hot, adjust the flange to the target position. After the flange cools down, re-inspect the installation dimensions. If the re-inspection is qualified, spot weld it at the joint between the center pipe and the flange.

[0082] Preheat the flange welding area to 280℃ in summer and 320℃ in winter. Use an infrared thermometer to monitor the temperature difference between the welding area and adjacent areas to ensure it is ≤25℃. After preheating, use an NBC-500 automatic welding machine to weld the outer seam. Use ER70-G welding wire with a diameter of 1.2mm. Use 45° pull welding. Use CO2 shielding gas with a flow rate of 12.5L / min. Use 230A welding current and 33V welding speed. Use 350cm / min welding speed. Start the powder feeding device simultaneously during welding to feed alloy powder into the molten pool along with the shielding gas. The powder feeding nozzle is aligned with the center of the shielding gas stream in front of the molten pool. The alloy powder ratio is 50wt% chromium powder and 50wt% nickel powder with a particle size of 75μm. Use a powder feeding rate of 20g / min and a wire feeding rate of 10m / min. When finishing the arc, the overlap should exceed the arc starting point by at least 10mm.

[0083] After the outer weld is completed, the temperature is lowered to 165℃ and held for 2 minutes. Then the temperature is raised to 300℃ and held for 5 minutes before being cooled to room temperature in the furnace. Then the inner weld is welded. The method and parameters for the inner weld are the same as those for the outer weld.

[0084] Repeat the entire process of flange assembly and welding described above to complete the assembly and welding of the flange on the other side of the workpiece. After both flanges are welded, heat the entire workpiece to 260°C and perform a stepped cooling process: the first stage is to keep it at 80°C for 4 hours, the second stage is to keep it at 60°C for 4 hours, and the third stage is to cool it to room temperature with the furnace.

[0085] After the workpiece is welded, one piece is randomly selected from each production batch for inspection. The weld position of the selected workpiece is cross-sectioned for inspection. The hardness of the weld is tested using a Vickers hardness tester, and internal defects of the weld are detected using flaw detection equipment. The penetration depth is then calculated.

[0086] The remaining steps, parameters, and detection methods are the same as in Example 1.

[0087] Comparative Example 3 This comparative example is based on Example 1, except that after spraying the zinc-aluminum coating in step S6, the penetrating epoxy sealing paint is not sprayed; instead, the same epoxy primer and polyester topcoat are directly sprayed. Specifically: Step S6: Surface spraying, After plasma cleaning of the workpiece surface, a special protective sleeve is installed in the non-sprayed area. Then, an arc spraying process is used to spray a zinc-aluminum coating on the surface of the central tube and accessories. The zinc content is 85wt%, the aluminum content is 15wt%, and the coating thickness is 55μm.

[0088] After the zinc-aluminum coating is applied, two coats of epoxy primer are applied evenly, each 30μm thick. Each coat is surface dry for 30 minutes, and the coating is uniform and free of drips. After the epoxy primer is applied, two coats of polyester topcoat are applied in sequence, each 30μm thick. Each coat is surface dry for 30 minutes, and the coating is uniform and free of drips.

[0089] After the coating is completely dry, it is tested, and workpieces that meet the testing standards are packaged and put into storage.

[0090] The remaining steps, parameters, and detection methods are the same as in Example 1.

[0091] Comparative Example 4 This comparative example is based on Example 1, except that after arc spraying the zinc-aluminum coating in step S6, the workpiece is completely cooled to room temperature and left for 2 hours, and then a penetrating epoxy sealing paint with the same formulation is sprayed on. Specifically: Step S6: Surface spraying, After plasma cleaning of the workpiece surface, a special protective sleeve is installed in the non-sprayed area. Then, an arc spraying process is used to spray a zinc-aluminum coating on the surface of the central tube and accessories. The zinc content is 85wt%, the aluminum content is 15wt%, and the coating thickness is 55μm.

[0092] After the zinc-aluminum coating is applied, the workpiece is completely cooled to room temperature and left for 2 hours before being sprayed with a low-viscosity sealing paint. This sealing paint is a penetrating epoxy sealing coating, which is made by uniformly mixing components A and B in a weight ratio of 4:1.

[0093] Component A is prepared by mixing 30 parts by weight of bisphenol A type epoxy resin, 8 parts by weight of phenolic epoxy resin, 12 parts by weight of C12-C14 glycidyl ether reactive diluent, 32 parts by weight of xylene, and 8 parts by weight of n-butanol; Component B is prepared by mixing 45 parts by weight of modified polyamide curing agent, 3 parts by weight of accelerator DMP-30, 40 parts by weight of xylene, and 10 parts by weight of n-butanol; wherein the modified polyamide curing agent is prepared by adding 15% by weight of bisphenol A type epoxy resin to polyamide resin and reacting at 70°C for 2 hours.

[0094] This penetrating epoxy sealing varnish has a solids content of 20wt% and a thickness of 18μm after drying.

[0095] After the sealing paint has fully dried, apply two coats of epoxy primer evenly, each coat being 30μm thick. Each coat should be surface dry for 30 minutes after application, and should be uniform without sagging. After the epoxy primer is applied, apply two coats of polyester topcoat in sequence, each coat being 30μm thick. Each coat should be surface dry for 30 minutes after application, and should be uniform without sagging.

[0096] After the coating is completely dry, it is tested, and workpieces that meet the testing standards are packaged and put into storage.

[0097] The remaining steps, parameters, and detection methods are the same as in Example 1.

[0098] Comparative Example 5 This comparative example is based on Example 1, except that after the inner and outer welds are welded in step S2, the entire structure is not heated to 260°C and then cooled in stages. Instead, it is directly air-cooled to room temperature. Specifically: Step S2: Flange assembly and welding. The flange inner diameter and the center pipe outer diameter are designed to be an interference fit with an interference of 0.20mm. Before assembly, heat the flange to 375℃ and quickly fit it onto the end of the bridge pipe. While it is hot, adjust the flange to the target position. After the flange cools down, re-inspect the installation dimensions. If the re-inspection is qualified, spot weld it at the joint between the center pipe and the flange.

[0099] Preheat the flange welding area to 280℃ in summer and 320℃ in winter. Use an infrared thermometer to monitor the temperature and ensure that the temperature difference between the welding area and the adjacent area is ≤25℃. After preheating, use an NBC-500 automatic welding machine to weld the outer seam. The welding wire is ER70-G with a diameter of 1.2mm. Use 45° pull welding. The shielding gas is CO2 with a flow rate of 12.5L / min. The welding current is 230A, the voltage is 33V, and the welding speed is 350cm / min. During the welding process, start the powder feeding device simultaneously to send the alloy powder into the molten pool along with the shielding gas. The powder feeding nozzle is aligned with the center of the shielding gas stream in front of the weld pool. The alloy powder ratio is 50wt% chromium powder, 38wt% nickel powder, and 12wt% molybdenum powder with a particle size of 75μm. The powder feeding rate is 20g / min, the welding wire feed rate is 10m / min, and the overlap at the arc termination point exceeds the arc initiation point by at least 10mm.

[0100] After the outer weld is completed, the temperature is lowered to 165℃ and held for 2 minutes. Then the temperature is raised to 300℃ and held for 5 minutes before being cooled to room temperature in the furnace. Then the inner weld is welded. The method and parameters for the inner weld are the same as those for the outer weld.

[0101] Repeat the entire process of flange assembly and welding described above to complete the assembly and welding of the flange on the other side of the workpiece. After both flanges are welded, no overall heating or stepped cooling treatment is performed.

[0102] After the workpiece is welded, one piece is randomly selected from each production batch for inspection. The weld position of the selected workpiece is cross-sectioned for inspection. The hardness of the weld is tested using a Vickers hardness tester, and internal defects of the weld are detected using flaw detection equipment. The penetration depth is then calculated.

[0103] The remaining steps, parameters, and detection methods are the same as in Example 1.

[0104] Comparative Example 6 This comparative example is based on Example 1, but in step S2, a 0.05mm clearance fit is used between the flange inner hole and the outer diameter of the central pipe, replacing the original interference fit. Specifically: Step S2: Flange assembly and welding. The flange inner hole and the outer diameter of the center pipe are designed for clearance fit, with a clearance of 0.05mm. Before assembly, heat the flange to 375℃, quickly fit it onto the end of the bridge pipe, and adjust the flange to the target position while it is hot. After the flange cools down, re-inspect the installation dimensions. If the re-inspection is qualified, spot weld it at the joint between the center pipe and the flange.

[0105] Preheat the flange welding area to 280℃ in summer and 320℃ in winter. Use an infrared thermometer to monitor the temperature and ensure that the temperature difference between the welding area and the adjacent area is ≤25℃. After preheating, use an NBC-500 automatic welding machine to weld the outer seam. The welding wire is ER70-G with a diameter of 1.2mm. Use 45° pull welding. The shielding gas is CO2 with a flow rate of 12.5L / min. The welding current is 230A, the voltage is 33V, and the welding speed is 350cm / min. During the welding process, start the powder feeding device simultaneously to send the alloy powder into the molten pool along with the shielding gas. The powder feeding nozzle is aligned with the center of the shielding gas stream in front of the weld pool. The alloy powder ratio is 50wt% chromium powder, 38wt% nickel powder, and 12wt% molybdenum powder with a particle size of 75μm. The powder feeding rate is 20g / min, the welding wire feed rate is 10m / min, and the overlap at the arc termination point exceeds the arc initiation point by at least 10mm.

[0106] After the outer weld is completed, the temperature is lowered to 165℃ and held for 2 minutes. Then the temperature is raised to 300℃ and held for 5 minutes before being cooled to room temperature in the furnace. Then the inner weld is welded. The method and parameters for the inner weld are the same as those for the outer weld.

[0107] Repeat the entire process of flange assembly and welding described above to complete the assembly and welding of the flange on the other side of the workpiece. After both flanges are welded, heat the entire workpiece to 260°C and perform a stepped cooling process: the first stage is to keep it at 80°C for 4 hours, the second stage is to keep it at 60°C for 4 hours, and the third stage is to cool it to room temperature with the furnace.

[0108] After the workpiece is welded, one piece is randomly selected from each production batch for inspection. The weld position of the selected workpiece is cross-sectioned for inspection. The hardness of the weld is tested using a Vickers hardness tester, and internal defects of the weld are detected using flaw detection equipment. The penetration depth is then calculated.

[0109] The remaining steps, parameters, and detection methods are the same as in Example 1.

[0110] For the workpieces in the above embodiments and comparative examples, the runout of the flange end faces and the coaxiality of the flange center holes were tested. The total coating thickness, coating adhesion, and corrosion resistance were also tested. The total coating thickness was measured using a paint film thickness gauge, and the coating adhesion was tested using the cross-cut adhesion test. Corrosion resistance was tested using a 2000-hour neutral salt spray test chamber. The test data are summarized below:

[0111] In addition, surface and internal hardness tests, weld penetration depth detection, and internal defect detection were performed on the workpieces of the above embodiments and comparative examples. The hardness test was conducted using a Vickers hardness tester, and the weld penetration depth and internal defects were detected using flaw detection equipment. The test results are summarized below:

[0112] By analyzing and comparing the above data, the following conclusions can be drawn: Example 1, as the preferred embodiment, exhibits the best performance across all aspects. The surface hardness of the weld is 282 HV, and the internal hardness is 279 HV, with stable values ​​that meet the requirements of 190–380 HV. This is mainly due to the alloy powder ratio of 50 wt% chromium powder, 38 wt% nickel powder, and 12 wt% molybdenum powder, which forms a uniform solid solution strengthening and dispersed strengthening phase in the weld pool. The matching of the powder feeding rate of 20 g / min and the wire feeding rate of 10 m / min ensures the effective transition of alloying elements. The weld penetration is 2.53 mm, and the weld is free of any defects, proving that the parameter combination of welding current of 230 A, voltage of 33 V, and speed of 350 cm / min provides sufficient heat input and good weld pool fluidity.

[0113] The flange end face runout is 0.03mm and the coaxiality is 0.05mm, which is attributed to the pre-stress generated by the 0.20mm interference fit, which effectively suppressed welding deformation. The total coating thickness is 193μm, the adhesion is grade 0, and the salt spray resistance exceeds 2500h. This is because the 55μm zinc-aluminum coating provides sufficient sacrificial anode protection, and the sealing paint sprayed within 30 minutes at the workpiece residual temperature of 65℃, with the help of the capillary effect and the temperature gradient, allows the 18μm penetrating epoxy sealing paint to fully penetrate into the pores of the zinc-aluminum layer after drying, forming a tight sealing layer and a mechanically anchored substrate. This allows the subsequent epoxy primer and polyester topcoat to achieve optimal adhesion, and the composite coating system completely shields the corrosive medium.

[0114] Example 2 uses the upper limit combination of parameters, and the weld hardness increases to 328 HV on the surface and 331 HV inside, close to the required upper limit. This is because the ratio of 55wt% chromium, 40wt% nickel, and 15wt% molybdenum in the alloy powder brings more carbide-forming elements and solid solution strengthening elements into the molten pool. The high powder feeding rate of 25g / min and the high powder feeding rate of 12m / min further increases the alloying degree of the weld metal; the penetration depth is 2.78mm, which is deeper than that of Example 1, achieved with a welding current of 240A, a voltage of 34V, and a speed of 370°C. The direct result of the high linear energy input of m / min: the total coating thickness is 220μm, the adhesion remains at level 0, and the salt spray test exceeds 2100h, indicating that under the conditions of a 60μm zinc-aluminum layer and a workpiece temperature of 80℃, the sealing paint with a solid content of 20wt% can still effectively penetrate and form a tight sealing layer. However, the salt spray life is slightly lower than that of Example 1, indicating that under the upper limit combination of parameters, although the salt spray performance far exceeds the requirement of 2000h, it is slightly lower than that of Example 1. This confirms that the combination of process parameters used in Example 1 has a better overall effect.

[0115] Example 3 uses the lower limit combination of parameters, resulting in a weld hardness of 212 HV on the surface and 208 HV internally, falling within the low range of 190–380 HV. This is a result of the low alloying ratio of 40 wt% chromium, 30 wt% nickel, and 5 wt% molybdenum in the alloy powder, along with a low powder feeding rate of 15 g / min and a wire feed rate of 8 m / min. The reduced content of strengthening elements in the molten pool weakens both solid solution strengthening and precipitation strengthening effects. The weld penetration is 2.14 mm, slightly lower than in Example 1, corresponding to a welding current of 220 A, a voltage of 32 V, and a speed of 33. The low energy input was 0 cm / min; the total coating thickness was 150 μm, the adhesion was grade 1, and the salt spray test exceeded 2000 h but the margin was small. This was because the sacrificial anode capacity of the 40 μm zinc-aluminum layer was relatively limited, the penetration driving force of the sealing paint with 20 wt% solids content was weak at the workpiece temperature of 40℃, the density of the 10 μm sealing layer decreased, and the total thickness of the primer and topcoat of 25 μm just met the protection requirements. Therefore, the protection life window of the entire coating system was narrowed. Although all indicators still met the requirements, they were all at the boundary, which proved the rationality of the lower limit of the process parameter window.

[0116] In Comparative Example 1, after the powder feeding device was shut off, the weld hardness plummeted to 168 HV on the surface and 172 HV inside, far below the lower limit requirement of 190 HV. This was because the molten pool lacked the alloying effect of chromium, nickel, and molybdenum, and the weld microstructure was mainly low-carbon ferrite and a small amount of pearlite, lacking solid solution strengthening and carbide dispersion strengthening phases. Although the penetration depth was still 2.31 mm, microcracks were detected inside the weld. This was because the lack of alloying elements reduced the oxidation resistance and fluidity of the molten pool, and the weld metal had insufficient strength and toughness reserves, resulting in microcracks under cooling shrinkage stress. These microcracks became corrosion initiation points in the 500-hour salt spray test, leading to rust at the weld, far below the requirement of 2000 hours, fully demonstrating the importance of the synchronous powder feeding process for the mechanical properties and corrosion resistance of the weld.

[0117] In Comparative Example 2, after completely replacing molybdenum powder with nickel powder, the weld hardness decreased to 218 HV on the surface and 221 HV inside, which is still within the acceptable range. However, hot cracks were detected inside the weld. This is because molybdenum is a strong carbide-forming element. The stable carbides it forms at high temperatures can pin grain boundaries and inhibit grain boundary slip, making it a key element for improving the weld's resistance to hot cracking. In the absence of molybdenum, during the step cooling process after welding, low-melting-point eutectics agglomerate at the grain boundaries, and the grain boundary strength is insufficient to resist thermal shrinkage stress, thus generating hot cracks. Rust appeared in the weld area after 1200 hours of salt spray testing, which is significantly earlier than the 2500 hours in Example 1. This proves that molybdenum not only improves the resistance to hot cracking, but the stable carbides and solid solutions it forms also significantly enhance the weld's resistance to electrochemical corrosion.

[0118] Comparative Example 3 involved directly spraying an epoxy primer after the zinc-aluminum coating, omitting the penetrating epoxy sealer. The coating adhesion dropped to level 2, and blistering and rust spots appeared after 1000 hours of salt spray testing. The failure mechanism lies in the numerous micron-sized pores on and inside the arc-sprayed zinc-aluminum coating. Without sealing treatment, the subsequent primer cannot effectively penetrate and fill these pores, forming only a covering film on the zinc-aluminum surface. When corrosive media penetrate the epoxy primer, they directly contact the substrate through the pores of the zinc-aluminum layer. The corrosion products generated when the zinc-aluminum in the pores forms a corrosion galvanic cell with the substrate expand in volume, causing the primer to blister and peel off, thus losing its protective function. The absence of the sealer causes the coating system to lose its key functions of sealing pores, forming a dense transition layer, and mechanically anchoring the substrate, thus significantly shortening the protective lifespan.

[0119] In Comparative Example 4, after the zinc-aluminum coating was applied, the workpiece was completely cooled to room temperature and left for 2 hours before the sealing paint was applied. The coating adhesion dropped to level 1, and multiple pitted rust spots appeared on the surface after 1500 hours of salt spray testing. The failure was caused by the significant increase in viscosity of the sealing paint under low temperature conditions, which resulted in the loss of the temperature gradient driving force formed by the residual heat of the workpiece. At the same time, the capillary adsorption effect generated by the cooling and contraction of hot air in the pores of the zinc-aluminum layer was lost. The sealing paint mainly remained on the surface of the zinc-aluminum layer to form a film, failing to effectively penetrate into the pores to form a deep seal. Although it still had a thickness of 18μm after drying, most of it was a surface deposit layer and did not form an effective mechanical anchor. The pitted rust spots were randomly distributed in the weakly sealed pore areas, indicating that as long as the temperature and time window deviated from the requirements, even if the same sealing paint formula was used, it would be impossible to achieve deep penetration and effective sealing. This proves that the workpiece temperature of 40-80℃ and the spraying within 30 minutes are the two key synergistic process elements.

[0120] Comparative Example 5 eliminated post-weld stepped cooling and directly air-cooled the workpiece to room temperature. The weld hardness showed severe unevenness, fluctuating between 246 and 405 HV on the surface and between 251 and 398 HV internally. Discontinuous microcracks were also detected. This was due to the high local heat concentration during flange welding, resulting in significant thermal stress in the weld and heat-affected zone under rapid air cooling. This caused uneven phase transformation in the microstructure, with some areas forming coarse, brittle martensite leading to excessive hardness, while other areas experienced slower cooling and lower hardness. The discontinuous microcracks were the result of the combined effects of welding thermal stress and the brittle-hard microstructure. During the 1600-hour salt spray test, rust appeared at the weld. Although the coating system was normal, the microcracks inside the weld became channels for corrosive media to penetrate the substrate, leading to early corrosion. This demonstrates the necessity of post-weld stepped cooling for eliminating residual stress, refining the microstructure, and preventing cracking.

[0121] In Comparative Example 6, after replacing the interference fit with a 0.05mm clearance fit, the flange end face runout worsened to 0.07mm, and the coaxiality worsened to 0.10mm, both exceeding the accuracy requirements of 0.05mm and 0.08mm, respectively. This is because, without the pre-stress generated by the interference fit, there is a lack of rigid constraint between the flange and the central tube before welding. The thermal stress generated during welding causes the flange to undergo greater free deformation, and the machining allowance is insufficient to completely correct it. Weld inspection revealed a defect of incomplete fusion at the root of the joint surface. This was due to the small gap between the flange and the central tube caused by the clearance fit. During welding, the molten pool metal failed to effectively wet and fill this gap, resulting in incomplete fusion at the root. Rust appeared at the joint surface during the 1800h salt spray test. The corrosion originated and expanded from this incomplete fusion defect, proving that the interference fit is not only a means of assembly positioning but also a key factor in controlling welding deformation, ensuring the welding quality of the joint surface, and the long-term reliability of the structure.

[0122] It should be understood that the embodiments of the present invention described above do not exhaustively describe all details, nor does it mean that the present invention is limited to the specific embodiments disclosed. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A manufacturing process for high-strength, corrosion-resistant welded bridges, characterized in that, Includes the following steps: Step S1: Center tube forming, The central tube is placed in a bending die and bent into shape. After forming, the dimensions of the workpiece are inspected, and qualified products are transferred to the next stage. Step S2: Flange assembly and welding. After heating the flange, it is inserted into the end of the center tube. While it is still hot, the flange is adjusted to the target position. After the flange cools down, the installation dimensions are re-inspected. If the re-inspection is qualified, spot welding is performed to fix the center tube and flange at the joint. Preheat the flange welding area and use an automatic welding machine to weld the outer weld. The welding adopts gas metal arc welding. During the welding process, the powder feeding device is started simultaneously to send alloy powder containing chromium, nickel and molybdenum into the molten pool along with the shielding gas. When the arc is closed, the overlap exceeds the arc starting point by at least 10mm. After the outer weld is completed, the temperature is first lowered to 150-180℃ and held, then the temperature is raised to 280-320℃ and held, and then cooled to room temperature in the furnace. The welding method of the inner weld is the same as that of the outer weld. Repeat the entire process of flange assembly and welding described above to complete the assembly and welding of the flange on the other side of the workpiece. After both flanges are welded, heat the entire workpiece to 260°C and then cool it down to room temperature in stages. Step S3: Welding of accessories, The workpiece is transferred to the accessory welding station for spot welding and fixation. After spot welding is completed, additional welding is performed. After additional welding, the position of the accessory is inspected. After the inspection is passed, it is ground and cleaned. Step S4: End machining, The end faces of the flanges at both ends are machined, and the key dimensions are inspected after machining. Step S5: Shot blasting and aging treatment. The workpiece is shot blasted, and after shot blasting, it is subjected to low-temperature tempering. After tempering, the surface condition of the workpiece is checked. Step S6: Surface spraying, After plasma cleaning of the workpiece surface and installation of protective sleeves in non-sprayed areas, zinc-aluminum coating is sprayed onto the surface of the central tube and accessories using an electric arc spraying process. After the zinc-aluminum coating is applied, a penetrating epoxy sealant is sprayed within 30 minutes at a workpiece surface temperature of 40-80℃. After the sealant has dried completely, an epoxy primer and a polyester topcoat are sprayed in sequence. After the coating has dried completely, it is tested. If the test is qualified, it is packaged and put into storage.

2. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, In step S2, the flange inner hole and the outer diameter of the central pipe are designed to be an interference fit with an interference amount of 0.10 to 0.30 mm, and the flange heating temperature is 350 to 400℃.

3. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, In step S2, when preheating the flange welding area, the preheating temperature is: 260-300℃ in summer and 300-340℃ in winter. Infrared temperature gun is used for monitoring, and it is ensured that the temperature difference between the welding area and the adjacent area is not greater than 25℃.

4. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, In step S2, the alloy powder is formulated as follows: 40-55 wt% chromium powder, 30-40 wt% nickel powder, and 5-15 wt% molybdenum powder. The powder particle size is 50-100 μm, the powder feeding rate is 15-25 g / min, and the powder feeding nozzle is aligned with the center of the protective gas stream in front of the weld pool.

5. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, During welding in step S2, the welding machine model is NBC-500, the welding wire model is ER70-G, the welding wire diameter is 1.2mm, the welding wire feed rate is 8~12m / min, 45° pull welding is used, the shielding gas is CO2 with a flow rate of 10~15L / min, the welding current is 220~240A, the voltage is 32~34V, and the welding speed is 330~370cm / min.

6. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, In step S2, after the weld is completed, when the temperature is lowered to 150-180℃, it needs to be held for 2-5 minutes. Then, when the temperature is raised to 280-320℃, it needs to be held for 5-10 minutes. The stepped cooling process is as follows: the first stage involves cooling to 80°C and holding for 4 hours; the second stage involves cooling to 60°C and holding for 4 hours; and the third stage involves cooling the furnace to room temperature.

7. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, In step S5, the parameters for shot blasting are: steel shot size 0.6–1.0 mm, projection speed 80–120 m / s, shot blasting time 4–6 min, and coverage 200%–240%. The tempering temperature for low-temperature tempering is 180–220℃, the holding time is 1–2 hours, and the furnace is cooled to room temperature after the holding time is completed.

8. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, In step S6, the zinc-aluminum coating has a zinc content of 85 wt%, an aluminum content of 15 wt%, and a coating thickness of 40–60 μm.

9. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, In step S6, the penetrating epoxy sealing varnish is prepared by uniformly mixing component A and component B in a weight ratio of 4:

1. Component A is prepared by mixing 25-35 parts by weight of bisphenol A type epoxy resin, 5-10 parts by weight of phenolic epoxy resin, 10-15 parts by weight of C12-C14 glycidyl ether reactive diluent, 28-36 parts by weight of xylene and 7-9 parts by weight of n-butanol. Component B is prepared by mixing 40-50 parts by weight of modified polyamide curing agent, 2-5 parts by weight of accelerator DMP-30, 36-44 parts by weight of xylene and 9-11 parts by weight of n-butanol; The modified polyamide curing agent is prepared by adding 10% to 20% of bisphenol A type epoxy resin by mass to polyamide resin and reacting at 60 to 80°C for 2 hours. The solids content of the penetrating epoxy sealing varnish is 18-22 wt%, and the thickness after drying is 10-20 μm.

10. The manufacturing process for high-strength corrosion-resistant welded bridges according to claim 1, characterized in that, In step S6, the epoxy primer is sprayed in two coats, each coat being 25-35 μm thick, and each coat is surface dry for 25-35 minutes after spraying. The polyester topcoat is applied in two coats, each coat being 25–35 μm thick, and each coat takes 25–35 minutes to dry to dry.

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