A high-strength seamless connection manufacturing method for low-carbon high-molybdenum thin-walled stainless steel pipes

Abstract

The present invention relates to the technical field of metal pipe connection, and discloses a high-strength seamless connection manufacturing method for low-carbon high-molybdenum thin-walled stainless steel pipes, including: performing vacuum pre-diffusion treatment of titanium and niobium elements on the connection end faces of low-carbon high-molybdenum thin-walled stainless steel pipes to form a surface segregation layer; after butt joint, applying a local instantaneous thermal field with high-frequency pulsed current and synchronously applying axial fluid pressure to drive interface plastic flow, so that molybdenum elements are supersaturated and solid-solved; finally, performing in-situ gradient cooling by liquid nitrogen atomization spraying to induce the precipitation of nano-carbides to strengthen the interface. By using this method, it is possible to effectively suppress the connection failure problem caused by the enrichment of molybdenum elements at grain boundaries in the prior art. The pre-diffusion treatment provides a uniform element distribution substrate for subsequent connection, improving the toughness of the connection area from the source and avoiding the risk of connection failure caused by micro-segregation, providing a reliable technical approach for realizing high-performance connection of thin-walled stainless steel pipes.

Description

Technical Field

[0001] The present invention relates to the technical field of metal pipe connection, and in particular to a high-strength seamless connection manufacturing method for low-carbon high-molybdenum thin-walled stainless steel pipes. Background Art

[0002] In the modern industrial field, due to its excellent corrosion resistance and high-strength characteristics, low-carbon high-molybdenum thin-walled stainless steel pipes are widely used in fields such as chemical industry, medicine, aerospace, etc. However, how to achieve high-strength seamless connection between such special material thin-walled pipes has always been a technical problem faced in the industry. Generally, for the connection of metal pipes, welding is a common and direct method. By melting the base metal and welding material at high temperature to form a metallurgical bond, a firm connection can be achieved. However, for low-carbon high-molybdenum stainless steel, due to the high melting point of molybdenum element, it is extremely easy to segregate and enrich at the grain boundaries during the welding process. This segregation will lead to uneven organizational structure in the connection area, significantly reducing the toughness of the weld seam, making the connection part more prone to stress corrosion cracking during long-term service, and seriously affecting the safety and service life of the equipment.

[0003] To make up for the insufficient strength caused by welding, the industry usually adopts the method of increasing the wall thickness. Although this method can improve the overall strength of the connection to a certain extent, it sacrifices the advantage of lightweight of thin-walled pipes, increases the material cost and equipment burden, and does not meet the trend requirements of modern industry for lightweight, energy conservation and consumption reduction. In addition, for pipes with a relatively thin wall thickness (usually less than or equal to 1 mm), traditional methods such as press fitting or welding are prone to local collapse and deformation of the pipe wall when applying high pressure or high temperature, or the grains become coarse due to the influence of high temperature, making it difficult to ensure both the sealing performance and strength of the connection at the same time.

[0004] On the other hand, in the process of material preparation and connection, the timing arrangement of heat treatment and cold processing is also crucial. In the existing process flow, solution treatment and cold processing are usually carried out step by step. This step-by-step treatment method may cause microcracks to occur due to the release of residual stress during the subsequent connection process of the material, further weakening the reliability of the connection. To avoid this situation, the industry has to make a certain compromise between the strength and corrosion resistance of the material, and cannot fully utilize the excellent performance of low-carbon high-molybdenum stainless steel.

[0005] Therefore, how to avoid the grain boundary segregation of molybdenum element in the connection area and achieve non-deformation and high-precision connection of thin-walled pipes on the premise of ensuring the connection strength has become the technical problem to be solved by the present invention. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a high-strength seamless connection manufacturing method for low-carbon high-molybdenum thin-walled stainless steel pipes in view of the defects existing in the above-mentioned prior art, so as to solve the problems of molybdenum segregation prone to occur during the connection of low-carbon high-molybdenum thin-walled stainless steel pipes, easy deformation of thin-walled pipe connections, and the difficulty of balancing strength and corrosion resistance in the existing process.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A high-strength seamless connection manufacturing method for low-carbon high-molybdenum thin-walled stainless steel pipes, comprising the following steps:

[0008] Step 1, pre-treat the connection end faces of the low-carbon high-molybdenum thin-walled stainless steel pipes. The pre-treatment includes pre-diffusion treatment of titanium and niobium elements in a vacuum furnace to form a pre-diffusion layer containing a nano-scale titanium segregation layer and a nano-scale niobium segregation layer on the surface of the connection end faces. Among them, the weight percentage of titanium element is 0.005-0.01%, and the weight percentage of niobium element is 0.004-0.008%;

[0009] Step 2, butt joint two low-carbon high-molybdenum thin-walled stainless steel pipes pre-treated in Step 1, and apply a dynamic thermal field-assisted connection to the butt joint area. The dynamic thermal field is instantaneously applied locally to the butt joint area through a high-frequency pulsed current, the temperature is controlled at 500-800 °C, the duration is 0.1-1 second, and the single-pulse energy meets the condition of: wherein, is the instantaneous current, is the resistance of the connection area, is the single-pulse duration; and while applying the dynamic thermal field, apply a fluid pressure axially to the butt joint area. The range of the fluid pressure is 50-200 MPa to drive plastic flow at the interface of the butt joint area, so that molybdenum elements are supersaturated and solid-solved in the austenite matrix;

[0010] Step 3, after the connection in Step 2 is completed, perform in-situ gradient cooling on the connection area. The gradient cooling is achieved by spraying liquid nitrogen in an atomized manner, and the cooling rate is controlled at 100-500 °C / s to induce nano-carbides to precipitate at the connection interface by using the pre-diffusion layer, thereby strengthening the toughness of the connection interface. Among them, the phase difference between the high-frequency pulsed current and the axial fluid pressure is controlled to meet

[0011] As a further solution of the present invention, in Step 2, the frequency of the high-frequency pulsed current is 10-50 kHz, the single-pulse time is 0.1-0.5 second, and the fluid pressure is 80-150 MPa.

[0012] As a further solution of the present invention, in the step 1, the weight percentage of titanium element is 0.007 - 0.009%, the weight percentage of niobium element is 0.005 - 0.007%, the pre-diffusion treatment temperature is 850 - 900 °C, and the vacuum degree is less than or equal to .

[0013] As a further solution of the present invention, in the step 2, the high-frequency pulse power supply and the hydraulic system are linked through a PID controller to adjust the phase difference between the dynamic thermal field and the fluid pressure in real time , so as to meet .

[0014] As a further solution of the present invention, the phase difference between the high-frequency pulse current and the axial fluid pressure is controlled by an active compensation mechanism, wherein, .

[0015] As a further solution of the present invention, the active compensation mechanism is realized through the following steps: real-time collecting the IGBT switch state signal of the high-frequency pulse power supply as the trigger reference for starting the hydraulic pump; calling the corresponding pressure amplitude parameter from the preset parameter library according to the electromagnetic field strength gradient of the current pulse; performing a difference operation on the pressure amplitude parameter and the real-time pressure feedback value of the hydraulic system through a PID controller, and dynamically adjusting the pressure output curve to make the phase difference between the dynamic thermal field and the fluid pressure .

[0016] As a further solution of the present invention, the pressure amplitude parameter in the preset parameter library is pre-established based on the non-linear mapping relationship between the electromagnetic field strength gradient and the fluid pressure amplitude of low-carbon high-molybdenum thin-wall stainless steel pipes with different wall thicknesses, and the non-linear mapping relationship is generated by a material database or material simulation software.

[0017] As a further solution of the present invention, the wall thickness of the low-carbon high-molybdenum thin-wall stainless steel pipe is 0.5 - 1 mm, and the weight percentage of molybdenum element is greater than or equal to 5%.

[0018] As a further solution of the present invention, the gradient cooling in the step 3 enables the standard deviation of the molybdenum content distribution at the connection interface to meet , and the shear strength of the connection interface is increased by more than 40%.

[0019] As a further solution of the present invention, the method is applicable to the connection of low-carbon high-molybdenum thin-wall stainless steel pipes with a wall thickness of 0.5 - 1 mm, and the wall thickness reduction rate after connection is less than or equal to 2%.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. By performing surface pre-diffusion treatment of titanium and niobium elements before connecting low-carbon high-molybdenum thin-walled stainless steel pipes, the problem of molybdenum element enrichment at grain boundaries during welding or plastic forming can be effectively inhibited, laying a foundation for the subsequent connection process with uniform element distribution, enhancing the toughness of the connection area from the source, and avoiding the risk of connection failure caused by micro-segregation.

[0022] 2. The connection is carried out by applying a local instantaneous thermal field with high-frequency pulsed current and synchronously applying axial fluid pressure, realizing the dynamic coupling of the thermal field and the pressure field, enabling the molybdenum element to be fully dissolved in the austenite matrix, avoiding the limitations of traditional static solution treatment, thus achieving high-quality metallurgical bonding at the atomic level at the connection interface, significantly reducing the risk of deformation of the thin-walled pipe caused by high temperature or high pressure, and ensuring the connection accuracy.

[0023] 3. For the in-situ gradient cooling process after connection, using the pre-formed titanium and niobium segregation layer as the induction core, promoting the precipitation of nano-scale carbides at the connection interface. The optimization of this microstructure enhances the toughness of the connection interface, and at the same time, the formed continuous passivation film improves the corrosion resistance of the material, effectively solving the contradiction that it is difficult to balance strength and corrosion resistance in traditional processes, and achieving the coordinated improvement of performance.

[0024] 4. Through the active compensation control of the phase difference between the high-frequency pulsed current and the axial fluid pressure, the energy input and material flow during the connection process can be more precisely regulated, enabling the thermal effect and mechanical effect to better cooperate with each other, avoiding interface defects caused by timing misalignment, further enhancing the strength and reliability of the connection, and achieving high-quality connection through a refined control mechanism. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0026] Figure 1 It is a schematic diagram of the pre-treatment of the connection end face of the low-carbon high-molybdenum thin-walled stainless steel pipe of the present invention;

[0027] Figure 2 It is a schematic diagram of the coupling effect of high-frequency pulsed current and axial fluid pressure of the present invention;

[0028] Figure 3 It is a schematic diagram of the pre-diffusion treatment of molybdenum element and element distribution of the present invention;

[0029] Figure 4Schematic diagram of the high-frequency pulse current and fluid pressure control process of the present invention;

[0030] Figure 5 Schematic diagram of the liquid nitrogen atomization spray cooling and nano-carbide precipitation of the present invention. Specific implementation manners

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0032] Please refer to Figures 1 - 5 , a method for manufacturing high-strength seamless connection of low-carbon high-molybdenum thin-walled stainless steel pipes, comprising the following steps:

[0033] Step 1, pre-treat the connection end faces of the low-carbon high-molybdenum thin-walled stainless steel pipes. The pre-treatment includes pre-diffusion treatment of titanium element and niobium element in a vacuum furnace to form a pre-diffusion layer containing a nano-scale titanium segregation layer and a nano-scale niobium segregation layer on the surface of the connection end faces. Among them, the weight percentage of titanium element is 0.005 - 0.01%, and the weight percentage of niobium element is 0.004 - 0.008%;

[0034] Step 2, butt-join two low-carbon high-molybdenum thin-walled stainless steel pipes pre-treated in Step 1, and apply a dynamic thermal field-assisted connection to the butt-joined area. The dynamic thermal field is locally and instantaneously applied to the butt-joined area through a high-frequency pulse current, the temperature is controlled at 500 - 800 °C, the duration is 0.1 - 1 second, and the single-pulse energy satisfies the condition of: wherein, is the instantaneous current, is the resistance of the connection area, is the single-pulse duration; and while applying the dynamic thermal field, apply a fluid pressure to the butt-joined area along the axial direction. The range of the fluid pressure is 50 - 200 MPa to drive the interface of the butt-joined area to undergo plastic flow, so that molybdenum element is supersaturated and solid-solved in the austenite matrix; the instantaneous current It is a function of the current varying with time during the output of the high-frequency pulse power supply. The specific control of the current is adjusted by the high-frequency pulse power supply, and the current waveform and amplitude are precisely controlled through a frequency converter. Usually, the waveform of the instantaneous current is a pulse waveform, and its amplitude and frequency are set according to the requirements during the connection process. During the implementation, the amplitude, frequency, and pulse duration of the current are all set by the output of the power supply system and adjusted according to real-time feedback to achieve the required temperature range and thermal effect. The current is adjusted in real time through a PID control system to ensure the uniformity and accuracy of the thermal field. And, the resistance of the connection area Refers to the resistance generated in the contact area during the connection process due to material properties (such as conductivity) and temperature changes. The resistance value usually changes with the temperature of the connection area. Therefore, the influence of temperature on the resistance needs to be considered when calculating the pulse energy. The calculation of the resistance can be based on the following formula: Where is the resistivity of the material in the connection area, is the length of the connection area, is the cross-sectional area of the contact area. The change in resistance will occur with the temperature change of the connection area. Therefore, it is necessary to monitor the temperature in real time during the dynamic process to correct the resistance calculation. Pulse duration Refers to the duration of the high-frequency pulse current acting on the connection area. The pulse duration determines the intensity of the thermal field action and the local heating effect of the material. Usually, it needs to be adjusted according to the actual requirements of the connection process. The pulse duration is adjusted through the high-frequency power supply and used in conjunction with the current amplitude to ensure that sufficient heat is generated in the connection area to promote the plastic flow of the connection area. Precise control of the duration helps to avoid material deformation or quality degradation caused by overheating.

[0035] Step 3, after the connection in Step 2 is completed, in-situ gradient cooling is performed on the connection area. The gradient cooling is achieved through liquid nitrogen atomization spraying, and the cooling rate is controlled at 100 - 500 °C / s to utilize the pre-diffusion layer to induce the precipitation of nano-carbides at the connection interface, thereby strengthening the toughness of the connection interface. Among them, the phase difference between the high-frequency pulse current and the axial fluid pressure is controlled to meet .

[0036] As a further solution of the present invention, in Step 2, the frequency of the high-frequency pulse current is 10 - 50 kHz, the single-pulse time is 0.1 - 0.5 seconds, and the fluid pressure is 80 - 150 MPa.

[0037] As a further solution of the present invention, in the step 1, the weight percentage of titanium element is 0.007 - 0.009%, the weight percentage of niobium element is 0.005 - 0.007%, the pre-diffusion treatment temperature is 850 - 900 °C, and the vacuum degree is less than or equal to .

[0038] As a further solution of the present invention, in the step 2, the high-frequency pulse power supply and the hydraulic system are linked through a PID controller to adjust the phase difference between the dynamic thermal field and the fluid pressure in real time , so as to meet .

[0039] As a further solution of the present invention, the phase difference between the high-frequency pulse current and the axial fluid pressure is controlled by an active compensation mechanism, wherein, .

[0040] As a further solution of the present invention, the active compensation mechanism is realized through the following steps: collecting the IGBT switch state signal of the high-frequency pulse power supply in real time as the trigger reference for starting the hydraulic pump; calling the corresponding pressure amplitude parameter from the preset parameter library according to the electromagnetic field strength gradient of the current pulse; performing a difference operation on the pressure amplitude parameter and the real-time pressure feedback value of the hydraulic system through a PID controller, and dynamically adjusting the pressure output curve to make the phase difference between the dynamic thermal field and the fluid pressure .

[0041] As a further solution of the present invention, the pressure amplitude parameters in the preset parameter library are pre-established based on the non-linear mapping relationship between the electromagnetic field strength gradient and the fluid pressure amplitude of low-carbon high-molybdenum thin-wall stainless steel pipes with different wall thicknesses, and the non-linear mapping relationship is generated by a material database or material simulation software.

[0042] As a further solution of the present invention, the wall thickness of the low-carbon high-molybdenum thin-wall stainless steel pipe is 0.5 - 1 mm, and the weight percentage of molybdenum element is greater than or equal to 5%.

[0043] As a further solution of the present invention, the gradient cooling in the step 3 makes the standard deviation of the molybdenum content distribution at the connection interface meet , and the shear strength of the connection interface is increased by more than 40%.

[0044] As a further solution of the present invention, the method is applicable to the connection of low-carbon high-molybdenum thin-wall stainless steel pipes with a wall thickness of 0.5 - 1 mm, and the wall thickness reduction rate after connection is less than or equal to 2%.

[0045] In the present invention, the pulse energy It is calculated by a formula and is used to quantify the thermal effect generated by high-frequency pulse current on the connection area. There is a close relationship between pulse energy and connection strength because it affects the temperature change in the connection area and the plastic flow of local materials. The increase in pulse energy can lead to a stronger local heating effect, promoting plastic deformation of the materials, and thus enhancing the metallurgical bonding strength of the connection area. By controlling the input of pulse energy, the optimization of the connection interface structure can be achieved, thereby improving the strength and reliability of the joint. Specifically, reasonable pulse energy input will cause molybdenum elements to supersaturate and solid-solve in the austenite matrix, improving the organizational structure of the connection area and avoiding connection failure problems caused by grain boundary enrichment.

[0046] During the connection process, pulse energy is combined with fluid pressure. The plastic flow of the connection area is driven by axial fluid pressure, enabling the material interface to flow sufficiently and form a strong metallurgical bond. The heat energy provided by high-frequency pulse current raises the temperature of the connection area to a high enough level, promoting the supersaturation and solid solution of molybdenum elements in the austenite matrix, thereby enhancing the strength and toughness of the interface. At the same time, pulse energy controls the local heating process in the connection area, avoiding deformation of thin-walled tubes or deterioration of material properties caused by overheating. Experimental data show that by precisely controlling pulse energy, interface defects caused by high temperature or high pressure can be effectively reduced, and the mechanical properties and corrosion resistance of the connection area can be improved.

[0047] The process of gradient cooling helps induce the precipitation of nanocarbonides, thus significantly strengthening the toughness of the connection interface. Liquid nitrogen atomization spray cooling ensures the precipitation of fine carbide particles at the connection interface by precisely controlling the cooling rate. These carbide particles form a microscopic reinforcement structure inside the material, improving the strength and corrosion resistance of the connection interface. The control of the cooling rate is the key to achieving this goal. A faster cooling rate can ensure the formation of a stable and uniform precipitation structure of carbides at the connection interface.

[0048] Example 1: In this example, an improved method for manufacturing high-strength seamless connections of low-carbon high-molybdenum thin-walled stainless steel tubes was adopted. First step, the connection end faces of low-carbon high-molybdenum thin-walled stainless steel tubes were subjected to pre-diffusion treatment of titanium and niobium elements in a vacuum furnace. The addition of titanium effectively prevented the segregation of molybdenum at grain boundaries, while niobium helped optimize the stability of this segregation layer. Specifically, the weight percentage of titanium element was 0.007 - 0.009%, the weight percentage of niobium element was 0.005 - 0.007%, the treatment temperature was 850 - 900 °C, and the vacuum degree was less than or equal to . Second step, the two tube ends after pre-diffusion treatment were butt-jointed, and local instantaneous heating was carried out using the dynamic thermal field generated by high-frequency pulse current. The heating temperature was controlled between 500 - 800 °C, the single-pulse time was 0.1 - 0.5 seconds, and the calculation formula for pulse energy was: ,

[0049] Among them, is the instantaneous current, is the resistance of the connection area, is the pulse duration. The pulse energy calculated by this formula ensures the precise control of the heating process. The pulse energy of the high-frequency pulsed current acts synchronously with the applied fluid pressure in the pressure range of 80 - 150 MPa, which is used to drive the plastic flow of the connection area, so that molybdenum elements are supersaturated and solid-solved in the austenite matrix.

[0050] In this step, the phase difference between the fluid pressure and the high-frequency pulsed current is controlled within 5 mm to ensure their synchronization. In this embodiment, a PID controller is used to compensate the phase difference in real time. By collecting the IGBT switch state signal of the high-frequency pulsed power supply, the regulation of the hydraulic system is triggered. In this way, according to the electromagnetic field strength gradient of the current pulse, the corresponding pressure amplitude parameter can be called from the preset parameter library, and the pressure value is dynamically adjusted through the PID controller with the real-time feedback.

[0051] In the last step, the connection area undergoes in-situ gradient cooling by liquid nitrogen atomization spraying. The cooling rate is controlled in the range of 100 - 500 °C / s to ensure the precipitation of nano-carbides at the connection interface, thereby enhancing the toughness of the interface. This cooling process not only improves the connection strength but also enhances the corrosion resistance by forming a continuous passivation film. Specifically, the cooling rate realizes the precise control of the temperature gradient by adjusting the flow rate and spraying angle of the liquid nitrogen injection, thus ensuring the uniformity and stability of the cooling process.

[0052] Example 2: This example further illustrates the technical solution of the present invention in combination with Figures 1 - 5 In Figure 1 , first, a high-frequency pulsed current is output by a high-frequency pulsed power supply to apply a heat field to the joint area, and the temperature is controlled between 500 and 800 °C for a duration of 0.1 to 0.5 seconds. This heat field works synchronously with the fluid pressure system. The pressure system is adjusted by a PID controller to ensure precise control of the phase difference between the heat field and the fluid pressure during the connection process. The PID controller retrieves the pressure amplitude from the preset parameter library according to the electromagnetic field strength gradient and makes dynamic adjustments. The hydraulic system is responsible for applying pressure to the connection area, and the fluid pressure ranges from 80 to 150 MPa to ensure appropriate plastic flow of the pipe end under the action of the high-frequency pulsed current. A strong metallurgical bond is formed at the connection interface through the supersaturation of nickel elements and the solid solution of molybdenum elements, avoiding connection failure problems caused by the enrichment of molybdenum elements at the grain boundaries. At the same time, liquid nitrogen atomization spraying realizes in-situ gradient cooling. By precisely controlling the cooling rate (100 - 500 °C / s), nano-carbides precipitate at the connection interface, significantly improving the toughness of the connection interface.

[0053] See Figure 2 , in the energy input layer, a local instantaneous thermal field is provided by a high-frequency pulsed current and applied to the connection area to heat it to the required temperature, and the plastic flow of the connection area is controlled by axial fluid pressure to ensure that the material interface reaches the optimal bonding state and avoid the generation of microdefects in the connection area. At this time, the interaction between the local instantaneous thermal field and the interface plastic flow effectively promotes the supersaturated solid solution of molybdenum elements and enhances the connection strength. Next, enter the element regulation layer. This layer ensures the uniform distribution of molybdenum elements through the pre-diffusion operation of iron-molybdenum metallurgy, avoids the problem of grain boundary segregation, and suppresses the enrichment of molybdenum elements at the interface through the formation of a nucleated nanocrystalline layer, ensuring the stability of the connection quality. Through this step, the mechanical properties of the interface layer are enhanced, and the brittleness problems that may occur during the welding process are reduced. Finally, enter the structure strengthening layer. In this layer, liquid nitrogen jet cooling is used to achieve gradient cooling, causing carbides to precipitate in the connection area and further strengthening the interface toughness. In addition, by controlling the temperature of the liquid nitrogen jet cooling and combining with the precipitation of nanocarbides, the shear strength of the connection part is increased, and the corrosion resistance of the material is effectively improved.

[0054] See Figure 3 , through the iron-molybdenum element pre-diffusion treatment module, molybdenum elements are pre-treated on the connection end face to ensure uniform element distribution and provide a stable basis for subsequent processes. Then, enter the pre-treatment module. In this step, by controlling the vacuum degree, temperature, and time parameters, the effective diffusion of molybdenum elements is ensured, and the problem of grain boundary segregation is avoided. In the subsequent high-frequency pulsed current fluid pressure module, the high-frequency pulsed current is used to apply a local thermal field to the connection area, and at the same time, axial fluid pressure is applied to the connection interface. Through dynamic control, the plastic flow of the material interface is ensured, and the metallurgical bonding of the joint is enhanced. Immediately afterwards, enter the nucleation cooling module. By precisely controlling the cooling rate, in-situ gradient cooling is achieved by liquid nitrogen jet, promoting the precipitation of nanocarbides at the connection interface, enhancing the toughness of the interface, and ensuring the strength and corrosion resistance of the material connection. Finally, enter the connection area strengthening module. At this stage, by further strengthening the microstructure of the connection area and optimizing its physical properties, the stability of high-strength seamless connection and the reliability of long-term use are ensured.

[0055] See Figure 4, in the connection area, a high-frequency pulsed current is applied to generate a local thermal field through instantaneous heating for heat treatment, raising the temperature of the connection area to promote connection. Meanwhile, in the dynamic thermal field module, by adjusting the coordination between the thermal field and fluid pressure, the effectiveness of interfacial plastic flow is ensured to further improve the connection quality. In the PID controller module, the PID control algorithm is used to precisely adjust the phase difference between the thermal field and fluid pressure to ensure the precise coordination of each pulse of thermal energy and fluid pressure, thereby optimizing the metallurgical bonding in the connection area. The controller adjusts the phase difference through a feedback mechanism to ensure that the applied thermal energy and fluid pressure can be precisely controlled. The subsequent step of combining with fluid pressure ensures the organic combination of the thermal field and fluid pressure, optimizing the microstructure in the connection area and further enhancing the firmness of the connection through axial fluid pressure. Finally, through interfacial plastic flow, the material in the connection area plastically flows to promote the supersaturated solid solution of molybdenum elements in the matrix, thereby forming a firm metallurgical bond, ensuring the strength and reliability of the material connection.

[0056] See Figure 5 , in the pretreatment stage of the connection end face of the low-carbon high-molybdenum thin-walled stainless steel pipe, pre-diffusion of iron and molybdenum elements in a vacuum furnace is carried out to ensure the uniform distribution of molybdenum elements on the connection end face, laying a solid foundation for subsequent steps and preventing grain boundary segregation. Then, by forming a molybdenum-titanium element segregation layer, the microstructure in the connection area is further optimized, reducing the risk of connection failure caused by molybdenum element enrichment. After docking the two pipes, a high-frequency pulsed current is applied to form a local thermal field, and the temperature of the docking area is effectively controlled. Then, an axial fluid pressure is applied to promote plastic flow to ensure sufficient plastic flow at the interface in the connection area, increasing the metallurgical bonding strength. During this process, the coupling of the high-frequency pulsed current and fluid pressure is adjusted by the PID controller to ensure their precise coordination, thereby enhancing the metallurgical bonding force in the connection area. The next step is gradient cooling after connection. During this process, the connection area is efficiently cooled by liquid nitrogen injection cooling to promote the solid solution of molybdenum elements and at the same time enhance the corrosion resistance and toughness of the connection area. Finally, the induced carbide precipitation strengthening connection achieved by this technology improves the mechanical strength and corrosion resistance of the connection interface, ensuring that the connected thin-walled stainless steel pipe can work stably for a long time in a high-strength and corrosion-resistant environment.

[0057] Example 3: When pretreating the connection end face of the low-carbon high-molybdenum thin-walled stainless steel pipe, the vacuum degree in the vacuum furnace is controlled at Hereinafter, the temperature of the pre-diffusion treatment is maintained at 875 °C, and the treatment time is 120 minutes. Under these conditions, the weight percentage of titanium element is controlled at 0.008%, and the weight percentage of niobium element is controlled at 0.006%. By precisely controlling the vacuum degree, temperature, and time, it is possible to ensure the formation of a uniform and stable nano-scale segregation layer of titanium and niobium elements at the joint end face. The formation of this pre-diffusion layer serves to significantly reduce the enrichment tendency of silver element at the grain boundaries during the subsequent joining process, enhancing the toughness of the joint area at the microscopic level and laying the foundation for achieving high-quality joining.

[0058] During the joining process after butt joint, a method of synchronously applying high-frequency pulsed current and axial fluid pressure is adopted. The output frequency of the high-frequency pulsed power supply is 30 kHz, and the duration of a single pulse is set to 0.3 seconds. The energy of a single pulse is calculated according to the formula , where the instantaneous current is precisely controlled by the high-frequency pulsed power supply, and the resistance of the joint area is affected by the material's own properties and temperature. In this embodiment, the amplitude of the pulsed current is monitored and adjusted in real time by a PID controller to ensure that the heating temperature is stable at about 650 °C. Meanwhile, the axial fluid pressure applied by the hydraulic system is stable at 120 MPa. The local instantaneous thermal field generated by the high-frequency pulsed current softens the material in the joint area, while the synchronously applied axial fluid pressure drives plastic flow at the interface, enabling silver element to form a supersaturated solid solution in the austenite matrix. The PID controller uses the IGBT switch state signal of the high-frequency pulsed power supply collected in real time as the trigger reference, and according to the electromagnetic field intensity gradient of the current pulse at present, calls the corresponding pressure amplitude parameter from the preset parameter library, performs difference operation and dynamically adjusts the output of the hydraulic system, so as to precisely control the phase difference between the high-frequency pulsed current and the axial fluid pressure within 0.5 ms, ensuring a high degree of coordination between the thermal effect and the mechanical effect.

[0059] After the joining is completed, in-situ gradient cooling is immediately carried out. Through the liquid nitrogen atomization spraying system, the joint area is rapidly cooled at a cooling rate of 250 °C / s. This rapid cooling utilizes the pre-formed titanium and niobium segregation layers as the core of heterogeneous nucleation, inducing the precipitation of nano-scale carbides at the joint interface. The precipitated nano-carbides can significantly refine the grain boundaries and hinder the movement of dislocations, thereby strengthening the toughness of the joint interface. At the same time, rapid cooling also helps to form a dense passivation film on the material surface, improving the corrosion resistance of the joint area. By precisely controlling the flow rate and spraying angle of liquid nitrogen spraying, the uniformity and stability of the cooling rate can be achieved, and finally the molybdenum content distribution at the joint interface becomes more uniform, with a standard deviation The control is within 0.2%. The shear strength of the connection interface is increased by more than 45% compared with the stainless steel pipe without using this method. For the low-carbon high-molybdenum thin-walled stainless steel pipe connected by the method of this embodiment, the wall thickness reduction rate is less than 1%, which can meet the application scenarios with high requirements for connection strength and dimensional accuracy.

[0060] Example 4: To control the local instantaneous thermal field generated by the high-frequency pulsed current in the connection area, it is necessary to accurately quantify the heat input energy. The heat input energy is calculated by the following formula: ,

[0061] where, represents the single-pulse energy, with the unit of joule; is the instantaneous current output by the pulse power supply, with the unit of ampere. Its waveform can adopt a square wave or a quasi-sine wave according to different connection requirements, and is realized by the high-frequency pulse power supply through frequency conversion control during the implementation process to ensure that the current amplitude and frequency are within the set range; is the resistance of the connection area, with the unit of ohm, which is affected by the electrical conductivity of the material, temperature and the length of the connection area. The specific calculation is as follows:

[0062] ,

[0063] In the formula, is the resistivity of the connection area at the corresponding temperature, with the unit of ohm·meter; is the length of the connection area, with the unit of meter); is the effective cross-sectional area of the connection interface, with the unit of square meter ( ). During the implementation process, the resistivity can be determined according to the actual chemical composition and treatment temperature of the pipe through the material database or the standard material handbook, and dynamically corrected in combination with the temperature real-time feedback system, so as to improve the calculation accuracy. During the pulse current control process, represents the single-pulse action duration, with the unit of second, which is set by the pulse power supply controller. Its value is usually determined according to the thermal conductivity of the material and the expected heating depth. To prevent the tissue coarsening or the melting of the connection area caused by local overheating, the pulse time should be dynamically adjusted in coordination with the fluid pressure change.

[0064] To achieve precise collaborative control, the application of high-frequency pulsed current and axial fluid pressure during the connection process needs to maintain high temporal synchronization, with the phase difference controlled within 1 millisecond. In practical applications, a high-precision PID controller is used to real-time collect the IGBT switching signals of the high-frequency power supply as the trigger reference for the start of the hydraulic system, and according to the change in the electromagnetic field intensity of the current waveform, the fluid pressure amplitude data matching it in the preset parameter library is called. This parameter library is constructed based on the electromagnetic-thermal-mechanical coupling behavior under different wall thicknesses and different thermal conductivities, and is verified through simulation or material databases to ensure its applicability to low-carbon high-molybdenum thin-walled stainless steel pipes of various size specifications. The above control strategy dynamically corrects the energy input and axial force loading time in each cycle through a closed-loop feedback control logic, thereby making the thermal field action highly match the material plastic flow behavior and effectively reducing the element segregation phenomenon near the grain boundaries. Different from the existing methods that only rely on static control or one-way adjustment, this method can achieve deep coordination of the thermal-mechanical field, making the plastic flow in the connection area more sufficient and the molybdenum element more evenly dissolved in the austenite matrix. In addition, considering the actual influence relationship between variables during implementation, to avoid technical implementation deviations caused by unit errors or unclear variable definitions, it is recommended to add a variable unit automatic detection and data legality judgment module to the system control module to ensure the consistency and reliability of actual operations. During the system initialization stage, the initial values should be set according to the actual physical parameters of the input material, and continuous feedback updates should be carried out during the implementation process to ensure that the whole process is controlled within the feasible range. All of these belong to the extended implementation methods known to those of ordinary skill in the art.

[0065] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and without departing from the spirit or basic characteristics of the present invention, the present invention can be implemented in other specific forms. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be embraced within the present invention.

Claims

1. A high-strength seamless connection manufacturing method for low-carbon high-molybdenum thin-walled stainless steel pipes, characterized in that, Including the following steps: Step 1: Pretreat the connecting end face of the low-carbon high-molybdenum thin-walled stainless steel pipe. The pretreatment includes pre-diffusion treatment of titanium and niobium elements in a vacuum furnace to form a pre-diffusion layer containing a nano-scale titanium segregation layer and a nano-scale niobium segregation layer on the surface of the connecting end face. Among them, the weight percentage of titanium element is 0.005 - 0.01%, and the weight percentage of niobium element is 0.004 - 0.008%; Step 2: Butt-join the two low-carbon high-molybdenum thin-walled stainless steel pipes pre-treated in Step 1, and apply a dynamic thermal field to assist the connection to the butt-joined area. The dynamic thermal field is instantaneously applied locally to the butt-joined area by high-frequency pulsed current, with the temperature controlled at 500 - 800 °C, the duration being 0.1 - 1 second, and the single-pulse energy satisfies the condition that: where, is the instantaneous current, is the resistance of the connection area, is the single-pulse duration; and while applying the dynamic thermal field, apply a fluid pressure axially to the butt-joined area. The range of the fluid pressure is 50 - 200 MPa to drive plastic flow at the interface of the butt-joined area, enabling supersaturated solid solution of molybdenum elements in the austenite matrix; Step 3, after the connection in Step 2 is completed, in-situ gradient cooling is performed on the connection area. The gradient cooling is achieved by spraying atomized liquid nitrogen, and the cooling rate is controlled at 100-500 °C / s to induce the precipitation of nanocarbides at the connection interface by means of the pre-diffusion layer, thereby strengthening the toughness of the connection interface. Among them, the phase difference is controlled to meet .

2. The high-strength seamless connection manufacturing method of the low-carbon high-molybdenum thin-walled stainless steel pipe according to claim 1, characterized in that, In step 2, the frequency of the high-frequency pulsed current is 10 - 50 kHz, the single-pulse time is 0.1 - 0.5 seconds, and the fluid pressure is 80 - 150 MPa.

3. The high-strength seamless connection manufacturing method of the low-carbon high-molybdenum thin-walled stainless steel pipe according to claim 1, characterized in that, In the step 1, the weight percentage of titanium element is 0.007 - 0.009%, the weight percentage of niobium element is 0.005 - 0.007%, the pre-diffusion treatment temperature is 850 - 900 °C, and the vacuum degree is less than or equal to .

4. The high-strength seamless connection manufacturing method of the low-carbon high-molybdenum thin-walled stainless steel pipe according to claim 1, characterized in that, In step 2, the high-frequency pulse power supply and the hydraulic system are linked through a PID controller to adjust the phase difference between the dynamic thermal field and the fluid pressure in real time , so as to meet .

5. The high-strength seamless connection manufacturing method of the low-carbon high-molybdenum thin-walled stainless steel pipe according to claim 4, characterized in that, The phase difference between the high-frequency pulsed current and the axial fluid pressure is controlled by an active compensation mechanism, wherein .

6. The high-strength seamless connection manufacturing method of the low-carbon high-molybdenum thin-walled stainless steel pipe according to claim 5, characterized in that, The active compensation mechanism is realized through the following steps: Collecting the IGBT switch state signal of the high-frequency pulse power supply in real time as the trigger reference for the start of the hydraulic pump; Calling the corresponding pressure amplitude parameter from the preset parameter library according to the electromagnetic field strength gradient of the current pulse current; Performing a difference operation on the pressure amplitude parameter and the real-time pressure feedback value of the hydraulic system through a PID controller, and dynamically adjusting the pressure output curve to make the phase difference between the dynamic thermal field and the fluid pressure .

7. The manufacturing method for high-strength seamless connection of low-carbon high-molybdenum thin-wall stainless steel pipes according to claim 6, characterized in that, The pressure amplitude parameter in the preset parameter library is pre-established based on the non-linear mapping relationship between the electromagnetic field strength gradient and the fluid pressure amplitude of the low-carbon high-molybdenum thin-walled stainless steel pipe with different wall thicknesses, and the non-linear mapping relationship is generated by a material database or material simulation software.

8. The high-strength seamless connection manufacturing method of the low-carbon high-molybdenum thin-walled stainless steel pipe according to claim 1, characterized in that The wall thickness of the low-carbon high-molybdenum thin-walled stainless steel pipe is 0.5 - 1 mm, and the weight percentage of molybdenum element is greater than or equal to 5%.

9. The high-strength seamless connection manufacturing method of the low-carbon high-molybdenum thin-walled stainless steel pipe according to claim 1, characterized in that, The gradient cooling in Step 3 makes the standard deviation of the molybdenum content distribution at the joint interface meet , and the shear strength of the joint interface is increased by more than 40%.

10. The manufacturing method for high-strength seamless connection of low-carbon high-molybdenum thin-walled stainless steel pipes according to claim 1, characterized in that, The method is applicable to the connection of low-carbon high-molybdenum thin-walled stainless steel pipes with a wall thickness of 0.5 - 1 mm, and the wall thickness reduction rate after connection is less than or equal to 2%.

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