Stainless steel-carbon steel composite pipe and manufacturing process thereof

Abstract

The invention discloses a stainless steel-carbon steel composite pipe and a manufacturing process thereof. The manufacturing process comprises the following steps: providing a carbon steel base layer pipe and a stainless steel metal belt; carrying out cleaning and sand blasting roughening treatment on the to-be-compounded surfaces of the two; the stainless steel band is curled and embedded into the base layer pipe, and a gap is reserved; the gap is subjected to spot welding sealing; after a sealing end cover is installed, high-pressure liquid is injected for hydraulic bulging, and double metal is tightly attached; and finally, solid-state diffusion bonding is carried out under the protective atmosphere to form metallurgical bonding. The composite pipe has a complete metallurgical bonding interface, a Cr, Ni and Fe element interdiffusion zone exists at the interface, the width is 10-30 [mu] m, the interface bonding strength is not lower than 280 MPa, carbides are distributed in a dispersed mode, and the average size is not larger than 4 [mu] m. The problem of brittle phase generated by a fusion casting method is avoided through an all-solid-state process, the bonding strength is far higher than that of a mechanical composite method, equipment is simple, the method is suitable for large-specification pipe production, and the contradiction between interface quality and production efficiency in the prior art is effectively solved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of composite metal pipe processing and preparation, in particular, relates to a stainless steel-carbon steel composite pipe and a manufacturing process thereof. BACKGROUND

[0002] The stainless steel-carbon steel composite pipe has a wide application demand in the fields of petroleum, chemical industry, marine engineering, etc. due to the advantages of carbon steel structural strength and stainless steel corrosion resistance. However, the existing composite pipe preparation technology still has many limitations, and an innovative method considering high performance, high efficiency and low cost is urgently needed.

[0003] At present, the mainstream preparation technology of bimetallic composite pipe can be divided into two categories: mechanical forming method and metallurgical forming method. The mechanical forming method includes mechanical expanding method, explosive composite method, etc., which has a high production efficiency, but generally has the problem of insufficient interface bonding strength. For example, the method of using a conical die to extrude and expand the lining pipe, the interface is only mechanically embedded, which is easy to crack or peel off due to stress concentration in subsequent processing or service.

[0004] The metallurgical forming method such as hot rolling and centrifugal casting can realize metallurgical bonding, but each has obvious shortcomings. The hot rolling method needs multiple rolling and heat treatment, which is complicated and has high energy consumption; the centrifugal casting method has complex equipment, large investment, and is not suitable for the preparation of large-diameter or super-long pipes, and the outer layer pipe is easy to deform and the inner layer pipe is not uniform in thickness due to the centrifugal force. In addition, the solid-liquid composite casting method (such as lost foam casting) can directly pour the composite layers, but the interface is easy to form brittle phases such as chromium carbide, which deteriorates the interface performance, and needs to be matched with subsequent hot rolling and expanding, which has a long process and poor environment.

[0005] Especially important is that the existing methods cannot effectively solve the contradiction between interface quality control and production efficiency in the production of large-size composite pipes. Therefore, it has become an urgent technical requirement in this field to develop a solid-state composite forming technology without melting and casting, high interface bonding strength, simple process and suitable for large-size pipes. SUMMARY

[0006] The purpose of the present application is to overcome the shortcomings of the prior art, and to provide a stainless steel-carbon steel composite pipe and a manufacturing process thereof. The specific content is as follows: A manufacturing process of a stainless steel-carbon steel composite pipe, the composite pipe is composed of a carbon steel base layer and a stainless steel composite layer, comprising the following steps: (1) providing a carbon steel base layer pipe and an austenitic stainless steel metal strip; (2) cleaning the inner surface of the carbon steel base layer pipe and the surface to be compounded of the stainless steel metal strip, and then performing surface sand blasting treatment to make the surface roughness Ra reach 3.2-6.3 μm; (3) The stainless steel metal strip is rolled up tightly against the inner surface of the carbon steel base tube to form a composite body, with a gap of 0.8 to 2.5 mm reserved at the joint of the stainless steel metal strip; (4) Temporarily seal the gap of the stainless steel metal strip to prevent high pressure liquid from entering the interface between the metal strip and the base tube during the expansion process. The temporary sealing treatment is to seal the gap at the joint of the stainless steel metal strip by continuous welding or dense spot welding. (5) Install a fixed sealing end cap at one end of the assembly and install a movable sealing plug with a liquid inlet at the other end; (6) High-pressure liquid is injected into the assembly through the liquid inlet on the movable sealing plug. The stainless steel metal strip is tightly bonded to the inner surface of the carbon steel base tube by hydraulic expansion. The expansion pressure is 180-350 MPa and the pressure holding time is 15-90 s. (7) Solid diffusion bonding of the assembly is carried out under a protective atmosphere at a temperature of 850-1050℃ and a holding time of 1-3h, followed by cooling to room temperature at a cooling rate of 15-45℃ / s. (8) Finishing and dimensional correction of the composite pipe.

[0007] In step (4), spot welding is used for temporary sealing, which solves the contradiction between high-pressure sealing and interface purity. The spot welding seal works synergistically through a clever mechanism: First, in the initial stage of hydraulic expansion, the metal barrier formed can effectively seal the gap and ensure the purity of the interface to be composited; Second, when the hydraulic pressure rises to the working pressure, its strength design allows it to be neatly torn, making way for the free expansion of the stainless steel strip; Finally, in the subsequent high-temperature diffusion process, its material properties are consistent with the matrix, which allows the residual part to be completely melted into the interface diffusion area without leaving any trace.

[0008] Furthermore, in step (1), the stainless steel strip is 304, 316, or 2205 duplex stainless steel with a thickness of 0.8–3.0 mm and a width of 1.02–1.15 times the inner circumference of the carbon steel base tube. By limiting the specific material, thickness, and width range of the stainless steel strip, the composite layer is ensured to have excellent corrosion resistance (such as the versatility of 304 and 316, and the strength of 2205 duplex steel). At the same time, the precise width ratio (1.02–1.15 times) ensures that the gap can be effectively closed after hydraulic expansion and will not wrinkle due to excessive interference, which is a key guarantee for achieving a high-strength, high-quality composite interface.

[0009] Furthermore, in step (5), the hydraulic bulging employs multi-stage pressure control. First, the pressure is increased to 80-120 MPa at a rate of 60-80 MPa / s and held for 8-12 seconds, then increased to the working pressure at a rate of 15-25 MPa / s. This multi-stage pressure control, with initial low-pressure holding, effectively eliminates air from the system, checks the seal, and allows the metal strip to initially expand before increasing to the working pressure to complete the bulging. This process effectively avoids uneven bonding or sealing failure caused by impact effects from single-stage pressurization, significantly improving process stability and yield.

[0010] Furthermore, in step (6), the solid-state diffusion connection is performed in a vacuum environment with a vacuum level not exceeding 5 × 10⁻⁶. -3 The process can be carried out under a vacuum or in an argon protective atmosphere with an argon purity of not less than 99.999%. By limiting the ultra-high vacuum or high-purity inert protective atmosphere, it is ensured that the interface to be composited is not oxidized during the solid-state diffusion bonding process, creating an absolutely clean environment for the full interdiffusion of elements such as Cr, Ni, and Fe. This is a core prerequisite for obtaining an interdiffusion zone with a width of 10-30 μm, uniform composition, and a high-strength metallurgical bonding interface.

[0011] Further, in step (6), the solid-state diffusion bonding involves heating to 650–750°C at a rate of 8–12°C / min and holding at that temperature for 25–35 min; then continuing to heat to the target temperature at a rate of 4–8°C / min; and finally cooling at a rate of 20–35°C / s after the holding period. This temperature-controlled diffusion bonding system, with its intermediate temperature range (650–750°C) holding period facilitating stress relaxation and initial element diffusion, followed by a slow rise to the target temperature to allow for sufficient diffusion, and finally rapid cooling to suppress excessive carbide precipitation and growth, results in an excellent interfacial structure with dispersed carbide distribution and no continuous network structure.

[0012] Furthermore, in step (6), before solid-state diffusion bonding, a pressure of 80–120 MPa is applied to the assembly at a temperature of 400–480 °C for a holding time of 10–25 min. Preheating and pressurizing at medium temperature before diffusion bonding can further smooth out microscopic irregularities at the interface before the atoms diffuse violently, resulting in tighter interface contact. Simultaneously, it can excite atomic diffusion dynamics in advance, helping to reduce the temperature and time of the subsequent main diffusion bonding stages, improving production efficiency and optimizing the interface structure.

[0013] Furthermore, in step (7), the finishing process includes polishing the inner surface of the composite pipe to a surface roughness Ra of no more than 1.2 μm; and straightening the composite pipe to a straightness deviation of no more than 0.8 mm / m. By quantitatively limiting the surface roughness and straightness of the composite pipe after finishing, the final product is ensured to have extremely high dimensional accuracy and a smooth inner surface, fully meeting the stringent requirements of high-end fluid transportation pipelines for fluid resistance, corrosion resistance, and installation accuracy.

[0014] A stainless steel-carbon steel composite pipe comprises a carbon steel base layer and a stainless steel composite layer, with a complete metallurgical interface formed between the two layers. At the interface, there exists a 10–30 μm wide interdiffusion zone of Cr, Ni, and Fe elements, and the interface bonding strength is not less than 280 MPa. This composite pipe features a metallurgical bond formed by elemental interdiffusion at the interface, resulting in high strength (≥280 MPa). Furthermore, the carbides are dispersed and finely distributed, without a continuous network structure, effectively avoiding stress concentration. This allows the interface to remain intact even when subjected to deformation (such as flattening tests), making its reliability far superior to traditional composite pipes.

[0015] Furthermore, the carbides at the interface are diffusely distributed, with an average carbide size not exceeding 4 μm, no continuous network of carbides present, and a carbon migration layer thickness of less than 15 μm. By clearly defining the size, distribution morphology, and carbon migration layer thickness of the interface carbides, the superiority of the interface structure of the product of this invention is quantitatively demonstrated. This structure ensures that the interface performance of the composite pipe remains stable during long-term service under high-temperature conditions and is not easily degraded due to carbon migration.

[0016] Furthermore, after the composite pipe is pressed to a spacing of 2 / 3 of the pipe diameter, there is no separation between the stainless steel composite layer and the carbon steel base layer, and the interfacial bonding strength retention rate is not less than 95%. The performance indicators after the specified flattening test (no separation, strength retention rate ≥95%) directly and powerfully demonstrate that the composite pipe prepared by this invention achieves extremely high levels of interfacial bonding strength, toughness, and peel resistance, enabling it to withstand extreme processing and service conditions and meet the most stringent application standards.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: The most significant advantage of this invention lies in its innovative step-by-step composite strategy of "hydraulic bulging + solid-state diffusion bonding." First, high-strength mechanical interlocking of the interface is achieved at room temperature through hydraulic pressure, providing excellent basic contact conditions for subsequent diffusion. Then, through precisely temperature-controlled solid-state diffusion bonding, atomic-level metallurgical bonding of the interface is achieved without melting any material. This strategy fundamentally avoids the brittle phase problem caused by the solidification process in casting methods, while its bonding strength far surpasses that of purely mechanical composite methods.

[0018] Second, in this invention, the entire process is solid-state connection, and the interface temperature is always controlled within a range far below the melting point of the casting liquid. This fundamentally eliminates the problem of the formation of brittle phases such as coarse and continuous carbides due to the contact between stainless steel liquid and carbon steel. It solves the technical pain point in the background technology that "the intermediate compound near the interface is a hard phase carbide, which easily causes stress concentration and induces cracks during drawing".

[0019] Third, this invention uses conventional hydraulic equipment and heat treatment furnaces, eliminating the reliance on expensive and specialized equipment such as centrifuges and large rolling mills (the background technology points out that the centrifugal method is "complex equipment" and "limited by pipe diameter"). It achieves uniform expansion through internal hydraulic pressure, which is particularly suitable for the preparation of large-diameter and ultra-long pipes, and solves the industry problem of "not being able to produce bimetallic composite pipes with a diameter exceeding a certain limit". Detailed Implementation

[0020] To enable those skilled in the art to understand the features and effects of this application, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art in this application, and in case of conflict, the definitions in this specification shall prevail.

[0021] The present application will be described below with reference to specific embodiments and examples. It should be understood that these specific embodiments and examples are merely illustrative and are not intended to limit the scope or use of the present application.

[0022] Unless otherwise stated, the methods, reagents, and conditions used in the preparation examples, comparative examples, and embodiments described below are conventional methods, reagents, and conditions in the art.

[0023] Unless otherwise specified, in this application, parts by weight represent the relative number of mass parts in the composition, which can be any mass unit, such as, but not limited to, kilograms, grams, etc.

[0024] The specific embodiments are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Unless otherwise specified, the raw materials and reagents used in the examples are commercially available.

[0025] Example 1 1. Material preparation: Prepare 20# carbon steel pipe as the base pipe, with specifications of Φ108mm×8mm×1000mm (outer diameter×wall thickness×length). Then prepare 304 austenitic stainless steel strip as the composite layer strip, with a stainless steel strip thickness of 1.5mm and a width of 305mm. 2. Surface treatment: The inner wall of the carbon steel pipe and the stainless steel strip to be composited are ultrasonically cleaned and degreased with acetone, and sandblasted with brown corundum. After treatment, the surface roughness Ra is measured with a surface roughness meter and confirmed to be 3.8μm. 3. Rolling and Spot Welding Sealing: The stainless steel strip is rolled tightly against the inner wall of the carbon steel pipe, leaving a 1.0mm gap at the joint. A WS-200 argon arc welding machine is used, with Φ1.0mm 304 welding wire as the filler material. To control heat input and ensure sealing, a lower current parameter (80A) and a faster welding speed are used during the welding process, ultimately forming a thin and continuous sealing weld along the entire gap. This weld must be smooth and flat to ensure that it can crack neatly along the predetermined path during the subsequent expansion process.

[0026] 4. End sealing and hydraulic expansion: A fixed sealing end cap is installed at one end of the assembly, and a movable sealing plug with a liquid inlet and a pressure sensor is installed at the other end; Connect to an ultra-high pressure hydraulic system for hydraulic bulging: (1) First stage (sealing test and pre-tightening): pressurize to 100 MPa at a rate of 70 MPa / s and hold for 10 s. The pressure is stable during the holding period without drop, indicating that the system (including end seals and axial seal welds) is well sealed; (2) Second stage (expansion): Continue to increase the pressure at a rate of 20 MPa / s. When the pressure rises to about 165 MPa, the monitoring system detects a brief small fluctuation in pressure before it continues to rise. This indicates that the axial sealing weld has been neatly torn. Continue to increase the pressure to the final expansion pressure of 200 MPa and hold the pressure for 30 s to allow the stainless steel strip to fully plastically deform and fit tightly against the inner wall of the carbon steel pipe.

[0027] 5. Solid-state diffusion bonding: Place the bulged assembly into a vacuum diffusion welding furnace and evacuate to a vacuum level of 3×10⁻⁶. -3 Pa, perform diffusion bonding heat treatment, that is, heat up to 700℃ at a rate of 10℃ / min, hold for 30min, then continue to heat up to 900℃ at a rate of 6℃ / min, hold for 2h, after the holding is completed, turn on the high-purity nitrogen cooling system, and cool the workpiece to room temperature at a cooling rate of 25℃ / s.

[0028] 6. Finishing: The inner surface of the composite tube is polished using an inner wall polishing machine. After completion, the surface roughness Ra is measured to be 1.0μm using a roughness tester. The composite tube is straightened using a seven-roll straightening machine and then checked with a straightness measuring instrument. Its straightness deviation is 0.5mm / m.

[0029] Example 2 1. Material preparation: Prepare 20# carbon steel pipe as the base pipe, with specifications of Φ159mm×10mm×1500mm (outer diameter×wall thickness×length). Then prepare 316 austenitic stainless steel strip as the composite layer strip, with a stainless steel strip thickness of 2.0mm and a width of 445mm. 2. Surface treatment: The inner wall of the carbon steel pipe and the stainless steel strip to be composited are ultrasonically cleaned and degreased with acetone, and sandblasted with brown corundum. After treatment, the surface roughness Ra is measured with a surface roughness meter and confirmed to be 4.5μm. 3. Rolling and Spot Welding Sealing: The stainless steel strip is rolled tightly against the inner wall of the carbon steel pipe, leaving a 1.5mm gap at the joint. A WS-200 argon arc welding machine is used, with Φ1.0mm 304 welding wire as the filler material. To control heat input and ensure sealing, a lower current parameter (80A) and a faster welding speed are used during the welding process, ultimately forming a thin and continuous sealing weld along the entire gap. This weld must be smooth and flat to ensure that it can crack neatly along the predetermined path during the subsequent expansion process.

[0030] 4. End sealing and hydraulic expansion: A fixed sealing end cap is installed at one end of the assembly, and a movable sealing plug with a liquid inlet and a pressure sensor is installed at the other end; Connect to an ultra-high pressure hydraulic system for hydraulic bulging: (1) First stage (sealing test and pre-tightening): pressurize to 90MPa at a rate of 65MPa / s and hold for 11s. The pressure is stable during the holding period without drop, indicating that the system (including end seals and axial seal welds) is well sealed; (2) Second stage (expansion): Continue to increase the pressure at a rate of 18 MPa / s. When the pressure rises to about 155 MPa, the monitoring system detects a brief small fluctuation in pressure before it continues to rise. This indicates that the axial sealing weld has been neatly torn. Continue to increase the pressure to the final expansion pressure of 260 MPa and hold the pressure for 50 s to allow the stainless steel strip to fully plastically deform and fit tightly against the inner wall of the carbon steel pipe.

[0031] 5. Solid-state diffusion bonding: The expanded assembly is placed in a heat treatment furnace filled with high-purity argon (99.999% purity) for diffusion bonding heat treatment, i.e., the temperature is raised to 680℃ at a rate of 9℃ / min and held for 28min, and then the temperature is raised to 950℃ at a rate of 5℃ / min and held for 1.5h. After the holding period, the high-purity nitrogen cooling system is turned on and the workpiece is cooled to room temperature at a cooling rate of 30℃ / s.

[0032] 6. Finishing: The inner surface of the composite tube is polished using an inner wall polishing machine. After completion, the surface roughness Ra is measured to be 1.1μm using a roughness tester. The composite tube is straightened using a seven-roll straightening machine and then checked with a straightness measuring instrument. Its straightness deviation is 0.7mm / m.

[0033] Example 3 1. Material preparation: Prepare 20# carbon steel pipe as the base pipe, with specifications of Φ325mm×15mm×2000mm (outer diameter×wall thickness×length). Then prepare 2205 duplex stainless steel strip as the composite layer strip, with a stainless steel strip thickness of 3.0mm and a width of 1050mm. 2. Surface treatment: The inner wall of the carbon steel pipe and the stainless steel strip to be composited are ultrasonically cleaned and degreased with acetone, and sandblasted with brown fused alumina. After treatment, the surface roughness Ra is measured with a surface roughness meter and confirmed to be 6.0 μm. 3. Rolling and Spot Welding Sealing: The stainless steel strip is rolled tightly against the inner wall of the carbon steel pipe, leaving a 2.2mm gap at the joint. A WS-200 argon arc welding machine is used, with Φ1.0mm 304 welding wire as the filler material. To control heat input and ensure sealing, a lower current parameter (80A) and a faster welding speed are used during the welding process, ultimately forming a thin and continuous sealing weld along the entire gap. This weld must be smooth and flat to ensure that it can crack neatly along the predetermined path during the subsequent expansion process.

[0034] 4. End sealing and hydraulic expansion: A fixed sealing end cap is installed at one end of the assembly, and a movable sealing plug with a liquid inlet and a pressure sensor is installed at the other end; Connect to an ultra-high pressure hydraulic system for hydraulic bulging: (1) First stage (sealing test and pre-tightening): pressurize to 120MPa at a rate of 80MPa / s and hold for 8 seconds. The pressure is stable during the holding period without drop, indicating that the system (including end seals and axial seal welds) is well sealed; (2) Second stage (expansion): Continue to increase the pressure at a rate of 25 MPa / s. When the pressure rises to about 220 MPa, the monitoring system detects a brief small fluctuation in pressure before it continues to rise. This indicates that the axial sealing weld has been neatly torn. Continue to increase the pressure to the final expansion pressure of 340 MPa and hold the pressure for 15 seconds to allow the stainless steel strip to fully plastically deform and fit tightly against the inner wall of the carbon steel pipe.

[0035] 5. Solid-state diffusion bonding: Performed in a vacuum environment (vacuum degree 2×0). -3The component is placed in a hot isostatic pressing furnace and subjected to a pressure of 100 MPa at 450°C for 20 min. Then, diffusion bonding heat treatment is performed in a vacuum furnace, i.e., the temperature is increased to 750°C at a rate of 12°C / min and held for 35 min, and then the temperature is increased to 1020°C at a rate of 4°C / min and held for 3 h. After the holding period, the high-purity nitrogen cooling system is turned on and the workpiece is cooled to room temperature at a cooling rate of 40°C / s.

[0036] 6. Finishing process: The inner surface of the composite tube is polished using an inner wall polishing machine. After completion, the surface roughness Ra is measured to be 0.9μm using a roughness tester. The composite tube is straightened using a seven-roll straightening machine and then checked with a straightness measuring instrument. Its straightness deviation is 0.6mm / m.

[0037] Comparative Example 1 (Comparison of Mechanical Forming Methods) Stainless steel-carbon steel composite pipes are manufactured using the mechanical expansion method. 1. Materials preparation: Same as in Example 1, using 20# carbon steel pipe (Φ108mm×8mm×1000mm) and 304 stainless steel strip (thickness 1.5mm, width 305mm).

[0038] 2. Surface treatment: Same as in Example 1, sandblasting treatment is performed, Ra=3.8μm.

[0039] 3. Assembly: Curl the stainless steel strip and place it into the carbon steel pipe.

[0040] 4. Mechanical expansion: The composite tube blank is cold expanded using a conical die mechanical expansion machine, with an expansion amount of 1.5%. No heat treatment is performed in this process.

[0041] 5. Finishing: Same as in Example 1, polishing and straightening are performed.

[0042] Comparative Example 2 (Metallurgical Forming Method - Solid-Liquid Composite Comparison) Stainless steel-carbon steel composite pipes are manufactured using a solid-liquid composite method. 1. Materials preparation: Carbon steel pipes are the same as in Example 1. Prepare 304 stainless steel ingots.

[0043] 2. Preheating of the base pipe: Preheat the carbon steel pipe to 300℃.

[0044] 3. Casting: Molten 304 stainless steel (superheated to 100°C) is poured into the carbon steel pipe and solidifies to form an inner lining.

[0045] 4. Heat treatment: Perform solution treatment at 1050℃ for 2 hours.

[0046] 5. Finishing: Same as Example 1.

[0047] Comparative Example 3 (Metallurgical Forming Method - Centrifugal Casting Comparison) 1. Prepare the mold: Use a large horizontal centrifugal casting machine.

[0048] 2. Casting: First, pour molten carbon steel into the rotating mold to form the outer tube; after it solidifies, pour molten 304 stainless steel to form the inner tube. The centrifugal speed is set relatively high in an attempt to improve the bonding.

[0049] 3. Post-processing: The inner and outer diameters of the cast composite pipe are machined to remove the uneven cast surface layer and obtain the final dimensions. Then, the same heat treatment (900℃×2h) and finishing are performed as in Example 1.

[0050] The following methods were used to measure Examples 1-3 and Comparative Examples 1-4: 1. Interface Bond Strength: Multiple rectangular specimens were cut axially from the composite pipe, ensuring the interface was centered within the specimen. The test was conducted using a dedicated shear fixture on a universal testing machine. The fixture was designed so that the loading force was parallel to the interface, thus applying pure shear force. The maximum shear load F (N) was recorded and divided by the original cross-sectional area A (mm²) of the interface. 2 The shear strength τ = F / A (MPa) was obtained, and the result was the average value of multiple samples.

[0051] 2. Flattening strength retention rate: First, test the interfacial shear strength (average value, τ0) of a group of unflattened samples. Cut a pipe section of a specified length and flatten it on a press until the distance between the two plates is 2 / 3 of the original pipe outer diameter. Hold for 10-15 seconds and then unload. Take a sample from the flattened pipe section and test its interfacial shear strength (τ1) again. Calculate the strength retention rate = (τ1 / τ0) × 100%.

[0052] 3. Interface structure / metallographic analysis: Take a cross-sectional sample of the tube containing the complete interface, and after mounting, rough grinding, fine grinding and polishing, lightly etch it with an appropriate etchant (such as FeCl3 hydrochloric acid solution for displaying austenitic stainless steel, and nitric acid alcohol solution for displaying carbon steel). Observe the interface morphology, compound distribution and defects under a metallographic microscope or scanning electron microscope (SEM).

[0053] 4. Interface structure / metallographic analysis: Randomly select no less than 5 fields of view on each sample and measure the equivalent circle diameter or maximum chord length of at least 50 carbide particles. The arithmetic mean of all measurements is the average size.

[0054] 5. Carbon migration layer thickness: The concentration distribution of carbon element was quantitatively analyzed by performing a line scan perpendicular to the interface using an electron probe. The width of the region where the carbon element concentration was significantly higher than the normal value of the carbon steel substrate was defined as the carbon migration layer thickness.

[0055] 6. Inner surface roughness: Measured using a portable surface roughness tester. At least five different locations are randomly selected along both the axial and circumferential directions on the inner surface of the composite tube for measurement. The arithmetic mean of all measurement results is taken. The instrument must be calibrated on a standard sample before measurement.

[0056] The measurement results are shown in the table below: By employing the measurement methods described above according to international or national standards, all performance data from the embodiments and comparative examples of this invention demonstrate a high degree of scientific validity, reliability, and comparability. The test data fully validates the superiority of this invention. The interfacial shear strength and performance retention rate after flattening of the prepared composite pipe significantly surpass those of the traditional methods represented by the comparative examples. The root cause of this performance improvement lies in the invention's successful suppression of the formation of brittle interfacial phases and carbon migration, resulting in a clean, dispersed, and ideal interfacial structure. Furthermore, the equipment used is highly versatile and cost-effective, making it particularly suitable for the industrial production of large-scale composite pipes.

[0057] It should be understood that the above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. It should not be considered that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A manufacturing process for a stainless steel-carbon steel composite pipe, characterized in that, The composite pipe is composed of a carbon steel base layer and a stainless steel composite layer, and includes the following steps: (1) Provide carbon steel base tubes and austenitic stainless steel metal strips; (2) Clean the inner surface of the carbon steel base tube and the stainless steel metal strip to be composited. After cleaning, perform surface sandblasting to make the surface roughness Ra reach 3.2 to 6.3 μm. (3) The stainless steel metal strip is rolled up tightly against the inner surface of the carbon steel base tube to form a composite body, with a gap of 0.8 to 2.5 mm reserved at the joint of the metal strip; (4) Seal the gap at the joint of the stainless steel metal strip by continuous welding or dense spot welding; (5) Install a fixed sealing end cap at one end of the assembly and install a movable sealing plug with a liquid inlet at the other end; (6) High-pressure liquid is injected into the assembly through the liquid inlet on the movable sealing plug. The stainless steel metal strip is tightly bonded to the inner surface of the carbon steel base tube by hydraulic expansion. The expansion pressure is 180-350 MPa and the pressure holding time is 15-90 s. (7) Solid diffusion bonding of the assembly is carried out under a protective atmosphere at a temperature of 850-1050℃ and a holding time of 1-3h, followed by cooling to room temperature at a cooling rate of 15-45℃ / s. (8) Finishing and dimensional correction of the composite pipe.

2. The manufacturing process according to claim 1, characterized in that: In step (1), the stainless steel metal strip is 304, 316 or 2205 duplex stainless steel, with a thickness of 0.8 to 3.0 mm and a width of 1.02 to 1.15 times the inner circumference of the carbon steel base tube.

3. The manufacturing process according to claim 1, characterized in that: In step (5), the hydraulic expansion adopts multi-stage pressure control. First, the pressure is increased to 80-120MPa at a rate of 60-80MPa / s and held for 8-12s. Then, the pressure is increased to the working pressure at a rate of 15-25MPa / s.

4. The manufacturing process according to claim 1, characterized in that: In step (6), the solid-state diffusion connection is performed in a vacuum environment with a vacuum level not exceeding 5 × 10⁻⁶. -3 Pa, or carried out under an argon protective atmosphere, with argon purity not less than 99.999%.

5. The manufacturing process according to claim 1, characterized in that: In step (6), the solid-state diffusion connection is achieved by heating to 650-750°C at a rate of 8-12°C / min and holding for 25-35 min; then heating to the target temperature at a rate of 4-8°C / min; and finally cooling at a rate of 20-35°C / s after holding.

6. The manufacturing process according to claim 1, characterized in that: In step (6), before solid-state diffusion bonding, the assembly is subjected to a pressure of 80-120 MPa at a temperature of 400-480°C for 10-25 minutes.

7. The manufacturing process according to claim 1, characterized in that: In step (7), the finishing process includes polishing the inner surface of the composite tube, with a surface roughness Ra of no more than 1.2 μm; and straightening the composite tube, with a straightness deviation of no more than 0.8 mm / m.

8. A stainless steel-carbon steel composite pipe manufactured by the process described in any one of claims 1 to 7, characterized in that: It includes a carbon steel base layer and a stainless steel composite layer, with a complete metallurgical bonding interface between the two layers. There is a Cr, Ni and Fe interdiffusion zone with a width of 10 to 30 μm at the interface, and the interface bonding strength is not less than 280 MPa.

9. The stainless steel-carbon steel composite pipe according to claim 8, characterized in that: The carbides at the interface are diffusely distributed, with an average size of no more than 4 μm, no continuous network of carbides, and a carbon migration layer thickness of less than 15 μm.

10. The stainless steel-carbon steel composite pipe according to claim 8, characterized in that: After the composite pipe is pressed to a spacing of 2 / 3 of the pipe diameter, there is no separation between the stainless steel composite layer and the carbon steel base layer, and the interface bonding strength retention rate is not less than 95%.

Images