Basic Knowledge of Spiral Pipes in Guangxi

Guangxi spiral pipes are manufactured from coiled steel strip, formed by room-temperature extrusion, and welded using an automated double-wire, double-sided submerged-arc welding process.

Production process

(1) The raw materials—steel strip coils, welding wire, and flux—are all subjected to rigorous physicochemical testing prior to use.

(2) The steel strip is butt-welded at its ends using either single‑wire or double‑wire submerged arc welding, and after coiling into a steel pipe, an automatic submerged arc weld is applied for repair.

(3) Prior to forming, the strip steel undergoes leveling, edge trimming, edge planing, surface cleaning and conveying, as well as pre‑bending of the edges.

(4) An electrical contact pressure gauge is used to regulate the pressure of the hydraulic cylinders on both sides of the conveyor, ensuring smooth strip steel conveyance.

(5) Employ either external‑control or internal‑control roller forming.

(6) A weld gap control device is employed to ensure that the weld gap meets welding requirements, with strict control over pipe diameter, misalignment, and weld gap.

(7) Both the internal and external welds are performed using Lincoln electric welding machines from the United States, employing either single‑wire or dual‑wire submerged arc welding to ensure consistent weld quality.

(8) All welded seams are inspected using an online, continuous ultrasonic automatic flaw detection system, ensuring 100% nondestructive testing coverage of the spiral welds. If defects are detected, the system triggers an automatic alarm and applies a spray‑on marking; production personnel use this information to adjust process parameters in real time and promptly eliminate any defects.

(9) Use an air plasma cutter to cut the steel pipes into individual lengths.

(10) After the steel pipes are cut into individual lengths, each batch must undergo a rigorous first‑article inspection, verifying the weld’s mechanical properties, chemical composition, fusion quality, and surface condition, as well as passing nondestructive testing. Only after confirming that the pipe‑manufacturing process meets all specifications may production proceed officially.

(11) Areas on the weld seam marked by continuous ultrasonic testing shall be re-examined by manual ultrasonic and X-ray inspection. If defects are confirmed, they shall be repaired and then subjected to nondestructive testing again until it is verified that the defects have been eliminated.

(12) All pipes containing steel strip butt welds and T-joints intersecting spiral welds shall undergo X-ray television or radiographic inspection.

(13) Each steel pipe undergoes a hydrostatic pressure test, with radial sealing employed to ensure pressure integrity. Both the test pressure and duration are strictly controlled by an automated hydraulic testing system. Test parameters are automatically printed and recorded.

(14) Machining of pipe ends ensures precise control of end-face perpendicularity, bevel angle, and root face.

Weld seam treatment

The spiral welded pipe line feeds the strip steel into the welding unit; as it passes through multiple sets of rolls, the strip gradually curls up to form a circular tube blank with an open seam. By adjusting the penetration of the squeezing rolls, the weld gap is maintained within 1–3 mm, and the two ends of the weld are brought into flush alignment.

　　1. If the gap is too large, the proximity effect is reduced, resulting in insufficient eddy current heating and poor intergranular bonding in the weld, which can lead to lack of fusion or cracking.

　　2. If the gap is too small, the proximity effect increases, leading to excessive welding heat and burn‑through of the weld; alternatively, after extrusion or roll‑forming, the weld may develop deep pits, compromising its surface quality.

　　After heating the two edges of the tube blank to the welding temperature, the extrusion rollers press them together, causing the metal grains to interpenetrate and crystallize, ultimately forming a strong weld. For spiral welded pipes, if the extrusion force is too low, fewer common crystals form, resulting in reduced weld strength and susceptibility to cracking under load; conversely, excessive extrusion force can squeeze molten metal out of the weld, not only weakening the weld but also generating substantial internal and external burrs and potentially leading to defects such as weld overlap.

Craftsmanship Features

Main process characteristics of spiral welded steel pipe:

a. During the forming process, the steel plate undergoes uniform deformation, exhibits low residual stress, and remains free of surface scratches. The fabricated spiral welded pipes offer greater flexibility in terms of diameter and wall‑thickness dimensions, particularly excelling in the production of high‑grade thick‑walled pipes—especially small and medium‑diameter thick‑walled pipes—where they provide advantages unmatched by other manufacturing processes, thereby meeting a wider range of customer requirements for spiral welded pipe specifications.

b. By employing an advanced double-sided submerged arc welding process, welding can be performed in the optimal position, minimizing defects such as edge misalignment, weld misalignment, and incomplete penetration, thereby facilitating precise control of weld quality.

c. Conduct 100% quality inspections on steel pipes, ensuring that the entire production process is under effective testing and monitoring, thereby effectively guaranteeing product quality.

d. All equipment along the entire production line is equipped with network connectivity to the computer-based data acquisition system, enabling real-time data transmission and allowing the central control room to monitor and manage process‑related technical parameters.

Stacking principles require

1. The principle for stacking spiral steel pipes is to ensure stable and secure stacking while organizing materials by grade and specification; materials of different grades must be stacked separately to prevent mixing and mutual corrosion.

2. It is prohibited to store materials that are corrosive to steel around the stacking areas of spiral welded steel pipes.

3. The base of the spiral steel pipe stack shall be elevated, stable, and level to prevent the material from becoming damp or deforming.

4. Materials of the same type shall be stacked separately according to their order of receipt.

5. For spiral‑welded steel pipes and structural steel stacked outdoors, wooden pallets or stone strips must be placed underneath; the stack should be slightly inclined to facilitate drainage, and the materials must be laid evenly and straight to prevent bending or deformation.

6. Stacking height for spiral steel pipes: no more than 1.2 m for manual handling and no more than 1.5 m for mechanical handling; stack width shall not exceed 2.5 m.

7. A certain clearance shall be maintained between stacks; inspection aisles are typically 0.5 m wide, while access aisles depend on the size of the materials and the handling equipment, generally ranging from 1.5 to 2.0 m.

8. Open-air stockpiling of angle steel and channel steel should be laid flat with the open side facing down; I-beams should be stored upright, and the flanges of the steel members must not face upward to prevent water accumulation and rusting.

9. Elevate the stack base: if the warehouse has a sun‑exposed concrete floor, raise it by 0.1 m; if it is an earthen floor, elevate it by 0.2–0.5 m. For open‑air storage areas, raise the concrete surface by 0.3–0.5 m and the sand‑clay surface by 0.5–0.7 m.

Before leaving the factory, spiral welded steel pipes shall undergo mechanical property testing, flattening tests, and flaring tests, and must meet the requirements specified in the relevant standards. The quality inspection methods for straight-seam steel pipes are as follows:

1. On the surface, this involves visual inspection. Visual inspection of welded joints is a simple and widely used method, serving as an important part of final‑product inspection, primarily aimed at identifying surface defects and dimensional deviations. Typically, it is performed by naked eye, with the aid of standard templates, gauges, and magnifying glasses. If surface defects are detected, there is a corresponding possibility that internal defects may also be present.

2. Physical Testing Methods: Physical testing methods rely on specific physical phenomena to perform measurements or inspections. For examining internal defects in materials or workpieces, nondestructive testing (NDT) is typically employed. Common NDT techniques include ultrasonic testing, radiographic testing, penetrant testing, and magnetic particle testing.

3. Strength Testing of Pressure Vessels: In addition to leak‑tightness testing, pressure vessels must also undergo strength testing. The two most common methods are hydrostatic testing and pneumatic testing. Both can verify the weld integrity of vessels and pipelines operating under pressure. Pneumatic testing is more sensitive and faster than hydrostatic testing, and it eliminates the need for draining after the test, making it particularly suitable for products that are difficult to drain. However, pneumatic testing carries greater risks than hydrostatic testing. During testing, appropriate safety measures must be strictly observed to prevent accidents.

4. Tightness Testing: For welded vessels used to store liquids or gases, non‑tight defects in the welds—such as through‑going cracks, porosity, slag inclusions, incomplete penetration, and porosity—can be detected by tightness tests. Common tightness testing methods include the kerosene test, the water‑filling test, and the hydrostatic pressure test.

5. Hydrostatic Test: Each steel pipe shall undergo a hydrostatic test without any leakage. The test pressure shall be calculated according to the following formula: P = 2ST/D, where S is the test stress in MPa; the test stress for the hydrostatic test shall be taken as 60% of the minimum yield strength specified in the relevant steel strip standard (235 MPa for Q235). Pressure‑holding time: For pipes with D < 508 mm, the test pressure shall be maintained for no less than 5 seconds; for pipes with D ≥ 508 mm, the test pressure shall be maintained for no less than 10 seconds. 4. Nondestructive Testing: Repair welds on steel pipes, butt joints of steel strips, and circumferential seams shall be inspected by X-ray or ultrasonic testing. For spiral-welded steel pipes used for conveying flammable ordinary fluids, 100% X-ray or ultrasonic inspection shall be performed. For spiral-welded steel pipes used for conveying ordinary fluids such as water, sewage, air, and heating steam, X-ray or ultrasonic inspection shall be conducted on a sampling basis (20%).

According to the quality inspection results of spiral welded steel pipes, they are typically classified into three categories:合格品 (qualified products), 返修品 (repaired products), and 废品 (scrap). Qualified products are those whose appearance and internal quality meet the relevant standards or the technical specifications for delivery and acceptance. Repaired products are those whose appearance and internal quality do not fully comply with the standards and acceptance criteria but may be repaired to meet these requirements after rework. Scrap refers to products that fail to meet the required standards in either appearance or internal quality and cannot be repaired, or whose repaired condition still does not satisfy the standards and acceptance criteria.

Scrap is further classified into two types: internal scrap and external scrap. Internal scrap refers to defective spiral steel pipes identified within the foundry or casting workshop, while external scrap consists of defects discovered after delivery—often becoming apparent only during machining, heat treatment, or service—and typically results in far greater economic losses than internal scrap. To minimize external scrap, it is advisable to conduct sample‑based trial heat treatments and rough machining on batch‑produced spiral steel pipes prior to shipment, thereby identifying potential defects at the manufacturing facility and enabling timely implementation of corrective measures.