Research on LASW Laser-Arc Hybrid Welding Technology and Performance of Large-Diameter Straight Seam Steel Pipes
Traditional submerged arc welding (SAW) processes for large-diameter, thick-walled straight seam steel pipes generally suffer from drawbacks such as high total heat input, large welding deformation, coarse weld grains, and low forming efficiency. These drawbacks make it difficult to meet the service requirements of high-end oil and gas transportation and marine engineering equipment for pipeline components with low stress, high toughness, and high dimensional accuracy. Hebei haiqianwei Steel Pipe Co., Ltd., a leading domestic manufacturer of JCOE straight seam submerged arc welded pipes, is conducting research on the adaptation of laser-arc hybrid welding (LASW) technology based on its fully automated PID forming production line. This research explores composite heat source welding schemes based on the existing submerged arc welding system. By adjusting key process parameters such as laser power, arc current, welding speed, and the distance between the laser and arc heat sources, the weld formation characteristics, microstructure, mechanical properties, and residual stress distribution patterns of traditional submerged arc welding and laser-arc hybrid welding are systematically compared. Experimental results show that the LASW dual-heat source coupling effect can effectively reduce the range of the welding heat-affected zone, refine the metal grains in the weld and fusion zone, and significantly reduce the probability of welding defects such as porosity, lack of fusion, and slag inclusions. The tensile strength, low-temperature impact toughness, and bending performance of the composite welded joint are all superior to those of traditional submerged arc welded joints. Furthermore, the single-pass welding penetration is greater, which can effectively reduce the number of welding passes and shorten the processing steps. This welding process can significantly improve the welding quality of large-diameter, thick-walled straight seam steel pipes, control welding residual stress and structural deformation, and is suitable for the large-scale precision production of haiqianwei's existing JCOE production line, possessing extremely high engineering application and process promotion value.
Keywords : Straight seam steel pipe; JCOE forming; LASW; Laser-arc hybrid welding; Microstructure; Mechanical properties; Hebei haiqianwei
I. Introduction
Large-diameter straight seam steel pipes are core pressure-bearing components in long-distance pressure transmission pipelines, offshore platform steel structures, and large-scale water conservancy infrastructure projects. The microstructure, dimensional accuracy, and structural stability of the welded joints directly determine the overall project's service safety and lifespan. Hebei haiqianwei Steel Pipe Co., Ltd., located in the Yanshan Pipeline Equipment Industrial Base in Hebei Province, possesses multiple JCOE-formed straight seam submerged arc welded pipe production lines. It can stably produce pipeline steel pipes with outer diameters ranging from 406.4 to 1422.4 mm and wall thicknesses up to 50 mm. Its products cover oil and gas transportation, new energy pipelines, and large steel structures, and comply with domestic and international standards such as API 5L and GB/T 9711. The company is one of the leading suppliers of high-end straight seam steel pipes in China. While its existing production system primarily utilizes double-sided submerged arc welding with multi-layer and multi-pass processes, which are mature and reliable, inherent shortcomings remain in the production of thick-walled, high-grade steel pipes.
Traditional submerged arc welding (SAW) involves long heat input durations and wide heat coverage. During multi-layer welding of thick plates, heat accumulation is prone to occur, leading to problems such as overall pipe deformation, excessive ellipticity, and residual stress concentration. The multi-layer weld stacking process is also susceptible to hidden defects such as interlayer inclusions and localized incomplete fusion. Furthermore, repeated welding heating can cause abnormal grain growth in the weld microstructure, resulting in decreased low-temperature toughness of the joint and making it unsuitable for complex and harsh operating conditions such as high pressure, low temperature, and strong corrosion. In addition, the complex multi-layer, multi-pass welding process and long processing cycle restrict the improvement of production efficiency in the intelligent manufacturing of high-end steel pipes. Facing the continuous upgrading of pipe quality requirements from emerging pipeline projects such as oil and gas and hydrogen energy transportation in China, haiqianwei has been continuously exploring new welding processes and attempting to introduce laser-arc hybrid welding technology to transform its existing welding system.
Laser-Arc Hybrid Welding (LASW) is a novel hybrid welding technology that spatially couples a high-energy-density laser heat source with a pulsed electric arc heat source, enabling complementary performance between the two heat sources. The laser heat source offers concentrated energy and strong penetration, allowing for rapid forming with large weld penetration; the electric arc heat source provides a large molten pool volume and high tolerance for assembly gaps, overcoming the limitations of single-laser welding, which demands stringent assembly precision and has low process error tolerance. The synergistic effect of the dual heat sources effectively optimizes the welding heat input field, improves the molten pool flow state, and suppresses the formation of welding defects.
Currently, LASW technology has been initially applied in the welding of medium and heavy plate steel structures, pressure vessels, and engineering machinery. However, systematic research on the complete process adaptability, microstructure evolution, and mechanical properties of JCOE-formed large-diameter thick-walled straight seam steel pipes is still insufficient, lacking experimental data support based on the operating conditions of mass production lines. This study conducts LASW welding experiments based on the production conditions of haiqianwei's JCOE forming production line to explore the influence mechanism of process parameters on weld formation, microstructure, and mechanical properties. Establishing a high-quality welding process system suitable for large-diameter straight seam steel pipes can provide theoretical support and technical reference for enterprises to upgrade high-end pipeline welding processes and optimize quality.
II. Test Materials and Equipment
2.1 Test base material
The test materials were general-purpose Q345B low-alloy high-strength structural steel plates, with a thickness ranging from 12 to 28 mm, matching the mainstream large-diameter straight seam steel pipe specifications of the haiqianwei JCOE production line. This steel possesses excellent plasticity, toughness, and weldability, and is widely used in pressure pipelines and large steel structure load-bearing components. The chemical composition and basic mechanical properties of the material meet the requirements of the national standard GB/T 1591-2018. All test specimens were processed using haiqianwei's industrial standard JCOE bending and forming process. Milling, pre-bending, and seam joining processes were consistent with the production line, with standardized bevel forms and assembly gaps, completely replicating actual production conditions to ensure the authenticity and engineering applicability of the test data.
2.2 Test Equipment
The experiment employed an integrated laser-arc hybrid welding system. The equipment is equipped with a high-precision beam adjustment module and a pulsed arc control system, enabling precise spatial coupling between the laser and arc heat sources. Core parameters such as laser power, welding speed, arc current, defocusing amount, and arc spacing can be precisely controlled via programmed parameters. An automatic walking mechanism ensures the stability and consistency of the welding process. The entire welding system is compatible with haiqianwei's existing JCOE forming and joining station, facilitating subsequent production line integration and evaluation.
Post-weld performance testing employs precision testing equipment such as metallographic microscopes, computer-controlled universal testing machines, low-temperature impact testing machines, and residual stress testers. This allows for the observation of the weld's macroscopic morphology, characterization of its microstructure, testing of its mechanical properties, and quantitative analysis of residual stress, comprehensively characterizing the overall performance of the welded joint. Non-destructive testing utilizes phased array ultrasonic testing equipment consistent with the haiqianwei production line, maintaining consistency with the company's existing quality inspection system.
2.3 Test Plan
To clarify the performance advantages of the LASW process compared to traditional welding, traditional submerged arc welding was set up as the control group and laser-arc hybrid welding as the experimental group. The material, thickness, groove structure, welding environment temperature, and shielding gas parameters of the two groups of specimens were kept completely consistent. The control group adopted the current mass production submerged arc welding process parameters of haiqianwei.
Orthogonal experimental design was used to optimize the core process parameters of LASW, with weld formation quality, defect rate, and mechanical properties as evaluation indicators to select the optimal combination of process parameters. After welding, all specimens underwent appearance quality inspection and phased array non-destructive testing. Subsequently, metallographic and mechanical property specimens were prepared, and microstructure observation, tensile testing, low-temperature impact testing, bending testing, and residual stress detection were performed. The advantages and disadvantages of the two welding processes and their feasibility for implementation on the haiqianwei production line were compared and analyzed.
III. Mechanism of LASW Composite Welding Process
The core technical feature of LASW welding is the synergistic coupling of two heat sources. The two heat sources work synchronously and mutually enhance each other in the same welding area, completely breaking through the performance bottleneck of single welding processes. During the welding process, the front laser beam acts at high speed on the joint area of the base material. The high-energy-density beam can quickly melt and penetrate the metal substrate, forming a stable laser deep-penetration keyhole, achieving rapid forming of large-depth welds. At the same time, the metal plasma excited by the laser beam can stabilize the arc combustion state, suppressing problems such as arc drift, arc breakage, and unstable combustion, significantly improving the process stability of the arc heat source.
The large molten pool formed by the post-arc heat source can effectively encompass the tiny gaps in plate assembly, compensating for the poor gap adaptability of laser welding. Simultaneously, the flowing molten pool can fully backfill and stir the laser-deep penetration area, homogenizing the weld metal composition and optimizing the weld morphology. Compared to the large-area, long-duration heat input mode of traditional submerged arc welding, LASW welding features concentrated heat input, short heat treatment time, strong heat controllability, and a reasonable temperature gradient in the welding area. This effectively suppresses the formation of coarse grain structures and reduces deformation and residual stress problems caused by accumulated welding heat. From a process mechanism perspective, it achieves low heat input, deep penetration, and high-quality joint welding results. This feature precisely solves the problem of excessive pipe ellipticity caused by heat accumulation in the production of thick-walled steel pipes from haiqianwei.
IV. Experimental Results and Analysis
4.1 Macroscopic Formation and Defect Analysis of Welds
Macroscopic morphology and phased array nondestructive testing results indicate that traditional submerged arc welding (SAW) results in excessive weld width, blurred heat-affected zone boundaries, poor uniformity at multi-layer weld joints, uneven weld reinforcement, and extensive post-weld grinding and finishing. Due to the heat accumulation in multi-layer welding and poor molten pool fluidity, defects such as micro-slag inclusions, interlayer incomplete fusion, and point porosity are prone to occur inside the weld, resulting in a high random defect incidence and poor joint quality stability.
LASW composite welding produces a smooth, uniform, and symmetrical weld surface with minimal spatter and highly controllable weld reinforcement, eliminating the need for extensive post-processing. The dual heat source coupling significantly enhances molten pool stirring, ensuring sufficient gas escape during welding and resulting in denser metal fusion. Non-destructive testing reveals no internal cracks, large-area porosity, or slag inclusions in the composite welded joint, exhibiting a significantly lower defect rate than traditional submerged arc welding. This results in a substantial improvement in joint quality and consistency, reducing the rework rate of haiqianwei finished steel pipes.
4.2 Microstructure Evolution Characteristics of Welds
Metallographic observation results show that the heat-affected zone of traditional submerged arc welding joints is relatively wide, the high temperature of welding lasts for a long time, and the grains of the base metal are significantly coarsened. The weld structure is mainly composed of coarse ferrite and lamellar pearlite, with a large grain size range and poor uniformity. The coarse grain structure will significantly reduce the low-temperature toughness and fatigue resistance of the welded joint, becoming a weak area in the long-term performance of pipeline components.
The LASW composite welding process offers precise and controllable heat input and a reasonable welding cooling rate, effectively suppressing abnormal grain growth. The weld and fusion zone exhibit a uniform and dense microstructure, with fine and uniform ferrite grains and a regular distribution of pearlite, without the formation of coarse and brittle structures. This refined grain structure effectively inhibits crack initiation and propagation, optimizes grain boundary mechanical properties, and enhances the overall strength, toughness, and structural stability of the welded joint at the microscopic level, making it more suitable for haiqianwei's pipeline product needs in high-altitude and marine environments.
4.3 Mechanical Property Analysis of Welded Joints
4.3.1 Tensile properties
The results of room temperature tensile tests show that the tensile strength of the welded joints produced by both processes meets the pipe design standards. The tensile strength of the traditional submerged arc welded joint fluctuates significantly, with some specimens breaking in the weld area. The LASW composite welded joint exhibits higher overall tensile strength and less data dispersion. All specimens fractured within the base material region, indicating that the composite weld has a higher weld strength than the base material, excellent joint strength matching, no weak areas, and more reliable pressure-bearing performance.
4.3.2 Low-temperature impact toughness
Low-temperature (-20℃) impact test data shows that the impact energy of the LASW composite welded joint is significantly higher than that of the traditional submerged arc welded joint. The coarse grains and embrittlement caused by the traditional welding process result in a significant reduction in the low-temperature impact resistance of the weld. The LASW process refines the grains and optimizes the microstructure, effectively improving the low-temperature embrittlement characteristics of the weld. This allows the joint to maintain excellent toughness reserves under low-temperature conditions, which can support haiqianwei's expansion into the pipeline supply market in cold regions.
4.3.3 Bending performance
During the forward and reverse bending tests, the LASW composite welded joint showed no defects such as cracks, peeling, or fissures in the weld, fusion zone, and heat-affected zone after bending at a specified angle. The joint exhibited excellent plastic deformation capacity, demonstrating that the weld is densely fused internally without internal defects, and that its structural plasticity and deformation compatibility meet engineering requirements. In contrast, some traditional welded specimens showed micro-cracks during bending, indicating weaker plastic stability compared to the LASW composite welded joint.
4.4 Analysis of Welding Deformation and Residual Stress
In the welding process of large-diameter, thick-walled straight seam steel pipes, deformation and residual stress caused by uneven heat distribution are key issues affecting the dimensional accuracy and service life of the pipes. Test data shows that traditional submerged arc welding has a large heat accumulation, resulting in greater ellipticity and radial deformation of the pipe body after welding. High residual tensile stress exists in the weld area, which can easily lead to later stress corrosion, crack propagation, and other hidden dangers. These are also key indicators that need to be controlled in the production of haiqianwei thick-walled pipes.
LASW composite welding features low total heat input and a narrow heat-affected zone, concentrating welding heat in the joint area. This significantly improves the overall heating uniformity of the pipe, resulting in a marked reduction in post-weld pipe deformation and higher dimensional accuracy. Simultaneously, the composite weld joint exhibits lower residual stress and a more uniform stress distribution, effectively mitigating the structural failure risk caused by stress concentration and enhancing the long-term service stability of pipeline components.
4.5 Process Production Efficiency Analysis
Traditional thick-walled straight seam steel pipes require multi-layer, multi-pass welding processes, resulting in numerous welding steps, long processing cycles, and significant manual intervention and post-processing workload, thus limiting production efficiency. The LASW process, with its single-pass, large-penetration-depth technology, can significantly reduce the number of welding passes for thick plates, simplify processing steps, and dramatically increase welding speed. Simultaneously, this process boasts a low welding defect rate, minimal rework, and simple post-processing. After modification and application on haiqianwei's existing JCOE production line, it can effectively improve the overall production line capacity and reduce comprehensive production costs such as consumables, labor, and equipment energy consumption, demonstrating excellent industrialization potential.
V. Conclusion
Based on the conditions of the JCOE forming production line at Hebei haiqianwei, an experimental study and performance characterization of LASW laser-arc hybrid welding process for large-diameter straight seam steel pipes was conducted. Combined with multi-dimensional comparative analysis of macro-forming properties, microstructure, mechanical properties, and structural stress, the following conclusions can be drawn:
1. The laser-arc dual heat source coupling effect can achieve process complementarity, effectively optimize the flow state of the weld pool in the welding of large-diameter thick-walled steel pipes, reduce typical welding defects such as porosity, slag inclusion, and lack of fusion, and the weld formation is flat and uniform. The overall forming quality and consistency of the welded joint are significantly better than the traditional submerged arc welding process, which helps to reduce the rework rate of the finished product flaw detection at haiqianwei.
2. LASW's precise and controllable low heat input characteristics can effectively suppress weld grain coarsening behavior, refine the microstructure of the weld and heat-affected zone, optimize the uniformity of the microstructure, and fundamentally improve the mechanical properties of the welded joint. The joint strength, low-temperature toughness and plasticity are all effectively improved, which can meet the performance requirements of high-end pipeline products of enterprises.
3. Composite welding technology can significantly reduce the problems of hot deformation and residual stress concentration in steel pipe welding, improve the dimensional processing accuracy of pipes, and avoid safety hazards such as stress corrosion and crack propagation during long-term service. It can be used in the manufacturing of high-pressure, low-temperature, and marine engineering pipeline products.
4. The LASW welding process is simple, has high forming efficiency, and low rework rate. It has the potential to be upgraded and integrated on the basis of haiqianwei's existing JCOE automated production line. While ensuring welding quality, it can improve industrial production efficiency and reduce production costs. It can be used as an alternative process for the high-end and intelligent production of large-diameter straight seam steel pipes and has broad engineering application prospects.
Further research could focus on the adaptation and optimization of LASW process parameters for ultra-thick-walled steel pipes and high-grade steel pipes such as X80, complete the debugging of equipment interfaces and production line linkage, establish a standardized welding process parameter database compatible with haiqianwei's existing quality inspection system, and further promote the large-scale and standardized application of laser composite welding technology in the pipeline manufacturing field.
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Disclaimer: The content of this article is for informational purposes only and does not constitute any investment advice. For practical application of the products, please follow the guidance of professional institutions.
