A duplex stainless steel submerged arc welding (SAW) process

Abstract

The application discloses a duplex stainless steel submerged arc welding (SAW) process and relates to the technical field of duplex stainless steel welding processes. The process identifies three key metallurgical stages in the welding cooling process, namely, a high-temperature coarse-grain control stage, a medium-temperature phase change regulation stage and a low-temperature precipitation inhibition stage, and sequentially applies three different modes of electromagnetic fields, namely, a high-strength intermittent alternating magnetic field, a low-frequency slowly-varying swing magnetic field, a weak steady-state static magnetic field or a zero magnetic field. The switching of the modes is automatically triggered by a control system according to temperature feedback. The system comprises an excitation coil integrated with a welding torch, a temperature sensor and a central controller. Through the dynamic and accurate switching of the electromagnetic field modes, the application synchronously solves the contradiction among the refined grains, the optimized two-phase ratio and the inhibited harmful phase, stabilizes the ferrite content of a joint at 45-55%, increases the impact energy at -40 DEG C by 25-50%, and significantly improves the corrosion resistance, thereby breaking through the heat input paradox of the traditional submerged arc welding.

Description

Technical Field

[0001] This invention relates to the field of duplex stainless steel welding technology, specifically a submerged arc welding (SAW) process for duplex stainless steel. Background Technology

[0002] The superior properties of duplex stainless steel depend on a balanced microstructure in which ferrite (α) and austenite (γ) phases each account for approximately 50%. Submerged arc welding (SAW), as a highly efficient welding method, is prone to problems such as phase imbalance, coarse grains, and precipitation of harmful phases in welded joints due to its high heat input. These problems are mainly manifested in the following ways: Weld metal: To ensure deposition efficiency, the heat input is usually high, resulting in a long high-temperature residence time in the molten pool and a slow cooling rate. Although this is beneficial for austenite transformation, it can also easily cause excessive growth of proeutectoid ferrite grains, and brittle σ phases may precipitate within the ferrite grains during later cooling.

[0003] The heat-affected zone (HAZ) of the weld is subjected to high-temperature cycling, resulting in severe grain coarsening, which leads to a significant reduction in toughness and corrosion resistance, making it the weakest link in the entire joint.

[0004] Traditional processes mainly improve microstructure by adjusting welding heat input, optimizing welding material alloy composition, or performing post-weld solution treatment. However, these methods all have limitations: reducing heat input can refine grains and reduce the precipitation of harmful phases, but it weakens the precipitation motive force of austenite, leading to excessive ferrite content and decreased toughness; increasing heat input can promote austenitization, but it inevitably exacerbates grain coarsening and the risk of harmful phase precipitation. This contradiction is known as the "heat input paradox" of duplex stainless steel welding. Summary of the Invention

[0005] This invention provides a submerged arc welding (SAW) process for duplex stainless steel to solve the above-mentioned problems.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A submerged arc welding (SAW) process for duplex stainless steel includes the following steps: Based on the real-time temperature during the cooling process of the welded joint, three different modes of electromagnetic field are automatically switched and applied. The three modes are as follows: (1) High temperature coarse grain control stage: When the temperature of the welded joint is in the range of solidus temperature to 1000℃, a high-intensity intermittent alternating magnetic field is applied to ensure the effective electromagnetic force on the high temperature molten pool / semi-molten pool. The magnetic induction intensity is 80-150 millitalas (mT) and the frequency is 5-15Hz. An intermittent working mode with an on-off ratio between 1:1 and 5:1 is adopted. The intermittent mode can enhance the stirring effect and break dendrites through instantaneous strong impact, and also allow the surface tension of the molten pool to recover during the intermittent period, avoiding excessive fluctuation of the molten pool surface and ensuring the weld bead formation quality. The solidus temperature is determined based on the specific grade of duplex stainless steel, and the value specified in the national standard GB / T 20878 or industry standard is preferred (e.g., the solidus temperature of S32205 is 1300℃±20℃, and that of S32750 is 1320℃±20℃). If there is no standard data, it can be calculated by thermodynamic software such as Thermo-Calc, or obtained by deduction based on welding thermal cycle test, and preset in the corresponding process package of the control system.

[0007] (2) Mid-temperature phase transformation control stage: When the temperature of the welded joint is in the austenite precipitation-dominant range of 1000℃ to 550℃ (its core metallurgical action range is 950℃ to 750℃), a low-frequency slowly changing oscillating magnetic field is applied with a magnetic induction intensity of 30-60mT and a frequency of 0.5-2Hz. The ultra-low frequency periodic, slowly varying magnetic field (0.5-2 Hz) is set to match the bulk diffusion timescale of major alloying elements (such as Cr and Ni) in duplex stainless steel within the core temperature range of 950℃ to 750℃. Studies show that the bulk diffusion coefficient of Cr in this temperature range is approximately 10-1. -12 ~10 -11 cm 2 / s, Ni element is approximately 5×10 -13 ~5×10 -12 cm 2 The diffusion characteristic time is approximately 0.5-2 seconds. A low-frequency oscillating magnetic field matching this diffusion period can induce periodic micro-region Lorentz forces through the magnetic field, thereby generating a directional electromagnetic stirring effect on the solute atoms, promoting their short-range diffusion, thus optimizing the compositional fluctuations of austenite nucleation sites, and accelerating the phase interface migration. This allows austenite to fill the ferrite matrix in a fine and dispersed form, resulting in an ideal two-phase microstructure. Specific experiments (approximately 430 nuclei / mm² on the S32205 sample at 950℃ with a 50mT magnetic field) confirmed that the austenite nucleation density reaches its peak in the 1.0-1.5Hz range (approximately 430 nuclei / mm²). 2 The nucleation density decreased significantly at frequencies below 0.5 Hz or above 2.0 Hz, demonstrating the effectiveness of this frequency range. (3) Low temperature precipitation suppression stage: When the temperature of the welded joint drops to the dangerous precipitation range of 550℃ to 350℃, apply a weak steady-state static magnetic field with a magnetic induction intensity of 5-20mT, or turn off the magnetic field; In stage (3), the choice between applying a weak steady-state static magnetic field or turning off the magnetic field depends on the alloy content and precipitation characteristics of the duplex stainless steel. For standard duplex stainless steel (such as S32205), its alloy content is relatively low, and the driving force for the precipitation of harmful phases (such as σ phase) is strong in the 550-350℃ range. Applying a weak steady-state static magnetic field of 5-20mT can slightly delay the nucleation of harmful phases by affecting the interface energy or atomic diffusion activation energy. For super duplex stainless steel (such as S32750), its high Cr, Mo, and N content improves the stability of the microstructure. In this temperature range, the driving force for the precipitation of harmful phases is significantly reduced. Turning off the magnetic field can avoid any disturbance to the already formed fine two-phase microstructure and simplify control.

[0008] The switching between the three electromagnetic field modes is automatically triggered by the control system based on the real-time monitoring feedback signal of the weld joint temperature.

[0009] Electromagnetic stirring technology, as a known method for refining the molten pool, is conventionally applied by applying a magnetic field of constant intensity, constant frequency, or a single mode during welding. Its main goal is to uniformly break up dendrites and refine the solidification structure throughout the entire process. However, this "consistent" stirring mode is inefficient or even detrimental to addressing the differentiated and even contradictory metallurgical requirements faced in different temperature stages (such as high-temperature solidification, intermediate-temperature phase transformation, and low-temperature cooling) in the welding of duplex stainless steel. For example, in the critical temperature range of solid-state phase transformation, excessive mechanical stirring may interfere with the ordered nucleation and directional growth of austenite; while in the low-temperature precipitation-sensitive range, unnecessary alternating magnetic fields may even promote the nucleation of harmful phases. This application abandons the traditional constant or single magnetic field application mode in electromagnetic stirring. Through time-programmed dynamic electromagnetic field control, it identifies three specific temperature ranges that have a decisive influence on the formation of the microstructure of duplex stainless steel in the welding thermal cycle. For each range with distinct metallurgical requirements, it designs and applies magnetic field modes with clear functions and different parameters in a timely manner. The three modes work together in a precise time / temperature sequence to form a complete dynamic metallurgical control program.

[0010] Preferably, the control system integrates a non-contact infrared temperature measurement unit or an embedded thermocouple for real-time monitoring of the temperature near the weld fusion line or a specific point in the heat-affected zone; the control system compares the monitored real-time temperature with a preset stage switching threshold temperature and dynamically triggers the switching of the electromagnetic field mode accordingly.

[0011] More preferably, the response delay time of the control system from receiving the temperature feedback signal to issuing the magnetic field mode switching command is ≤0.3 seconds; the control accuracy of the switching time point between the stages (1), (2), and (3) is within ±0.5 seconds; wherein, the trigger temperature threshold for switching from stage (1) to stage (2) is 1000℃±10℃, and the trigger temperature threshold for switching from stage (2) to stage (3) is 550℃±10℃.

[0012] In this invention, the trigger temperature thresholds for stage switching (1000℃±10℃ and 550℃±10℃) are the signal points at which the control system issues mode switching commands based on temperature feedback, aiming to ensure the continuity and accuracy of the control logic; while the temperature range of each stage (such as 950℃ to 750℃ in stage (2)) is the key core action range corresponding to the metallurgical physical process; when the temperature drops to 1000℃ to trigger and apply the swing magnetic field of stage (2), the temperature of the welded joint will naturally cool and pass through the austenite precipitation-dominant range of 950℃ to 750℃, and the magnetic field will continue to act in this core range to achieve optimal phase transformation control; the range of 750℃ to 550℃ serves as a temperature transition zone, maintaining the magnetic field mode of stage (2) until the 550℃ trigger switching, avoiding unnecessary mode disturbances.

[0013] Preferably, in stage (2), the direction of application of the low-frequency slowly varying oscillating magnetic field is set to a horizontal or vertical direction perpendicular to the welding direction, and its magnetic field direction is periodically reversed in a sinusoidal or square wave manner at the frequency of 0.5-2Hz.

[0014] Furthermore, in stage (2), the specific parameters of the low-frequency slowly varying oscillating magnetic field are optimized and matched according to the specific grade of duplex stainless steel: for standard duplex stainless steel (such as S32205), a parameter combination of frequency 1.0-1.5Hz and magnetic induction intensity 40-50mT is used; for super duplex stainless steel (such as S32750), a parameter combination of frequency 0.5-1.0Hz and magnetic induction intensity 50-60mT is used.

[0015] Preferably, before the start of stage (1), an arc stabilization preheating stage is added: within the initial 1-3 seconds after arc ignition, a weak steady-state static magnetic field with a magnetic induction intensity of 10-30mT is applied, and after the arc and molten pool stabilize, the high-intensity intermittent alternating magnetic field is switched to the high-intensity intermittent alternating magnetic field. Before the start of stage (1), the added arc stabilization preheating stage plays a crucial role; the 10-30mT weak steady-state static magnetic field applied within the initial 1-3 seconds after arc ignition can effectively constrain the arc shape and promote the rapid formation of a stable molten pool; the control system monitors the welding arc voltage in real time, and when the voltage fluctuation is stable within the range of ≤±1V and the wire feeding mechanism provides stable feedback, it is determined that the arc and molten pool have reached a stable state, and then automatically switches to the high-intensity intermittent alternating magnetic field of stage (1); in addition, in the intermittent working mode (on / off ratio of 1:1 to 5:1) adopted in stage (1), the designed "interruption period" provides a time window for the arc and molten pool to recover stability, thereby avoiding arc blow or excessive fluctuation of the molten pool that may be caused by a continuous strong magnetic field.

[0016] The present invention also provides an electromagnetic control system for implementing the above-mentioned process. The system includes an electromagnetic field generating device, a temperature monitoring unit, a control system, and a programmable power supply. The electromagnetic field generating device preferably adopts a coaxial nested double coil structure and is integrated behind the submerged arc welding torch, forming a closed-loop control system together with the temperature monitoring unit and the central controller.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. By using time-series programming, the contradictory metallurgical objectives (grain refinement, austenitization promotion, and suppression of harmful phases) in traditional processes are decomposed into different temperature stages and given optimal electromagnetic intervention, thereby achieving decoupling and synergistic resolution of the contradictions; the weld can obtain fine original grains and achieve sufficient and uniform austenitization, while effectively suppressing harmful phases.

[0018] 2. Compared with traditional processes and conventional electromagnetic stirring, the method of this invention can stably control the ferrite content in the weld center within the ideal range of 45%-55%, and the austenite morphology is finer and more dispersed; the low-temperature impact toughness of the joint (including HAZ) can be increased by 25%-50%, and the pitting corrosion resistance is significantly improved. Attached Figure Description

[0019] Figure 1 A flowchart of a submerged arc welding (SAW) process for duplex stainless steel provided by the present invention.

[0020] Figure 2 This is a schematic diagram of the structure of the excitation coil assembly provided by the present invention.

[0021] Figure 3 A block diagram of the electromagnetic control system provided by the present invention.

[0022] Figure label annotations: 2a, main coil; 2b, auxiliary coil; 2c, outer casing. Detailed Implementation

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein in the specification of the application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing drawings of this application are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification, claims, or foregoing drawings of this application are used to distinguish different objects, not to describe a particular order.

[0024] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0025] To achieve the process described in this invention, the electromagnetic control system is integrated with conventional submerged arc welding equipment in the following manner: 1. Physical composition of the system The system mainly consists of the following components: Main welding unit: includes submerged arc welding machine host, wire feeding mechanism, flux conveying and recycling device and standard submerged arc welding torch.

[0026] Electromagnetic field generating device: Its core is an excitation coil assembly; this assembly preferably adopts a coaxial nested double coil structure; the inner layer is the main coil 2a, which is made of a rectangular cross-section copper tube wound into a solenoid shape and cooled by water, used to generate a high-intensity alternating magnetic field; the outer layer is the auxiliary coil 2b, which is coaxially wound outside the main coil 2a at a certain interval, independently controlled, used to generate a steady-state static magnetic field or to assist in adjusting the magnetic field distribution; the two coils are jointly encapsulated in a protective shell 2c made of insulating and heat-resistant material (such as glass fiber reinforced epoxy resin); preferably, the shell 2c has a window on the side closer to the workpiece to reduce the air gap in the magnetic circuit; the width of the excitation coil assembly should cover the weld and part of the heat-affected zone on both sides; In the coaxial nested double-coil structure, the inner main coil is connected to a programmable AC power supply to generate a high-intensity intermittent alternating magnetic field in stage (1) and a low-frequency slowly changing oscillating magnetic field in stage (2). The outer auxiliary coil is connected to an independent programmable DC power supply to generate a weak steady-state static magnetic field required for the preheating stage and stage (3). The control system independently controls the output of the programmable AC power supply and the programmable DC power supply according to the process stage instructions, so as to realize the coordinated or independent operation of the inner and outer coils, thereby accurately and seamlessly switching between different magnetic field modes.

[0027] Temperature monitoring unit: includes at least one non-contact infrared thermometer for real-time temperature signal acquisition; its optical probe adopts a side-mounted avoidance design, pointing to the weld edge or heat-affected zone surface 15-40mm behind the welding torch, where the area is not covered by flux; for conditions with severe flux coverage (such as thick plate multi-layer welding), an "infrared temperature measurement and embedded thermocouple combined monitoring" scheme can be adopted: before welding, a micro-hole with a depth of 2-3mm is drilled at a preset position in the heat-affected zone, and a high-temperature resistant thermocouple is embedded, which is cross-compared and calibrated with the infrared temperature measurement data in real time to ensure the accuracy and reliability of the temperature signal.

[0028] Control system: includes a programmable logic controller (PLC) or industrial computer, which internally stores and runs the timing programming control program described in this invention.

[0029] Programmable power supply: supplies power to the excitation coil, and its output waveform (current, frequency, on / off mode) can be precisely controlled by digital commands from the control system.

[0030] 2. Spatial layout and mechanical connections of each unit In the welding direction (i.e., the direction of welding torch movement), each component is fixedly installed along the welding line in the following order: At the very front is the submerged arc welding torch, whose protruding wire maintains a normal welding distance from the workpiece surface; The excitation coil assembly is fixedly mounted behind the welding torch via a rigid connecting bracket. This bracket ensures that the central axis of the coil is substantially parallel to the central axis of the welding torch and aligned with the center line of the weld. The distance between the lower surface of the coil assembly and the upper surface of the workpiece can be adjusted via the bracket, with an optimal adjustment range of 20-50 mm.

[0031] An infrared thermometer is mounted on a separate bracket behind or to the side of the excitation coil assembly. Its probe is pointed precisely at a certain angle to the surface of the weld or heat-affected zone about 15-40 mm behind the welding torch. This point is the temperature monitoring point.

[0032] 3. Electrical and signal connections of the system The units are connected in the following logical sequence to form a closed-loop control system: Signal input path: The output signal line of the infrared thermometer is connected to the analog input module of the control system.

[0033] Control core: The timing control program running inside the control system processes temperature signals in real time and compares and makes decisions with the preset process model.

[0034] Command output path: The control system sends real-time control commands to the programmable power supply unit through a digital communication interface or analog output module.

[0035] Power execution path: The programmable power supply unit drives the excitation coil assembly to generate a magnetic field that meets the requirements according to the received instructions.

[0036] Optional signals: The welding current and voltage signals of the submerged arc welding host can be fed back to the control system as an auxiliary judgment basis.

[0037] Furthermore, multiple process packages for different duplex stainless steel grades, different plate thicknesses, and different bevel types can be pre-stored in the control system. Each process package includes matching welding parameter recommendations and specific parameters for the three stages of time-programmed electromagnetic fields coupled with them.

[0038] Example 1: Butt welding of 12mm thick S32205 duplex stainless steel plates In this embodiment of the invention, a submerged arc welding (SAW) process for duplex stainless steel is described, such as... Figures 1-3 As shown, it includes the following steps: Pre-welding preparation: Test plate dimensions 300mm×150mm×12mm, with a 60° V-groove and a 1mm blunt edge. Cleaning before welding; use ER2209 welding wire (Φ4.0mm) and a matching alkaline sintering flux; Welding parameters: welding current 480A, voltage 30V, welding speed 35cm / min, interpass temperature controlled below 150℃; Electromagnetic control technology: The control system uses an infrared thermometer to monitor the temperature 20mm behind the fusion line in real time.

[0039] Phase switching: When the temperature is >1000℃, enter phase (1); when the temperature drops to 1000℃, switch to phase (2); when the temperature continues to drop and passes through the core range of 950℃-750℃; when the temperature drops to 550℃, switch to phase (3), and the system response delay is ≤0.3s.

[0040] Stage (1) parameters: magnetic induction intensity 100mT, frequency 10Hz, intermittent mode (on for 0.6s, off for 0.2s). Stage (2) parameters: magnetic induction intensity 45mT, frequency 1.2Hz, horizontal sinusoidal oscillating magnetic field; Stage (3) parameters: Apply a steady-state static magnetic field of 15mT until the temperature is below 200℃.

[0041] Performance test results: Metallographic structure: The weld seam consists of fine equiaxed crystals, and austenite is uniformly and diffusely distributed in the ferrite matrix in the form of slender strips and blocks.

[0042] Ferrite content: 49.2% at the weld center.

[0043] -40℃ Impact Energy: The average impact energy of the weld metal is 89J.

[0044] Pitting potential: +1120mV in 3.5% NaCl solution vs. SCE.

[0045] Example 2: Butt welding of 20mm thick S32750 super duplex stainless steel plates Pre-welding preparation: Test plate dimensions 400mm×200mm×20mm, with double-sided U-shaped bevels. Use ER2594 welding wire (Φ4.0mm) and ultra-high basicity flux. Welding parameters: Welding current 520A, voltage 32V, welding speed 30cm / min, multi-layer multi-pass welding, strictly control interpass temperature ≤120℃; Electromagnetic control technology: A combination of open-loop and closed-loop control is adopted; firstly, the thermal cycle is theoretically calculated based on welding parameters, and the switching time of each stage is preset; at the same time, miniature thermocouples are embedded in the HAZ for temperature calibration.

[0046] Stage (1) parameters: magnetic induction intensity 130mT, frequency 8Hz, intermittent mode (on for 0.7s, off for 0.15s); the welding process was stable. According to monitoring, the arc voltage fluctuation value was ±0.8V, the molten pool had no abnormal surge, the weld bead was smooth and uniform, and there were no defects such as undercut or lack of fusion, which proved the stability of the arc under high-intensity intermittent magnetic field. Stage (2) parameters: Based on the characteristics of high alloy content and slow diffusion of S32750, lower frequency parameters are adopted: magnetic induction intensity 55mT, frequency 0.8Hz, vertical oscillating magnetic field to optimize thickness direction uniformity; Stage (3) Parameters: Turn off the magnetic field (For super duplex stainless steel S32750, due to its extremely high alloy content, turning off the magnetic field in the 550-350℃ range is sufficient to stabilize the structure, inhibit precipitation, and make it easier to simplify control; for standard duplex stainless steel, applying a weak static magnetic field is the preferred safeguard).

[0047] Performance test results: Ferrite content: 46.8% at the weld center.

[0048] PREN value: Calculated based on Cr%+3.3Mo%+16N%, the PREN value of the weld metal is ≥41, which is comparable to that of the base metal.

[0049] Corrosion resistance: Potentiodynamic polarization tests in 3.5% NaCl solution showed that its pitting breakdown potential (Eb) was +1250mV vs. SCE, which was significantly higher than +1080mV of Comparative Example 3.

[0050] σ phase detection: No σ phase precipitation was observed after sensitization treatment at 675℃ for 15min and metallographic etching.

[0051] Example 3: Butt welding of 8mm thick S32101 economical duplex stainless steel plates Operation process: The operator selects the "S32101_8mm_I type bevel" process package on the control system interface; the system automatically recommends parameters of welding current 340A, voltage 28V, and speed 50cm / min, and loads the corresponding three-stage electromagnetic field program.

[0052] Electromagnetic field program: stage (1) (high-intensity intermittent field), stage (2) (low-frequency oscillating field, frequency 1.5Hz), stage (3) (weak static magnetic field); the system performs closed-loop fine-tuning through real-time temperature measurement.

[0053] Results: The weld formation is aesthetically pleasing, the ferrite content is controlled at 52%, the performance fully meets the product requirements, and the operation is simple.

[0054] Example 4 Test plate: S32205, 14mm thick, V-shaped bevel.

[0055] Welding parameters: current 460A, voltage 31V, speed 38cm / min.

[0056] Electromagnetic control technology: Stage (1): A lower magnetic induction intensity boundary value of 80mT, frequency of 15Hz, and on / off ratio of 1:1 are adopted.

[0057] Stage (2): A lower magnetic induction intensity boundary value of 30mT and a frequency of 2.0Hz are adopted.

[0058] Stage (3): Use a lower static magnetic field strength of 5mT.

[0059] The control system employs open-loop sequential control.

[0060] Performance results: Ferrite content: 52.1%.

[0061] Impact energy at -40℃: 78J.

[0062] Compared with 61J in Comparative Example 2 (constant magnetic field), the impact energy is still 28% higher; the metallographic structure is significantly better than that of Comparative Example 2, showing a more uniform austenite distribution.

[0063] Example 5 Test plate: S32205, 16mm thick.

[0064] Welding parameters: current 500A, voltage 32V, speed 35cm / min.

[0065] Electromagnetic control technology: Stage (1): Magnetic induction intensity 120mT, frequency 8Hz, on / off ratio 4:1.

[0066] Stage (2): Magnetic induction intensity 50mT, frequency 1.0Hz, using a vertically oscillating magnetic field.

[0067] Phase (3): Turn off the magnetic field.

[0068] Performance results: Ferrite content: 48.5%.

[0069] Impact energy at -40℃: 86J.

[0070] Pitting potential: +1105mV vs. SCE.

[0071] Comparative Example 1 (No magnetic field) The welding parameters were exactly the same as in Example 1, but no electromagnetic field was applied throughout the process.

[0072] Metallographic structure: The weld seam consists of coarse columnar crystals, and austenite is mainly distributed in a continuous network along the grain boundaries.

[0073] Ferrite content: up to 68.5% in the weld center.

[0074] Impact energy at -40℃: only 32J.

[0075] Pitting potential: +865mV vs. SCE.

[0076] Comparative Example 2 (Constant Magnetic Field) The same welding parameters as in Example 1 were used, but a constant alternating magnetic field (magnetic induction intensity 60mT, frequency 10Hz) was applied throughout the process to simulate existing conventional electromagnetic stirring technology.

[0077] Metallographic structure: Compared with Comparative Example 1, the columnar crystals are refined, but the distribution of austenite is still uneven, with blocky aggregations in some areas.

[0078] Ferrite content: 58.3% at the weld center.

[0079] Impact energy at -40℃: 61J.

[0080] Pitting potential: +980mV vs. SCE.

[0081] Comparative Example 3 (Constant Magnetic Field Welding of S32750) The same welding parameters as in Example 2 were used, and a constant magnetic field (70mT, 10Hz) was applied throughout the process.

[0082] Ferrite content: 53.1% at the weld center.

[0083] PREN value: The PREN value of the weld is approximately 39.5.

[0084] σ phase detection: After the same sensitization treatment, a small amount of particulate σ phase was observed to precipitate in the metallographic structure.

[0085] Comparative Example 4: Demonstration of Incorrect Hyperparameters Conditions: Except for the electromagnetic field parameters, the rest are the same as in Example 1.

[0086] Electromagnetic field parameters: Stage (1): Applying an excessively strong constant magnetic field (180mT, >150mT) causes severe churning of the molten pool and deterioration of the weld bead formation.

[0087] Stage (2): The high-frequency alternating magnetic field (10Hz, >2Hz) was still incorrectly used for stirring.

[0088] Stage (3): A strong alternating magnetic field is still applied.

[0089] Performance results: The weld formation is poor and there are surface defects.

[0090] Ferrite content: 61% (insufficient austenitization).

[0091] Impact energy at -40℃: 48J (far lower than 89J in Example 1).

[0092] Performance testing Performance tests were performed on the welds of all embodiments and comparative examples, and the test methods and standards are as follows: Ferrite content was determined by metallographic method according to GB / T 13305; Charpy V-notch impact energy at -40℃ was determined according to GB / T 229; The pitting potential was determined in 3.5 wt% NaCl solution using the potentiodynamic polarization method according to ASTM G150. The precipitation of the σ phase was determined by sensitizing the sample at 675℃ for 15 min, followed by metallographic etching and microscopic observation according to GB / T 10561. The room temperature tensile properties (tensile strength Rm and elongation after fracture A) are determined according to GB / T 2651-2008 "Tension Test Method for Welded Joints". The specimen is cut transversely from the center of the weld and processed into a standard plate-shaped tensile specimen. It is loaded at a specified rate on a universal testing machine until fracture. The Rm and A values ​​are obtained by the testing machine recording system or by measurement and calculation.

[0093] The results are shown in Table 1 below: Table 1 Performance Test Results As can be seen from the table above, the method and system described in this invention (examples) are significantly and consistently superior to the traditional non-magnetic field process (Comparative Example 1) and the existing constant electromagnetic stirring technology (Comparative Examples 2 and 3) in all test performances.

[0094] It should be noted that, for the sake of simplicity, the foregoing embodiments are all described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to the present invention. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to the present invention.

[0095] It should be understood that the disclosed apparatus can be implemented in other ways, as illustrated in the embodiments provided in this application. For example, the apparatus embodiments described above are merely illustrative; the division of units described above is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or communication connections shown or discussed may be through some interfaces; the indirect coupling or communication connections between devices or units may be telecommunications or other forms.

[0096] The units described above as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0097] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still combine, add, delete, or otherwise adjust the features of the various embodiments of the present invention according to the circumstances without conflict or creative effort, thereby obtaining different technical solutions that do not fundamentally depart from the concept of the present invention. These technical solutions also fall within the scope of protection of the present invention.

Claims

1. A submerged arc welding (SAW) process for duplex stainless steel, characterized in that, Includes the following steps: Based on the real-time temperature during the cooling process of the welded joint, three different modes of electromagnetic fields are automatically switched and applied: (1) When the temperature is in the range of solidus temperature to 1000℃, apply a high-intensity intermittent alternating magnetic field with a magnetic induction intensity of 80-150mT, a frequency of 5-15Hz, and an on / off ratio of 1:1 to 5:

1. (2) When the temperature range is from 1000℃ to 550℃ (the core metallurgical action range is from 950℃ to 750℃), a low-frequency slowly changing oscillating magnetic field is applied with a magnetic induction intensity of 30-60mT and a frequency of 0.5-2Hz. (3) When the temperature is in the range of 550℃ to 350℃, apply a weak steady-state static magnetic field with a magnetic induction intensity of 5-20mT or turn off the magnetic field.

2. The submerged arc welding (SAW) process for duplex stainless steel according to claim 1, characterized in that, The trigger temperature threshold for switching from stage (1) to stage (2) is 1000℃±10℃, and the trigger temperature threshold for switching from stage (2) to stage (3) is 550℃±10℃.

3. The submerged arc welding (SAW) process for duplex stainless steel according to claim 1, characterized in that, The low-frequency slowly varying oscillating magnetic field described in stage (2) is applied in a horizontal or vertical direction perpendicular to the welding direction, and its magnetic field direction is periodically reversed in a sine wave or square wave pattern at a frequency of 0.5-2Hz.

4. The submerged arc welding (SAW) process for duplex stainless steel according to claim 3, characterized in that, In stage (2), the specific parameters of the low-frequency slowly varying oscillating magnetic field are optimized and matched according to the specific grade of duplex stainless steel: For standard duplex stainless steel, a parameter combination of frequency 1.0-1.5Hz and magnetic induction intensity 40-50mT is used. For super duplex stainless steel, a parameter combination of frequency 0.5-1.0Hz and magnetic induction intensity 50-60mT is used.

5. The submerged arc welding (SAW) process for duplex stainless steel according to claim 1, characterized in that, The magnetic field selection for stage (3) is based on the alloy content of duplex stainless steel: for standard duplex stainless steel, a weak steady-state static magnetic field with a magnetic induction intensity of 5-20mT is applied; for super duplex stainless steel, the magnetic field is turned off.

6. The submerged arc welding (SAW) process for duplex stainless steel according to claim 1, characterized in that, Before the start of stage (1), an arc stabilization preheating stage is added: within the initial 1-3 seconds after arc ignition, a weak steady-state static magnetic field with a magnetic induction intensity of 10-30mT is applied. After the arc and molten pool stabilize, the high-intensity intermittent alternating magnetic field is switched to the high-intensity intermittent alternating magnetic field.

7. An electromagnetic control system for implementing the process described in any one of claims 1-6, characterized in that, include: An electromagnetic field generating device includes a coaxial nested double coil structure, with the inner main coil used to generate an alternating magnetic field and the outer auxiliary coil independently controlled. Temperature monitoring unit, used to monitor the temperature of weld or heat-affected zone in real time; The control system has a built-in timing programming control program that receives temperature signals and sends commands to the programmable power supply accordingly. The programmable power supply drives the electromagnetic field generator to output a magnetic field of the corresponding mode according to the instructions.

8. The electromagnetic control system according to claim 7, characterized in that, The electromagnetic field generating device is mounted behind the submerged arc welding torch via a rigid connecting bracket, with the lower surface of its coil 20-50mm from the surface of the workpiece; the infrared thermometer probe of the temperature monitoring unit points to a monitoring point 15-40mm behind the welding torch.

9. The electromagnetic control system according to claim 8, characterized in that, The electromagnetic field generating device adopts a coaxial nested double coil structure: the inner layer is a main coil wound with water-cooled copper tubes, which is used to generate a high-intensity alternating magnetic field; the outer layer is an auxiliary coil, which is coaxial with the main coil but independently controlled, and is used to generate the weak steady-state static magnetic field or to assist in adjusting the spatial distribution of the magnetic field.

10. The electromagnetic control system according to claim 7, characterized in that, The control system contains multiple pre-stored process packages. Each process package is designed for a specific duplex stainless steel grade, plate thickness, and bevel type, and includes a complete preset scheme of matching welding parameters and three-stage electromagnetic field parameters.

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