Single-sided welding and double-sided forming composite welding process for thick-walled pipe

Abstract

The application discloses a kind of thick-walled pipe single-sided welding double-sided forming composite welding process, S1, to be welded thick-walled pipe joint is beveled, and single-sided bevel is formed on the outer wall of pipeline, and root gap and land are reserved;The thickness of the land and the root gap are set according to the wall thickness of the pipeline, material and laser welding penetration capacity, so that the land can be completely penetrated during subsequent laser backing welding and form a continuous and uniform back weld on the inner wall of the pipeline;S2, backing welding is carried out on the front of the bevel using a laser beam, the laser beam is centered with the bevel before welding, and by adjusting the laser power, welding speed, defocusing amount and other parameters, the laser beam energy is concentrated on the land area, and the land is completely penetrated.Compared with the prior art, the application has the advantages that the complex processes such as back cleaning, polishing and back welding in the traditional process are avoided, the construction process is simplified, and the welding efficiency is improved.

Description

Technical Field

[0001] This invention belongs to the field of welding technology, specifically relating to a composite welding process for single-sided welding and double-sided forming of thick-walled pipes. Background Technology

[0002] Thick-walled pipes are widely used in petroleum, chemical, and power industries, and their butt joint circumferential welds typically require full penetration. Traditional processes often employ argon arc welding or shielded metal arc welding for the root pass, followed by filling and capping. To ensure root quality, carbon arc gouging and grinding are often performed on the back side before welding. This method is cumbersome, time-consuming, and costly; the root cleaning environment is poor, and quality depends heavily on the welder's skill level. For small-diameter pipes or pipes inaccessible to personnel, back-side root cleaning and welding are difficult to implement.

[0003] Specifically, while double-sided submerged arc welding avoids manual root pass welding, it requires sufficient space inside the pipe for welding and is not suitable for small-diameter pipes or fixed joints in the field. Laser welding has high energy density and can achieve single-sided welding with double-sided forming, making it suitable for root pass welding, but its deposition efficiency is low and its cost is high when used alone for thick-walled filler welds. Submerged arc welding has high deposition efficiency and is suitable for filler and cover welds, but direct root pass welding is prone to burn-through or incomplete penetration.

[0004] In other words, the existing laser and submerged arc welding technology is not adaptable to the circumferential seams of thick-walled pipes. Back-side forming often still requires root cleaning or backing, the process parameters are complex to match, and improper bevel design can easily lead to back-side oxidation, incomplete fusion, or burn-through.

[0005] Therefore, in order to address the above-mentioned technical problems, it is necessary to provide a composite welding process for single-sided welding and double-sided forming of thick-walled pipes. Summary of the Invention

[0006] The purpose of this invention is to provide a composite welding process for single-sided welding and double-sided forming of thick-walled pipes, which can solve the technical problems mentioned in the background art.

[0007] To achieve the above objectives, a specific embodiment of the present invention provides the following technical solution:

[0008] A composite welding process for single-sided welding and double-sided forming of thick-walled pipes includes the following steps: S1, beveling the joint of the thick-walled pipe to be welded, forming a single-sided bevel on the outer wall of the pipe, and reserving a blunt edge and root gap; the thickness of the blunt edge and the root gap are set according to the pipe wall thickness, material, and laser welding penetration capability, so that the blunt edge can be completely melted during subsequent laser root pass welding and a continuous and uniform back weld is formed on the inner wall of the pipe; S2, performing root pass welding on the front side of the bevel using a laser beam, aligning the laser beam with the center of the bevel before welding, and adjusting the laser power, welding speed, defocusing amount, etc. The parameters are set so that the laser beam energy is concentrated on the blunt edge area, completely melting the blunt edge and forming a continuous and uniform back weld on the back of the inner wall of the pipe, without incomplete penetration or fusion defects, and the back weld reinforcement and weld width meet the quality requirements; S3, without any root cleaning, grinding or back welding treatment on the back of the root weld, multi-layer and multi-pass filling and capping welding are directly performed on the front of the root weld using submerged arc welding. By controlling the heat input and interpass temperature of submerged arc welding, burn-through or structural damage to the root weld and back weld is avoided, and finally the single-sided welding and double-sided forming of the thick-walled pipe joint is completed.

[0009] In one or more embodiments of the present invention, before step S1, there are also assembly and tack welding steps: assembling the butt joints of the thick-walled pipes to be welded, controlling the assembly gap, misalignment and edge angle, so that the joint meets the assembly accuracy requirements of laser root welding; then, tack welding is performed by manual tungsten inert gas welding or laser spot welding, the tack weld avoids the starting position of laser root welding, and the two ends of the tack weld are ground into a gentle slope to ensure continuous and stable laser root welding.

[0010] In one or more embodiments of the present invention, in step S1, after the beveling is completed, the beveling surface and the inner and outer walls of the pipe are cleaned within a certain range to remove oil, rust, scale, moisture and other contaminants; for beveling processed by thermal cutting, the surface hardened layer and cutting burrs need to be removed by mechanical methods to expose the metallic luster of the beveling surface.

[0011] In one or more embodiments of the present invention, in step S2, the pipe joint is preheated before laser root pass welding. The preheating temperature is determined according to the pipe material, wall thickness and carbon equivalent to ensure that cold cracks are not generated during laser root pass welding. During the laser root pass welding process, an inert protective gas, which is argon or helium, is introduced into the back of the pipe to form a gas protection zone on the back of the pipe to prevent high-temperature oxidation of the back weld and ensure the quality of the back forming.

[0012] In one or more embodiments of the present invention, in step S2, the laser root pass welding adopts laser self-fusion welding or laser filler wire welding; when laser filler wire welding is adopted, the welding wire is fed into the molten pool from the front of the laser beam. By controlling the wire feeding speed, welding speed and welding wire alignment position, the welding wire is uniformly melted and fills the root gap, controlling the reinforcement height and weld width of the back weld, while preventing undercut and lack of fusion.

[0013] In one or more embodiments of the present invention, after step S2 and before step S3, the method further includes: visual inspection and non-destructive testing of the laser-cut root weld. The visual inspection includes checking whether the back weld is continuous and whether there are defects such as surface porosity, cracks, lack of fusion, undercut, and burn-through. The non-destructive testing adopts radiographic testing or phased array ultrasonic testing. Only after confirming that there are no defects such as incomplete penetration, lack of fusion, cracks, and oxide inclusions in the interior and root of the root weld can the submerged arc welding filler and cover weld be performed. If defects are found, the defective parts are locally repaired and re-inspected.

[0014] In one or more embodiments of the present invention, in step S3, submerged arc welding adopts multi-pass multi-layer welding. After each weld is completed, the slag and spatter are cleaned and the surface quality of the interpass weld is checked. The interpass temperature is controlled within a preset range, which is determined according to the pipe material and welding process evaluation to prevent the root pass weld and heat-affected zone from becoming coarse and tough due to excessively high interpass temperature, or cold cracks from occurring due to excessively low interpass temperature.

[0015] In one or more embodiments of the present invention, in step S3, during the submerged arc welding filling and cover process, by controlling the welding heat input, interpass temperature and welding sequence, the temperature of the back side of the root weld does not exceed the allowable value, thereby avoiding secondary oxidation, overheating or burn-through of the back weld; at the same time, the submerged arc welding pool does not directly act on the root of the root weld, ensuring that the back weld is not damaged.

[0016] In one or more embodiments of the present invention, after the submerged arc welding capping is completed in step S3, the weld is subjected to post-weld heat treatment, which includes hydrogen removal treatment or stress relief heat treatment to reduce residual welding stress and improve weld microstructure and properties. After the post-weld heat treatment is completed, the weld is subjected to non-destructive testing, including one or more of radiographic testing, ultrasonic testing, magnetic particle testing or penetrant testing, to verify that the internal and surface quality of the weld meets the design requirements.

[0017] In one or more embodiments of the present invention, the thick-walled pipe is a carbon steel, low alloy steel, heat-resistant steel or stainless steel pipe; the beveling in step S1 is performed by machining, or by hot cutting followed by mechanical finishing; machining includes turning, milling or boring, hot cutting includes plasma cutting or flame cutting, and mechanical finishing after hot cutting removes the heat-affected zone and surface oxide layer.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: the composite welding method combining laser root pass welding and submerged arc welding is adopted. Taking advantage of the high energy density and strong penetration of laser welding, single-sided penetration can be achieved on the outside of the pipe to form a continuous and uniform back weld. Then, the high deposition efficiency of submerged arc welding is used to complete multi-layer and multi-pass filling and cover welding, thereby avoiding the complicated processes of back root cleaning, grinding and back repair welding in traditional processes, simplifying the construction process and improving welding efficiency.

[0019] Meanwhile, this invention does not rely on the welding space inside the pipe, making it particularly suitable for fixed joints on site, small-diameter pipes, and thick-walled pipe joints where internal welding conditions are not available, thus expanding the scope of application of the process. In addition, by controlling the misalignment of the assembly, the root gap, the thickness of the blunt edge, and the quality of the bevel cleaning, and by combining it with tack welding, laser beam alignment, preheating, and back inert gas protection, the forming stability of the root pass weld can be improved, and the probability of defects such as incomplete penetration, incomplete fusion, burn-through, and oxidation can be reduced.

[0020] Furthermore, by setting up visual inspection and non-destructive testing after laser rooting, root quality can be controlled in advance, preventing defects from being covered by subsequent welds and reducing the risk of rework.

[0021] Meanwhile, controlling the interpass temperature, welding heat input, and welding sequence during the submerged arc welding filling and capping processes can reduce welding deformation and residual stress while ensuring efficient filling, and avoid thermal damage to the root pass weld. Combined with post-weld heat treatment, the risk of cold cracking and hydrogen-induced cracking can be further reduced, and the microstructure and properties of the welded joint can be improved. Thus, this invention can improve construction efficiency while taking into account weld formation quality, joint reliability, and engineering adaptability, making it particularly suitable for field applications of circumferential butt welding of thick-walled pipes. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a flowchart of a composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to an embodiment of the present invention.

[0024] Figure 2 This is a flowchart of the thick-walled pipe assembly and beveling process in one embodiment of the present invention;

[0025] Figure 3 This is a flowchart of laser root pass welding and back pass forming in one embodiment of the present invention;

[0026] Figure 4 This is a flowchart of the submerged arc welding filler welding and capping welding processes according to one embodiment of the present invention;

[0027] Figure 5 This is a flowchart of post-weld inspection and heat treatment in one embodiment of the present invention. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions in this disclosure, the technical solutions in the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. Based on the embodiments in this disclosure, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this disclosure.

[0029] This invention discloses a composite welding process for thick-walled pipes, involving single-sided welding and double-sided forming, applicable to circumferential butt welding of thick-walled pipes made of carbon steel, low-alloy steel, heat-resistant steel, or stainless steel. It is particularly suitable for on-site fixed joints, small-diameter pipes, or thick-walled pipe joints where internal welding space is limited. Thick-walled pipes refer to pipe structures with significant wall thickness where conventional welding methods alone cannot simultaneously achieve both root formation quality and filling efficiency. The core idea of ​​this process is to first utilize the high energy density and strong penetration of laser welding to achieve stable single-sided penetration on the outer side of the pipe, thus forming a continuous and uniform back weld on the inner wall. Then, leveraging the high deposition efficiency of submerged arc welding, high-efficiency multi-layer, multi-pass filling and capping are completed from the front side. This composite welding method avoids the complex operations of back root cleaning, grinding, or back repair welding required in traditional processes, significantly improving construction efficiency and reducing reliance on on-site working conditions and welder skills.

[0030] like Figure 1 As shown, the composite welding process for single-sided welding and double-sided forming of thick-walled pipes in one embodiment of the present invention includes the following steps:

[0031] like Figure 2As shown, the thick-walled pipe joint to be welded is assembled. During assembly, the butt joint surfaces of the two pipe sections are aligned, and the misalignment, gap, and edge angle are controlled within the range that can be stably formed by laser root welding. Generally, the root gap can be controlled between 0.2mm and 1.5mm, and the specific adjustment is made according to the wall thickness, bevel angle, and laser welding capability; the misalignment is preferably no more than 10% of the wall thickness, and more preferably no more than 1.0mm, to ensure the stability of the molten pool and the continuous formation of the back side when the laser beam acts on the blunt edge area. To prevent joint displacement during welding after assembly, it is preferable to use rigid clamps or internal and external alignment tools for temporary fixation, and to evenly arrange several positioning points around the joint to keep the pipe end roundness consistent and reduce the impact of root gap fluctuations on the stability of root welding. Tack welding can be performed by manual tungsten inert gas welding or laser spot welding. The tack weld is set in an area far away from the starting position of laser root welding, and its two ends are ground into a gentle slope to avoid molten pool disturbance, energy reflection, or incomplete fusion caused by tack weld protrusions during laser root welding. This process ensures the stability of the assembly and provides a smooth transition for the subsequent continuous movement of the laser beam.

[0032] The beveling process then proceeds. The beveling is preferably located on the outer wall of the pipe, forming a single-sided beveling structure. The beveling angle can be determined based on the wall thickness and process capability, generally ranging from 25° to 45°. The blunt edge thickness is preferably 0.5mm to 3.0mm. The root gap and blunt edge thickness work together to ensure that the laser root pass can effectively penetrate the blunt edge without causing burn-through due to excessive penetration. The design principle is that the beveling does not pursue the large opening of traditional submerged arc welding, but rather takes into account the high energy density and concentrated penetration of laser welding, allowing the laser energy to preferentially concentrate on the root of the blunt edge, thus forming a stable back-side profile under smaller opening conditions. Beveling can be completed by mechanical processing methods such as turning, milling, or boring. When using thermal cutting to form the initial beveling, it is preferable to further remove the surface hardened layer and cutting burrs through mechanical finishing to avoid high-hardness structures and oxide layers affecting laser coupling efficiency. The bevel and a certain area on both sides should be thoroughly cleaned to remove oil, rust, moisture, scale and other contaminants, and to expose the metallic luster of the bevel surface. This can reduce reflection and impurity inclusion during laser welding, and reduce the risk of porosity, lack of fusion and root inclusion defects.

[0033] Preferably, acetone or anhydrous ethanol can be used for degreasing before welding to further improve welding stability.

[0034] like Figure 3As shown, after assembly and beveling, the laser root pass welding step begins. During root pass welding, a laser beam continuously scans the circumference of the pipe's circumferential seam, ensuring precise alignment between the laser beam center and the bevel center. The laser source can be a fiber laser or a disk laser, and the laser power, welding speed, and defocusing amount are set according to the material type, wall thickness, and bevel size. For example, for medium-to-high strength thick-walled carbon steel pipes, the laser power can be selected from 3kW to 12kW, the welding speed from 0.5m / min to 2.5m / min, and the defocusing amount from -2mm to +3mm to form a deeper and narrower molten pool, improving penetration of the blunt edge. Its mechanism lies in controlling the energy density of the laser spot in the blunt edge region, ensuring complete melting of the blunt edge while simultaneously forming a continuous and uniform back weld on the inner wall of the pipe. This back weld must be free of incomplete penetration, lack of fusion, and obvious burn-through defects, and the weld reinforcement and width must be within acceptable limits to ensure sufficient load-bearing capacity and redundancy at the root during subsequent submerged arc welding. Compared to conventional tungsten inert gas welding for root pass, laser root pass has the advantages of concentrated heat input, less deformation, and high consistency of root formation, making it especially suitable for single-sided welding and double-sided forming scenarios for thick-walled pipes.

[0035] Preferably, laser autofusion welding or laser filler wire welding can be used in laser root pass welding. Laser autofusion welding is suitable for situations with small bevel sizes and high assembly precision, offering advantages such as simpler process, lower heat input, and denser weld microstructure. Laser filler wire welding involves feeding the welding wire into the molten pool from the front or side-front of the laser beam. The welding wire material is preferably matched to or slightly higher grade than the base metal composition to improve root metal filling capacity and crack resistance. The wire feed speed, feed angle, and welding wire alignment position must be coordinated with the laser focus position to ensure uniform melting of the welding wire in the molten pool and filling of the root gap. This not only controls the weld reinforcement and width on the back side but also provides additional metal compensation when there are slight fluctuations in bevel assembly, reducing the risk of undercut, collapse, and lack of fusion. Preferably, real-time monitoring devices can be installed on both sides of the laser root pass area to observe the molten pool radiation, weld surface morphology, or back side formation online, allowing for timely adjustments to the laser power or welding speed, thereby further improving process stability.

[0036] Before laser root pass welding, the joint can be preheated according to the pipe material, wall thickness, and carbon equivalent. The preheating temperature is typically set between 80℃ and 200℃, with the specific value determined based on the material's hardening tendency and the ambient temperature. Preheating reduces the cooling rate of the weld joint, decreasing the tendency for hardening and cold cracking in the heat-affected zone, which is particularly significant for low-alloy high-strength steel and pipes with large wall thicknesses. Simultaneously, during laser root pass welding, it is preferable to introduce argon, helium, or a mixture of inert gases into the back of the pipe to form a protective shield. This protects the back weld from air intrusion at high temperatures, preventing oxidation, blackening, intergranular contamination, and embrittlement. The flow rate of the back shielding gas can be adjusted according to the pipe diameter and internal volume to ensure complete coverage of the protective shield without disturbing the molten pool. Through the synergistic effect of preheating and back shielding, the metal purity and toughness of the back weld can be further improved, enabling the laser root pass welding to truly form a stable root capable of withstanding subsequent thermal cycles.

[0037] After laser root pass welding, it is preferable to first perform visual and non-destructive testing on the root pass weld before proceeding to the submerged arc welding filling and capping steps. Visual inspection mainly checks for continuity of the back weld and the presence of defects such as surface porosity, cracks, undercut, burn-through, or localized collapse. Non-destructive testing can use radiographic testing or phased array ultrasonic testing to confirm the presence of incomplete penetration, lack of fusion, cracks, or oxide inclusions inside and at the root of the root pass weld. If local defects are found, only the defective area can be repaired and re-inspected to avoid excessive heat input due to blind rework. The significance of this inspection step is that it moves root quality control forward to before submerged arc welding, ensuring that subsequent high-efficiency filling is based on a reliable root, thus preventing root defects from being covered under multiple weld passes and guaranteeing the long-term reliability of the entire weld.

[0038] Next, submerged arc welding (SAW) is performed for filling and capping. SAW is preferably performed using a multi-pass, multi-layer welding method, utilizing flux to form a stable arc and good weld formation. After each pass, slag and spatter should be cleaned, and the surface quality between passes should be checked to ensure there are no inclusions, cracks, or lack of fusion before proceeding to the next pass. SAW parameters should be matched according to the pipe material, wall thickness, bevel size, and welding position. Welding current, welding voltage, and travel speed should be controlled within a reasonable heat input range. Preferably, the interpass temperature is controlled within the range determined by the welding procedure qualification, such as 100°C to 250°C, to prevent excessively high temperatures from causing coarse grains and reduced toughness in the root pass and heat-affected zone, and to prevent excessively low temperatures from increasing cold cracking susceptibility. The mechanism is that although the root pass has formed the back side shape, its microstructure may still soften, temper, or undergo secondary oxidation during subsequent thermal cycles. Therefore, it is necessary to control the heat input and interpass temperature to avoid excessive thermal shock to the root from the molten pool. Preferably, during the filler and cover welding processes, the welding can be carried out in a symmetrical sequence from both sides to the middle or in segments to reduce welding shrinkage stress and circumferential deformation, thereby maintaining the roundness of the pipe and the stability of the joint.

[0039] like Figure 4 As shown, furthermore, during the submerged arc welding filling and capping processes, the temperature on the back side of the root pass can be controlled by adjusting the welding heat input, interpass temperature, and welding sequence to ensure that the temperature does not exceed the allowable value, thus preventing secondary oxidation, overheating, or burn-through of the back pass weld. The allowable value here can be determined based on the material type and welding procedure qualification, with the aim of ensuring that the back pass weld formed by laser root pass is not damaged by subsequent thermal cycling. Compared to the traditional method of first root pass, then root cleaning, and finally back pass welding, this invention eliminates the need for secondary fusion deposition from the back of the pipe, thus eliminating the environmental risks of root cleaning operations and avoiding positioning difficulties and quality fluctuations caused by re-welding on the back side. Preferably, the submerged arc welding flux can be a low-hydrogen sintered flux or a fused flux to reduce the hydrogen content of the weld and improve the toughness of the weld metal; the welding wire can be a solid wire or a flux-cored wire with a strength matching the base metal strength grade, thereby improving the ductility and toughness coordination of the filler metal.

[0040] like Figure 5As shown, after the submerged arc welding is completed, post-weld heat treatment can be performed on the weld according to service requirements. This post-weld heat treatment can be hydrogen removal treatment or stress relief heat treatment, and the treatment temperature and holding time can be determined based on material standards, wall thickness, and welding procedure qualification. Its function is to eliminate residual welding stress, reduce the risk of hydrogen-induced delayed cracking, and promote the homogenization of the weld microstructure, thereby improving the overall mechanical properties of the welded joint. After post-weld heat treatment, the weld is subjected to one or more of the following: radiographic testing, ultrasonic testing, magnetic particle testing, or penetrant testing, to verify that the internal and surface quality of the weld meets design requirements. By combining post-weld heat treatment with final inspection, this composite welding process can be applied not only to general industrial pipelines but also to pressure pipelines or important process pipelines with high service reliability requirements.

[0041] Preferably, to further improve welding adaptability and process tolerance, the present invention may also introduce several non-essential additional measures. For example, a high-temperature resistant anti-spatter coating can be locally sprayed on the inner wall of the bevel to reduce metal spatter adhesion during the initial laser arc initiation; temperature sensing patches or infrared monitoring points can also be installed on the outer surface of the pipe to track the temperature of the welding area in real time, thereby more accurately controlling the preheating temperature and interpass temperature. For another example, when implementing ultra-thick-walled pipes, a short-term natural cooling or forced temperature control step can be added between laser root pass and submerged arc welding to avoid excessive heat accumulation; or a simple gas guiding device can be installed inside the pipe to make the back-side shielding gas distribution more uniform. These measures do not constitute a limitation on the basic process of the present invention, but rather serve as preferred optimization methods to further improve forming stability and quality consistency under different materials and wall thicknesses.

[0042] In summary, this invention establishes a continuous welding process suitable for thick-walled pipes by using a composite path of laser single-sided root pass to form a double-sided root and submerged arc welding for efficient filling and capping. Structurally, this process eliminates the traditional back-side root cleaning and back-side repair welding steps. Mechanistically, it leverages the complementary advantages of high penetration and low deformation of laser welding with high deposition rate and low cost of submerged arc welding. Effectively, it achieves a balance between root pass quality, construction efficiency, and field adaptability, making it particularly suitable for small-diameter thick-walled pipes, fixed-end welding, and complex conditions where internal welding is impossible.

[0043] It will be apparent to those skilled in the art that this disclosure is not limited to the details of the exemplary embodiments described above, and that this disclosure can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of this disclosure is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within this disclosure. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0044] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A composite welding process for single-sided welding and double-sided forming of thick-walled pipes, characterized in that, Includes the following steps: S1. Perform beveling on the thick-walled pipe joint to be welded, forming a single-sided bevel on the outer wall of the pipe, and reserving a blunt edge and root gap; the thickness of the blunt edge and the root gap are set according to the pipe wall thickness, material and laser welding penetration capability, so that the blunt edge can be completely melted during subsequent laser root pass welding and a continuous and uniform back weld is formed on the inner wall of the pipe. S2. Laser beam is used for root pass welding on the front side of the bevel. Before welding, the laser beam is aligned with the center of the bevel. By adjusting parameters such as laser power, welding speed, and defocusing amount, the laser beam energy is concentrated on the blunt edge area to completely melt the blunt edge and form a continuous and uniform back weld without incomplete penetration or fusion defects on the back side of the inner wall of the pipe. The back weld reinforcement and weld width meet the quality requirements. S3. Without any root cleaning, grinding, or back welding treatment on the back of the root weld, multi-layer, multi-pass filling and cover welding are performed directly on the front of the root weld using submerged arc welding. By controlling the heat input and interpass temperature of submerged arc welding, burn-through or structural damage to the root weld and back weld is avoided, and finally, the single-sided welding and double-sided forming of the thick-walled pipe joint is completed.

2. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, Before step S1, there are also assembly and tack welding steps: assemble the butt joints of the thick-walled pipes to be welded, control the assembly gap, misalignment and edge angle, so that the joint meets the assembly accuracy requirements of laser root welding; then use manual tungsten inert gas welding or laser spot welding for tack welding, the tack weld avoids the starting position of laser root welding, and the two ends of the tack weld are ground into a gentle slope to ensure the continuous and stable laser root welding.

3. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, In step S1, after the beveling is completed, the bevel surface and the inner and outer walls of the pipe are cleaned within a certain range to remove oil, rust, scale, moisture and other contaminants. For bevels processed by thermal cutting, the surface hardening layer and cutting burrs need to be removed by mechanical methods to expose the metallic luster of the bevel surface.

4. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, In step S2, the pipe joint is preheated before laser root pass welding. The preheating temperature is determined according to the pipe material, wall thickness and carbon equivalent to ensure that cold cracks are not generated during laser root pass welding. During the laser root pass welding process, an inert protective gas, which is argon or helium, is introduced into the back of the pipe to form a gas protection zone on the back of the pipe to prevent high-temperature oxidation of the back weld and ensure the quality of the back forming.

5. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, In step S2, the laser root pass welding adopts either laser autofusion welding or laser filler wire welding. When laser filler wire welding is used, the welding wire is fed into the molten pool from the front of the laser beam. By controlling the wire feeding speed, welding speed and welding wire alignment position, the welding wire is made to melt evenly and fill the root gap, control the reinforcement height and weld width of the back weld, and prevent undercut and lack of fusion.

6. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, The process after step S2 and before step S3 includes: visual inspection and non-destructive testing of the laser-cut root weld. Visual inspection includes checking whether the back weld is continuous and whether there are defects such as surface porosity, cracks, lack of fusion, undercut, and burn-through. Non-destructive testing uses radiographic testing or phased array ultrasonic testing. Only after confirming that there are no defects such as incomplete penetration, lack of fusion, cracks, and oxide inclusions in the interior and root of the root weld can submerged arc welding for filling and capping be performed. If defects are found, the defective areas are locally repaired and re-inspected.

7. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, In step S3, submerged arc welding adopts multi-pass multi-layer welding. After each weld is completed, the slag and spatter are cleaned and the surface quality of the interpass weld is checked. The interpass temperature is controlled within a preset range, which is determined according to the pipe material and welding process evaluation to prevent the root pass weld and heat-affected zone from becoming coarse and tough due to excessively high interpass temperature, or cold cracks from occurring due to excessively low interpass temperature.

8. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, In step S3, during the submerged arc welding filling and capping processes, by controlling the welding heat input, interpass temperature and welding sequence, the temperature on the back side of the root pass weld is kept below the allowable value to avoid secondary oxidation, overheating or burn-through of the back pass weld; at the same time, the submerged arc welding pool is prevented from directly acting on the root of the root pass weld to ensure that the back pass weld is not damaged.

9. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, After completing the submerged arc welding capping in step S3, the weld is subjected to post-weld heat treatment, which includes hydrogen removal treatment or stress relief heat treatment to reduce residual welding stress and improve weld microstructure and properties. After the post-weld heat treatment is completed, the weld is subjected to non-destructive testing, including one or more of radiographic testing, ultrasonic testing, magnetic particle testing or penetrant testing, to verify that the internal and surface quality of the weld meets the design requirements.

10. The composite welding process for single-sided welding and double-sided forming of thick-walled pipes according to claim 1, characterized in that, The thick-walled pipe is made of carbon steel, low alloy steel, heat-resistant steel or stainless steel; the beveling process in step S1 is carried out by machining or by mechanical finishing after thermal cutting; machining includes turning, milling or boring, thermal cutting includes plasma cutting or flame cutting, and mechanical finishing after thermal cutting removes the heat-affected zone and surface oxide layer.

Images