Welding method of high-nickel finned tube

Abstract

The invention discloses a welding method of a high-nickel finned tube and relates to the technical field of high-nickel finned tube welding. The high-nickel finned tube comprises fins, a transition layer and a base tube, the fins are made of high-nickel alloy 2.4879 or 2.4878, the transition layer is made of austenitic stainless steel 304, and the base tube is made of carbon steel Rst37-2; the welding method adopts direct current positive connection TIG welding, namely tungsten electrode inert gas shielded welding, and comprises the following steps: S1, preparation before welding; s2, layered welding; s3, post-welding treatment; and S4, quality detection. According to the welding method, the problems of thermal stress cracks, cold shut and air holes of dissimilar material welding are effectively solved through stepped groove, standardized cleaning, layered welding and precise postweld treatment. The high-plasticity welding wire and an adaptive process absorb shrinkage stress, the fusion compatibility is optimized, and whole-course protection is strengthened. The mechanical property and corrosion resistance of the finished finned tube reach the standard, stable welding quality is guaranteed, and the finished finned tube is adaptive to complex working conditions.

Description

Technical Field

[0001] This application relates to the field of high-nickel finned tube welding technology, and in particular to a welding method for high-nickel finned tubes. Background Technology

[0002] High-nickel finned tubes are high-efficiency heat exchange elements specifically designed for extreme hot and corrosive environments. Their core structure consists of high-nickel alloy fins, an austenitic stainless steel transition layer, and a carbon steel base tube. The fins significantly improve heat transfer efficiency by increasing the heat exchange surface area. They can withstand temperatures up to 1150℃ and are widely used in critical devices such as petrochemical cracking furnaces, nuclear power steam generators, and waste heat recovery systems. They are a core component ensuring equipment safety and energy efficiency under high-temperature operating conditions.

[0003] During the production of high-nickel finned tubes, specialized welding equipment is required. First, the metal fins are tightly wound around the surface of the base tube at specific intervals and angles. Then, the tube is fixed using a welding base and welded. The welding process typically employs efficient and reliable methods such as high-frequency welding and argon arc welding to ensure uniform welds and good sealing, thereby improving the overall durability and heat exchange efficiency of the finned tube.

[0004] Existing welding methods suffer from significant differences in the thermal expansion coefficients of dissimilar materials during welding. This leads to uneven thermal stress during the cooling process, which in turn induces welding cracks and affects the structural integrity of the components. Furthermore, a single bevel cannot meet the fusion requirements of different materials. In addition, the rapid heat loss on the carbon steel side can easily result in insufficient fusion of materials, leading to cold shut defects. During welding, metallurgical defects such as porosity can easily occur due to oxidation and decomposition of impurities, which significantly reduces the density and mechanical properties of the weld. Summary of the Invention

[0005] The purpose of this application is to provide a welding method for high-nickel finned tubes in order to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, this application specifically adopts the following technical solution:

[0007] A welding method for a high-nickel finned tube, the high-nickel finned tube comprising fins, a transition layer, and a base tube, wherein the fins are made of high-nickel alloy 2.4879 or 2.4878, the transition layer is made of austenitic stainless steel 304, and the base tube is made of carbon steel Rst37-2; the welding method employs direct current positive polarity TIG welding, i.e., tungsten inert gas welding, and the method includes the following steps:

[0008] Preparations before S1 welding:

[0009] a. Beveling: A stepped transition bevel is adopted and formed by milling. The overall depth of the stepped transition bevel is 2.0mm-2.5mm and does not exceed 1 / 2 of the thickness of the base material. Beveling includes the bottom mating surface of the high-nickel alloy fins, the upper and lower mating surfaces of the austenitic stainless steel 304 transition layer, and the top mating surface of the carbon steel base tube.

[0010] b. Surface cleaning: Mechanically clean the bevel and the area within 20mm on both sides, and then perform chemical cleaning as needed to ensure that the surface is free of oxide scale, oil, moisture and residual impurities;

[0011] c. Assembly and positioning: Heat-resistant alloy tooling is used to coaxially fix the fins, transition layer and base tube. Stainless steel shims are placed at the contact points between the clamping tooling and the base material. Fixing is done by tack welding. The weld point spacing is 50-80mm and the weld point length is ≤8mm.

[0012] S2 layered welding:

[0013] a. First self-fusion welding: No welding wire welding, using ultra-low heat input to form a transition layer, welding current 50-60A, arc voltage 8-9V, welding speed 100-120mm / min, welding torch shielding gas flow rate 6-8L / min, back shielding gas is circulated inside the base tube, flow rate 4-5L / min, forced air cooling after welding until interpass temperature ≤100℃;

[0014] b. Secondary fine welding: Use nickel-based transition welding wire ERNiCr-3 for cover welding to form a stress buffer layer. The welding current is 80-90A, the arc voltage is 10-11V, the welding speed is 70-80mm / min, the filler wire speed is 35-40mm / min, the shielding gas flow rate of the welding torch is 10-12L / min, the back shielding gas flow rate is 5-6L / min, and argon is continuously passed through until the weld cools to below 200℃.

[0015] S3 post-welding treatment:

[0016] The welded finned tubes are subjected to stress-relief annealing, followed by surface cleaning to remove oxide scale, spatter, and residual impurities.

[0017] S4 Quality Inspection:

[0018] The welded finned tubes are subjected to visual inspection, radiographic inspection, and weld shear strength testing.

[0019] By adopting the above technical solution, this welding method clearly defines the materials of the fins, transition layer, and tube. DC positive polarity TIG welding is used during welding. Before welding, a stepped transition beveling process, standardized surface cleaning, and positioning and fixing with heat-resistant alloy tooling are performed. Layered welding is conducted during welding, including an initial self-fusion welding without welding wire and a secondary precision welding with ERNiCr-3 welding wire. Pure argon gas protection is used throughout the process. After welding, stress-relief annealing and surface cleaning are performed. This welding method effectively solves the problems of welding cracks, cold shuts, and porosity in dissimilar materials, ensuring the quality and stability of the weld base and improving the overall quality of the finned tube.

[0020] Furthermore, in S1a, the specific parameters of the stepped transition bevel are as follows: the bevel angle on the carbon steel base tube side is 30°, the step height is 1.5±0.1mm, and the blunt edge dimension is 0.5±0.1mm; the bevel angle on the high-nickel alloy fin side is 25°, and the assembly gap is 0.05-0.10mm; the bevel angle of the austenitic stainless steel 304 transition layer is 35°, and the overlap with the carbon steel base tube side is 1.0±0.1mm.

[0021] By adopting the above technical solutions and precisely quantifying the geometric parameters of the bevel, stress concentration and insufficient fusion caused by manual processing or dimensional deviations are avoided. The bevel is designed to address the differences in thermal properties of different materials, further optimizing the stress dispersion effect of the gradient interface. This reduces the occurrence of cracks and cold shut defects at the structural level, ensuring the consistency of bevel processing accuracy and improving process repeatability and standardization.

[0022] Furthermore, in S1b, mechanical cleaning involves polishing with a stainless steel wire brush or 180-grit sandpaper until a metallic luster is exposed, followed by wiping with anhydrous ethanol; chemical cleaning involves acid washing with a mixture of nitric acid and hydrofluoric acid in a volume ratio of 3:1 for 5-8 minutes, followed by repeated rinsing with clean water and drying at 80-100℃.

[0023] By adopting the above technical solutions and standardizing cleaning procedures, impurities such as oxide scale, oil, and moisture on the bevel surface can be thoroughly removed, thereby reducing porosity and cold shuts. Eliminating impurities at the source enhances the surface activity of the base material, improves the fusion compatibility between dissimilar materials, and provides a clean and stable bonding surface for subsequent welding, thus facilitating the welding process.

[0024] Furthermore, in S1c, the welding current for tack welding is controlled at 60-70A. After tack welding, the weld joint is inspected, and formal welding can only be carried out after confirming that there are no cracks or pores.

[0025] By adopting the above technical solutions and limiting the welding current, it is possible to avoid excessive tack welding current that could lead to local overheating and prevent cracks from forming in the tack welding itself. Through pre-inspection of weld quality, defects introduced in the tack welding process can be eliminated in advance, preventing the defects from expanding during the formal welding process. At the same time, it further stabilizes the coaxial position of the workpiece, provides a precise benchmark for layered welding, and ensures the uniformity of subsequent weld formation.

[0026] Furthermore, the tungsten electrode used in the TIG welding is a cerium tungsten electrode (WCe20) with a diameter of 2.4 mm and a 30° cone angle at the front end; the purity of the pure argon gas is ≥99.99%.

[0027] By adopting the above technical solutions, cerium-tungsten electrodes have stronger arc stability and better burn-off resistance than pure tungsten electrodes, which can reduce tungsten inclusion defects caused by tungsten electrode burn-off during welding; high-purity argon gas can enhance the protection effect of the molten pool and high-temperature weld, avoid porosity and oxide scale defects caused by air oxidation, and adapt to the stringent requirements of heat source stability and protection effect for welding dissimilar materials.

[0028] Furthermore, in S2a, the first self-fusion welding uses high-frequency arc initiation and gradual arc attenuation, filling the arc crater when the arc is terminated; during the welding process, the arc length is ≤1.5mm, and it quickly passes through the sensitization temperature zone of 600-1000℃.

[0029] By adopting the above technical solutions, high-frequency arc initiation avoids contact between the tungsten electrode and the base material, eliminating tungsten inclusion defects; arc attenuation can fill the arc crater and prevent the formation of arc crater cracks; short-arc welding improves the controllability of the molten pool and avoids cold shuts caused by molten pool deviation; rapid passage through the sensitization temperature zone reduces the precipitation of carbides at the grain boundaries of high-nickel alloys and stainless steel, reduces the tendency of intergranular corrosion, and indirectly improves the crack resistance of the weld.

[0030] Furthermore, in the S2b, the elongation of the nickel-based transition welding wire ERNiCr-3 is ≥35%, and the S and P contents are ≤0.01%. Before use, the welding wire is dried by keeping it at 200-250℃ for 1 hour. After drying, it is placed in an insulated container and taken out as needed. The exposure time to air does not exceed 30 minutes. When filling the wire, it is fed in from the front edge of the molten pool to avoid contact with the electric arc.

[0031] By adopting the above technical solutions, the high-plasticity welding wire ensures the deformation capacity of the stress buffer layer, effectively absorbs shrinkage stress, and prevents crack propagation; the low sulfur and phosphorus content reduces the induction of hot cracks; the drying treatment removes moisture from the surface of the welding wire and avoids porosity; the standardized wire filling method prevents wire jamming defects, while ensuring that the welding wire is fully melted, achieving complete coverage and defect filling of the self-fusion weld, and strengthening the weld density.

[0032] Furthermore, in S3, the stress-relief annealing process involves holding at 700-800℃ for 1-2 hours followed by air cooling; for applications requiring extremely high corrosion resistance, a solution treatment is performed by holding at 1050-1100℃ for 1 hour followed by water cooling, and the annealing fixture is made of heat-resistant alloy material.

[0033] By adopting the above technical solutions, the above annealing process can accurately eliminate welding residual stress and avoid the generation of delayed cracks after welding; solution treatment can solve the problem of intergranular corrosion and improve the corrosion resistance of the weld; heat-resistant alloy tooling avoids carburizing and contamination reaction between the tooling and the base material, prevents impurities from being introduced into the pores, and adapts to the performance requirements of welds under different working conditions.

[0034] Furthermore, in S3, the surface cleaning includes mechanical cleaning and chemical cleaning: mechanical cleaning uses a stainless steel wire brush to remove oxide scale and spatter, and grinds the weld seam; chemical cleaning uses a mixture of nitric acid and hydrofluoric acid with a volume ratio of 3:1 for pickling, followed by rinsing with clean water and drying at 80-100℃.

[0035] By adopting the above technical solution, the above treatment method can remove oxide scale and spatter from the weld surface, ensuring the surface quality of the weld; avoid oxide scale covering affecting the accuracy of subsequent non-destructive testing, ensuring that defects can be effectively identified; and at the same time improve the corrosion resistance of the weld surface, avoiding corrosion failure caused by surface impurities during subsequent use.

[0036] Furthermore, in S4, the visual inspection includes weld smoothness, absence of cracks, porosity, cold shuts and other defects, and the perpendicularity deviation between the fins and the base tube is ≤0.5°; radiographic testing (RT) is performed in accordance with JB / T4730.2, the weld quality grade is ≥II; the weld shear strength is ≥80% of the lowest value of the base material, and the tensile strength is ≥450MPa.

[0037] By adopting the above technical solution and conducting multiple tests on the finned tubes, it is possible to directly verify whether the process meets the requirements. Through multi-dimensional testing, it is ensured that the mechanical properties and working conditions of the weld meet the standards, thus avoiding the use of unqualified products and ensuring the consistency of mass production quality.

[0038] In summary, this application includes at least one of the following beneficial effects;

[0039] 1. In this application, the stepped transition bevel transforms the traditional abrupt interface between dissimilar materials into a gradual interface, dispersing stress concentration during cooling and contraction. Furthermore, the thermal expansion coefficient of the 304 austenitic stainless steel transition layer falls between that of high-nickel alloys and carbon steel, mitigating stress abrupt changes caused by differences in thermal expansion coefficients. The initial self-fusion welding employs ultra-low heat input and high welding speed to reduce heat accumulation and grain coarsening. The secondary precision welding uses high-ductility nickel-based welding wire ERNiCr-3 to form a stress buffer layer, absorbing contraction stress through plastic deformation. Combined with strict control of interpass temperature ≤100℃, the occurrence and propagation of hot cracks and delayed cracks are prevented from the source. Through the structural design of the fins, transition layer, and base tube, as well as the layered welding process and the use of suitable materials, welding cracks caused by uneven thermal stress can be eliminated.

[0040] 2. In this application, the stepped bevel design specifically addresses the angles, gaps, and overlaps between the carbon steel and high-nickel alloy sides, increasing the fusion area on the carbon steel side, mitigating rapid heat loss, and preventing insufficient fusion. The initial self-fusion welding without welding wire allows the high-nickel alloy, stainless steel, and carbon steel substrates to melt and form a mixed solid solution transition layer, reducing the wetting angle difference between dissimilar materials and improving weld pool spreadability. The secondary precision welding uses a suitable welding wire to cover and fill, ensuring the weld completely encapsulates the self-fusion transition layer, compensating for any shortcomings in the initial fusion and achieving dense fusion across the entire interface. By optimizing fusion conditions and interface compatibility, full fusion of dissimilar materials can be achieved, eliminating cold shut defects.

[0041] 3. In this application, a combination of mechanical and chemical cleaning is used before welding to effectively remove oxide scale, oil, and moisture from the bevel surface. Simultaneously, the welding wire is dried at 200-250℃ to eliminate gas sources caused by impurities. Furthermore, pure argon gas with a purity ≥99.99% is used throughout the process for protection. Welding torch protection and back-side protection inside the base tube work in tandem. A drag shield is added during the secondary precision welding to extend protection in the high-temperature zone, preventing gas generation from high-temperature oxidation in the weld and heat-affected zone. The initial self-fusion welding avoids metallurgical reactions between the external welding wire and the base material. Low-sulfur, low-phosphorus welding wire is used in the secondary precision welding to reduce harmful gases generated by metallurgical reactions. Simultaneously, the solidification rate of the molten pool is controlled to ensure sufficient gas escape, achieving a weld without porosity. Throughout the welding process, impurities are controlled and welding is optimized, effectively suppressing defects such as porosity. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the finished high-nickel finned tube in this application;

[0043] Figure 2 This is a simplified schematic diagram of the overall process of welding high-nickel finned tubes in this application;

[0044] Figure 3 This is a detailed schematic diagram of the pre-welding preparation process in this application;

[0045] Figure 4 This is a detailed schematic diagram of the layered welding process in this application;

[0046] Figure 5 This is a detailed schematic diagram of the post-weld processing steps in this application;

[0047] Figure 6 This is a schematic diagram showing the breakdown of the quality inspection process in this application. Detailed Implementation

[0048] The following is in conjunction with the appendix Figure 1 -6 provides further details regarding this application.

[0049] This application discloses a welding method for high-nickel finned tubes.

[0050] Reference Figure 1 - Figure 6 A welding method for a high-nickel finned tube, the high-nickel finned tube comprising fins, a transition layer, and a base tube, wherein the fins are made of high-nickel alloy 2.4879 or 2.4878, the transition layer is made of austenitic stainless steel 304, and the base tube is made of carbon steel Rst37-2; the welding method is DC positive polarity TIG welding, i.e., tungsten inert gas welding, and the method includes the following steps:

[0051] Preparations before S1 welding:

[0052] a. Beveling: A stepped transition bevel is adopted and formed by milling. The overall depth of the stepped transition bevel is 2.0mm-2.5mm and does not exceed 1 / 2 of the thickness of the substrate.

[0053] b. Surface cleaning: Mechanically clean the bevel and the area within 20mm on both sides, and then perform chemical cleaning as needed to ensure that the surface is free of oxide scale, oil, moisture and residual impurities;

[0054] c. Assembly and positioning: Heat-resistant alloy tooling is used to coaxially fix the fins, transition layer and base tube. Stainless steel shims are placed at the contact points between the clamping tooling and the base material. Fixing is done by tack welding. The weld point spacing is 50-80mm and the weld point length is ≤8mm.

[0055] S2 layered welding:

[0056] a. First self-fusion welding: No welding wire welding, using ultra-low heat input to form a transition layer, welding current 50-60A, arc voltage 8-9V, welding speed 100-120mm / min, welding torch shielding gas flow rate 6-8L / min, back shielding gas is circulated inside the base tube, flow rate 4-5L / min, forced air cooling after welding until interpass temperature ≤100℃;

[0057] b. Secondary fine welding: Use nickel-based transition welding wire ERNiCr-3 for cover welding to form a stress buffer layer. The welding current is 80-90A, the arc voltage is 10-11V, the welding speed is 70-80mm / min, the filler wire speed is 35-40mm / min, the shielding gas flow rate of the welding torch is 10-12L / min, the back shielding gas flow rate is 5-6L / min, and argon is continuously passed through until the weld cools to below 200℃.

[0058] S3 post-welding treatment:

[0059] The welded finned tubes are subjected to stress-relief annealing, followed by surface cleaning to remove oxide scale, spatter, and residual impurities.

[0060] S4 Quality Inspection:

[0061] The welded finned tubes are subjected to visual inspection, radiographic inspection, and weld shear strength testing.

[0062] Before welding, a stepped transition bevel is machined using CNC milling. The bevel and surrounding area undergo standardized cleaning. The workpiece is then coaxially fixed using heat-resistant alloy tooling, and tack welding is performed. During welding, a layered process is employed: first, a transition layer is formed through self-fusion without welding wire and ultra-low heat input; then, an ERNiCr-3 welding wire is used for precision welding to form a stress buffer layer, all under pure argon gas protection. Finally, stress-relief annealing and surface cleaning complete the welding process. This method features a closed-loop process with smooth transitions, effectively solving problems such as cracking, cold shuts, and porosity in welding dissimilar materials, ensuring the basic strength and stability of the weld, and adapting to complex working conditions.

[0063] In S1a, the specific parameters of the stepped transition bevel are as follows: the bevel angle of the carbon steel base tube side is 30°, the step height is 1.5±0.1mm, and the blunt edge dimension is 0.5±0.1mm; the bevel angle of the high nickel alloy fin side is 25°, and the assembly gap is 0.05-0.10mm; the bevel angle of the austenitic stainless steel 304 transition layer is 35°, and the overlap with the carbon steel base tube side is 1.0±0.1mm.

[0064] By clearly defining the bevel angles, step heights, blunt edge dimensions, assembly gaps, and tolerance requirements for the carbon steel base tube side, the high-nickel alloy fin side, and the 304 stainless steel transition layer side, the welding conditions for dissimilar materials are further optimized at the structural level. This avoids stress concentration caused by bevel dimensional errors (reducing the risk of cracking), and also designs bevel parameters according to the differences in the thermal properties of different materials, increasing the fusion area and improving the molten pool spreading space, effectively avoiding the problem of insufficient fusion (cold shut), while ensuring the consistency of bevel machining accuracy, providing a stable structural benchmark for subsequent layered welding.

[0065] In S1b, mechanical cleaning involves polishing with a stainless steel wire brush or 180-grit sandpaper until the metal luster is exposed, followed by wiping with anhydrous ethanol. Chemical cleaning involves acid washing with a mixture of nitric acid and hydrofluoric acid in a volume ratio of 3:1 for 5-8 minutes, followed by repeated rinsing with clean water and drying at 80-100℃.

[0066] By clearly defining the specific operations and parameters for mechanical cleaning (using a stainless steel wire brush or 180-grit sandpaper for polishing and wiping with anhydrous ethanol) and chemical cleaning (using a 3:1 mixture of nitric acid and hydrofluoric acid for pickling, rinsing, and drying), impurities such as oxide scale, oil, and moisture on the bevel surface can be thoroughly removed, reducing gas generation (porosity) during welding and addressing poor wettability of the weld pool (cold shut). Simultaneously, it enhances the surface activity of the base material, improves the fusion compatibility between dissimilar materials, and creates a clean and stable bonding surface for layered welding, reducing the probability of defects from the source.

[0067] In S1c, the welding current for tack welding is controlled at 60-70A. After tack welding, the weld joint is inspected, and formal welding can only be carried out after confirming that there are no cracks or pores.

[0068] The tungsten electrode used in TIG welding is a cerium tungsten electrode (WCe20) with a diameter of 2.4 mm and a 30° cone angle at the front end; the purity of the pure argon gas is ≥99.99%.

[0069] In addition, in S2a, the first self-fusion welding uses high-frequency arc initiation and gradual arc decay, filling the arc crater when the arc is extinguished; the arc length during welding is ≤1.5mm, and it quickly passes through the sensitization temperature range of 600-1000℃.

[0070] Furthermore, in S2b, the elongation of the nickel-based transition welding wire ERNiCr-3 is ≥35%, and the S and P contents are ≤0.01%. Before use, the welding wire is dried by keeping it at 200-250℃ for 1 hour. After drying, it is placed in an insulated container and taken out as needed. The exposure time to air should not exceed 30 minutes. When filling the wire, it is fed from the front edge of the molten pool to avoid contact with the electric arc.

[0071] By controlling the tack welding current and strengthening the pre-inspection of weld quality, cracks and porosity are avoided during the tack welding process, while the workpiece position is stabilized to ensure the accuracy of subsequent welding references. Parameter limitations on high-purity argon gas and dedicated cerium-tungsten electrodes improve arc stability, reduce tungsten electrode burn-off, avoid tungsten inclusion defects, strengthen the protection of the molten pool and high-temperature weld, and reduce the risk of porosity. Requirements such as high-frequency arc ignition, short-arc welding, and rapid passage through the sensitization temperature zone eliminate tungsten inclusions and crater cracks, reduce intergranular corrosion tendency, further optimize the transition layer fusion effect, and avoid cold shut issues. Limitations on the performance, processing methods, and filler specifications of ERNiCr-3 welding wire ensure the plasticity and density of the stress buffer layer, effectively absorb shrinkage stress, prevent cracking, and avoid problems such as moisture introduction and wire inclusion, reducing porosity and slag inclusions. Through meticulous control of the entire process, including tack welding, TIG welding consumables, the initial self-fusion welding operation, and the secondary precision welding wire stage, precise defect prevention and control are achieved.

[0072] In S3, the stress-relief annealing process involves holding at 700-800℃ for 1-2 hours followed by air cooling; for applications requiring extremely high corrosion resistance, a solution treatment is performed by holding at 1050-1100℃ for 1 hour followed by water cooling. The annealing fixtures are made of heat-resistant alloy material.

[0073] In S3, surface cleaning includes mechanical cleaning and chemical cleaning: mechanical cleaning uses a stainless steel wire brush to remove oxide scale and spatter, and grinds the weld seams; chemical cleaning uses a mixture of nitric acid and hydrofluoric acid in a volume ratio of 3:1, followed by pickling with clean water and drying at 80-100℃.

[0074] In addition, in S4, the visual inspection includes weld smoothness, absence of cracks, porosity, cold shuts and other defects, and the perpendicularity deviation between the fins and the base tube is ≤0.5°; radiographic testing (RT) shall be performed in accordance with JB / T4730.2, the weld quality grade shall be ≥II; the weld shear strength shall be ≥80% of the lowest value of the base material, and the tensile strength shall be ≥450MPa.

[0075] By differentiating the parameters of conventional annealing and solution treatment under high corrosion resistance conditions, residual welding stress can be precisely eliminated, delayed cracking can be avoided, and corrosion resistance can be improved. By thoroughly removing oxide scale and spatter, the surface quality of the weld can be guaranteed, facilitating subsequent non-destructive testing, while also improving the corrosion resistance of the weld. After welding, by conducting visual inspection, radiographic inspection, and mechanical property testing of the finned tube, it is possible to determine whether there are cracks, cold shuts, or porosity issues in the finned tube, ensuring that the weld performance meets the standards and providing a quality basis for mass production.

[0076] Working Principle: Before welding, the bottom mating surfaces of the high-nickel alloy fins, the upper and lower mating surfaces of the austenitic stainless steel 304 transition layer, and the top mating surface of the carbon steel base tube are beveled by milling. Then, the bevel and a 20mm area on both sides are mechanically cleaned, followed by chemical cleaning as needed. Heat-resistant alloy fixtures are used to coaxially fix the fins, transition layer, and base tube. Stainless steel shims are placed at the contact points between the fixtures and the base material, and the parts are fixed by tack welding. The first self-fusion welding is then performed, using ultra-low heat input to form the transition layer. After the self-fusion welding, a second precision welding is performed, using nickel-based transition welding wire ERNiCr-3 for a cover welding to form a stress buffer layer. After welding, the welded finned tube undergoes stress-relief annealing, followed by surface cleaning to remove oxide scale, spatter, and residual impurities. Finally, the welded finned tube undergoes visual inspection, radiographic inspection, and weld shear strength testing. This yields a high-nickel finned tube that meets the requirements.

Claims

1. A welding method for high-nickel finned tubes, characterized in that: The high-nickel finned tube includes fins, a transition layer, and a base tube. The fins are made of high-nickel alloy 2.4879 or 2.4878, the transition layer is made of austenitic stainless steel 304, and the base tube is made of carbon steel Rst37-2. The welding method is DC positive polarity TIG welding, i.e., tungsten inert gas welding, which includes the following steps: Preparations before S1 welding: a. Beveling: A stepped transition bevel is adopted and formed by milling. The overall depth of the stepped transition bevel is 2.0mm-2.5mm and does not exceed 1 / 2 of the thickness of the substrate. b. Surface cleaning: Mechanically clean the bevel and the area within 20mm on both sides, and then perform chemical cleaning as needed to ensure that the surface is free of oxide scale, oil, moisture and residual impurities; c. Assembly and positioning: Heat-resistant alloy tooling is used to coaxially fix the fins, transition layer and base tube. Stainless steel shims are placed at the contact points between the clamping tooling and the base material. Fixing is done by tack welding. The weld point spacing is 50-80mm and the weld point length is ≤8mm. S2 layered welding: a. First self-fusion welding: No welding wire welding, using ultra-low heat input to form a transition layer, welding current 50-60A, arc voltage 8-9V, welding speed 100-120mm / min, welding torch shielding gas flow rate 6-8L / min, back shielding gas is circulated inside the base tube, flow rate 4-5L / min, forced air cooling after welding until interpass temperature ≤100℃; b. Secondary fine welding: Use nickel-based transition welding wire ERNiCr-3 for cover welding to form a stress buffer layer. The welding current is 80-90A, the arc voltage is 10-11V, the welding speed is 70-80mm / min, the filler wire speed is 35-40mm / min, the shielding gas flow rate of the welding torch is 10-12L / min, the back shielding gas flow rate is 5-6L / min, and argon is continuously passed through until the weld cools to below 200℃. S3 post-welding treatment: The welded finned tubes are subjected to stress-relief annealing, followed by surface cleaning to remove oxide scale, spatter, and residual impurities. S4 Quality Inspection: The welded finned tubes are subjected to visual inspection, radiographic inspection, and weld shear strength testing.

2. The welding method for a high-nickel finned tube according to claim 1, characterized in that: In S1a, the specific parameters of the stepped transition bevel are as follows: the bevel angle on the carbon steel base tube side is 30°, the step height is 1.5±0.1mm, and the blunt edge dimension is 0.5±0.1mm; the bevel angle on the high-nickel alloy fin side is 25°, and the assembly gap is 0.05-0.10mm; the bevel angle of the austenitic stainless steel 304 transition layer is 35°, and the overlap with the carbon steel base tube side is 1.0±0.1mm.

3. The welding method for a high-nickel finned tube according to claim 1, characterized in that: In S1b, mechanical cleaning involves polishing with a stainless steel wire brush or 180-grit sandpaper until the metal luster is exposed, followed by wiping with anhydrous ethanol; chemical cleaning involves acid washing with a mixture of nitric acid and hydrofluoric acid in a volume ratio of 3:1 for 5-8 minutes, followed by repeated rinsing with clean water and drying at 80-100℃.

4. The welding method for a high-nickel finned tube according to claim 1, characterized in that: In S1c, the welding current for tack welding is controlled at 60-70A. After tack welding, the weld joint is inspected, and formal welding can only be carried out after confirming that there are no cracks or pores.

5. The welding method for a high-nickel finned tube according to claim 1, characterized in that: The tungsten electrode used in the TIG welding is a cerium tungsten electrode (WCe20) with a diameter of 2.4 mm and a 30° cone angle at the front end; the purity of the pure argon gas is ≥99.99%.

6. The welding method for a high-nickel finned tube according to claim 1, characterized in that: In S2a, the first self-fusion welding uses high-frequency arc initiation and gradual arc decay, filling the arc crater when the arc is extinguished; during the welding process, the arc length is ≤1.5mm, and it quickly passes through the sensitization temperature zone of 600-1000℃.

7. The welding method for a high-nickel finned tube according to claim 1, characterized in that: In the S2b, the elongation of the nickel-based transition welding wire ERNiCr-3 is ≥35%, and the S and P contents are ≤0.01%. Before use, the welding wire is dried by keeping it at 200-250℃ for 1 hour. After drying, it is placed in an insulated container and taken out as needed. The exposure time to air should not exceed 30 minutes. When filling the wire, it is fed in from the front edge of the molten pool to avoid contact with the electric arc.

8. The welding method for a high-nickel finned tube according to claim 1, characterized in that: In S3, the stress-relief annealing process involves holding at 700-800℃ for 1-2 hours followed by air cooling; for applications requiring extremely high corrosion resistance, a solution treatment is performed by holding at 1050-1100℃ for 1 hour followed by water cooling, and the annealing fixture is made of heat-resistant alloy material.

9. The welding method for a high-nickel finned tube according to claim 1, characterized in that: In S3, surface cleaning includes mechanical cleaning and chemical cleaning: mechanical cleaning uses a stainless steel wire brush to remove oxide scale and spatter, and grinds the weld seam; chemical cleaning uses a mixture of nitric acid and hydrofluoric acid with a volume ratio of 3:1, followed by pickling with clean water and drying at 80-100℃.

10. The welding method for a high-nickel finned tube according to claim 1, characterized in that: In S4, the visual inspection includes weld smoothness, absence of cracks, porosity, cold shuts and other defects, and the perpendicularity deviation between the fins and the base tube is ≤0.5°; radiographic testing (RT) is performed in accordance with JB / T4730.2, the weld quality grade is ≥II, the weld shear strength is ≥80% of the lowest value of the base material, and the tensile strength is ≥450MPa.

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