From steel plate to steel tube: How does LSAW welding process create the energy "steel artery"?

What exactly is LSAW process? What are the key features hidden in the core principle?

The full name of LSAW is "longitudinal submerged arc welding". As the name implies, its core feature is that the weld is parallel to the axis of the steel pipe, and the welding process is completed under the "covering" of granular flux. Different from spiral welding of spiral submerged arc welding (SSAW) and resistance heating of high frequency straight seam welding (ERW), LSAW process takes a single hot-rolled medium-thick steel plate as raw material, and then completes longitudinal welding by submerged arc welding technology, and finally forms a complete straight seam steel pipe.

The core advantages of this process come from two points: first, the stress on the weld is more reasonable-the longitudinal weld is in the same direction as the pipeline transportation pressure, which can effectively disperse the stress and avoid the cracking risk of spiral weld caused by stress superposition; Second, the welding quality is more stable-during submerged arc welding, the flux will form a closed cavity to wrap the arc, which will not only isolate the air to protect the molten pool, but also slow down the heat loss, so that the weld metal can be fully fused and the compactness can reach 99.9%. According to API 5L international standard, the steel grade of LSAW steel pipe covers X42 to X120, the maximum diameter can reach 1422 mm, and the wall thickness can exceed 45 mm, which fully meets the requirements of ultra-high pressure and large diameter pipelines.

Compared with other processes, the uniqueness of LSAW is particularly obvious. Taking ERW steel pipe as an example, it relies on high-frequency current heating and welding, so it can't handle thick-walled steel plates; Although the cost of SSAW steel pipe is low, the fatigue resistance of spiral weld is only 60% of that of LSAW, so it is difficult to be competent in extreme environments such as deep sea and cold area. It is these characteristics that make LSAW steel pipe the first choice for major projects such as west-to-east gas transmission and oil and gas pipelines in Central Asia.

From steel plate to steel pipe: What are the core steps of LSAW welding?

The production process of LSAW is like a "precision practice". From the steel plate entering the factory to the finished product leaving the factory, it needs to go through 16 core processes, and each step has strict parameter control. The first thing is the pretreatment of steel plate. The steel plate entering the production line must pass the ultrasonic flaw detection of the whole plate to remove the raw materials with defects such as looseness and slag inclusion, and then the surface oxide scale is removed by shot blasting and rust removal, and then the edge of the plate is processed into an accurate groove with an edge milling machine. The angle error of the groove must be controlled within 0.5, otherwise the fusion quality of the weld will be directly affected.

Forming is the key link to determine the size of steel pipe. At present, there are two mainstream forming processes, JCOE and UOE. JCOE process can be called "gradual shaping". The edge curvature of the steel plate is first treated by a pre-bending machine, and then it is gradually stamped into "J →C →O" on a forming machine. This method is flexible and can be adapted to the production of steel pipes with different specifications. In 2024, the global market share has reached 88.36%. The UOE process adopts "compression molding", which first presses the steel plate into a "U" shape and then presses it into an "O" shape with a mold, with higher molding accuracy, but the equipment investment is 2-3 times that of JCOE, and it is mainly used in high-end markets such as the European Union and Japan.

Welding is the core of the process, which is divided into two steps: pre-welding and fine welding. In the pre-welding, the formed tube blank is preliminarily welded by gas shielded welding (MAG) to prevent the wrong edge in the subsequent welding; For finishing welding, multi-wire submerged arc welding is used, and at most four wires can be used at the same time. After the inner weld is completed, the steel pipe is turned over to weld the outer side. Multi-wire welding technology can improve welding efficiency by 30%, and the weld penetration is uniform, avoiding incomplete penetration defects. Post-welding treatment is also indispensable. The residual stress of steel pipe needs to be eliminated by overall expanding to control the diameter tolerance within 0.2%, and then the post-welding heat treatment at 200-300℃ can promote the escape of hydrogen atoms and reduce the risk of cracking.

Each molding process has its own advantages and disadvantages: how to choose JCOE, UOE and CE?

In addition to the mainstream JCOE and UOE, LSAW also has CE (Roll Bending Forming) and other processes. Different technical paths are suitable for different scenarios, and the core of selection lies in "specification requirements" and "cost balance". The advantage of JCOE process is its flexibility, which can produce steel pipes with a diameter of 406-1422 mm and a wall thickness of 6-40 mm, and the equipment is easy to debug, which is suitable for multi-specification and small-batch orders. Most steel pipe factories in China are mainly based on JCOE process.

UOE process is the "king of large diameter and thick wall", and its molding method can ensure that the roundness error of steel pipe is less than 0.5% and the weld performance is uniform, which is especially suitable for manufacturing super-large steel pipes with a diameter of more than 1016 mm and a wall thickness of more than 30 mm, and is widely used in deep-sea oil and gas pipelines. However, the shortcomings of this process are also very prominent-a set of UOE production line needs an investment of more than 1 billion yuan, and the change time is as long as 8 hours, which is only suitable for large-scale standardized production.

CE process is roll bending forming, in which steel plates are gradually bent into tube blanks through multiple sets of rollers. The equipment cost is low, and the production efficiency is high. The daily output of single wire can reach 150 tons. However, due to the forming principle, only small and medium-sized steel pipes with diameters less than 610 mm can be produced, which are mainly used in urban water supply networks. According to the data of an East China Steel Pipe Factory, the unit cost of CE process is 12% lower than that of JCOE when producing φ 508 mm steel pipes, but it can't accept orders with φ 1016 mm or more.

Quality is the lifeline: How to keep the bottom line of "zero defect" in LSAW welding?

The service environment of LSAW steel pipe often involves extreme conditions such as high pressure and corrosion, and any tiny defect in weld may cause leakage accidents, so the quality control of the whole process is very important. In the raw material link, the chemical composition of steel plate must be strictly controlled, and the sulfur content must be lower than 0.01%, otherwise manganese sulfide inclusions will form and become the "starting point" of hydrogen-induced cracking. In the welding process, the drying of flux is the key-when the moisture content of the flux that is not dried exceeds 0.1%, hydrogen will be decomposed at the high temperature of arc, resulting in the hydrogen content in the weld soaring to 15ppm, far exceeding the safety threshold of 5ppm.

Post-weld inspection is "double insurance", and steel pipes need to undergo two nondestructive inspections: the first one is ultrasonic and X-ray inspection of welds to check whether there are pores and slag inclusions inside; The second time was conducted after expanding and hydrostatic test, and the new defects caused by stress release were investigated. According to API 5L standard, there are no cracks, undercut and other defects in Class I weld, and the number of blowholes in Class II weld shall not exceed 2 within 50 mm.. A steel pipe factory once missed a slag inclusion defect of 0.1 mm, which caused the steel pipe to burst during the water pressure test, and the direct loss exceeded one million yuan.

Hydrostatic test is the last checkpoint before leaving the factory. Steel pipes should be kept at 1.5 times the design pressure for 10 minutes, and the pressure drop should not exceed 0.5MPa. At the same time, each steel pipe should be tested by magnetic powder at the end to prevent the crack at the end caused by forming stress-these testing links are interlocking and jointly construct the "quality defense line" of LSAW steel pipe.

Coping with new demands: What technical upgrades are LSAW process undergoing?

With the rise of new scenarios such as hydrogen storage and transportation, carbon capture and storage (CCUS), LSAW process is also upgrading to "high-end, intelligent and green". In the field of hydrogen energy pipelines, the traditional LSAW steel pipes need to be improved by low-hydrogen welding materials, post-welding dehydrogenation and other processes to reduce the risk of hydrogen-induced cracking. JFE in Japan has achieved mass production of 2.5GPa hydrogen-resistant LSAW steel pipes, and the cost per ton is 18% lower than that of traditional products.

Intelligent transformation has become the key to improve efficiency. Laser vision seam tracking system has been popularized in 35% production lines, which can adjust the position of welding torch in real time and control the seam misalignment within 0.3 mm; The application of welding robot will improve the production efficiency by 40%, and the accuracy of defect identification will reach 99.8%. It is estimated that by 2030, the penetration rate of digital welding technology will increase to 55%. In the intelligent factory of a leading enterprise in China, the digital twinning technology was used to simulate the welding process, which reduced the trial welding cost by 25% and shortened the production cycle by 15%.

Green transformation is equally urgent. Using electric furnace steelmaking instead of traditional converter steelmaking can reduce the carbon emission of LSAW steel pipe by 60%, and this kind of low-carbon products has obtained an export premium of 12%-15%. At the same time, the popularization and application of recyclable flux increased the flux utilization rate from 70% to 95%, further reducing the production energy consumption.

From the initial application in the 1940s to the core technology of energy infrastructure, the development of LSAW welding technology has always resonated with industrial progress. In the future, with the popularization of X120 steel pipes and the full coverage of intelligent production lines, LSAW technology will continue to exert its strength in new fields such as hydrogen energy and deep-sea engineering, and continue to guard the "steel artery" of global energy transmission.