Low-temperature steel structure welding method

Abstract

The invention belongs to the technical field of steel structure welding, and particularly relates to a low-temperature steel structure welding method. The method aims at solving the technical problems that when the low-alloy high-strength steel is welded in the low-temperature environment of-40 DEG C to-10 DEG C, cold cracks are easily generated due to too high cooling speed and concentrated thermal stress, and the toughness of a welding joint is reduced. According to the technical scheme, the method comprises the steps that a workpiece is placed in a low-temperature environment till the temperature is balanced; preheating the preheating areas on the two sides of the welding joint and keeping constant temperature; gas shielded welding is adopted for multi-layer and multi-pass welding, and welding heat input and interlayer temperature are strictly controlled; post-heat treatment is carried out after welding; and finally, staged controlled cooling is executed, in the first stage, cooling is conducted from the after-heating temperature to 100 DEG C, the cooling rate is controlled to be 8-20 DEG C / h, in the second stage, cooling continues to be conducted till the temperature is balanced with the environment temperature, and the cooling rate is controlled to be 1.0-2.0 DEG C / h. The method is mainly used for conducting high-quality welding construction on the low-alloy high-strength steel structure in the low-temperature environment.

Description

Technical Field

[0001] This invention relates to the field of steel structure welding technology. More specifically, this invention relates to a method for low-temperature steel structure welding. Background Technology

[0002] Welding low-alloy high-strength steel in low-temperature environments is a challenging technical task. When the ambient temperature is tens of degrees below zero Celsius, the plasticity and toughness of the steel decrease significantly, while its brittle transition temperature increases, making the material more prone to crack initiation during the welding thermal cycle and subsequent cooling. This is especially true for thick steel structural components, where complex temperature gradients and thermal stresses are generated internally during welding. Improper process control can easily lead to cold cracks in the weld or heat-affected zone. Cold cracking is one of the most common defects in the welding of low-alloy high-strength steel. Its occurrence is delayed, potentially appearing only after welding and a period of time, posing a serious threat to the safety and reliability of the structure.

[0003] Traditional welding processes often prove inadequate in low-temperature environments. A major challenge lies in setting and precisely executing a complete temperature control procedure adapted to low-temperature conditions. This refers not only to the molten pool temperature at the moment of welding but, more importantly, to the overall condition of the workpiece before welding begins, the balance between continuous heat input and dissipation during welding, and the cooling behavior of the joint after welding. If the workpiece temperature is not sufficiently balanced with the low-temperature environment, and preheating and welding begin directly, the significant temperature difference between the workpiece's core and surface will become a major stress source. Preheating is a conventional method to prevent cold cracking, but the selection of the preheating temperature range, the width of the preheating zone, the heating rate, and the determination of the holding time all require fine-tuning based on the specific steel type, thickness, and ambient temperature. Inappropriate preheating parameters may not effectively improve the microstructure and stress state of the weld area and may even lead to new thermal stress problems due to excessively rapid heating.

[0004] The control of heat input during the welding process itself is equally crucial. In low-temperature environments, a higher welding heat input is sometimes used to maintain necessary molten pool fluidity and ensure fusion. However, excessively high heat input can expand the heat-affected zone, leading to coarse grains and reduced toughness in this area. Simultaneously, excessively fast welding speeds or improper interpass temperature management can result in weld metal remaining at high temperatures for too short a time or cooling too quickly, hindering hydrogen escape and potentially promoting the formation of hardened structures—both significant factors inducing cold cracking. Therefore, limiting the heat input of each weld pass to a reasonable range and stably maintaining the interpass temperature within a range higher than ambient temperature is the core challenge of process control, requiring overcoming the objective condition of rapid heat loss in low-temperature environments.

[0005] Post-weld heat treatment and cooling control are another crucial yet often overlooked aspect. If a welded joint is directly exposed to cold air for rapid cooling after high-temperature welding, the intense shrinkage will generate extremely high tensile residual stress. This stress, combined with structural stress and diffusing hydrogen, can easily lead to cracking. Therefore, timely post-weld heat treatment to remove hydrogen and soften the microstructure is essential. However, the temperature and holding time of the post-heat treatment need to be precise; too low a temperature or too short a time will be ineffective, while too high a temperature may damage material properties. The subsequent cooling process is even more complex. If the welded joint is allowed to cool naturally from the post-heat treatment temperature to ambient temperature, especially in low-temperature environments, the cooling rate is often too rapid and uncontrollable. This uncontrolled rapid cooling will reintroduce thermal stress and may cause adverse microstructural transformations in the post-heat-treated joint area, thus partially or completely negating the beneficial effects of the post-heat treatment. Designing a gentle, controllable cooling procedure, particularly limiting the cooling rate within a safe range in critical temperature zones, has been a long-standing technical challenge in low-temperature welding processes. In the past, due to the lag in temperature measurement methods and the rudimentary nature of temperature control equipment, it was very difficult to achieve precise, segmented rate control of the entire cooling process. It often relied on the experience of operators, resulting in poor process stability and large fluctuations in welding quality.

[0006] In summary, ensuring welding quality when welding low-alloy high-strength steel structures at low temperatures hinges on the precise and coordinated control of the entire temperature curve, encompassing the initial workpiece temperature, preheating, welding heat input, interpass temperature, post-heating, and subsequent cooling. Any malfunction or mismatch in any of these stages can lead to decreased joint performance and defects such as cold cracking. Therefore, developing a systematic and stable process for managing this complex temperature history is a problem that needs to be addressed in this field. Summary of the Invention

[0007] The purpose of this invention is to provide a systematic low-temperature welding process, aiming to solve the problem of cold cracking in low-alloy high-strength steel during low-temperature welding by synergistically controlling the temperature field throughout the welding process. Specifically, this invention aims to achieve the following objectives: providing a complete temperature control process suitable for low-temperature environments, encompassing workpiece balancing, preheating, welding, post-heating, and cooling; ensuring workpiece stability before welding begins by setting reasonable preheating parameters and isothermal time; controlling the thermal cycle during welding by limiting welding heat input and maintaining specific interpass temperatures; eliminating hydrogen and improving microstructure through post-heat treatment at specified temperatures and times; and finally, designing a phased and strictly controlled cooling program, particularly controlling the cooling behavior of the weld joint in the critical temperature range, enabling the joint to cool smoothly and controllably to ambient temperature, thereby avoiding harmful stress and microstructure caused by excessively rapid or uneven cooling, fundamentally improving the cold crack resistance and overall reliability of welded joints in low-temperature environments.

[0008] To address the aforementioned problems and achieve the objectives and other advantages of this invention, a low-temperature steel structure welding method is provided, applicable to welding low-alloy high-strength steel at ambient temperatures ranging from -40°C to -10°C, comprising the following steps: Place the steel structure workpiece to be welded in an environment with a temperature of -40 to -10℃ until the temperature of the workpiece body reaches equilibrium with the ambient temperature. Preheat the preheating zones on both sides of the welded joint. The width of the preheating zone is 3-5 times the thickness of the steel plate. Control the preheating temperature at 180-220℃ and the preheating rate should not exceed 100℃ / h. After reaching the preheating temperature, maintain the temperature for 60-90 minutes. Gas shielded welding is used for multi-layer and multi-pass welding. The welding heat input of each pass is controlled within the range of 15-25 kJ / cm, while the interpass temperature of each pass is maintained at 150-200℃. After welding, the welded joint is subjected to post-heat treatment by heating it to 280-320℃ and holding it at this temperature for 120-180 minutes. After the post-heat treatment, the welded joint is cooled according to a preset cooling procedure; the cooling procedure includes: The cooling process from the post-heating temperature to 100℃: When the temperature of the welded joint is higher than 300℃, allow it to cool naturally to 300℃; when the temperature of the welded joint is not higher than 300℃ or after cooling from higher than 300℃ to 300℃, control the cooling rate within the range of 8-20℃ / h until it cools to 100℃. The cooling stage from 100℃ to ambient temperature: The welded joint is cooled from 100℃ to an equilibrium state where the temperature difference with the ambient temperature does not exceed 5℃, and the cooling rate is controlled within the range of 1.0-2.0℃ / h.

[0009] Preferably, in the low-temperature steel structure welding method, an auxiliary gas supply device independent of the protective gas path of the welding torch is added during the gas shielded welding step. During the welding process, the auxiliary gas supply device sprays additional inert protective gas onto the surface of the preheating area in the direction of the welding torch's travel, forming an inert gas barrier that isolates welding fumes and spatter. The auxiliary inert protective gas injected by the auxiliary gas supply device is preheated before being injected, and the temperature of the gas after preheating is controlled within the range of 50-100℃. The gas injection path of the auxiliary gas supply device is configured to avoid the airflow blowing directly into the measurement area where the temperature sensor used to monitor the interlayer temperature is located; A gas diffuser is fixedly installed at the end of the nozzle of the auxiliary gas supply device. The gas diffuser is a cylindrical cavity filled with a porous medium. The inlet end of the cylindrical cavity is connected to the nozzle of the auxiliary gas supply device, and the outlet end faces the surface of the preheating area of ​​the welding joint. The porosity of the porous medium is in the range of 40%-60%, and the average pore size is 100-150μm. A gas insulation cover is installed around the gas diffuser, extending from the gas inlet end of the gas diffuser to a position close to the surface of the preheating area of ​​the weld joint.

[0010] Preferably, in the low-temperature steel structure welding method, after placing the steel structure workpiece to be welded in an environment with a temperature of -40 to -10°C, and before preheating the preheating areas on both sides of the welding joint, the welding joint and the surrounding area are dried. The nearby area refers to the metal surface area covered by extending 100-150mm to both sides of the weld seam, with the center line of the weld joint as the reference. When the drying process uses hot air purging, the hot air temperature is controlled within the range of 60-120℃, and the surrounding area is continuously purged. Alternatively, when infrared heating is used for the drying process, the surface temperature of the metal in the vicinity is raised to 40-80°C using an infrared heating device and maintained at that temperature. The endpoint of the drying process is determined as follows: after the drying process, a humidity test strip is attached to the metal surface of the nearby area, and the test strip shows a dry state.

[0011] Preferably, in the low-temperature steel structure welding method, after the drying process is completed, the weld joint and the surrounding area, which are covered by thermal insulation material or a movable protective cover extending 100-150mm to both sides of the weld joint centerline as a reference, are isolated from the surrounding humid air. The covering and isolation are maintained until the preheating process begins, and the thermal insulation material or movable protective cover is removed after the preheating temperature rises above the ambient dew point temperature.

[0012] Preferably, in the low-temperature steel structure welding method, the temperature control in the preheating, interpass temperature maintenance, post-heat treatment and cooling process steps is performed by a flexible heating and insulation system. The flexible heating and insulation system includes an electric heating unit attached to the surface of the workpiece, an adjustable insulation layer covering the electric heating unit, a controller connected to and controlling the electric heating unit, a temperature sensor for real-time monitoring of the temperature of the weld joint, and an actuator driven by the controller to change the unfolded or covered area of ​​the adjustable insulation layer. During the preheating process, the controller controls the power output of the electric heating unit based on the feedback signal from the temperature sensor in order to achieve and maintain the preset preheating temperature. During the interlayer temperature maintenance step, the controller controls the electric heating unit to supplement heating when the temperature is below 150°C, and controls the actuator to open part of the adjustable insulation layer to accelerate heat dissipation when the temperature is above 200°C, based on the interlayer temperature data monitored by the temperature sensor. In the post-heat treatment step, the controller controls the electric heating unit according to the signal from the temperature sensor, so that the temperature of the welded joint reaches and stabilizes within the set post-heat temperature range. During the cooling process, the controller performs the following closed-loop control: During the first stage of cooling, the controller calculates the current cooling rate of the welded joint in real time based on the temperature data continuously collected by the temperature sensor, and compares and judges the calculation result with the preset upper limit of the cooling rate of 20℃ / h and the lower limit of 8℃ / h in the first stage. When the real-time cooling rate is higher than 20℃ / h, the controller sends a command to the electric heating unit to start low-power heat supplementation, and at the same time sends a command to the actuator to drive the adjustable insulation layer to reduce the unfolded area and enhance the insulation. When the real-time cooling rate is below 8℃ / h, the controller sends a command to the electric heating unit to stop heating, and at the same time sends a command to the actuator to drive the adjustable insulation layer to increase the unfolded area and enhance heat dissipation. During the second-stage cooling process, the controller calculates the current cooling rate of the welded joint in real time based on the temperature data continuously collected by the temperature sensor, and compares and judges the calculation result with the preset upper limit of the cooling rate of 2.0℃ / h and the lower limit of 1.0℃ / h for the second stage. When the real-time cooling rate is higher than 2.0℃ / h, the controller sends a command to the electric heating unit to start low-power heat supplementation, and at the same time sends a command to the actuator to drive the adjustable insulation layer to reduce the unfolded area and enhance the insulation. When the real-time cooling rate is below 1.0℃ / h, the controller sends a command to the electric heating unit to stop heating, and at the same time sends a command to the actuator to drive the adjustable insulation layer to increase the unfolded area and enhance heat dissipation.

[0013] Preferably, in the low-temperature steel structure welding method, the temperature sensor is respectively set in the following three areas: the weld metal area with the center line of the weld joint as the reference, the heat-affected zone extending 10-15mm outward from the edge of the weld metal area, and the base material area extending more than 50mm outward from the edge of the weld metal area.

[0014] Preferably, in the aforementioned low-temperature steel structure welding method, during the entire process of preheating, welding, and post-heat treatment, continuous local environmental dew point monitoring and anti-condensation control are performed on the welded joint and its surrounding area. At least two ambient temperature and humidity sensors are installed within a range of 200-300mm on each side of the center line of the welded joint to monitor the air temperature and relative humidity in the area in real time. The controller calculates and displays the dew point temperature of the monitored area in real time based on the air temperature and relative humidity data collected by the ambient temperature and humidity sensor. When the calculated dew point temperature is not lower than 95% of the surface temperature of the welded joint metal measured by the temperature sensor, the controller automatically activates anti-condensation measures. Anti-condensation measures include: activating the far-infrared radiation heater installed above the welded joint to irradiate and heat the air within a range of 50-100mm above the welded joint, thereby raising the air temperature at that location and making its dew point lower than the surface temperature of the welded joint.

[0015] Preferably, in the aforementioned low-temperature steel structure welding method, gradient heating is simultaneously applied to the base material areas on both sides of the weld joint throughout the entire process of preheating, welding, and post-heat treatment. The gradient heating area extends from the boundary of the preheating area outwards from the base material by a distance of 2-4 times the thickness of the steel plate. The target temperature control for gradient heat tracing is as follows: in the preheating stage, the temperature of the heat tracing area reaches 30%-50% of the lower limit of the preheating temperature; in the interpass temperature maintenance stage, the temperature of the heat tracing area reaches 20%-40% of the lower limit of the interpass temperature; and in the post-heat treatment stage, the temperature of the heat tracing area reaches 15%-30% of the lower limit of the post-heat treatment temperature.

[0016] Preferably, in the aforementioned low-temperature steel structure welding method, during the second stage of the cooling process, controllable mechanical constraints are applied, specifically as follows: At the start of the second stage of cooling, multiple pairs of adjustable pressure dynamic clamps are arranged at intervals of 300-500mm along the length of the welded joint. The two clamping ends of each pair of dynamic clamps act symmetrically on the same point in the base material area 80-120mm away from the center line of the welded joint. The dynamic fixture applies pressure perpendicular to the surface of the base material via hydraulic or servo motor drive. The initial pressure applied by the dynamic clamp is set to 8%-12% of the yield strength of the base material at room temperature; The pressure adjustment of the dynamic fixture covers the entire second-stage cooling process, continuing until the welded joint cools to an equilibrium state where the temperature difference with the ambient temperature does not exceed 5°C. During the second stage of cooling, the real-time temperature of the welded joint is continuously monitored using a temperature sensor; Based on the monitored real-time temperature, the applied pressure of the dynamic fixture is dynamically adjusted so that the applied pressure increases as the temperature of the welded joint decreases. For every 10°C decrease in the temperature of the welded joint, the pressure applied by the dynamic fixture increases by 5%-8% of the initial pressure value.

[0017] The present invention has at least the following beneficial effects: This invention systematically solves the technical problem of cold cracking in low-alloy high-strength steel during welding at -40 to -10°C by implementing a full-process temperature coordination control that covers workpiece low-temperature equalization, preheating, welding heat input and interpass temperature control, post-heat treatment, and staged programmed cooling, and significantly improves the crack resistance of welded joints.

[0018] This invention effectively avoids the formation of coarse grains and hardened structures by strictly controlling the welding heat input (15-25 kJ / cm) and interpass temperature (150-200℃), and by combining it with post-heat treatment at a specified temperature and time (280-320℃, 120-180 min), which significantly reduces the diffusible hydrogen content and improves the microstructure, resulting in welded joints with excellent strength, plasticity and low-temperature impact toughness.

[0019] This invention designs a phased and strictly controlled cooling process to cool the joint from the post-heating temperature to the ambient temperature at a gradual and controllable rate. The cooling rate in the first stage (300-100℃) is controlled at 8-20℃ / h, and the cooling rate in the second stage is controlled at 1.0-2.0℃ / h, thereby significantly reducing welding residual stress and deformation, and stabilizing the microstructure and properties of the joint.

[0020] This invention creates a gas barrier that isolates fumes and spatter by adding an independent auxiliary gas supply device and spraying preheated inert protective gas into the welding area. This avoids the rapid cooling effect of low-temperature gas on the preheating area, while ensuring the accuracy of the temperature monitoring area and enhancing the stability of the welding process in low-temperature environments.

[0021] This invention effectively removes moisture from the metal surface and prevents secondary condensation by drying the weld joint and surrounding area before preheating and covering them with insulating material. This significantly reduces the initial hydrogen source for hydrogen-induced cracking and improves the moisture-proof reliability of the process.

[0022] This invention utilizes a flexible heating and insulation system that integrates an electric heating unit, an adjustable insulation layer, a controller, and a temperature sensor. This system enables automated closed-loop control of the entire process, including preheating, interlayer temperature maintenance, post-heating, and cooling. It significantly reduces human error and ensures the accuracy and consistency of process parameter execution.

[0023] This invention continuously monitors the local environmental dew point temperature throughout the welding process and automatically activates anti-condensation measures (such as far-infrared radiation heating) when the dew point approaches the metal surface temperature. This dynamically prevents moisture from condensing on the joint surface during welding, further cuts off the hydrogen intrusion pathway, and enhances the process adaptability in humid environments.

[0024] This invention constructs a temperature buffer zone for a smooth transition from high-temperature weld to low-temperature base material by implementing gradient heating on the base material regions on both sides of the welded joint during preheating, welding, and post-heat treatment. This reduces the temperature gradient and thermal stress peak, promotes hydrogen diffusion and escape, and improves the uniformity of the joint structure.

[0025] This invention actively compensates for the tensile stress generated by the cooling shrinkage of the joint by applying a controllable mechanical constraint that dynamically increases with the decrease in temperature during the second stage of cooling. This more effectively reduces the final residual stress level and improves the dimensional stability and crack resistance of the joint.

[0026] This invention integrates a distributed temperature sensing network, on-site sensor calibration, and an intelligent closed-loop control system to ensure that complex low-temperature welding temperature procedures can be strictly, stably, and repeatedly executed, significantly improving the reliability and consistency of welding quality, and is suitable for high-standard, large-scale engineering applications.

[0027] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to embodiments, so that those skilled in the art can implement it based on the description.

[0029] It should be noted that, unless otherwise specified, the experimental methods described in the following implementation plan are all conventional methods, and the reagents and materials described are all commercially available unless otherwise specified.

[0030] This invention provides a low-temperature steel structure welding method, applicable to welding of low-alloy high-strength steel in ambient temperatures ranging from -40°C to -10°C, comprising the following steps: Place the steel structure workpiece to be welded in an environment with a temperature of -40 to -10℃ until the temperature of the workpiece body reaches equilibrium with the ambient temperature. Preheat the preheating zones on both sides of the weld joint. The width of the preheating zone should be 3-5 times the thickness of the steel plate. Control the preheating temperature at 180-220℃, with a preheating rate not exceeding 100℃ / h. After reaching the preheating temperature, maintain it at a constant temperature for 60-90 minutes. The preheating rate of no more than 100℃ / h is a general guideline. When the ambient temperature is below -20℃, to prevent excessive thermal shock, it is recommended to reduce the initial heating rate (e.g., from ambient temperature to 0℃) to 50-70℃ / h, and then gradually increase it to the upper limit rate after the overall temperature of the workpiece recovers.

[0031] Gas shielded welding is used for multi-layer and multi-pass welding. The welding heat input of each pass is controlled within the range of 15-25 kJ / cm, while the interpass temperature of each pass is maintained at 150-200℃. After welding, the welded joint is subjected to post-heat treatment. The welded joint is heated to 280-320℃ and held at this temperature for 120-180 minutes. The post-heat temperature of 280-320℃ is suitable for common low-alloy high-strength steels such as Q345 and Q390. For special steel grades, it is necessary to conduct separate tests to determine the appropriate temperature.

[0032] After the post-heat treatment, the welded joint is cooled according to a preset cooling procedure; the cooling procedure includes: The cooling process from the post-heating temperature to 100℃: When the temperature of the welded joint is higher than 300℃, allow it to cool naturally to 300℃; when the temperature of the welded joint is not higher than 300℃ or after cooling from higher than 300℃ to 300℃, control the cooling rate within the range of 8-20℃ / h until it cools to 100℃. The cooling process from 100℃ to ambient temperature involves continuing to cool the welded joint from 100℃ until it reaches an equilibrium state where the temperature difference with the ambient temperature does not exceed 5℃, with the cooling rate controlled within the range of 1.0-2.0℃ / h. This cooling rate range is suitable for common components with a plate thickness ≤50mm. For plate thicknesses >50mm or cases with extremely high restraint, a more suitable rate needs to be determined through process qualification tests. An equilibrium state where the temperature difference with the ambient temperature does not exceed 5℃ means that within 30 minutes, the temperature fluctuation of the welded joint does not exceed ±2℃, and the difference from the ambient temperature does not exceed 5℃. The core of this cooling procedure is that during the cooling process from the post-heating temperature to 100℃, the system intelligently selects a cooling strategy based on the real-time temperature. If the temperature is higher than 300℃, natural heat dissipation is relied upon; once the temperature sensor detects that the temperature has dropped to 300℃ or lower, the system immediately initiates closed-loop control, precisely maintaining the cooling rate at 8-20℃ / h until the temperature reaches 100℃. Subsequent cooling is always carried out at a slower rate of 1.0-2.0℃ / h.

[0033] When welding low-alloy high-strength steel in low-temperature environments, the main technical challenge lies in effectively preventing cold cracks caused by rapid cooling and thermal stress concentration, as well as the decrease in weld joint toughness. Conventional welding processes often struggle to systematically manage the complete temperature journey from pre-weld to post-weld when facing extreme environments of tens of degrees below zero Celsius, making it easy for delayed cracks to occur in the weld or heat-affected zone, posing a threat to structural safety.

[0034] The closest existing technologies typically involve preheating the workpiece before welding, controlling the interpass temperature during welding, and performing post-weld heat treatment. However, these existing technologies often lack the requirement for homogenization of the initial state of the workpiece, the preheating parameters may be too general, and most importantly, the post-weld cooling process mostly relies on natural cooling or simple heat preservation, lacking precise and segmented programmed control of the cooling rate, especially within the critical temperature range. This results in the weld joint potentially experiencing significant thermal stress and unfavorable microstructural transformations during cooling, failing to completely eliminate the risk of cold cracking.

[0035] Compared to existing technologies, this solution first requires placing the workpiece to be welded in a target low-temperature environment until its body temperature reaches equilibrium with the ambient temperature. This step aims to eliminate the initial temperature difference between the inside and surface of the workpiece, and between the workpiece and the environment, laying the foundation for subsequent uniform heating and avoiding additional stress sources caused by the temperature difference between the core and the surface.

[0036] Subsequently, the area on both sides of the weld joint, with a width 3-5 times the thickness of the steel plate, is preheated. The preheating temperature is controlled at 180-220℃, and the preheating rate does not exceed 100℃ / h. After reaching the preheating temperature, it is held at a constant temperature for 60-90 minutes. The purpose of this step is to bring the metal in the area to be welded to a stable thermal state with improved plasticity and enhanced hydrogen diffusion capacity, thereby reducing thermal shock and hardening tendency during welding.

[0037] When performing multi-layer, multi-pass gas shielded welding, the welding heat input for each pass must be strictly controlled within the range of 15-25 kJ / cm, while maintaining the interpass temperature between each pass at 150-200℃. This aims to avoid overheating leading to coarse grains or undercooling resulting in hardened structures, while ensuring that the weld zone is always within a temperature window conducive to hydrogen escape.

[0038] After welding, the weld joint undergoes post-heat treatment, heating it to 280-320℃ and holding it at this temperature for 120-180 minutes. The purpose is to significantly reduce the hydrogen content in the joint and improve the microstructure. It is important to note that after post-heat treatment, during the cooling process from the holding temperature to 300℃, the material still retains good plasticity, and internal stresses can be relaxed through slight creep. Therefore, the cooling rate in this range is not strictly limited, allowing for relatively rapid natural cooling. However, when the temperature drops to 300℃ and below, the material enters a temperature range highly sensitive to hydrogen-induced cracking and stress concentration, requiring active rate control. Therefore, in the critical first stage of the subsequent cooling procedure from 300-100℃, the cooling rate is strictly controlled within the range of 8-20℃ / h. This aims to allow the joint to pass through the sensitive range at a gradual rate, allowing for sufficient stress relaxation. The second stage involves cooling the joint from 100°C to an equilibrium state with a temperature difference of no more than 5°C from the ambient temperature at an extremely slow rate of 1.0-2.0°C / h. The purpose is to minimize the residual stress generated by cooling shrinkage when the material's plasticity further decreases, ensuring a smooth transition of the joint to the ambient temperature, thereby stabilizing the effect of the post-heat treatment and fundamentally preventing the generation of delayed cracks.

[0039] When placing the workpiece to be welded in a low-temperature environment, the specific ambient temperature can be -30℃, -20℃, or -10℃. A walk-in high and low temperature test chamber can be used to achieve a constant environment. The threshold for determining the equilibrium between the workpiece temperature and the environment can be set when the difference between the temperature measured at multiple points on the workpiece surface and the ambient temperature is less than 2℃ for two consecutive hours. During preheating, the specific value for the width of the preheating zone can be 3, 4, or 5 times the thickness of the steel plate. The specific preheating temperature can be 180℃, 200℃, or 220℃. The heating rate threshold can be set to no more than 80℃ / h, 90℃ / h, or 100℃ / h. The specific holding time after reaching the preheating temperature can be 60min, 75min, or 90min. A flexible ceramic fiber heating blanket combined with a high-precision PID temperature controller can be used for preheating. The heating blanket can be laid flat on the surface of the base material, extending a predetermined width to both sides based on the weld centerline, and covered with high-temperature resistant insulation cotton. During operation, the controller controls the heating blanket power according to the set program, first heating it to the target temperature at a rate not exceeding the set threshold, and then switching to constant temperature mode and starting the timer.

[0040] The welding method can be gas metal arc welding (GMAW). The specific heat input can be selected as 16 kJ / cm, 20 kJ / cm, or 24 kJ / cm. The specific range for maintaining the interpass temperature can be selected as 150-170℃, 170-190℃, or 180-200℃. A digital gas metal arc welding machine can be used for welding. The heat input is controlled by adjusting the welding current, voltage, and torch travel speed, ensuring the product of these parameters divided by the travel speed falls within a predetermined range. A type K thermocouple can be used for interpass temperature monitoring. Immediately after the weld pass is completed, the thermocouple's measuring end should be in close contact with the base material surface 10-15 mm from the edge of the previous weld fusion line. The preheating heating blanket and its control system can be reused to maintain the interpass temperature. During welding intervals, the controller continuously monitors the interpass temperature. If it falls below the lower limit of the set range, the heating blanket is automatically activated for low-power compensation heating. If the temperature exceeds the upper limit during welding, the shielding gas flow can be temporarily increased to assist in heat dissipation.

[0041] The specific selection of the post-heating temperature can be 280℃, 300℃, or 320℃. The specific holding time can be 120min, 150min, or 180min. The specific cooling rate range for the first stage can be 8-12℃ / h, 10-15℃ / h, or 15-20℃ / h. The specific cooling rate range for the second stage can be 1.0-1.5℃ / h or 1.5-2.0℃ / h. The equipment for achieving post-heating and controlled cooling can reuse the flexible heating and holding system from the preheating stage, with the addition of a thicker insulation layer. Temperature sensors can be installed at the weld center, the heat-affected zone, and a slightly more distant base material area. During operation, in the post-heating stage, the controller drives the heating system to heat the weld area to the set temperature and maintain that temperature. After the constant temperature period ends, the cooling process begins. The first stage involves reducing the temperature from the post-heating temperature to 100℃. When the temperature enters the 300-100℃ range, the controller calculates the cooling rate based on real-time data from the temperature sensor and dynamically adjusts the heating power and the coverage of the insulation layer to keep the rate within a predetermined range. When the temperature reaches 100℃, the second stage begins, cooling at a slower, controlled rate until the temperature reaches equilibrium with the ambient temperature.

[0042] The technical effect achieved by this method is to improve the cold crack resistance and overall mechanical properties of welded joints of low-alloy high-strength steel in low-temperature environments. This effect is achieved through a series of synergistically controlled steps: First, low-temperature homogenization and sufficient preheating of the workpiece reduce thermal stress and temperature difference at the start of welding, thus lowering the tendency for hardening. Second, strictly controlled welding heat input and stable interpass temperature management prevent excessive grain growth or excessively rapid cooling, which facilitates hydrogen escape. Finally, timely post-heat treatment effectively reduces the diffusible hydrogen content in the weld and improves the microstructure, while the subsequent staged slow cooling process allows the joint to pass through the stress-sensitive temperature range at an extremely low rate, minimizing the generation of residual stress, thereby stabilizing the joint's microstructure and properties and inhibiting the formation of delayed cold cracks.

[0043] Example 1 The workpiece was constructed using Q345D low-alloy high-strength steel plate, 30mm thick, with a welding environment temperature of -20℃. First, the workpiece was placed in the -20℃ environment for 24 hours to allow its temperature to equalize with the environment. The preheating zone width was set to 120mm, and the temperature was increased to 200℃ at a rate of 85℃ / h, then held at that temperature for 80 minutes. The welding process employed gas metal arc welding (GMAW), controlling the heat input of each weld pass between 19.5-21.0 kJ / cm and maintaining the interpass temperature at 170-185℃. Immediately after welding, post-heat treatment was performed, heating the joint to 300℃ and holding it at that temperature for 150 minutes. The cooling process then proceeds as follows: the first stage cools from 300℃ to 100℃, with the cooling rate controlled at 14.2℃ / h (the cooling rate can be calculated in real time by the controller of the flexible heating and insulation system, and the heating power and insulation layer opening can be dynamically adjusted to precisely control the cooling rate at 14.2℃ / h); the second stage cools from 100℃ to -20℃, with the cooling rate controlled at 1.6℃ / h.

[0044] Post-weld testing, conducted 72 hours later, revealed no cracks using either ultrasonic or magnetic particle inspection. The diffusible hydrogen content in the weld metal was 2.0 mL / 100 g. Tensile specimens from the joint all fractured at the base metal, with a tensile strength of 558 MPa. Charpy V-notch impact testing at -40℃ showed an average J of 65 J at the weld center and an average of 72 J in the heat-affected zone. The maximum principal residual stress of the welded joint was 248 MPa. Metallographic observation indicated that the microstructure of the heat-affected zone was predominantly fine lath bainite.

[0045] Example 1 shows that by fully executing the workpiece balancing, preheating, controlled welding, post-heating, and programmed cooling process, a welded joint with no cracks, low hydrogen content, high toughness, low residual stress, and good microstructure can be obtained, effectively solving the problems of cold cracking and reduced joint toughness in low-temperature welding in the prior art.

[0046] Comparative Example 1 The same steel plate and environment as in Example 1 were used, but the preheating parameters were adjusted: the preheating temperature was changed to 160°C and the holding time was changed to 60 min. During the welding process, the interpass temperature dropped to 125°C at one point. After welding, the same post-heating and procedural cooling were performed as in Example 1.

[0047] Magnetic particle inspection 72 hours post-weld revealed two surface microcracks, approximately 2-4 mm in length, at the weld toe. The diffusible hydrogen content in the weld metal was 3.8 mL / 100 g. The tensile strength of the joint was 545 MPa. In the -40℃ impact energy test, the average value at the weld center was 42 J, and the lowest value in the heat-affected zone was 29 J. The maximum principal residual stress of the welded joint was 332 MPa. Metallographic observation revealed the presence of lath martensite in the heat-affected zone. This indicates that insufficient preheating led to a poor weld initiation condition, inadequate hydrogen removal, and uncontrolled interpass temperature, promoting the formation of hardened structures and initiating microcracks, severely impairing the low-temperature toughness of the joint.

[0048] Comparative Example 2 The preheating, welding, and post-heating parameters were exactly the same as in Example 1, but all temperature control devices were removed after the post-heat treatment, allowing the joint to cool naturally in an environment of -20°C. Monitoring showed that the average cooling rate from 300°C to 100°C was approximately 48°C / h, and the average cooling rate from 100°C to -20°C was approximately 8.5°C / h.

[0049] Ultrasonic testing 72 hours post-weld revealed a reflection signal resembling a suspected microcrack in the fusion line region. The diffuse hydrogen content in the weld was 2.4 mL / 100 g. The tensile strength of the joint was 550 MPa. In the -40℃ impact energy test, the average impact value in the heat-affected zone was 48 J. The maximum principal residual stress of the welded joint reached 388 MPa. This indicates that although post-heat treatment reduced the hydrogen content, subsequent uncontrolled rapid cooling reintroduced extremely high residual stress and may have induced micro-defects, leading to a deterioration in joint toughness. This demonstrates that programmed slow cooling after welding is crucial for controlling residual stress.

[0050] Comparative Example 3 The process is the same as in Example 1 during the preheating and welding stages, but the post-heat treatment and all subsequent cooling steps are completely omitted. Heating is stopped immediately after welding, and the joint is allowed to cool freely in air at -20°C.

[0051] Magnetic particle inspection 72 hours post-weld revealed clear delayed cold cracks in the heat-affected zone (HAZ) of multiple weld joints, with a maximum length of approximately 8 mm. The diffusible hydrogen content in the weld was as high as 6.2 mL / 100 g. The tensile strength of the joint was 538 MPa. In the -40℃ impact energy test, the average impact value of the HAZ was only 35 J. Metallographic observation showed a large amount of coarse hardened martensite in the HAZ. This proves that omitting the post-heat hydrogen removal and stress relief steps leads to a large amount of residual diffusible hydrogen in the weld joint, forming a harmful hardened structure. Under the combined action of cooling stress, this inevitably triggers severe delayed cold cracks, significantly deteriorating the overall performance of the joint. The welding process parameters and results of Example 1 and Comparative Examples 1-3 are shown in Table 1.

[0052] Table 1 Comparison of welding process parameters and results between Example 1 and Comparative Examples 1-3 As shown in Table 1, Example 1, by executing the complete process control flow, obtained a welded joint with optimal overall performance. It exhibited no post-weld cracks, the lowest diffusible hydrogen content, the highest low-temperature impact toughness, and the lowest residual stress level, with its microstructure primarily composed of bainite, which has good toughness. This confirms that this method can systematically solve the core problems of cold cracking and decreased joint toughness in low-temperature welding in the background art.

[0053] Comparative Example 1 suffered from poor weld initiation due to insufficient preheating, inadequate hydrogen removal, and uncontrolled interpass temperature, which ultimately resulted in microcracks and a significant decrease in toughness.

[0054] Although Comparative Example 2 underwent post-heating, the subsequent natural cooling rate was too fast, resulting in extremely high residual stress and causing defects and deterioration of toughness.

[0055] In Comparative Example 3, the complete omission of post-heating and controlled cooling resulted in a large amount of diffusible hydrogen remaining in the joint and forming a hardened structure, which directly led to severe delayed cold cracking and a comprehensive deterioration of performance.

[0056] Comparative Examples 1-3, from different perspectives, demonstrate the necessity and synergistic effect of each process step in Example 1 (sufficient preheating and interpass temperature control, effective post-heating hydrogen removal, and strict programmed slow cooling) in preventing cold cracking in low-temperature welding and ensuring joint toughness.

[0057] In another embodiment, in the low-temperature steel structure welding method, an auxiliary gas supply device independent of the protective gas path of the welding gun body is added during the gas shielded welding step. During the welding process, the auxiliary gas supply device sprays additional inert protective gas onto the surface of the preheating area in the direction of the welding torch's travel, forming an inert gas barrier that isolates welding fumes and spatter. The auxiliary inert protective gas injected by the auxiliary gas supply device is preheated before being injected, and the temperature of the gas after preheating is controlled within the range of 50-100℃. The gas injection path of the auxiliary gas supply device is configured to avoid the airflow blowing directly into the measurement area where the temperature sensor used to monitor the interlayer temperature is located; A gas diffuser is fixedly installed at the end of the nozzle of the auxiliary gas supply device. The gas diffuser is a cylindrical cavity filled with a porous medium. The inlet end of the cylindrical cavity is connected to the nozzle of the auxiliary gas supply device, and the outlet end faces the surface of the preheating area of ​​the welding joint. The porosity of the porous medium (such as sintered metal or ceramic) is controlled within the range of 40%-60%, corresponding to an average pore size of about 100-150μm. A gas insulation cover is installed around the gas diffuser, extending from the gas inlet end of the gas diffuser to a position close to the surface of the preheating area of ​​the weld joint.

[0058] In the welding of low-alloy high-strength steel in low-temperature environments, in addition to controlling the overall temperature flow to prevent cold cracking, the welding process itself also faces problems such as shielding gas loss due to low-temperature air convection, welding fumes and spatter interfering with the molten pool, and contaminating the surface of the preheating zone. These factors may impair the gas shielding effect, leading to porosity or slag inclusions in the weld, and may also affect the accuracy of temperature sensor measurements, thereby interfering with the precise control of interpass temperature. The basic welding process manages the macroscopic thermal process through preheating, temperature-controlled welding, and post-heat treatment, but no specific arrangements are made for the control of the local gaseous environment and cleanliness during the instantaneous welding process.

[0059] In the multi-layer, multi-pass welding process using gas shielded welding, an auxiliary gas supply device, independent of the welding torch's protective gas path, is added. This device injects additional inert shielding gas onto the surface of the preheated area along the welding torch's travel direction during welding. The injected gas is preheated before reaching the workpiece surface, with its temperature controlled within the range of 50-100°C. The purpose of this step is to provide a suitable inert gas barrier to the welding area. The preheated gas prevents the direct blowing of low-temperature gas onto the preheated base material surface, avoiding localized rapid cooling and thus reducing temperature fluctuations and thermal stress. Simultaneously, this additional gas barrier helps isolate and disperse fumes and spatter generated during welding, preventing them from depositing and contaminating the preheated area surface, creating cleaner conditions for the fusion of subsequent weld passes.

[0060] The gas injection path of the auxiliary gas supply device is specifically configured to avoid the airflow directly blowing onto the measurement area of ​​the temperature sensor used to monitor interpass temperature. This is to prevent high-speed airflow from directly blowing onto the sensing end of the temperature sensor, avoiding forced convection cooling that could cause the sensor reading to be lower than the actual workpiece temperature, thus ensuring the authenticity and reliability of the interpass temperature monitoring data and providing accurate feedback for precise temperature control. A gas diffuser is fixedly installed at the end of the injection port of the auxiliary gas supply device. This gas diffuser is a cylindrical cavity filled with a porous medium, with its inlet end connected to the injection port and its outlet end facing the surface of the preheating area of ​​the weld joint. The porosity of the porous medium (such as sintered metal or ceramic) is controlled within the range of 40%-60%. The purpose of this structural design is to transform the ejected high-pressure concentrated airflow into a uniform, gentle, and wider-coverage laminar or steady-flow gas curtain. Uniform airflow distribution can more effectively cover and protect a larger area and reduce disturbance to the molten pool. A gas insulation cover is installed outside the gas diffuser, extending from the gas diffuser's inlet end to a position close to the surface of the preheating area of ​​the weld joint. The purpose is to reduce the heat exchange between the preheated inert protective gas and the surrounding low-temperature environment during the process of delivering it to the workpiece surface, maintain the gas temperature as much as possible, and ensure that it still has the expected preheating effect when it reaches the workpiece surface.

[0061] By implementing the above steps, this scheme enhances the control over the local microenvironment during welding, building upon the basic welding process. The preheated and uniformly diffused auxiliary inert gas barrier effectively isolates external interference in low-temperature environments, stabilizing the local atmosphere and surface condition of the welding area and reducing welding defects caused by fume contamination and inadequate gas protection. Simultaneously, the airflow design, which avoids temperature sensors, ensures the accuracy of monitoring key process parameters, enabling reliable execution of precise interpass temperature control. These measures work together to further improve the stability of the welding process and the intrinsic quality of the weld in low-temperature environments, providing a more comprehensive guarantee for obtaining high-performance welded joints.

[0062] In another embodiment, in the low-temperature steel structure welding method, after placing the steel structure workpiece to be welded in an environment with a temperature of -40 to -10°C, and before preheating the preheating areas on both sides of the welding joint, the welding joint and the surrounding area are dried. The nearby area refers to the metal surface area covered by extending 100-150mm to both sides of the weld seam, with the center line of the weld joint as the reference. When the drying process uses hot air purging, the hot air temperature is controlled within the range of 60-120℃, and the surrounding area is continuously purged. Alternatively, when infrared heating is used for the drying process, the surface temperature of the metal in the vicinity is raised to 40-80°C using an infrared heating device and maintained at that temperature. The endpoint of the drying process is determined as follows: after drying, a humidity test strip is attached to the metal surface in the vicinity of the drying area, and the test strip shows a dry state. The endpoint of the drying process is determined as follows: when the humidity test strip displays the dry state color mark on the metal surface, drying is complete.

[0063] In the welding of low-alloy high-strength steel in low-temperature environments, besides managing the temperature history to prevent cold cracking, the surface condition of the workpiece is also a crucial but easily overlooked factor. Low-temperature environments are often accompanied by high humidity. When moving low-temperature workpieces from storage to the welding station or pre-treating their surfaces, moisture in the air easily condenses on the metal surface, which is below the dew point temperature. If this condensation or moisture is not removed before preheating and welding, it will decompose under subsequent high temperatures, producing hydrogen, which dissolves into the weld pool and heat-affected zone, significantly increasing the risk of hydrogen-induced cold cracking. The basic welding process removes hydrogen through preheating and post-heat treatment; however, if the metal surface itself has already adsorbed or condensed a significant amount of moisture before welding, it will place a greater initial burden on the hydrogen removal process, potentially exceeding the capacity of subsequent heat treatment and leading to potential cracking.

[0064] While preheating can remove some moisture through increased temperature, its core purpose is to heat the metal to a specific temperature to improve its weldability, not specifically for dehumidification. If there is significant condensation on the surface, the intense vaporization of moisture during preheating can disrupt localized temperature uniformity. Furthermore, the generated water vapor decomposes into hydrogen at high temperatures, and these hydrogen atoms can directly dissolve into the metal during the heating process, thus introducing an additional hydrogen source before welding even begins. This essentially creates new risks while attempting to solve a problem.

[0065] Adding a separate drying step before preheating aims to proactively cut off this initial source of hydrogen at its source. This step, performed at a low temperature before the welding thermal cycle begins, uses specialized hot air or infrared heating to gently and thoroughly remove condensation and adsorbed moisture from the metal surface. This provides a dry, clean starting surface for subsequent preheating processes, ensuring that the preheating process purely serves the goal of temperature increase. It avoids the complex situation of interference between heating and dehumidification, thus more systematically and reliably reducing the overall risk of hydrogen-induced cooling cracks.

[0066] To address the issue of pre-welding moisture, after placing the steel structure workpiece in a low-temperature environment and ensuring its temperature equalization, a drying step is added before preheating to treat the weld joint and its surrounding area. The surrounding area refers to the metal surface region extending 100-150mm to both sides of the weld joint centerline. Defining this drying range ensures that the main areas directly affected by the welding thermal cycle, as well as a certain range of the base material surface outside these areas, are in a low-humidity state before welding begins, reducing hydrogen introduction at the source.

[0067] The drying process can be carried out in two specific ways. When using hot air purging, the hot air temperature is controlled within the range of 60-120℃, and the surrounding area is continuously purged. This method aims to directly blow warm, flowing gas onto the metal surface, using forced convection to heat and remove surface moisture and humidity, resulting in direct and uniform coverage. Alternatively, when using infrared heating, the surface temperature of the metal in the surrounding area is raised to 40-80℃ using an infrared heating device and maintained at this temperature. This method aims to use infrared radiation to penetrate the air and directly heat the metal body, causing its surface temperature to rise uniformly and promoting the evaporation of adsorbed moisture. This method is suitable for areas sensitive to airflow or with complex structures. The endpoint of the drying process is determined by: after drying, a humidity test strip is attached to the metal surface in the surrounding area; the test strip should indicate a dry state. This intuitive physical determination method aims to provide a clear, operable, and rapid on-site quality acceptance standard, ensuring that the drying effect meets the standards and avoiding the subjectivity and uncertainty of judgment based on experience.

[0068] By implementing the aforementioned drying process, this approach adds a pretreatment step targeting the microenvironment of the workpiece surface before macroscopic temperature control. Its direct effect is a significant reduction in the hydrogen source introduced by moisture on the workpiece surface during the initial welding stage, alleviating the burden on hydrogen diffusion and escape during subsequent welding and heat treatment. The dried metal surface is more stable, which also facilitates a more uniform temperature rise during preheating, avoiding micro-airflow disturbances or temperature unevenness caused by sudden vaporization of localized moisture. Overall, this step, by actively controlling pre-welding conditions, further reduces the initial driving force of hydrogen-induced cooling cracks, enhances the overall welding process's ability to cope with low-temperature and humid environments, and results in a welded joint with higher intrinsic quality reliability and crack resistance margin.

[0069] In another embodiment, in the low-temperature steel structure welding method, after the drying process is completed, thermal insulation material or a movable protective cover is used to cover and isolate the weld joint and the surrounding area, extending 100-150mm to both sides of the weld joint centerline as a reference, so as to isolate the area from the surrounding humid air; the covering and isolation is maintained until the preheating process begins, and the thermal insulation material or movable protective cover is removed after the preheating temperature rises above the ambient dew point temperature.

[0070] In the welding of low-alloy high-strength steel in low-temperature environments, pre-weld drying of the welding area can effectively remove surface condensation and reduce hydrogen sources. However, during the interval between drying and preheating, the dried metal surface is exposed to the low-temperature, humid workshop environment, posing a risk of re-absorbing moisture from the air and even secondary condensation. If the drying effect is weakened before welding begins, the efforts of the initial drying process will be significantly reduced, and the risk of hydrogen-induced cracking remains. While the basic welding process and pre-weld drying address the initial preparation, no protection measures are in place for the critical transition phase from drying to high-temperature preheating. Maintaining the drying effect depends on environmental contingencies, resulting in insufficient process stability.

[0071] To address the issue of maintaining the drying effect, an additional step of covering and isolating the area with thermal insulation materials or a movable protective cover is added after the drying process. The covered and isolated area is the same as the drying area, that is, the weld joint and its surrounding area extending 100-150mm to both sides of the weld centerline. The purpose of clearly defining the same coverage area as the drying area is to ensure that all dried surfaces are physically isolated and protected, avoiding any blind spots.

[0072] The materials used for the insulation covering can be flexible insulating blankets or protective covers made of thin metal sheets. The purpose of covering is to create a physical barrier, temporarily isolating the dry area from the surrounding humid air. This prevents the dry metal surface from reabsorbing moisture or condensing during preheating due to continuous contact with cold, humid air. This insulation covering must be maintained until the preheating process officially begins. After preheating starts, the metal temperature in the covered area begins to rise as the heating system operates. Once the preheating temperature exceeds the ambient dew point, the metal surface temperature is above the air dew point, and the risk of moisture condensing on the surface is largely eliminated. At this point, the insulation material or movable protective cover can be removed. This removal timing is chosen to ensure safety while avoiding the insulation covering from affecting the uniform heat dissipation and proper temperature sensor monitoring during the subsequent preheating process.

[0073] By implementing the aforementioned covering and isolation steps, this approach establishes an effective connection and protection link between the drying process and the preheating process. Its direct effect is to lock in the results of the drying process, minimizing fluctuations in the hydrogen content of the metal surface in critical areas before welding, and providing a controllable and stable starting condition for subsequent welding. This measure improves the robustness of the entire pre-welding preparation process, reduces the potential impact of environmental humidity fluctuations or uncertain operation intervals on welding quality, makes the hydrogen-induced crack prevention measures based on drying treatment more reliable and thorough, and further enhances the quality stability of the welded joint in harsh low-temperature and humid environments.

[0074] In another embodiment, the temperature control in the preheating, interpass temperature maintenance, post-heat treatment, and cooling process steps of the low-temperature steel structure welding method is performed by a flexible heating and insulation system. The flexible heating and insulation system includes an electric heating unit attached to the surface of the workpiece, an adjustable insulation layer covering the electric heating unit, a controller connected to and controlling the electric heating unit, a temperature sensor for real-time monitoring of the weld joint temperature, and an actuator driven by the controller to change the unfolded or covered area of ​​the adjustable insulation layer. The adjustable insulation layer may be composed of multiple layers of high-temperature resistant ceramic fiber felt and a temperature-resistant slide rail mechanism, which is driven by a servo motor to unfold or retract along the length of the weld to change the heat dissipation area.

[0075] During the preheating process, the controller controls the power output of the electric heating unit based on the feedback signal from the temperature sensor in order to achieve and maintain the preset preheating temperature. During the interlayer temperature maintenance step, the controller controls the electric heating unit to supplement heating when the temperature is below 150°C, and controls the actuator to open part of the adjustable insulation layer to accelerate heat dissipation when the temperature is above 200°C, based on the interlayer temperature data monitored by the temperature sensor. In the post-heat treatment step, the controller controls the electric heating unit according to the signal from the temperature sensor, so that the temperature of the welded joint reaches and stabilizes within the set post-heat temperature range. During the cooling process, the controller performs the following closed-loop control: During the first stage of cooling, the controller calculates the current cooling rate of the welded joint in real time based on the temperature data continuously collected by the temperature sensor, and compares and judges the calculation result with the preset upper limit of the cooling rate of 20℃ / h and the lower limit of 8℃ / h in the first stage. When the real-time cooling rate is higher than 20℃ / h, the controller sends a command to the electric heating unit to start low-power heat supplementation, and at the same time sends a command to the actuator to drive the adjustable insulation layer to reduce the unfolded area and enhance the insulation. When the real-time cooling rate is below 8℃ / h, the controller sends a command to the electric heating unit to stop heating, and at the same time sends a command to the actuator to drive the adjustable insulation layer to increase the unfolded area and enhance heat dissipation. During the second-stage cooling process, the controller calculates the current cooling rate of the welded joint in real time based on the temperature data continuously collected by the temperature sensor, and compares and judges the calculation result with the preset upper limit of the cooling rate of 2.0℃ / h and the lower limit of 1.0℃ / h for the second stage. When the real-time cooling rate is higher than 2.0℃ / h, the controller sends a command to the electric heating unit to start low-power heat supplementation, and at the same time sends a command to the actuator to drive the adjustable insulation layer to reduce the unfolded area and enhance the insulation. When the real-time cooling rate is below 1.0℃ / h, the controller sends a command to the electric heating unit to stop heating, and at the same time sends a command to the actuator to drive the adjustable insulation layer to increase the unfolded area and enhance heat dissipation.

[0076] In the welding of low-alloy high-strength steel at low temperatures, the basic welding process relies on manual monitoring or decentralized heating and holding equipment to perform preheating, interpass temperature maintenance, post-heat treatment, and cooling procedures. This approach makes precise, stable, and consistent temperature control difficult to achieve at low temperatures. Temperature control at each step may be inconsistent; the heating, holding, and cooling processes depend on operator experience and adjustments, resulting in delayed responses and difficulty in real-time closed-loop control of the cooling rate. This leads to large process fluctuations, with temperature parameters easily deviating from predetermined ranges, thus affecting the stability and consistency of weld joint performance and making the control of cold cracking risk unreliable.

[0077] To address the challenges of automated temperature control and closed-loop regulation throughout the entire process, a flexible heating and insulation system was employed to control temperature during preheating, interpass temperature maintenance, post-heat treatment, and cooling. This system comprises an electric heating unit attached to the workpiece surface, an adjustable insulation layer covering the heating unit, a controller connecting to and controlling the heating unit, a temperature sensor for real-time monitoring of the weld joint temperature, and an actuator driven by the controller to adjust the area of ​​the adjustable insulation layer. The integration of these components aims to create a physical control system capable of real-time sensing, intelligent judgment, and proactive adjustment, replacing the reliance on manual experience in traditional operations.

[0078] In the system's operation, the controller plays a crucial decision-making role. During the preheating stage, the controller continuously adjusts the power output of the electric heating unit based on feedback signals from the temperature sensor. This ensures the weld joint area reaches the target preheating temperature at a preset heating rate and automatically compensates for heat loss throughout the constant temperature maintenance period, keeping temperature fluctuations within a minimal range. During the interpass temperature maintenance stage, the controller, based on real-time data monitored by the temperature sensor, automatically controls the electric heating unit to provide low-power supplementary heating when the temperature is below 150℃, and controls the actuator to open part of the adjustable insulation layer to accelerate heat dissipation when the temperature is above 200℃. This aims to automatically and precisely maintain the joint temperature within the target range of 150-200℃ during the welding interval, without manual intervention. In the post-heat treatment stage, the controller again automatically controls the electric heating unit based on temperature sensor signals, ensuring the weld joint temperature reaches and stabilizes within the set post-heat treatment temperature range of 280-320℃, ensuring that hydrogen removal treatment is carried out at a constant temperature for a sufficient duration.

[0079] In the cooling process, the closed-loop control function of the controller is crucial. During the first stage of cooling, the controller calculates the current cooling rate in real time based on data continuously collected by the temperature sensor and compares it to the preset rate range of 8-20℃ / h for the first stage. When the real-time cooling rate exceeds 20℃ / h, the controller instructs the electric heating unit to activate low-power heat supplementation, and simultaneously instructs the actuator to drive the adjustable insulation layer to reduce its unfolded area to enhance insulation. When the real-time cooling rate falls below 8℃ / h, the controller instructs the electric heating unit to stop heating, and simultaneously instructs the actuator to increase the unfolded area of ​​the adjustable insulation layer to enhance heat dissipation. The aim is to forcibly control the cooling rate within a safe range by dynamically adjusting heating and heat dissipation within the critical temperature range of 300-100℃. During the second stage of cooling, the controller executes similar closed-loop logic, but the judgment benchmark is a more stringent rate range of 1.0-2.0℃ / h preset for the second stage. The aim is to achieve extremely slow, near-constant cooling in a lower temperature range, minimizing thermal stress.

[0080] By deploying this flexible heating and insulation system and its closed-loop control logic, this solution achieves automated and precise management of core temperature parameters throughout the welding process. The effect is to transform critical process steps that previously relied on manual judgment and operation into a stable and repeatable process completed collaboratively by sensors, controllers, and actuators. This not only significantly reduces process deviations caused by human factors, but more importantly, it enables real-time feedback and dynamic adjustment of the cooling rate, a key and difficult-to-control parameter, ensuring the smoothness and controllability of the entire cooling process from post-heating temperature to ambient temperature. Systematic automatic control allows complex low-temperature welding temperature procedures to be strictly and consistently executed, thereby significantly improving the stability of weld quality and the reliability of joint resistance to cold cracking.

[0081] In another embodiment, the low-temperature steel structure welding method comprises temperature sensors located in the following three regions: the weld metal region with the weld joint centerline as a reference, the heat-affected zone extending 10-15 mm outward from the edge of the weld metal region, and the base material region extending more than 50 mm outward from the edge of the weld metal region.

[0082] Before initiating the preheating process, all temperature sensors should be calibrated and verified in an ambient temperature range of -40°C to -10°C using a separate portable temperature calibration device, if conditions permit. All temperature sensors should be calibrated in the field before preheating. It is recommended to use a portable dry-trap furnace or a high-precision blackbody furnace capable of operating in low-temperature environments for calibration. If field conditions are severely limited, at least a calibrated precision platinum resistance thermometer should be used for comparative verification.

[0083] When using a flexible heating and insulation system to perform closed-loop control of the entire welding process of low-alloy high-strength steel in a low-temperature environment, the system's control accuracy and reliability are highly dependent on the accuracy and representativeness of the data fed back by the temperature sensors. In a low-temperature environment, the temperature sensors themselves may have zero-point drift or accuracy deviations, and measurements from a single measuring point or at an inappropriate location cannot fully reflect the true temperature state of different characteristic areas of the weld joint (such as the weld metal, the heat-affected zone, and the relatively unheated base material) during complex thermal cycling. If the sensor data contains systematic errors or cannot represent the overall temperature field, then automatic control based on this data, including the adjustment of preheating temperature, interpass temperature, and critical cooling rate, will lose its accurate basis, potentially causing actual process parameters to deviate from the safety window, thereby introducing quality risks.

[0084] To improve the accuracy and data representativeness of the temperature monitoring system, specific regulations were established for the arrangement and calibration of temperature sensors. Temperature sensors were positioned in three key areas: the weld metal area, referenced to the weld joint centerline, for direct monitoring of the molten metal temperature; the heat-affected zone (HAZ), extending 10-15 mm outward from the weld metal area edge, where microstructure changes most drastically and is the focus of temperature monitoring; and the base material area, extending more than 50 mm outward from the weld metal area edge, where the temperature reflects the base material's temperature state away from the heat source. This zoned arrangement aims to construct a distributed monitoring network capable of simultaneously capturing the highest temperature at the weld center, the temperature gradient in the HAZ, and the heat transfer status of the base material. The acquired data is used not only for real-time control but also for a comprehensive assessment of the range and extent of the welding thermal cycle's impact on the material.

[0085] Before the preheating process begins, all temperature sensors must be calibrated and verified in an actual ambient temperature environment of -40 to -10°C using an independent portable temperature calibration device. The purpose of this step is to calibrate the accuracy and verify the functionality of each temperature sensor before it is put into use under actual operating conditions. By comparing the temperature sensor readings with the readings of a portable temperature calibration device of known accuracy, systematic errors in the temperature sensor can be corrected or identified, ensuring that its measurements are accurate even at low temperatures. This process eliminates measurement deviations that may arise from differences between the factory calibration environment and the actual operating environment, and also eliminates damage or performance degradation caused by transportation or installation.

[0086] By implementing the aforementioned zoned arrangement and on-site calibration measures for temperature sensors, this scheme provides a high-quality data foundation for the closed-loop temperature control system. The effect is a significant improvement in the spatial representativeness and numerical reliability of temperature feedback information. The multi-point arrangement allows the control system to more comprehensively perceive the temperature field of the weld joint, avoiding overall control errors caused by inaccurate single-point measurements. Rigorous on-site calibration ensures the accuracy of the measurement chain from the source, making heating power adjustment, insulation layer action control, and cooling rate calculation based on this data reliable. This enhances the scientific nature of managing complex thermal processes, making the execution of key aspects such as preheating, interpass temperature control, and programmed cooling more precise and reliable. This, in turn, improves welding process stability while further ensuring the optimization of the weld joint's microstructure and mechanical properties, enhancing the overall scheme's ability to cope with harsh low-temperature environments.

[0087] In another embodiment, the low-temperature steel structure welding method involves continuous local environmental dew point monitoring and anti-condensation control of the welded joint and its surrounding area throughout the entire process of preheating, welding, and post-heat treatment. At least two ambient temperature and humidity sensors are installed within a range of 200-300mm on each side of the center line of the welded joint to monitor the air temperature and relative humidity in the area in real time. The controller calculates and displays the dew point temperature of the monitored area in real time based on the air temperature and relative humidity data collected by the ambient temperature and humidity sensor. When the calculated dew point temperature is not lower than 95% of the surface temperature of the welded joint metal measured by the temperature sensor, the controller automatically activates anti-condensation measures. Anti-condensation measures include: activating the far-infrared radiation heater installed above the welded joint to irradiate and heat the air within a range of 50-100mm above the welded joint, thereby raising the air temperature at that location and making its dew point lower than the surface temperature of the welded joint.

[0088] When using a flexible heating and insulation system to implement closed-loop temperature control throughout the welding process of low-alloy high-strength steel in a low-temperature environment, the system can effectively manage the temperature of the workpiece. However, in a low-temperature, high-humidity workshop environment, the air conditions surrounding the welding area are an independent and uncontrollable variable. When the workpiece is locally heated, its surface temperature is much higher than the surrounding cold air temperature, which may cause moisture in the air to condense on the preheated metal surface—a phenomenon known as condensation. Pre-weld drying can remove initial moisture, but it cannot prevent secondary condensation induced by localized high temperatures during welding. Once this condensate comes into contact with the high-temperature metal surface, it will rapidly vaporize and decompose into hydrogen, directly invading the weld pool and heat-affected zone, becoming an immediate hydrogen source for hydrogen-induced cracking. The basic solution lacks monitoring and response to this dynamic environmental risk.

[0089] To address the risk of dynamic condensation due to ambient humidity during welding, continuous localized environmental dew point monitoring and anti-condensation control are implemented simultaneously throughout the preheating, welding, and post-heat treatment processes, focusing on the welded joint and its surrounding area. At least two ambient temperature and humidity sensors are installed within a 200-300mm radius on either side of the welded joint's centerline to monitor the air temperature and relative humidity in real time. These sensors directly capture the ambient air conditions closest to the heat source, providing real-time data for dew point calculation. This monitoring range covers the main air areas potentially affected by the temperature field.

[0090] The controller calculates and displays the dew point temperature of the monitored area in real time based on air temperature and relative humidity data collected by ambient temperature and humidity sensors. This step converts the air's temperature and humidity status into a key physical indicator, the dew point temperature, which represents the critical temperature at which moisture in the air begins to condense. Continuous calculation and display provide clear early warning indicators for operators or automated systems. When the calculated dew point temperature is not lower than 95% of the surface temperature of the welded joint measured by the temperature sensor, the controller automatically activates anti-condensation measures. Setting this 95% threshold aims to intervene early, establishing a safety buffer, before the metal surface temperature drops very close to the dew point and condensation is imminent.

[0091] Automatic anti-condensation measures include activating a far-infrared radiation heater positioned above the weld joint to irradiate and heat the air within a 50-100mm range above the weld joint. The purpose of using far-infrared radiation heating is to directly heat the air molecules in the designated area, rather than relying primarily on convection, thereby rapidly and directionally raising the temperature of the air layer immediately above the metal surface. The direct effect of this measure is to increase the temperature of the air in that area, thus changing its calculated dew point value and ultimately ensuring that the dew point temperature of the local air remains lower than the temperature of the metal surface below, fundamentally eliminating the physical conditions for condensation.

[0092] By implementing continuous dew point monitoring and proactive anti-condensation control, this solution adds a dynamic protective layer against the gaseous conditions of the welding microenvironment. Its effect is to effectively prevent secondary condensation on the workpiece surface caused by fluctuations in ambient humidity or localized heating during the preheating, welding, and post-heat treatment processes, which can last for several hours. This is equivalent to achieving dynamic drying assurance during the welding process, building upon pre-weld drying, and further cutting off an immediate and active hydrogen source for hydrogen-induced cracking. This measure, in conjunction with temperature control and post-weld hydrogen removal, constitutes a more comprehensive hydrogen damage prevention system, significantly improving the process robustness and joint reliability of high-quality welding in complex, humid, and low-temperature environments.

[0093] Example 2 Based on the complete process of Example 1, dew point monitoring and anti-condensation control of the local welding environment were added, while all other process parameters and execution steps remained strictly consistent with Example 1.

[0094] During implementation, ambient temperature and humidity sensors were installed 250mm on each side of the weld joint centerline to monitor the air temperature and relative humidity in real time. The ambient temperature was maintained at -20℃ throughout the welding process. During the preheating phase, when the preheating temperature reached 200℃, the monitoring showed a relative humidity of 85%, and the calculated dew point temperature was approximately -21.6℃. At this time, the surface temperature of the weld joint metal measured by the temperature sensor was 200℃, and the calculated dew point temperature was lower than the surface temperature; therefore, the system did not trigger anti-condensation measures. During the second stage of slow cooling, when the weld joint temperature cooled to -17℃, the monitored ambient air temperature remained at -20℃, but the relative humidity rose to 92%, and the calculated dew point temperature was approximately -20.8℃. At this point, the ratio of the calculated dew point temperature to the joint surface temperature exceeded the preset 95% intervention threshold. The controller then automatically activated the far-infrared radiation heater positioned above the weld joint to irradiate and heat the air within an area of ​​approximately 80mm above the weld joint. After approximately 30 minutes, the air temperature at this location rose to -18℃. Since the absolute humidity of the air did not change significantly, its dew point temperature remained at approximately -20.8°C. However, the air temperature was now higher than the dew point temperature, thus eliminating the conditions for condensation on the metal surface at -17°C. Subsequently, the heater stopped working.

[0095] The weld joint was inspected 72 hours post-weld. No cracks were detected by ultrasonic and magnetic particle testing. The diffusible hydrogen content of the weld metal was measured to be 1.9 mL / 100g. All tensile specimens from the joint fractured at the base metal, with a tensile strength of 561 MPa. Charpy V-notch impact energy testing at -40℃ showed an average of 67 J at the weld center and 73 J in the heat-affected zone. The maximum principal residual stress of the weld joint was 240 MPa. Metallographic observation indicated that the microstructure of the heat-affected zone was predominantly fine lath bainite.

[0096] Results Description: Example 2, while fully inheriting the systematic temperature control process of Example 1, adds continuous monitoring of the local environmental dew point and active anti-condensation measures. In this experiment, when the ambient humidity increased during the later stages of cooling, causing the ratio of the calculated dew point temperature to the joint surface temperature to exceed the set threshold, the system successfully activated the far-infrared heater, raising the air temperature near the weld and effectively preventing condensation on the metal surface. The final test results, including no cracks, low diffusible hydrogen content, excellent low-temperature toughness, low residual stress, and good microstructure, showed slight improvements in diffusible hydrogen content and toughness compared to Example 1, and a further reduction in residual stress. This data indicates that under relatively humid and harsh working conditions, the added dew point monitoring and active anti-condensation measures can effectively block additional hydrogen sources introduced by the dynamic intrusion of air moisture, thereby providing more active protection for the weld joint quality and improving the adaptability and reliability of the process in complex and variable environments.

[0097] In another embodiment, in the low-temperature steel structure welding method, gradient heating is simultaneously applied to the base material areas on both sides of the weld joint throughout the entire process of preheating, welding, and post-heat treatment. The gradient heating area extends from the boundary of the preheating area outwards from the base material by a distance of 2-4 times the thickness of the steel plate. The target temperature control for gradient heat tracing is as follows: during the preheating stage, the temperature of the heat-tracing area should reach 30%-50% of the lower limit of the preheating temperature; during the interpass temperature maintenance stage, the temperature of the heat-tracing area should reach 20%-40% of the lower limit of the interpass temperature; and during the post-heat treatment stage, the temperature of the heat-tracing area should reach 15%-30% of the lower limit of the post-heat treatment temperature. The gradient heat tracing temperature target is 30%-50% of the lower limit of the preheating temperature. If the preheating temperature is 180℃, then the heat tracing temperature should be 54-90℃, which is sufficient to act as a buffer zone.

[0098] When welding low-alloy high-strength steel at low temperatures, controlling the temperature of the weld joint and its near-weld zone is crucial to preventing cold cracking. Existing techniques typically involve preheating a certain width of area on both sides of the weld joint, the core of which is to directly raise the temperature of the area to be welded to improve its weldability. However, this method mainly focuses on a localized area near the weld, paying insufficient attention to the temperature of the base material further away. In welding, especially thick plate welding, a significant temperature gradient forms between the weld area and the cooler base material further away. This steep temperature change is a major cause of significant thermal stress and deformation, potentially indirectly exacerbating the tendency for cold cracking.

[0099] Compared with existing technologies, this solution simultaneously applies gradient heating to the base material areas on both sides of the weld joint throughout the entire process of preheating, welding, and post-heat treatment. The gradient heating area extends from the boundary of the preheating zone outwards to a distance of 2-4 times the thickness of the steel plate. The purpose of this step is to further expand the scope of active temperature management from the traditional preheating zone, establishing a smooth temperature transition zone between the weld heat source and the distant low-temperature base material, thereby effectively flattening the temperature distribution curve of the entire component and reducing the temperature gradient near critical areas.

[0100] The target temperature for gradient heating is controlled separately according to different stages of the welding process. In the preheating stage, the temperature of the heated area reaches 30%-50% of the lower limit of the preheating temperature. The purpose is to ensure that the base material in the extended area has a certain initial temperature before welding begins, reducing the initial temperature difference between it and the high-temperature preheating zone, creating smoother conditions for the subsequent transfer of welding heat. In the interpass temperature maintenance stage, the temperature of the heated area reaches 20%-40% of the lower limit of the interpass temperature. The purpose is to maintain the temperature of this extended area at a moderately elevated level during the continuous welding thermal cycle, allowing it to act as a "thermal buffer." This slows down the rapid loss of welding heat to the distant base material, facilitating interpass temperature stability, and reduces transverse stress caused by the cyclic heating and cooling of the weld. In the post-heat treatment stage, the temperature of the heated area reaches 15%-30% of the lower limit of the post-heat treatment temperature. The aim is to allow the extended region to undergo a mild thermal process during the post-heating hydrogen removal process, promoting the diffusion of any trace hydrogen that may be present in the region, and enabling the entire component to shrink at a more coordinated rate during subsequent cooling, thereby further reducing the overall residual stress.

[0101] By implementing the aforementioned gradient heating, this scheme creates a controlled temperature buffer zone at the outer edge of the core temperature control zone. The effect is a significant improvement in the overall temperature field uniformity of the welded component, resulting in a smoother transition from the high-temperature weld to the low-temperature base material. This helps reduce peak thermal stress caused by localized, intense thermal expansion and contraction, decreases welding deformation, and creates a more favorable overall temperature environment for hydrogen diffusion and escape. Mechanistically, a gentler temperature gradient implies lower internal stress and more coordinated deformation, fundamentally weakening the driving force for cold crack initiation and propagation.

[0102] Example 3 The same Q345D steel plate as in Example 1 was used, with a thickness of 30mm, and the ambient temperature was -20℃. Gradient heat tracing was implemented while performing all the steps of Example 1. The preheating zone was 120mm wide, and the gradient heat tracing zone extended 90mm outward from the boundary of the preheating zone. During the preheating stage, the preheating zone temperature was 200℃, and the gradient heat tracing zone temperature was controlled at 75℃. During the interpass temperature maintenance stage, the interpass temperature was maintained at 170-185℃, and the gradient heat tracing zone temperature was controlled at 40℃. During the post-heat treatment stage, the post-heating temperature was 300℃, and the gradient heat tracing zone temperature was controlled at 50℃.

[0103] Post-weld inspection (72 hours) revealed no cracks under both ultrasonic and magnetic particle testing. The diffusible hydrogen content in the weld metal was 1.8 mL / 100 g. Tensile specimens from the joint all fractured at the base metal, with a tensile strength of 562 MPa. Charpy V-notch impact testing at -40℃ showed an average J of 68 J at the weld center and an average of 74 J in the heat-affected zone. The maximum principal residual stress of the welded joint was 235 MPa. Metallographic observation indicated that the microstructure of the heat-affected zone consisted of uniform, fine lath bainite.

[0104] Results: In Example 3, after implementing gradient heating, the performance indicators of the welded joint obtained showed a further optimization trend compared with Example 1, particularly with lower diffusible hydrogen content, higher impact toughness, and lower residual stress. This indicates that gradient heating, by improving the overall temperature field uniformity of the welded component, effectively reduces thermal stress and deformation during the welding process, creating better conditions for hydrogen diffusion, thereby obtaining a welded joint with more uniform microstructure and superior performance. This example confirms that adding gradient heating on the basis of systematic temperature control can further improve the overall quality and reliability of welded joints under low-temperature conditions.

[0105] In another embodiment, in the low-temperature steel structure welding method, during the second stage of the cooling process, controlled mechanical constraints are applied, specifically as follows: At the start of the second stage of cooling, multiple pairs of adjustable pressure dynamic clamps are arranged at intervals of 300-500mm along the length of the welded joint. The two clamping ends of each pair of dynamic clamps act symmetrically on the same point in the base material area 80-120mm away from the center line of the welded joint. The dynamic fixture applies pressure perpendicular to the surface of the base material via hydraulic or servo motor drive. The initial pressure applied by the dynamic clamp is set to 8%-12% of the yield strength of the base material at room temperature; The pressure adjustment of the dynamic fixture covers the entire second-stage cooling process, continuing until the welded joint cools to an equilibrium state where the temperature difference with the ambient temperature does not exceed 5°C. During the second stage of cooling, the real-time temperature of the welded joint is continuously monitored using a temperature sensor; Based on the monitored real-time temperature, the applied pressure of the dynamic fixture is dynamically adjusted so that the applied pressure increases as the temperature of the welded joint decreases. For every 10°C decrease in weld joint temperature, the pressure applied by the dynamic fixture increases by 5%-8% of the initial pressure value. The pressure adjustment of the dynamic fixture employs a closed-loop control algorithm. Based on the real-time weld joint temperature fed back by the temperature sensor, the pressure is adjusted according to a preset proportional-integral-derivative (PID) algorithm or linear proportional algorithm, ensuring that the pressure increases as the temperature decreases. Preliminary tests have shown that this proportional range effectively compensates for the shrinkage stress of Q345D steel under the aforementioned process. For other materials or plate thicknesses, a suitable proportional relationship can be determined through finite element simulation or process qualification tests.

[0106] In the welding of low-alloy high-strength steel at low temperatures, a systematic temperature control process can significantly reduce the risk of cold cracking. However, during the later cooling process of the welded joint, especially in the stage of cooling from a higher temperature to ambient temperature, tensile residual stress is inevitably generated due to the constraint of the surrounding cooler base material on the thermal contraction of the metal. This residual stress, together with structural stress and possibly residual hydrogen, remains a potential factor inducing cracking. The basic approach reduces the temperature difference and shrinkage rate through extremely slow cooling, thereby reducing the stress peak, but it is essentially a passive adaptation to the cooling process, lacking active intervention and compensation for the shrinkage deformation itself.

[0107] To address the issue of proactive compensation for shrinkage stress during cooling, a controlled mechanical constraint step was added during the second stage of the cooling process. At the start of the second stage of cooling, multiple pairs of adjustable-pressure dynamic clamps were arranged at intervals of 300-500 mm along the length of the weld joint. The two clamping ends of each pair of dynamic clamps act symmetrically on the same point in the base material region, 80-120 mm from the weld joint centerline. This arrangement aims to establish symmetrical mechanical constraint points on both sides of the weld, ensuring uniform force application and preventing additional bending moments. The location of the constraint points from the weld centerline was chosen to effectively influence the weld area while avoiding direct damage to the heat-affected zone.

[0108] The dynamic fixture applies pressure perpendicular to the base material surface via hydraulic or servo motor drive. This drive method allows for continuous and precise digital control of the pressure. The initial pressure applied by the dynamic fixture is set to 8%-12% of the base material's yield strength at room temperature. This initial pressure range is chosen to establish a moderate pre-pressure field in the early stages of cooling. This pressure value induces slight plastic pre-deformation in the base material to allow for shrinkage compensation, while remaining well below the material's yield strength to ensure no damage occurs during pressure application. Pressure regulation covers the entire second-stage cooling process, continuing until the weld joint cools to an equilibrium state where the temperature difference with ambient temperature does not exceed 5°C, ensuring active mechanical intervention throughout the main stress generation phase.

[0109] During the second-stage cooling process, the real-time temperature of the welded joint is continuously monitored using temperature sensors. Based on the monitored real-time temperature, the applied pressure of the dynamic fixture is dynamically adjusted, increasing as the welded joint temperature decreases. For every 10°C decrease in welded joint temperature, the applied pressure of the dynamic fixture increases by 5%-8% of the initial pressure value. The purpose of this dynamic adjustment mechanism is to achieve real-time matching between mechanical compensation and thermal shrinkage. As the temperature decreases, the amount of metal shrinkage increases, resulting in increased tensile stress. At this time, proportionally increasing the externally applied compressive pressure can actively offset some of the tensile stress generated by shrinkage, thereby keeping the actual net tensile stress level borne by the joint within a low range.

[0110] By implementing the aforementioned controllable mechanical constraint steps, this scheme introduces an active mechanical control dimension on top of temperature control. The effect is that during the cooling and shrinkage process of the welded joint, the externally applied dynamic compensation pressure directly counteracts and offsets the internally generated tensile stress. This not only further reduces the final residual stress, especially the transverse residual tensile stress, which is most dangerous for crack initiation, but also promotes more coordinated micro-plastic deformation of the joint during cooling, which is beneficial for stress relaxation and homogenization. In conclusion, this provides a new process guarantee for obtaining welded joints with lower residual stress and higher dimensional stability.

[0111] Example 4 The same Q345D steel plate as in Example 1, with a thickness of 30 mm, was used. The ambient temperature was -20°C, and the temperature control process was exactly the same as in Example 1. Based on this, controlled mechanical constraints were applied at the start of the second stage of cooling. A pair of hydraulically driven dynamic clamps were arranged every 400 mm along the weld, with the clamping points 100 mm from the weld centerline. The base material's yield strength at room temperature was 345 MPa, and the initial pressure was set to 35 MPa (approximately 10%). During the cooling process from 100°C to -20°C, the pressure was adjusted in real time according to the temperature drop; for every 10°C decrease in temperature, the pressure increased by 6% of the initial value.

[0112] Post-weld inspection (72 hours) revealed no cracks under both ultrasonic and magnetic particle testing. The diffusible hydrogen content in the weld metal was 1.9 mL / 100 g. Tensile specimens from the joint all fractured at the base metal, with a tensile strength of 565 MPa. Charpy V-notch impact testing at -40℃ showed an average J of 67 J at the weld center and an average of 73 J in the heat-affected zone. The maximum principal residual stress of the welded joint was 220 MPa. Metallographic observation revealed that the heat-affected zone consisted of uniform, fine lath bainite.

[0113] Results: In Example 4, after implementing controllable mechanical constraints, the welded joint exhibited improved low-temperature impact toughness compared to Example 1, while maintaining crack-free, low-hydrogen-content, and high-strength characteristics. Furthermore, the maximum principal residual stress was significantly reduced. This demonstrates that, based on precise temperature control, superimposed dynamic mechanical stress compensation can more effectively regulate the final stress state of the welded joint. By actively counteracting cooling shrinkage stress, this method creates a more favorable internal stress distribution within the joint, providing better conditions for the stability of the microstructure and the full utilization of toughness, thus achieving a synergistic enhancement effect in improving the overall performance of the joint.

[0114] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and embodiments shown and described herein.

Claims

1. A low-temperature steel structure welding method, applicable to welding of low-alloy high-strength steel in an ambient temperature range of -40 to -10°C, characterized in that, Includes the following steps: Place the steel structure workpiece to be welded in an environment with a temperature of -40 to -10℃ until the temperature of the workpiece body reaches equilibrium with the ambient temperature. Preheat the preheating zones on both sides of the welded joint. The width of the preheating zone is 3-5 times the thickness of the steel plate. Control the preheating temperature at 180-220℃ and the preheating rate should not exceed 100℃ / h. After reaching the preheating temperature, maintain the temperature for 60-90 minutes. Gas shielded welding is used for multi-layer and multi-pass welding. The welding heat input of each pass is controlled within the range of 15-25 kJ / cm, while the interpass temperature of each pass is maintained at 150-200℃. After welding, the welded joint is subjected to post-heat treatment by heating it to 280-320℃ and holding it at this temperature for 120-180 minutes. After the post-heat treatment, the welded joint is cooled according to a preset cooling procedure; the cooling procedure includes: The cooling process from the post-heating temperature to 100℃: When the temperature of the welded joint is higher than 300℃, allow it to cool naturally to 300℃; when the temperature of the welded joint is not higher than 300℃ or after cooling from higher than 300℃ to 300℃, control the cooling rate within the range of 8-20℃ / h until it cools to 100℃. The cooling stage from 100℃ to ambient temperature: The welded joint is cooled from 100℃ to an equilibrium state where the temperature difference with the ambient temperature does not exceed 5℃, and the cooling rate is controlled within the range of 1.0-2.0℃ / h.

2. The low-temperature steel structure welding method as described in claim 1, characterized in that, In the gas shielded welding process, an auxiliary gas supply device, independent of the protective gas path of the welding torch, is added. During the welding process, the auxiliary gas supply device sprays additional inert protective gas onto the surface of the preheating area in the direction of the welding torch's travel, forming an inert gas barrier that isolates welding fumes and spatter. The auxiliary inert protective gas injected by the auxiliary gas supply device is preheated before being injected, and the temperature of the gas after preheating is controlled within the range of 50-100℃. The gas injection path of the auxiliary gas supply device is configured to avoid the airflow blowing directly into the measurement area where the temperature sensor used to monitor the interlayer temperature is located; A gas diffuser is fixedly installed at the end of the nozzle of the auxiliary gas supply device. The gas diffuser is a cylindrical cavity filled with a porous medium. The inlet end of the cylindrical cavity is connected to the nozzle of the auxiliary gas supply device, and the outlet end faces the surface of the preheating area of ​​the welding joint. The porosity of the porous medium is in the range of 40%-60%, and the average pore size is 100-150μm. A gas insulation cover is installed around the gas diffuser, extending from the gas inlet end of the gas diffuser to a position close to the surface of the preheating area of ​​the weld joint.

3. The low-temperature steel structure welding method as described in claim 1, characterized in that, After placing the steel structure workpiece to be welded in an environment with a temperature of -40 to -10℃, and before preheating the preheating areas on both sides of the welding joint, the welding joint and the surrounding area are dried. The nearby area refers to the metal surface area covered by extending 100-150mm to both sides of the weld seam, with the center line of the weld joint as the reference. When the drying process uses hot air purging, the hot air temperature is controlled within the range of 60-120℃, and the surrounding area is continuously purged. Alternatively, when infrared heating is used for the drying process, the surface temperature of the metal in the vicinity is raised to 40-80°C using an infrared heating device and maintained at that temperature. The endpoint of the drying process is determined as follows: after the drying process, a humidity test strip is attached to the metal surface of the nearby area, and the test strip shows a dry state.

4. The low-temperature steel structure welding method as described in claim 3, characterized in that, After the drying process is completed, the weld joint and the surrounding area, which are covered by thermal insulation material or a movable protective cover extending 100-150mm to both sides of the weld joint centerline, are isolated from the surrounding humid air. The covering and isolation are maintained until the preheating process begins, and the thermal insulation material or movable protective cover is removed after the preheating temperature rises above the ambient dew point temperature.

5. The low-temperature steel structure welding method as described in claim 1, characterized in that, Temperature control in the preheating, interlayer temperature maintenance, post-heat treatment and cooling process steps is performed by a flexible heating and insulation system. The flexible heating and insulation system includes an electric heating unit attached to the surface of the workpiece, an adjustable insulation layer covering the electric heating unit, a controller connected to and controlling the electric heating unit, a temperature sensor for real-time monitoring of the temperature of the weld joint, and an actuator driven by the controller to change the unfolded or covered area of ​​the adjustable insulation layer. During the preheating process, the controller controls the power output of the electric heating unit based on the feedback signal from the temperature sensor in order to achieve and maintain the preset preheating temperature. During the interlayer temperature maintenance step, the controller controls the electric heating unit to supplement heating when the temperature is below 150°C, and controls the actuator to open part of the adjustable insulation layer to accelerate heat dissipation when the temperature is above 200°C, based on the interlayer temperature data monitored by the temperature sensor. In the post-heat treatment step, the controller controls the electric heating unit according to the signal from the temperature sensor, so that the temperature of the welded joint reaches and stabilizes within the set post-heat temperature range. During the cooling process, the controller performs the following closed-loop control: During the first stage of cooling, the controller calculates the current cooling rate of the welded joint in real time based on the temperature data continuously collected by the temperature sensor, and compares and judges the calculation result with the preset upper limit of the cooling rate of 20℃ / h and the lower limit of 8℃ / h in the first stage. When the real-time cooling rate is higher than 20℃ / h, the controller sends a command to the electric heating unit to start low-power heat supplementation, and at the same time sends a command to the actuator to drive the adjustable insulation layer to reduce the unfolded area and enhance the insulation. When the real-time cooling rate is below 8℃ / h, the controller sends a command to the electric heating unit to stop heating, and at the same time sends a command to the actuator to drive the adjustable insulation layer to increase the unfolded area and enhance heat dissipation. During the second-stage cooling process, the controller calculates the current cooling rate of the welded joint in real time based on the temperature data continuously collected by the temperature sensor, and compares and judges the calculation result with the preset upper limit of the cooling rate of 2.0℃ / h and the lower limit of 1.0℃ / h for the second stage. When the real-time cooling rate is higher than 2.0℃ / h, the controller sends a command to the electric heating unit to start low-power heat supplementation, and at the same time sends a command to the actuator to drive the adjustable insulation layer to reduce the unfolded area and enhance the insulation. When the real-time cooling rate is below 1.0℃ / h, the controller sends a command to the electric heating unit to stop heating, and at the same time sends a command to the actuator to drive the adjustable insulation layer to increase the unfolded area and enhance heat dissipation.

6. The low-temperature steel structure welding method as described in claim 5, characterized in that, Temperature sensors are installed in the following three areas: the weld metal area with the weld joint centerline as the reference, the heat-affected zone extending 10-15mm outward from the edge of the weld metal area, and the base material area extending more than 50mm outward from the edge of the weld metal area.

7. The low-temperature steel structure welding method as described in claim 5, characterized in that, Throughout the entire process of preheating, welding, and post-heat treatment, continuous local environmental dew point monitoring and anti-condensation control are carried out on the welded joint and its surrounding area. At least two ambient temperature and humidity sensors are installed within a range of 200-300mm on each side of the center line of the welded joint to monitor the air temperature and relative humidity in the area in real time. The controller calculates and displays the dew point temperature of the monitored area in real time based on the air temperature and relative humidity data collected by the ambient temperature and humidity sensor. When the calculated dew point temperature is not lower than 95% of the surface temperature of the welded joint metal measured by the temperature sensor, the controller automatically activates anti-condensation measures. Anti-condensation measures include: activating the far-infrared radiation heater installed above the welded joint to irradiate and heat the air within a range of 50-100mm above the welded joint, thereby raising the air temperature at that location and making its dew point lower than the surface temperature of the welded joint.

8. The low-temperature steel structure welding method as described in claim 1, characterized in that, During the entire process of preheating, welding and post-heat treatment, gradient heating is simultaneously applied to the base material areas on both sides of the weld joint. The gradient heating area extends from the boundary of the preheating area outwards from the base material by a distance of 2-4 times the thickness of the steel plate. The target temperature control for gradient heat tracing is as follows: in the preheating stage, the temperature of the heat tracing area reaches 30%-50% of the lower limit of the preheating temperature; in the interpass temperature maintenance stage, the temperature of the heat tracing area reaches 20%-40% of the lower limit of the interpass temperature; and in the post-heat treatment stage, the temperature of the heat tracing area reaches 15%-30% of the lower limit of the post-heat treatment temperature.

9. The low-temperature steel structure welding method as described in claim 5, characterized in that, During the second stage of the cooling process, a controlled mechanical constraint is applied, as follows: At the start of the second stage of cooling, multiple pairs of adjustable pressure dynamic clamps are arranged at intervals of 300-500mm along the length of the welded joint. The two clamping ends of each pair of dynamic clamps act symmetrically on the same point in the base material area 80-120mm away from the center line of the welded joint. The dynamic fixture applies pressure perpendicular to the surface of the base material via hydraulic or servo motor drive. The initial pressure applied by the dynamic clamp is set to 8%-12% of the yield strength of the base material at room temperature; The pressure adjustment of the dynamic fixture covers the entire second-stage cooling process, continuing until the welded joint cools to an equilibrium state where the temperature difference with the ambient temperature does not exceed 5°C. During the second stage of cooling, the real-time temperature of the welded joint is continuously monitored using a temperature sensor; Based on the monitored real-time temperature, the applied pressure of the dynamic fixture is dynamically adjusted so that the applied pressure increases as the temperature of the welded joint decreases. For every 10°C decrease in the temperature of the welded joint, the pressure applied by the dynamic fixture increases by 5%-8% of the initial pressure value.

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