Development and Application of a Continuous Production Line for Large-Diameter Double-Seam Welded Stainless Steel Pipes

Abstract: This paper reviews the current state of research on straight-seam welded pipe production lines in China and abroad, with a particular focus on the development and application of a continuous production line for large-diameter, double-seam welded stainless steel pipes. The production line incorporates several advanced technologies:

• Dual-station coil uncoiling
• Automated in-line strip centering and welding
• An integrated flexible roll-forming system
• Multi-torch hybrid welding with intelligent control
• Automated in-line pipe cutting and end beveling

The production line can manufacture large-diameter, double-seam welded stainless steel pipes with outer diameters of up to 820 mm, improving production efficiency and reducing manufacturing costs. In addition, a Manufacturing Execution System (MES) specifically designed for stainless steel welded pipe production has been integrated to support information-based product management and intelligent manufacturing.

Although China's manufacturing technology for seamless stainless steel pipes is well established and widely used across various industries, the development of welded stainless steel pipe manufacturing, particularly for large-diameter products, has lagged behind. One of the key constraints is the limited availability of suitable raw materials. Currently, domestic production lines for wide hot-rolled stainless steel strip include the 700 mm line at Jiuquan Iron & Steel (Group) Co., Ltd., the 1,780 mm lines at Angang Lianzhong and China Baowu, and the 2,250 mm line at Taiyuan Iron & Steel (TISCO). However, even the coils produced by TISCO's 2,250 mm line can only support the production of welded stainless steel pipes with a maximum outer diameter of 711 mm.

Constrained by the available strip width, welded pipe manufacturers in China and abroad generally design their production lines to accommodate conventional coil widths. Most straight-seam welded pipe mills are designed for pipes with outer diameters of Ø114 mm or less, while continuous in-line forming and welding lines for stainless steel pipes with outer diameters exceeding Ø219 mm remain relatively uncommon. For straight-seam welded stainless steel pipes with outer diameters greater than Ø711 mm, continuous in-line forming and welding are particularly challenging. Such pipes are therefore still produced mainly using individual pipe-forming processes, such as UOE and JCO. These processes involve multiple manufacturing steps, resulting in low production efficiency and high manufacturing costs. Consequently, manufacturers may struggle to meet project delivery schedules.

Internationally, manufacturers in Japan and Italy have developed continuous production lines for stainless steel welded pipes with maximum outer diameters of 630 mm and 635 mm, respectively, based on FFX flexible forming technology and continuous roll-forming technology. In the early 21st century, China introduced various types of stainless steel welded pipe manufacturing equipment, driving improvements in product variety, production processes, and product quality. Among the available technologies, high-frequency induction resistance welding (ERW) is primarily used to manufacture welded pipes for mechanical and structural applications with relatively modest technical requirements.

With advances in production equipment and manufacturing capabilities, the application of stainless steel welded pipes has continued to expand. Currently, small-diameter, thin-walled stainless steel welded pipes are mainly manufactured using in-line continuous forming processes, whereas large-diameter, thick-walled pipes are typically produced using off-line roll-forming processes, such as UOE and JCO forming. For welded pipes with outer diameters exceeding 508 mm, the manufacturing process generally involves joining two steel plates, followed by a series of operations, including forming, welding, heat treatment, nondestructive testing, and hydrostatic testing.

Constrained by the available steel coil width, traditional off-line, single-pipe forming processes are characterized by low production efficiency, high manufacturing costs, and limited automation. Therefore, there is an urgent need to develop a production line capable of processing steel coils of different widths and continuously forming large-diameter, double-seam welded stainless steel pipes in line. Such a production line would overcome the limitations of conventional manufacturing processes and address key technical challenges associated with the use of wide steel coils and the continuous production of large-diameter welded pipes. It would also improve production efficiency, reduce manufacturing costs, and enhance the level of automation, intelligence, and digitalization in manufacturing operations.

The continuous production line for large-diameter, double-longitudinal-seam welded stainless steel pipes is designed to manufacture pipes with outer diameters of up to 820 mm. The line primarily comprises the following equipment and systems: a staggered two-station uncoiler, a leveling unit, an in-line automatic strip centering and welding system, a cross-cutting unit, an in-line ultrasonic testing system for steel plates, a flexible roll-forming unit, an automatic welding system, an in-line heat treatment unit, a sizing unit, a straightening unit, an eddy-current testing unit, an integrated CNC cutting and beveling machine, and an intelligent, information-based control system. The production line layout is shown in Figure 1.

The core technology of this production line lies in the combination of a dual-uncoiler system and an in-line automatic strip centering and welding system. This configuration enables continuous joining of steel strips from successive coils, allowing uninterrupted in-line production of large-diameter, double-longitudinal-seam welded stainless steel pipes. In addition, the line is equipped with in-line nondestructive testing (NDT) equipment, an automatic weld seam tracking system, and a Manufacturing Execution System (MES), further enhancing the level of automation and intelligence.

Figure 1. Schematic diagram of the continuous production line for large-diameter, double-longitudinal-seam welded stainless steel pipes.

The production line uses two steel coils of different widths as feedstock. A dual-station in-line uncoiling system simultaneously unwinds and levels the steel strips, while an automatic centering device aligns their edges and centerlines with the production line. The strips are then joined by arc welding, using an automatic arc-tracking system and visual monitoring of the weld pool, to produce a continuous wide steel strip with a longitudinal weld seam. After undergoing multi-probe, immersion-type ultrasonic testing, the strip is fed into an integrated continuous flexible roll-forming unit. This unit uses CNC-controlled rollers with adjustable forming angles to progressively cold-form the strip into a pipe blank. The pipe blank is then welded using a hybrid process that combines high-penetration plasma arc welding with tungsten inert gas (TIG) welding, producing a large-diameter, double-longitudinal-seam stainless steel pipe. The pipe then undergoes in-line heat treatment and nondestructive testing. Throughout the manufacturing process, production, inspection, and equipment operating data are collected, analyzed, and stored in real time through a dedicated Manufacturing Execution System (MES). This system supports intelligent production management and information traceability, enabling effective control of the manufacturing process and product quality.

Table 1 summarizes the basic production capabilities of the continuous production line for large-diameter, double-longitudinal-seam welded stainless steel pipes, while Table 2 presents the key process parameters. The production process is illustrated in Figure 2. The line integrates all major manufacturing operations, from raw material uncoiling to cutting to length and end beveling, into a continuous in-line process. This highly automated configuration significantly improves production efficiency compared with conventional manufacturing methods.

To achieve high-efficiency, low-cost production of large-diameter stainless steel welded pipes, Zhejiang Dewei developed an intelligent continuous production line for stainless steel pipes with outer diameters of up to 820 mm, building on existing technologies developed both in China and abroad.

The production line features a staggered two-station uncoiler that simultaneously unwinds two steel coils of different widths. The strips are then automatically centered and welded together to form a continuous wide steel strip, which serves as the feedstock for subsequent pipe production. This configuration breaks away from the conventional single-coil production model. For pipe forming, the line combines FFX (Flexible Forming) technology with cluster-roll forming and employs a proprietary cold-bending process for the steel strip. Automated welding is then used to join the longitudinal seams. This integrated production approach, based on in-line double-longitudinal-seam welding and continuous cold forming, significantly improves production efficiency and reduces manufacturing costs.

In addition, to support the development of intelligent and digitalized manufacturing in line with national policies, Zhejiang Dewei developed a dedicated Manufacturing Execution System (MES) for stainless steel pipe production. The system supports production order management, process control, production scheduling, in-process quality control, and dynamic equipment management. It also enables the real-time collection, analysis, statistical processing, and storage of production, equipment, and process data. By transforming traditional production management practices, the system improves operational efficiency and provides effective product quality traceability.

Table 1. Basic Production Line Specifications

Table 2. Key Process Parameters

Production Process for Large-Diameter Stainless Steel Welded Pipes with Double Longitudinal Seams:

Steel Coil → Incoming Material Re-inspection → Loading and Uncoiling → In-line Strip Joining (Welding) → Ultrasonic Testing → Roll Forming → Longitudinal Seam Welding → Rough Sizing → In-line Solution Heat Treatment → Finish Sizing and Straightening → Eddy Current Testing → Cutting to Length and End Beveling → Radiographic Testing → Hydrostatic Testing → Pickling and Passivation → Cleaning and Drying → Surface Inspection → Final Product Inspection → Marking → Packaging

The key technical features of the production line are as follows:

(1) Development of a Staggered Two-Station Uncoiling System and an In-Line Continuous Automatic Strip Centering and Welding System

Due to the limited width of available stainless steel coils, the production of welded pipes with outer diameters exceeding Ø820 mm typically requires coils wider than 2,500 mm, placing considerable demands on upstream steel production facilities. To make effective use of standard-width raw materials, a staggered two-station uncoiling system was developed to simultaneously unwind two steel strips of different widths. An automatic centering and welding unit then aligns the strip edges and joins the strips by welding, forming a single wide steel strip with one longitudinal seam. This solution addresses the challenge of securing suitable raw materials for the continuous in-line production of large-diameter stainless steel pipes. It also significantly improves production efficiency and reduces intermediate material handling, making it a key technology for the continuous manufacture of large-diameter stainless steel pipes.

(2) Development of Integrated In-Line Forming Technology Combining Tandem Flexible Forming and Cluster-Roll Forming with CNC-Controlled Multi-Angle Adjustment

This technology integrates FFX (Flexible Forming) with cluster-roll forming. An independently developed, multi-stage progressive cold-forming system consists of five sets of flat rolls and five sets of adjustable vertical rolls. The system ensures tooling compatibility across the entire rough-forming section. When switching between pipe specifications, there is no need to replace the forming rolls; only their positions and angles need to be adjusted. This enables continuous in-line forming of large-diameter stainless steel pipes with outer diameters ranging from Ø406 to Ø820 mm.

In addition, the production line is equipped with an automated data acquisition system that monitors, analyzes, and stores the positions and angles of the forming rolls in real time. When production of a particular pipe specification is resumed, the recorded data can be used to quickly and accurately restore the corresponding forming settings, thereby reducing setup time and improving overall production efficiency.

(3) Development of a Multi-Torch Welding Process and an Intelligent Multi-Station Welding Control System

The production line is equipped with two welding stations: an upstream station for automatic in-line strip centering and welding, and a downstream station for longitudinal seam welding of the pipe blanks. This configuration enables the simultaneous in-line welding of two longitudinal seams. Each station is equipped with an automatic arc-tracking system and a weld-pool visualization system, enabling real-time monitoring and control of the welding process. These systems help maintain consistent weld quality, reduce welding defects, and support high-efficiency production.

(4) Development of Integrated In-Line Automatic Pipe Cutting and End-Beveling Technology

Traditionally, stainless steel welded pipes are cut to length using band saws or circular saws, followed by a separate off-line end-beveling operation. This process is characterized by low efficiency, significant cutting deformation, and excessive metal chip generation. To overcome these limitations, an integrated in-line automatic cutting and end-beveling technology has been developed. It uses a high-speed rotary cutting device equipped with multiple cutting heads arranged at different angles to perform precision cutting and end beveling in a single operation. This eliminates the need for a separate off-line beveling process and enables the efficient and reliable in-line cutting and end beveling of large-diameter welded pipes.

(5) Development of an MES for Stainless Steel Welded Pipe Production

A Manufacturing Execution System (MES) specifically designed for stainless steel welded pipe production has been developed. As shown in Figure 3, the system is seamlessly integrated with the production line's automation control systems, wireless communication infrastructure, and networked data systems. At key production stages, a PROFIBUS communication network and high-precision linear sensors are used to collect production data. The MES enables the real-time collection, analysis, storage, and retrieval of these data, supporting the digitalization and intelligent control of the manufacturing process.

The production line for large-diameter, double-longitudinal-seam welded stainless steel pipes has been successfully commissioned and is now in batch production at Zhejiang Dewei. Its application is shown in Figure 4.

To evaluate the quality of the pipes manufactured on this production line, representative samples were selected for comprehensive testing. The specimens were 06Cr19Ni10 (S30408) stainless steel pipes with dimensions of Ø820 × 10 mm, manufactured in accordance with GB/T 12771—2019, Stainless Steel Welded Pipes for Fluid Transport. The tests covered chemical composition, tensile properties, technological properties, impact toughness, corrosion resistance, and metallographic microstructure.

(1) Chemical Composition Analysis

Samples for chemical composition analysis were taken from the weld seam areas. The chemical composition was determined in accordance with GB/T 11170—2008, Stainless Steel—Determination of Multi-Element Contents—Spark Discharge Atomic Emission Spectrometric Method (Routine Method). The results for both weld seams are presented in Table 3, and all measured values met the specified requirements.

Figure captions:

Fig. 3. Manufacturing Execution System (MES) for the Production Line

Fig. 4. Application of the Large-Diameter, Double-Longitudinal-Seam Welded Stainless Steel Pipe Production Line

(2) Transverse Tensile Testing

Transverse tensile specimens were taken from the weld seams and tested at room temperature in accordance with GB/T 228.1—2010, Metallic Materials—Tensile Testing—Part 1: Method of Test at Room Temperature. The test results are presented in Table 4, and all specimens met the specified requirements.

(3) Transverse Bend Testing

Transverse bend specimens were taken from the weld seam regions. For each weld seam, one face-bend specimen and one root-bend specimen were prepared. The tests were conducted in accordance with GB/T 2653—2008, Bend Test Methods for Welded Joints. The mandrel diameter (\(D\)) was set to four times the specimen thickness, and the bending angle was 180°. No cracks or fissures were observed in the weld seam regions after testing.

(4) Charpy V-Notch Impact Testing

Transverse Charpy V-notch impact specimens were taken from the weld zones, with one set of three specimens prepared for each weld seam. The tests were conducted in accordance with GB/T 229—2007, Metallic Materials—Charpy Pendulum Impact Test Method. The specimens measured 55 mm × 10 mm × 7.5 mm, with a V-notch depth of 2 mm. The test temperature was 25 °C. The results are presented in Table 5.

(5) Intergranular Corrosion Testing

Intergranular corrosion tests were conducted on specimens taken from the weld zones, with two specimens tested for each weld seam. The tests were performed in accordance with Method E of GB/T 4334—2008, Corrosion of Metals and Alloys—Test Methods for Intergranular Corrosion of Stainless Steels. The polished specimens were continuously immersed for 16 h in a boiling aqueous solution of copper sulfate and sulfuric acid, followed by a 180° bend test. No intergranular corrosion cracks were observed in any of the specimens after bending, indicating that all specimens passed the test.

(6) Metallographic Examination

The microstructures of the welded joints were examined, and the results are presented in Figures 5–7. The base metal exhibits a uniformly distributed austenitic structure. The weld metal has a relatively uniform cellular austenitic structure containing a small amount of dendritic ferrite, while the heat-affected zone (HAZ) consists primarily of austenite with a small amount of chain-like ferrite.

In summary, the large-diameter, double-longitudinal-seam welded stainless steel pipes manufactured on this production line meet all the requirements of GB/T 12771—2019, Stainless Steel Welded Pipes for Fluid Transport, demonstrating consistent and reliable product quality.

Table 3. Chemical Composition of the Weld Seams in Stainless Steel Welded Pipes (Mass Fraction, %)

Table 4. Room-Temperature Tensile Properties of Stainless Steel Welded Pipes

Table 5. Charpy V-Notch Impact Test Results for Stainless Steel Welded Pipe Joints

(a)Weld I(b) Weld II

Fig. 5. Full-Field Scanning Metallographic Micrograph of a Stainless Steel Welded Joint

(a) Base metal
(b) Weld metal
(c) Heat-affected zone (HAZ)

Fig. 6. Metallographic Microstructure of Weld Seam I

(a) Base metal
(b) Weld metal
(c) Heat-affected zone (HAZ)

Fig. 7. Metallographic Microstructure of Weld Seam II

The development and successful implementation of a continuous production line for large-diameter, double-longitudinal-seam welded stainless steel pipes has overcome key limitations of conventional manufacturing methods, making continuous in-line production of such pipes possible for the first time in both China and internationally. The production line overcomes the constraints of traditional approaches, including the JCO (press-bending) process, which forms pipes from individual plates, and manufacturing methods based on single steel coils. It addresses key challenges associated with the availability of suitable raw materials for continuous production and the low efficiency of conventional off-line manufacturing processes, which involve multiple processing steps. As a result, it enables the mass production of large-diameter stainless steel welded pipes. The production line also meets downstream customers' requirements for high product quality, full-process quality traceability, and short delivery times. Furthermore, it promotes coordinated development across the stainless steel supply chain and helps drive the industry toward higher quality, greater efficiency, and intelligent manufacturing.

However, the relatively high tooling changeover costs and lengthy setup times for small-batch, multi-specification orders make this production line less economical than conventional single-pipe forming processes, such as JCO, for customized production in small and medium batches. Furthermore, further improvements are needed in real-time acquisition of process data throughout the production line, online prediction of forming quality, closed-loop control of process parameters, and the development of a digital twin platform.