A bimetallic composite pipe blank with an electroplated nickel layer and a method for producing the same

Abstract

The present application belongs to the technical field of bimetallic composite blank, and particularly relates to a bimetallic composite pipe blank with a nickel electroplated layer and a preparation method thereof. The method comprises the following steps: electroplating a nickel layer on the inner wall of an outer base pipe, constructing an electroslag remelting solid-liquid composite casting device, and preparing a bimetallic composite pipe blank through slag arc leading and electroslag remelting. In the method, an acid Watt nickel plating solution system is used for electroplating the nickel layer, and the thickness of the nickel layer is controlled to be 10-20 μm, so as to improve the wetting relationship between the 20G carbon steel and the 304L stainless steel; a low-melting-point and low-viscosity quaternary slag system is selected as a solid slag material to assist in slag discharge; during the composite process, the slag material not only serves as an arc leading medium, but also forms a refining slag pool after being melted, so as to not only improve the flowability of the slag pool and promote the floating and discharge of the interface molten slag, but also maintain the temperature field required for the stable melting of the 20G electrode by adjusting the resistivity of the slag system.

Description

Technical Field

[0001] This invention belongs to the field of bimetallic composite billet technology, specifically relating to a bimetallic composite tube blank with an electroplated nickel layer and its preparation method, and more particularly to a bimetallic composite tube blank formed by electroslag remelting solid-liquid composite casting technology between an electroplated nickel layer on the inner wall of an outer base tube material and an inner solid material, and its preparation method. Background Technology

[0002] With the rapid development of my country's basic industries towards high efficiency, low carbon emissions, and large capacity, the boiler industry is placing increasingly stringent requirements on the performance of pipeline materials. Taking alkali recovery or waste incineration boilers in paper mills as examples, the external pipes must resist high-temperature flue gas oxidation and sulfur corrosion, while the internal pipes must withstand the strong scouring of high-temperature, high-pressure steam. Under these complex conditions of combined high temperature, high pressure, and corrosive media, while single carbon steel pipes are low in cost and high in strength, they have extremely poor corrosion resistance in acidic media; while single stainless steel pipes are corrosion-resistant, they are expensive and pose a risk of stress corrosion cracking under certain conditions. Therefore, traditional single-metal materials are no longer sufficient to meet design requirements. Thus, bimetallic composite pipes with external corrosion resistance and internal pressure bearing capacity, combining the advantages of both materials, have become the best solution to this problem. The billet is the key and foundation of the composite pipe's quality, and the billet-making process plays a decisive role in the quality control of the composite and the interfacial bonding of the composite pipe.

[0003] There are several existing bimetallic composite billet preparation routes, mainly divided into mechanical composite and metallurgical composite. Mechanical composite technology is mature, but its interfacial bonding degree and stability are limited. Metallurgical composite can achieve higher bonding strength, but it generally suffers from problems such as uncontrollable interfacial transition layer microstructure and composition, segregation, and sensitivity to defects. Especially for high-quality metallurgical composites of stainless steel and carbon steel, there is currently no mature engineered composite process. Particularly concerning is 304L stainless steel, which contains a large number of easily oxidized elements. During preheating and high-temperature contact stages, its inner surface readily and rapidly forms a dense and stable oxide film (such as Cr2O3). This oxide film severely hinders the direct contact and wetting of the molten 20g steel with the 304L metal matrix, causing slag to be easily trapped between the interfaces. This failure in wetting directly leads to obvious delamination, severe slag inclusions, and incomplete fusion defects at the interface in the produced billet, failing to meet subsequent processing requirements. Therefore, there is an urgent need to develop a preparation process that can adapt to the high-temperature casting environment and solve the problems of interfacial wetting and slag inclusions. Summary of the Invention

[0004] Therefore, the purpose of this invention is to provide a bimetallic composite tube blank with an electroplated nickel layer and its preparation method, which aims to overcome the defects of obvious delamination, severe slag inclusion, and lack of fusion caused by the wetting failure at the interface of the tube blank produced by high-quality metallurgical composite of stainless steel and carbon steel.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A method for preparing a bimetallic composite tube blank with an electroplated nickel layer includes the following steps: S1. Electroplating a nickel layer on the inner wall of the outer base tube; grinding the inner wall of the outer base tube until a metallic luster is exposed, and after alkaline degreasing and acid pickling activation, electroplating a nickel layer on the activated inner wall; the electroplating nickel layer adopts an acidic Watt nickel plating solution system and controls the nickel layer thickness to be 10-20μm; S2. Construct an electroslag remelting solid-liquid composite casting equipment; vertically fix the electroplated outer base tube on a conductive rotating base and place a preheating ring around the outer base tube; then insert the inner solid bar material as a consumable electrode into the outer base tube to ensure that the annular gap is uniform; then fill the bottom of the annular gap in batches with dry CaF2-Al2O3-CaO-MgO quaternary slag system solid slag material. S3. Preparation of bimetallic composite tube blank by slag arc ignition and electroslag remelting: The bottom of the consumable electrode is kept in micro-contact with the solid slag layer. An arc ignition voltage and current are applied to generate an electric arc in the gaps between slag particles and melt the slag in situ to form a liquid slag pool. After the slag pool finally stabilizes, the rotating base is immediately driven to rotate the preheated outer base tube around its axis. At the same time, the working voltage and current of electroslag remelting are maintained so that the consumable electrode melts and drips uniformly and continuously in the slag pool. After the metal droplets pass through the slag layer, they quickly spread on the inner wall of the outer base tube under the wetting effect of the electroplated nickel layer to form a clean bimetallic interface. As the consumable electrode continues to melt and the liquid level of the slag pool rises synchronously, a dense solid-liquid composite molding is finally completed from bottom to top to obtain a bimetallic composite tube blank.

[0006] As a further embodiment of the present invention, the grinding is performed using 80-120 grit sandpaper or a mechanical boring tool to grind the inner wall until all visible oxide scale is removed and a metallic luster is exposed with a surface roughness Ra of 3.2-6.3 μm; the alkaline degreasing is performed using an alkaline degreasing solution at 60-80°C, which contains 40-60 g / L NaOH and 20-30 g / L Na2CO3, and the soaking time is 10-15 min. After rinsing with water, the tube is immediately subjected to acid pickling activation; the acid pickling activation is performed using a mixed acid solution of 20-30 wt% hydrochloric acid and 5-10 wt% sulfuric acid, and the base tube is soaked at room temperature for 1-3 min until uniform and vigorous bubbles are generated on the inner surface and the surface turns silvery-gray, so as to destroy the chromium-rich passivation film on the inner surface of the outer base tube.

[0007] As a further embodiment of the present invention, the acidic Watt nickel plating solution system includes: 250-300 g / L nickel sulfate, 40-50 g / L nickel chloride, and 35-40 g / L boric acid. The plating solution temperature is controlled at 45-55°C, the pH value is 4.0-4.8, and the current density is set at 2.0-4.0 A / dm². After electroplating, the solution is immediately rinsed with deionized water and dried with hot air to prevent secondary oxidation of the nickel layer surface.

[0008] As a further embodiment of the present invention, the mass percentage of each component in the solid slag is: CaF2 65-75%, Al2O3 25-35%, CaO 17-20%, MgO 3-5%, and the quaternary slag system is a low melting point and low viscosity slag system. During the composite process, the liquid slag attached to the interface is forcibly peeled off and squeezed to float to the surface of the slag pool, forming a clean bimetallic bonding interface.

[0009] As a further embodiment of the present invention, the solid slag material is added in batches. The first batch is 50-60% of the total slag volume, with a filling height of 30-50 mm, to form an initial liquid slag pool. The second batch is 25-35% of the total slag volume, to expand the volume of the initial liquid slag pool and maintain a stable resistive heating state. The third batch is 10-20% of the total slag volume, to adjust the depth of the slag pool to ensure that the end of the consumable electrode is always melting in a stable high-temperature slag pool.

[0010] In a further embodiment of the present invention, the micro-contact state is such that the bottom of the consumable electrode is 0-2 mm away from the solid slag layer, while the consumable electrode maintains contact with the conductive rotating base; the arc-starting voltage is AC45V, the arc-starting current is 800A, and the power supply frequency is 50Hz; the time for adding the first batch of slag and starting the arc is 5-10 minutes, and the time for forming the initial liquid slag pool is 10-20 minutes. After the initial liquid slag pool is formed, a second batch of slag is added, which is used to expand the volume of the initial liquid slag pool. After adding the first batch of slag, stabilize it for 5-10 minutes to allow it to fully melt in the initial liquid slag pool. Then, add a third batch of slag, which is used to adjust the liquid level and depth of the slag pool. After adding the third batch, stabilize it for 5-10 minutes to allow the conductivity, temperature field, and liquid level of the entire liquid slag pool to stabilize. After the slag pool finally stabilizes, a stable liquid slag pool with a depth of 45 mm is formed. The temperature of the stable liquid slag pool is maintained at 1500-1600℃, and the stable melting and recombination time of the self-consumable electrode continuous melting and dripping is 3-5 hours.

[0011] As a further embodiment of the present invention, the mechanism for driving the rotating base adopts a drive method of variable frequency motor and high-speed ratio reducer. The synchronous rotation of the rotating base and the outer base tube is achieved by stepless speed regulation of the frequency converter and the torque increase effect of the reducer, and the rotation speed is 3-5 r / min.

[0012] As a further embodiment of the present invention, no external forced water cooling is used in the entire solid-liquid composite process. The outer base tube is used as a crystallizer, and dynamic thermal equilibrium is achieved by relying on natural air convection heat dissipation and the continuous heat input of the molten metal pool inside the outer base tube.

[0013] As a further embodiment of the present invention, after the consumable electrode reaches the predetermined melting length, the electroslag heat input time is maintained for 20-40 minutes to allow the upper metal molten pool to complete the feeding and promote the floating of residual slag and oxide inclusions into the slag pool. Then the power supply is stopped, and the composite tube blank is left to stand in place for 30-60 minutes to allow the internal metal molten pool to completely solidify. Finally, it is naturally cooled in the air for 3-6 hours to room temperature to obtain the bimetallic composite tube blank.

[0014] Furthermore, the present invention also provides a bimetallic composite tube blank with an electroplated nickel layer, which is prepared by the above-mentioned preparation method. The outer base tube is made of 304L stainless steel, and the inner consumable electrode is made of 20G carbon steel. The interface between the 304L stainless steel and the 20G carbon steel is wavy and interlocked. At the same time, the interface forms a transition zone with a width of 2-4μm and an element diffusion influence zone of 80-100μm. In addition, the shear strength of the bimetallic composite tube blank reaches 220MPa and the tensile strength reaches 456MPa.

[0015] The beneficial effects of this invention are as follows: This invention is based on electroslag remelting solid-liquid composite casting technology, and specifically proposes to electroplate a 10-20μm nickel layer onto the inner wall of the outer 304L stainless steel to improve the wetting relationship between 20G carbon steel and 304L stainless steel, and to use a low-melting-point slag system to assist in slag removal, in order to prepare a composite tube blank with an outer 304L stainless steel / inner 20G carbon steel. During the composite process, the slag material serves as both an arc-igniting medium and forms a refining slag pool after melting. Furthermore, it isolates external air to prevent external oxygen from participating in the entire composite process, thereby improving the fluidity of the slag pool and promoting the upward floating and discharge of interfacial molten slag. Simultaneously, the temperature field required for stable melting of the 20G electrode is maintained by adjusting the slag resistivity. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a simplified schematic diagram of the electroslag remelting solid-liquid composite casting equipment constructed according to the present invention.

[0018] Figure 2 The image shows the macroscopic morphology and a magnified view of the unnickel-plated composite tube blank.

[0019] Figure 3 Macroscopic morphology and enlarged partial view of the nickel-plated composite tube blank.

[0020] Figure 4 This is an OM (Oxygen Oxide) microstructure characteristic diagram of the interface region of a nickel-plated 304L / 20G bimetallic composite tube blank. Figure 4 (a) is a diagram showing the overall tissue characteristics including the 20G side, the composite interface and the 304L side; Figure 4 (b) is a partial histological feature map of the 20G side, showing Widmanstätten histological features. Figure 4 (c) is a local austenitic microstructure diagram of the 304L side.

[0021] Figure 5 This is a SEM image of a nickel-plated 304L / 20G bimetallic composite tube blank. Figure 5 (a) is the overall interface morphology including the 20G side, the composite interface and the 304L side, where R1 is the 20G side region, R2 is the composite interface region and R3 is the 304L side region. Figure 5 (b) shows a magnified view of the local morphology of the R2 composite interface region. Figure 5 (c) shows the local microstructure of the R1 region, i.e. the 20G side, which is mainly composed of ferrite and pearlite; Figure 5 (d) shows the local tissue features of the R3 region, i.e., the 304L side.

[0022] Figure 6 This is an EDS surface scan result of a nickel-plated 304L / 20G bimetallic composite tube blank. Figure 6 (a) shows the surface distribution characteristics of Fe element; Figure 6 (b) shows the surface distribution characteristics of Ni element; Figure 6 (c) shows the surface distribution characteristics of Cr element; Figure 6 (d) represents the surface distribution characteristics of element C.

[0023] Figure 7 This is an EDS line scan image of the interface of a nickel-plated 304L / 20G bimetallic composite tube blank. Figure 7 (a) shows the distribution characteristics of Fe element along the scanning direction; Figure 7 (b) shows the distribution characteristics of Ni elements along the scanning direction; Figure 7 (c) shows the distribution characteristics of Cr element along the scanning direction; Figure 7 (d) shows the distribution characteristics of C elements along the scanning direction.

[0024] Figure 8 High-magnification scan of the interface region of a nickel-plated 304L / 20G bimetallic composite tube blank. Figure 8 (a) shows the distribution characteristics of Fe element in the interface region; Figure 8 (b) shows the distribution characteristics of Ni element in the interface region; Figure 8 (c) shows the distribution characteristics of Cr element in the interface region; Figure 8 (d) represents the distribution characteristics of C elements in the interface area.

[0025] Figure 9 This is a point scan image of the interface area of ​​a nickel-plated 304L / 20G bimetallic composite tube blank.

[0026] Figure 10 The mechanical property curves are for nickel-plated 304L / 20G bimetallic composite tube blanks. Figure 10 (a) is a tensile stress-displacement curve; Figure 10 (b) is a shear stress-displacement curve.

[0027] Figure 11 SEM image of the tensile fracture surface of a nickel-plated 304L / 20G bimetallic composite tube blank. Figure 11 (a) is a view of the overall morphology of the tensile fracture surface; Figure 11 (b) is a magnified view of the fracture surface. Figure 11 (c) is another magnified view of the topography of a local area.

[0028] Figure 12 SEM image of the shear fracture surface of a nickel-plated 304L / 20G bimetallic composite tube blank. Figure 12 (a) is a diagram of the overall morphology of the shear fracture surface; Figure 12 (b) is a magnified view of the local shear fracture surface.

[0029] In the diagram: 1. 304L stainless steel pipe; 2. Frame; 3. Rotating base; 4. Conductive plate; 5. Preheating system; 6. Preheating coil; 7. 20G carbon steel bar. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Given the common problems in bimetallic composite billet preparation, such as uncontrollable microstructure and composition of the interface transition layer, segregation, and sensitivity to defects, especially for high-quality metallurgical composites of stainless steel and carbon steel, there is currently no mature engineering composite process. Research has shown that solid-liquid composite technology has significant advantages in preparing high-strength bimetallic interfaces, while electroslag remelting technology can achieve metal purification and refining. Combining the two is expected to provide a new technical path to solve the aforementioned problems. Therefore, this invention addresses the defects of interfacial wettability failure and slag inclusion difficulties in bimetallic bonding by proposing an electroslag remelting solid-liquid composite casting technology. It also proposes electroplating a nickel layer on the inner wall of the outer base tube to improve the wetting relationship between the outer base tube and the inner solid bar, and using a low-melting-point slag system to assist in slag removal for preparing the bimetallic composite billet. During the composite process, the slag serves as both an arc-initiating medium and a refining slag pool after melting, while also isolating external air to prevent external oxygen from participating in the entire composite process. The present invention preferably uses a slag system with low melting point and low viscosity to improve the fluidity of the slag pool and promote the floating and discharge of interfacial molten slag. At the same time, the resistivity of the slag system is adjusted to maintain the temperature field required for the stable melting of solid bars as consumable electrodes.

[0032] The bimetallic composite tube blank involved in this invention consists of an outer base tube and an inner solid bar. Specifically, in this case, the outer base tube is 304L stainless steel, and the inner solid bar is 20G carbon steel. The solid bar serves as a consumable electrode. Under the high temperature conditions generated by electroslag arc initiation, the 20G carbon steel forms a molten metal liquid that wets the surface of the 304L stainless steel, thereby achieving metallurgical composite formation of the bimetallic composite tube blank.

[0033] The main chemical compositions of the 304L stainless steel and 20G carbon steel involved in this invention are shown in Table 1.

[0034] Table 1

[0035] It should be noted that, for the sake of simplicity, the present invention stipulates that the terms "304L stainless steel" and "304L" both refer to the same material, namely 304L austenitic stainless steel; the terms "20G carbon steel" and "20G" both refer to the same material, namely 20G carbon steel.

[0036] Next, the technical solution of the present invention will be described in detail based on 304L stainless steel and 20G carbon steel.

[0037] This invention proposes a method for preparing a bimetallic composite tube blank with an electroplated nickel layer, comprising the following steps: First step: Electroplating a nickel layer on the inner wall of the outer base tube; polishing the inner wall of the outer base tube until a metallic luster is exposed, and after alkaline degreasing and acid pickling activation, electroplating a nickel layer on the activated inner wall; the electroplating nickel layer adopts an acidic Watt nickel plating solution system and controls the nickel layer thickness to be 10-20μm.

[0038] As a further implementation of this step, a 304L stainless steel tube with an outer diameter of 219mm and an inner diameter of 180mm is selected as the outer tube blank. The inner wall of the 304L stainless steel tube is polished using 80-120 grit sandpaper or a mechanical boring tool until all visible oxide scale is removed and a metallic luster is exposed, so that the surface roughness Ra reaches 3.2-6.3μm to enhance the mechanical interlocking force. Subsequently, the polished 304L stainless steel tube is immersed in an alkaline degreasing solution at 60-80℃ for 10-15 minutes, and then immediately subjected to acid pickling activation after rinsing with water. The alkaline degreasing solution contains 40-60g / L NaOH and 20-30g / L Na2CO3. Then, a mixed acid solution of 20-30wt% hydrochloric acid and 5-10wt% sulfuric acid is used to immerse the 304L stainless steel tube as a whole at room temperature for 1-3 minutes until uniform and vigorous bubbles are generated on its inner surface and it turns silvery-gray, so as to destroy the chromium-rich passivation film on the inner surface of the 304L stainless steel tube. The purpose of overall immersion is to ensure that the acidic activation solution fills the inner cavity of the 304L stainless steel pipe, so as to avoid gas retention in the pipe cavity and insufficient activation of the inner wall in some areas.

[0039] After activation, nickel electroplating is performed rapidly using an acidic Watt's nickel plating bath system. The bath temperature is controlled at 45-55℃, the pH value is maintained at 4.0-4.8, and the current density is set at 2.0-4.0 A / dm². The plating time is precisely controlled to ensure the deposited nickel layer thickness is strictly controlled within the range of 10-20 μm. Preferably, the plating time is set at a current density of 3.0 A / dm² for 25 minutes. After plating, the layer is immediately rinsed with deionized water and dried with hot air to prevent secondary oxidation of the nickel surface, and then set aside for later use.

[0040] The acidic Watt's nickel plating solution system used in this invention comprises: nickel sulfate (NiSO4·6H2O) 250-300 g / L, nickel chloride (NiCl2·6H2O) 40-50 g / L, and boric acid (H3BO3) 35-40 g / L. Its purpose is to form a continuous, dense, and well-adhesive nickel layer on the inner wall surface of a 304L stainless steel pipe after acid pickling and activation treatment. The surface of 304L stainless steel typically has a stable chromium-rich passivation film, which hinders the direct deposition of metallic nickel. Therefore, before electroplating, the surface oxide scale is removed by mechanical grinding to create a certain roughness, followed by alkaline washing to remove oil stains. Finally, an activation treatment using a mixed acid solution of hydrochloric acid and sulfuric acid is performed to break down the chromium-rich passivation film on the 304L surface and expose the fresh metal surface. Electroplating is then immediately performed, allowing the Ni²⁺ in the solution to dissipate. + Electrons are gained at the cathode surface and reduced to metallic nickel: Ni² + +2e - →Ni.

[0041] The nickel layer deposited in this invention can achieve good bonding stability by forming a micro-mechanical interlock through the rough surface structure of the 304L inner wall and by continuously electrocrystallizing on the activated stainless steel surface. Subsequently, during the electroslag remelting solid-liquid composite process, the nickel layer can also participate in the diffusion of interfacial elements and the formation of a transition layer between 304L and 20G, improving the wetting and spreading state of the liquid 20G steel on the 304L inner wall surface. Furthermore, the rationale for choosing nickel plating in this invention is based on the good metallurgical compatibility between Ni and the Fe-Cr matrix; the Ni layer can mitigate the abrupt compositional change between 304L and 20G, promoting the formation of the interfacial transition zone; and compared to other highly reactive elements, Ni is less likely to introduce a significant brittle reaction layer, which is beneficial for maintaining interfacial toughness.

[0042] In the acidic Watt nickel plating bath system used in this invention, NiSO4·6H2O is the main salt, primarily used to provide Ni²⁺. + Its concentration is controlled at 250–300 g / L. This concentration range ensures that the plating bath contains sufficient and stable Ni²⁺. + The concentration is sufficient to meet the requirements of large-area electrodeposition on the inner wall of the 304L aluminum alloy, reducing the Ni² concentration near the cathode. + A concentration gradient reduces concentration polarization, allowing the nickel layer to deposit continuously at a stable rate. If the NiSO4·6H2O concentration is below 250 g / L, then Ni² + Insufficient supply can easily lead to a decrease in deposition rate, insufficient coating thickness, and local discontinuity in the coating. It may also cause pinholes, pitting, or decreased adhesion due to the relatively enhanced hydrogen evolution. If the concentration of NiSO4·6H2O is higher than 300 g / L, the ionic strength of the plating solution will be too high, the coating crystals will easily become coarser, the internal stress will increase, the maintenance of the plating solution will be more difficult, and it will not be conducive to obtaining a uniform and dense nickel layer.

[0043] In the acidic Watt nickel plating bath system used in this invention, NiCl2·6H2O is mainly used to improve the conductivity of the solution and promote the activation and dissolution of the nickel anode; its concentration is controlled at 40–50 g / L. An appropriate amount of Cl... - It can reduce the resistance of the plating solution, improve the current distribution on the inner curved surface of the 304L aluminum alloy, and prevent passivation of the nickel anode surface, allowing the anode to continuously dissolve and replenish Ni². + If the NiCl2·6H2O concentration is below 40 g / L, the plating solution will have insufficient conductivity, leading to increased tank voltage, uneven current distribution, and easy passivation of the nickel anode, resulting in Ni²⁺ degradation. + The supply is unstable; if the NiCl2·6H2O concentration is higher than 50 g / L, the Cl in the plating solution will be unstable. - High content of nickel content increases the corrosivity of the plating solution, which can easily lead to increased internal stress, more pinholes, or rough surfaces in the plating layer, thereby reducing the stability of the nickel layer in the subsequent high-temperature solid-liquid composite process.

[0044] In the acidic Watt's nickel plating bath system used in this invention, H3BO3 acts as a pH buffer, with its concentration controlled at 35–40 g / L. During the electroplating process, the inner wall of the 304L stainless steel plate serves as the cathode, and Ni²⁺... + The reduction deposition on the surface may be accompanied by hydrogen evolution, causing localized pH changes near the cathode. If the pH fluctuations near the cathode are too large, Ni(OH)₂ colloidal precipitates are easily formed and become trapped in the coating, resulting in defects such as darkening, roughness, pinholes, or pitting. An appropriate amount of H₃BO₃ can stabilize the pH near the cathode diffusion layer, inhibiting the formation of Ni(OH)₂ precipitates, thereby improving the density and smoothness of the coating. If the H₃BO₃ concentration is below 35 g / L, the buffering capacity is insufficient, and coating defects increase; if the H₃BO₃ concentration is above 40 g / L, the improvement in buffering capacity is limited, and it may increase solution crystallization and precipitation, as well as maintenance difficulties.

[0045] This invention controls the current density within the range of 2.0–4.0 A / dm², preferably 3.0 A / dm². This current density range achieves a balance between deposition rate, coating density, and adhesion. If the current density is too low, the nickel deposition rate is slow, the coating thickness is insufficient, and discontinuous coatings are easily formed, making it difficult to fully utilize its role in improving wetting and isolating the oxide film during subsequent high-temperature lamination processes. If the current density is too high, the Ni² on the cathode surface... + Excessive consumption intensifies concentration polarization and accelerates hydrogen evolution, easily leading to rough, burnt, dendritic, or high-stress coatings, which reduce the bonding stability between the coating and 304L stainless steel. Therefore, controlling the current density to 2.0–4.0 A / dm² and controlling the nickel layer thickness to 10–20 μm through electroplating time can yield a nickel layer of moderate thickness, continuous density, and suitable for subsequent electroslag remelting solid-liquid composite bonding.

[0046] The concentration ranges of NiSO4·6H2O, NiCl2·6H2O, and H3BO3 are coordinated to ensure that the plating bath maintains good nickel ion supply, conductivity, anodic activation ability, and cathodic pH stability under conditions of 45–55℃, pH 4.0–4.8, and current density of 2.0–4.0 A / dm². This results in the formation of a continuous and dense nickel layer with a thickness of 10–20 μm on the inner wall surface of 304L stainless steel after mechanical polishing and acid pickling activation. This nickel layer can improve the wetting and spreading of liquid 20G steel on the inner wall surface of 304L during subsequent electroslag remelting solid-liquid composite processes, and participate in interfacial element diffusion and transition layer formation, thereby reducing the risk of interfacial inclusions, incomplete fusion, and delamination defects.

[0047] The second step is to construct an electroslag remelting solid-liquid composite casting equipment. The electroplated outer base tube is vertically fixed on a conductive rotating base and a preheating ring is placed around the outer base tube. Then, the inner solid bar material is inserted into the outer base tube as a consumable electrode to ensure that the annular gap is uniform. Then, dry solid slag material of the CaF2-Al2O3-CaO-MgO quaternary slag system is filled into the bottom of the annular gap in batches.

[0048] As a further implementation of this step, such as Figure 1 As shown, firstly, the pre-treated 304L stainless steel tube 1 is vertically fixed on a rotating support device, which includes a frame 2 and a moving mechanism. The frame 2 is equipped with a rotatable rotating base 3 and a conductive plate 4. The rotating base 3 is driven by the rotating mechanism at the bottom of the frame 2 and maintains the synchronous rotation of the rotating base 3 and the 304L stainless steel tube 1. The moving mechanism includes an external preheating system 5 and a movable preheating ring 6. Next, a 20G carbon steel rod 7 with a diameter of 120mm and a length of 5m is used as a consumable electrode, connected to the negative terminal of the power supply through a clamping device, and inserted into the interior of the 304L stainless steel tube 1. The relative positions of the 20G carbon steel rod 7 and the 304L stainless steel tube 1 are adjusted to ensure that the annular gap is uniform.

[0049] After assembly, dry quaternary slag solid material is filled into the gap between the bottom of the 304L stainless steel tube and the 20G consumable electrode. The quaternary slag system is selected from the following components by mass percentage: calcium fluoride (CaF2) 65-75%, alumina (Al2O3) 25-35%, calcium oxide (CaO) 17%-20%, and magnesium oxide (MgO) 3%-5%, with a total slag weight of 2kg. The quaternary slag system used in this invention is a low-melting-point, low-viscosity slag system. During the composite process, the liquid slag adhering to the interface is forcibly peeled off and squeezed to float to the surface of the slag pool, forming a clean bimetallic bonding interface.

[0050] Furthermore, the solid slag material of the present invention is added in batches, preferably in three batches, and the slag should be evenly distributed to the surrounding area during addition, avoiding accumulation in one place. The first batch of slag material accounts for 50-60% of the total slag material, with a filling height of 30-50mm, and is used to cover the arc-starting area between the bottom of the 304L stainless steel pipe and the 20G consumable electrode, so that a layer with certain resistance characteristics is formed between the solid slag material particles, and local electric arc and resistance heat are quickly generated after energization, causing the bottom slag material to melt first to form an initial liquid slag pool; after the initial liquid slag pool is formed and the current and voltage tend to stabilize, the second batch of slag material is added, accounting for 25%-35% of the total slag material, to expand the volume of the initial liquid slag pool and maintain a stable resistance heating state; subsequently, the third batch of slag material is added according to the slag pool liquid level and the melting state of the consumable electrode, accounting for 10%-20% of the total slag material, to adjust the slag pool depth and ensure that the end of the 20G consumable electrode is always melted in a stable high-temperature slag pool.

[0051] Compared with adding all the slag at once, the batch addition of solid slag involved in this invention can avoid a sudden drop in slag pool temperature, arc extinction, or current fluctuation caused by a large amount of cold slag entering the arc-starting zone at once. At the same time, it can reduce the risk of incompletely melted solid slag being drawn into the 304L / 20G composite interface, which is beneficial to improving the stability of the electroslag remelting solid-liquid composite process and reducing interface slag inclusions and incomplete fusion defects.

[0052] It should be noted that the CaF2-Al2O3-CaO-MgO quaternary slag system used in this invention is not arbitrarily selected, but rather designed to meet the comprehensive requirements of "rapid arc initiation, stable heating, reduced slag viscosity, and slag inclusion removal" during the 304L / 20G solid-liquid composite process. This slag system must simultaneously meet the requirements of low melting point, suitable resistivity, good fluidity, and a certain oxide absorption capacity to ensure stable melting of the 20G consumable electrode and the formation of a clean metallurgical bonding interface on the inner wall surface of the 304L.

[0053] CaF2 is the main flux component in the slag system. Its function is to lower the melting point and viscosity of the slag, improve its fluidity, and enable the solid slag to quickly form an initial liquid slag pool after arc ignition. CaF2 can provide Ca²⁺ in high-temperature molten slag. + and F - F -It can weaken the continuity of the Al-O network structure in the aluminate melt, reduce the degree of polymerization of the slag structure, thereby reducing the viscosity and surface tension of the slag, which is conducive to the slag detaching from the 304L / 20G interface and floating upward. If the CaF2 content is too low, the melting point of the slag system will increase and the viscosity will increase, which will easily cause the slag to remain at the composite interface and form slag inclusion defects. If the CaF2 content is too high, the resistivity of the slag system will decrease, the Joule heating capacity will be insufficient, and the slag pool will be too active, which will easily cause slag pool fluctuations and instability in the smelting process.

[0054] Al₂O₃ is a key component for adjusting the resistivity and high-temperature stability of the slag system. An appropriate amount of Al₂O₃ can increase the resistivity of the molten slag, generating stable resistance heat when current passes through the liquid slag pool, thus maintaining the temperature field required for continuous melting of the 20G consumable electrode. Furthermore, Al₂O₃ can improve the thermal stability of the slag pool, preventing uneven heating caused by excessively thin slag. If the Al₂O₃ content is too low, the resistance heat of the slag pool is insufficient, leading to instability in the electrode melting process; if the Al₂O₃ content is too high, the melting point and viscosity of the slag system increase, which is detrimental to slag flow and the removal of interfacial inclusions.

[0055] CaO is an important component for regulating slag basicity and the absorption capacity of oxide inclusions. During preheating and high-temperature lamination, 304L stainless steel easily forms a dense oxide film, represented by Cr2O3. This oxide film hinders direct contact between molten 20G steel and 304L stainless steel. Adding an appropriate amount of CaO can increase slag basicity, improve the slag's absorption and transfer capacity for FeO, Al2O3, and composite oxide inclusions, and work in conjunction with CaF2's effect of reducing slag viscosity and improving slag fluidity. This makes it easier for oxides and inclusions at the interface to be encapsulated, peeled off, and floated out by the slag, thus reducing the risk of oxide films and slag remaining at the composite interface. If the CaO content is too low, the slag basicity is insufficient, resulting in weak absorption and transfer capacity for oxide inclusions; if the CaO content is too high, it may increase the slag melting point and crystallization tendency, increasing slag viscosity, which is detrimental to slag pool stability and inclusion flotation.

[0056] MgO, as a stabilizing component of the slag system, is mainly used to adjust the high-temperature stability, basicity, and viscosity of the molten slag. The addition of a small amount of MgO can improve the slag pool's resistance to fluctuations, inhibit excessive slag erosion and drastic changes in composition, and maintain a relatively stable electrical and thermal conductivity during electroslag remelting. If the MgO content is too low, the slag pool stability will be insufficient; if the MgO content is too high, it will significantly increase the melting point and viscosity of the slag system, which is detrimental to rapid slag melting and the removal of inclusions.

[0057] Therefore, this invention lowers the melting point and viscosity using CaF2, adjusts resistivity and thermal stability using Al2O3, enhances the slag's alkalinity and oxygen removal capacity using CaO, and improves the high-temperature stability of the slag pool using MgO, enabling this quaternary slag system to simultaneously perform arc initiation, heating, refining, and slag removal functions. This slag system, in synergy with the electroplated nickel layer, effectively reduces the obstruction of the oxide film on the inner wall of the 304L slag to the wetting and spreading of liquid 20G, thereby reducing interfacial inclusions, delamination, and incomplete fusion defects.

[0058] The third step involves slag arc ignition and electroslag remelting to prepare a bimetallic composite tube blank. The bottom of the consumable electrode is kept in micro-contact with the solid slag layer. An arc ignition voltage and current are applied to generate an arc in the gaps between slag particles, melting the slag in situ to form a liquid slag pool. Once the slag pool stabilizes, the rotating base is immediately driven to rotate the preheated outer tube around its axis. Simultaneously, the working voltage and current for electroslag remelting are maintained, allowing the consumable electrode to melt and drip uniformly and continuously in the slag pool. After passing through the slag layer, the molten metal droplets rapidly spread onto the inner wall of the outer tube under the wetting effect of the electroplated nickel layer, forming a clean bimetallic interface. As the consumable electrode continues to melt and the slag pool level rises synchronously, a dense solid-liquid composite molding is finally completed from bottom to top, yielding the bimetallic composite tube blank.

[0059] As a further implementation of this step, the quaternary slag system used in this invention has a large number of tiny air gaps in its particle packing structure. When a voltage under certain conditions is applied, the strong electric field between the consumable electrode and the rotating base is sufficient to break down the air gaps between the slag particles, triggering gas discharge and generating a high-energy electric arc.

[0060] The specific operation process of slag arc ignition and electroslag remelting is as follows: First, the 20G consumable electrode is slowly lowered so that its bottom is 0-2mm away from the solid slag layer filled at the bottom of the 304L tube. To create good arc ignition conditions, the 20G consumable electrode is kept in close contact with the conductive rotating base. Then, a 50Hz AC power supply is connected, the arc ignition voltage is set to AC45V, and the arc ignition current is limited to 800A. Under the action of this high-voltage electric field, a momentary short circuit or gas ionization breakdown occurs at the contact point at the bottom of the consumable electrode, igniting a high-energy arc in the gaps between the slag particles. The huge heat released by the arc rapidly melts the solid slag around the contact point into a liquid state. As the amount of liquid slag increases, the current conduction path rapidly changes from unstable gas arc discharge to stable liquid molten slag resistance conduction. Under the continuous resistance heating, the surrounding solid slag gradually melts completely. After solid slag is added in batches to form a stable liquid slag pool, a large amount of Joule heat is generated when a high current passes through the pool. This Joule heat rapidly raises the temperature of the pool and maintains it at 1500-1600℃, which is much higher than the melting point of 20G carbon steel. The 20G consumable electrode inserted into the high-temperature stable slag pool gradually melts and forms droplets. At this point, the depth of the stable liquid slag pool is 45mm. Once the slag pool is stabilized, the rotating mechanism at the bottom of the frame is immediately activated. This mechanism uses a variable frequency motor and a high-speed ratio reducer to drive the rotating base. Through the stepless speed regulation of the frequency converter combined with the torque-increasing effect of the reducer, the rotating mechanism maintains a speed of 3-5 r / min, thereby driving the 304L tube to rotate. At the same time, the working voltage and melting current of the electroslag remelting are maintained at 45V and 800A, respectively, to ensure that the 20G consumable electrode melts and drips evenly and continuously. During this process, the ultra-low speed rotating base drives the 304L stainless steel tube to rotate while a movable preheating ring preheats the rotating 304L stainless steel tube, thereby creating a uniform thermal field and effectively preventing the 304L tube wall from burning through due to local overheating. As the 20G steel droplets pass through the slag layer and converge at the bottom, thanks to the excellent wettability of the electroplated nickel layer, the 20G steel liquid quickly spreads on the inner surface of the 304L stainless steel. Furthermore, due to the use of a low-melting-point, low-viscosity slag system, the liquid slag originally attached to the interface can be forcibly peeled off and squeezed to float to the surface of the slag pool, thus forming a clean 304L / 20G bimetallic interface below the slag pool. The entire solid-liquid composite process does not require external forced water cooling. The 304L stainless steel tube acts as a crystallizer, relying on natural air convection for heat dissipation and the continuous heat input from the molten metal pool inside the 304L stainless steel tube to achieve dynamic thermal equilibrium. As the electrodes continue to melt and the liquid level in the slag pool rises synchronously, a dense solid-liquid composite is finally formed from bottom to top.

[0061] It should be noted that the process of adding solid slag in batches to form a stable liquid slag pool is as follows: the first batch of slag is added and the arc is ignited in 5-10 minutes, and the initial liquid slag pool is formed in 10-20 minutes. After the initial liquid slag pool is formed, the second batch of slag is added. This second batch of slag is used to expand the volume of the initial liquid slag pool. After being added, it is stabilized for 5-10 minutes to allow it to fully melt in the initial liquid slag pool. Then, the third batch of slag is added. This third batch of slag is used to adjust the liquid level and depth of the slag pool. After being added, it is stabilized for 5-10 minutes to allow the conductivity, temperature field, and liquid level of the entire liquid slag pool to tend to stabilize. After the slag pool is stabilized, the 20G consumable electrode is controlled to continuously melt and drip. The stabilization melting and bonding time is 3-5 hours, so that the 20G molten metal passes through the slag pool and fills the interior of the 304L stainless steel tube from bottom to top, forming a metallurgical bonding interface with the nickel-plated 304L inner wall. Once the 20G consumable electrode reaches the predetermined melting length, continue the electroslag heat input for 20-40 minutes to allow the upper molten metal pool to complete the feeding and promote the floating of residual slag and oxide inclusions into the slag pool. Then, stop the power supply and let the bimetallic composite tube blank stand in place for 30-60 minutes to ensure that the internal molten metal pool is completely solidified. Finally, let it cool naturally in the air for 3-6 hours to room temperature to obtain the bimetallic composite tube blank.

[0062] Furthermore, in this step, the rotating base speed is controlled at 3-5 r / min. This speed is not arbitrarily set, but is determined comprehensively based on the heating uniformity of the 304L stainless steel tube, the spreading stability of the 20G molten steel, the stability of the slag pool, and the interface slag removal requirements. During the electroslag remelting solid-liquid composite process, the 20G consumable electrode melts at the center position, and the droplets spread and solidify near the inner wall of the 304L stainless steel tube after passing through the slag pool. If the 304L stainless steel tube is completely stationary, the circumferential temperature field of the inner wall of the 304L stainless steel tube is prone to unevenness due to the eccentricity of the consumable electrode, local temperature fluctuations in the slag pool, and uneven flow of the molten steel. Overheating, ablation, or insufficient spreading of molten steel may occur in some areas, leading to inconsistent circumferential composite quality.

[0063] If the rotational speed of the rotating base is below 3 r / min, the circumferential movement of the rotating base is too slow, which can easily cause excessively long heating time in local areas of the 304L stainless steel pipe, resulting in uneven temperature distribution, leading to local overheating, increased oxidation, or uneven heating of the pipe wall. Simultaneously, the large temperature difference when the insufficiently preheated area comes into contact with the liquid 20G is detrimental to the continuous spreading of the molten steel and stable interfacial bonding. If the rotational speed of the rotating base is above 5 r / min, the residence time of the rotating base per unit circumferential position is too short, resulting in insufficient local heat input. This makes it difficult for the inner wall of the 304L stainless steel pipe to reach a stable preheated state, easily causing the 20G molten steel to rapidly cool and solidify upon contact with the inner wall of the 304L stainless steel pipe, leading to insufficient wetting and spreading, discontinuous interfacial bonding, or local incomplete fusion defects. Therefore, controlling the rotational speed of the rotating base at 3-5 r / min can ensure a uniform and stable circumferential preheating effect for the 304L stainless steel pipe while avoiding local overheating.

[0064] Next, the present invention will further verify the technical solution of the present invention through a series of experimental methods.

[0065] Figure 2 Macroscopic morphology and partial magnified views of the unnickel-plated comparative sample are shown. Except for the absence of nickel plating, the process parameters for this preform are consistent with those of the nickel-plated composite tube preform of this invention. It can be seen that after cooling and demolding, the 304L outer tube and the 20G inner core fail to achieve a metallurgical connection, resulting in a significant gap filled with a large amount of non-metallic electroslag material.

[0066] Figure 3 Macroscopic morphology and magnified views of the nickel-plated composite tube blank are shown, demonstrating successful bimetallic composite tube blank bonding. The volume of the characterization sample is 10mm × 10mm × 10mm. First, the sample was sequentially polished using 240#, 400#, 600#, 800#, 1000#, 1200#, 1500#, and 2000# sandpaper. Then, the 20G and 304L sides were etched using nitric acid alcohol solution and aqua regia, respectively. After cleaning and drying, the interfacial microstructure was observed using an optical microscope (OM, Leica Flexacam C3). Figure 4 As shown; the interface transition zone, interface microstructure, and fracture morphology were analyzed using a scanning electron microscope (SEM, ZEISS Sigma 300), such as... Figure 5 , 11 As shown in Figure 12; the interface region was analyzed using energy dispersive spectroscopy (EDS) to study the elemental distribution in the interface region, such as... Figure 6-9 As shown.

[0067] like Figure 4 (a) to Figure 4 (c) and Figure 5 (a) to Figure 5 As shown in (d), the interface between 304L and 20G exhibits a wavy interlocking structure with a dense bond. No microcracks, pores, or inclusions were observed within the field of view. The local microstructure on the 304L side displays typical austenitic characteristics with clear grain outlines and obvious twins within the grains. The local microstructure on the 20G side shows a distinct acicular and feather-like interlocking morphology, exhibiting Widmanstätten characteristics. The composite interface is not mechanically bonded, forming a transition zone with a width of 2-4 μm, demonstrating clear metallurgical bonding characteristics.

[0068] like Figure 6 (a)- Figure 6 As shown in (d), the overall EDS surface scan results reveal that Fe, Cr, and Ni elements exhibit distinct partitioning on both sides of the interface; within the interface transition region, the distribution of Fe, Cr, and Ni is not abrupt, but rather displays certain compositional transition characteristics. Figure 7 (a)- Figure 7 As shown in (d), the line scan results indicate that Fe, Cr, and Ni exhibit gradient changes near the interface, suggesting elemental migration and compositional transition between 304L and 20G, forming an elemental diffusion influence zone of 80-100 μm. Figure 8 (a)- Figure 8 As shown in (d), the EDS spot scan results indicate that the test point is located near the composite interface. The spectral results show that this region is dominated by Fe, with Cr, Ni, C, and small amounts of Mn and S also detected. The high Fe content indicates that this location is still predominantly Fe-based. The presence of Cr and Ni suggests that the 304L side elements and the electroplated Ni layer participated in element migration and compositional reconstruction near the interface during the high-temperature composite process. In summary, the EDS results indicate that the composite interface is not a simple abrupt transition interface, but rather a transitional region containing Fe, Cr, Ni, and C. Figure 5-9 SEM morphology, EDS surface scan, line scan and spot scan analysis showed that the area near the composite interface was not a single-component abrupt interface, but rather a transitional region containing elements such as Fe, Cr, Ni and C, which was different from the matrix structures on both sides.

[0069] like Figure 10 (a) to Figure 10 As shown in (b), the tensile shear test was conducted using a 200kN universal testing machine with a tensile speed of 3mm / min. The total length of the tensile shear specimen was 60mm, the gauge length was 18mm, and the tensile and shear cross-sectional area was 2mm × 1.5mm = 3mm². 2 The bimetallic composite tube blank exhibits excellent mechanical properties, with an overall shear strength of 220 MPa and a tensile strength of 456 MPa, indicating that the composite tube maintains good strength and toughness.

[0070] like Figure 11As shown, Figure 11 (a) shows the overall morphology of the tensile fracture surface. It can be seen that the fracture surface is relatively undulating and there are obvious tearing features in some areas, indicating that the specimen underwent a relatively complex fracture process during the tensile process. Figure 11 (b) shows the magnified morphology of the fracture surface, where a large number of dimples and tear ridges can be observed, indicating that the region is dominated by ductile fracture characteristics. Figure 11 (c) shows another magnified area of ​​the morphology, revealing relatively smooth cleavage-like steps and lamellar tearing features, indicating that in addition to ductile tearing, local quasi-cleavage fracture features also accompanied the fracture process. Therefore, the tensile fracture surface as a whole exhibits a mixed fracture mode characterized by ductile fracture as the main feature and local quasi-cleavage features.

[0071] like Figure 12 As shown, Figure 12 (a) shows the overall morphology of the shear fracture surface. It can be seen that the fracture surface is relatively rough, and the local area shows obvious lamellar peeling and step-like fracture characteristics, indicating that the interface under shear load has undergone tearing failure along the interface. Figure 12 (b) shows a magnified view of the local shear fracture surface, revealing tear ridges, voids, and irregular step-like morphology, indicating localized plastic deformation and crack propagation during shear failure. Compared to the tensile fracture surface, the shear fracture surface exhibits fewer dimples and more pronounced lamellar tearing and shear step features, suggesting that the fracture process is primarily dominated by shear stress.

[0072] The above are merely embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method for preparing a bimetallic composite tube blank with an electroplated nickel layer, characterized in that, Includes the following steps: S1. Electroplating a nickel layer on the inner wall of the outer base tube; grinding the inner wall of the outer base tube until a metallic luster is exposed, and after alkaline degreasing and acid pickling activation, electroplating a nickel layer on the activated inner wall; the electroplating nickel layer adopts an acidic Watt nickel plating solution system and controls the nickel layer thickness to be 10-20μm; S2. Construct an electroslag remelting solid-liquid composite casting equipment; vertically fix the electroplated outer base tube on a conductive rotating base and place a preheating ring around the outer base tube; then insert the inner solid bar material as a consumable electrode into the outer base tube to ensure that the annular gap is uniform; then fill the bottom of the annular gap in batches with dry CaF2-Al2O3-CaO-MgO quaternary slag system solid slag material. S3. Preparation of bimetallic composite tube blank by slag arc ignition and electroslag remelting: The bottom of the consumable electrode is kept in micro-contact with the solid slag layer. An arc ignition voltage and current are applied to generate an electric arc in the gaps between slag particles and melt the slag in situ to form a liquid slag pool. After the slag pool finally stabilizes, the rotating base is immediately driven to rotate the preheated outer base tube around its axis. At the same time, the working voltage and current of electroslag remelting are maintained so that the consumable electrode melts and drips uniformly and continuously in the slag pool. After the metal droplets pass through the slag layer, they quickly spread on the inner wall of the outer base tube under the wetting effect of the electroplated nickel layer to form a clean bimetallic interface. As the consumable electrode continues to melt and the liquid level of the slag pool rises synchronously, a dense solid-liquid composite molding is finally completed from bottom to top to obtain a bimetallic composite tube blank.

2. The method for preparing a bimetallic composite tube blank with an electroplated nickel layer according to claim 1, characterized in that, The grinding process involves using 80-120 grit sandpaper or a mechanical boring tool to grind the inner wall until all visible oxide scale is removed, revealing a metallic luster and a surface roughness Ra of 3.2-6.3 μm. The alkaline degreasing process uses an alkaline degreasing solution at 60-80℃, containing 40-60 g / L NaOH and 20-30 g / L Na2CO3, with an immersion time of 10-15 minutes. After rinsing with water, the tube is immediately subjected to acid pickling activation. The acid pickling activation process uses a mixed acid solution of 20-30 wt% hydrochloric acid and 5-10 wt% sulfuric acid, immersing the base tube at room temperature for 1-3 minutes until uniform and vigorous bubbles are generated on the inner surface, turning it silvery-gray, in order to destroy the chromium-rich passivation film on the inner surface of the outer base tube.

3. The method for preparing a bimetallic composite tube blank with an electroplated nickel layer according to claim 1, characterized in that, The acidic Watt nickel plating solution system includes: 250-300 g / L nickel sulfate, 40-50 g / L nickel chloride, and 35-40 g / L boric acid. The plating solution temperature is controlled at 45-55℃, the pH value is 4.0-4.8, and the current density is set at 2.0-4.0 A / dm². After electroplating, the solution is immediately rinsed with deionized water and dried with hot air to prevent secondary oxidation of the nickel layer surface.

4. The method for preparing a bimetallic composite tube blank with an electroplated nickel layer according to claim 1, characterized in that, The solid slag material contains the following components by mass percentage: CaF2 65-75%, Al2O3 25-35%, CaO 17-20%, MgO 3-5%. The quaternary slag system is a low-melting-point, low-viscosity slag system. During the composite process, the liquid slag attached to the interface is forcibly peeled off and squeezed to float to the surface of the slag pool, forming a clean bimetallic interface.

5. The method for preparing a bimetallic composite tube blank with an electroplated nickel layer according to claim 1, characterized in that, The solid slag material is added in batches. The first batch is 50-60% of the total slag volume, with a filling height of 30-50mm, to form an initial liquid slag pool. The second batch is 25-35% of the total slag volume, to expand the volume of the initial liquid slag pool and maintain a stable resistive heating state. The third batch is 10-20% of the total slag volume, to adjust the depth of the slag pool and ensure that the end of the consumable electrode is always melted in a stable high-temperature slag pool.

6. The method for preparing a bimetallic composite tube blank with an electroplated nickel layer according to claim 5, characterized in that, The micro-contact state is such that the bottom of the consumable electrode is 0-2 mm away from the solid slag layer, while the consumable electrode maintains contact with the conductive rotating base; the arc-starting voltage is AC45V, the arc-starting current is 800A, and the power supply frequency is 50Hz; the time for the first batch of slag to be added and the arc to start is 5-10 minutes, and the time for the initial liquid slag pool to form is 10-20 minutes. After the initial liquid slag pool is formed, a second batch of slag is added. This second batch of slag is used to expand the volume of the initial liquid slag pool and stabilizes after addition. The slag is allowed to fully melt in the initial liquid slag pool for 5-10 minutes. Then, a third batch of slag is added to adjust the liquid level and depth of the slag pool. After adding the third batch, the liquid is allowed to stabilize for 5-10 minutes to stabilize the conductivity, temperature field, and liquid level of the entire liquid slag pool. Once the slag pool is finally stabilized, a stable liquid slag pool with a depth of 45 mm is formed. The temperature of the stable liquid slag pool is maintained at 1500-1600℃. Meanwhile, the stable melting and recombination time of the self-consumable electrode continuously melting and dripping is 3-5 hours.

7. The method for preparing a bimetallic composite tube blank with an electroplated nickel layer according to claim 1, characterized in that, The mechanism for driving the rotating base adopts a drive method of variable frequency motor and high-speed ratio reducer. The synchronous rotation of the rotating base and the outer base tube is achieved by stepless speed regulation of the frequency converter and the torque increase effect of the reducer, and the rotation speed is 3-5 r / min.

8. The method for preparing a bimetallic composite tube blank with an electroplated nickel layer according to claim 1, characterized in that, No external forced water cooling is used in the entire solid-liquid composite process. The outer base tube acts as a crystallizer, and dynamic thermal equilibrium is achieved by relying on natural air convection for heat dissipation and the continuous heat input of the molten metal pool inside the outer base tube.

9. The method for preparing a bimetallic composite tube blank with an electroplated nickel layer according to claim 1, characterized in that, After the consumable electrode reaches the predetermined melting length, continue to maintain the electroslag heat input time for 20-40 minutes to allow the upper molten metal pool to complete the feeding and promote the floating of residual slag and oxide inclusions into the slag pool. Then, stop the power supply and let the composite tube blank stand in place for 30-60 minutes to allow the internal molten metal pool to completely solidify. Finally, let it cool naturally in the air for 3-6 hours to room temperature to obtain the bimetallic composite tube blank.

10. A bimetallic composite tube blank with an electroplated nickel layer, characterized in that, The bimetallic composite tube blank with an electroplated nickel layer is prepared by any one of the preparation methods of claims 1-9. The outer base tube is made of 304L stainless steel, and the inner consumable electrode is made of 20G carbon steel. The interface between the 304L stainless steel and the 20G carbon steel is wavy and interlocked. At the same time, the interface forms a transition zone with a width of 2-4μm and an element diffusion influence zone with a width of 80-100μm. In addition, the shear strength of the bimetallic composite tube blank reaches 220MPa and the tensile strength reaches 456MPa.

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