| Detailed Introduction

　　Spiral welded steel pipe is manufactured from coiled strip steel, formed by cold extrusion, and welded using an automatic double-wire, double-sided submerged-arc welding process. In the production of spiral welded steel pipe, the strip steel is fed into the pipe‑welding unit; as it passes through multiple sets of rollers, it gradually coils to form a circular tube blank with an open seam. By adjusting the downward pressure of the squeeze rolls, the weld gap is maintained within 1–3 mm, and the ends of the weld are brought into flush alignment.

　　(1) The raw materials—steel strip coils, welding wire, and flux—are all subjected to rigorous physicochemical inspections prior to use. (2) The ends of the steel strip are butted together and welded using single‑wire or dual‑wire submerged arc welding; after coiling into a steel pipe, automatic submerged arc welding is employed for seam repair. (3) Before forming, the steel strip 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 and stable strip transport. (5) Roll‑forming is carried out either with external control or internal control. (6) A weld gap‑control device is utilized to ensure that the weld gap meets welding requirements; pipe diameter, misalignment, and weld gap are all strictly controlled. (7) Both internal and external welds are performed using American Lincoln electric welding machines, employing either single‑wire or dual‑wire submerged arc welding, thereby achieving consistent and reliable weld quality. (8) All completed welds are inspected online by an automated continuous ultrasonic flaw detector, guaranteeing 100% nondestructive testing coverage of the spiral welds. If defects are detected, an automatic alarm is triggered and markings are applied; production personnel promptly adjust process parameters to eliminate defects. (9) Steel pipes are cut into individual lengths using an air‑plasma cutting machine. (10) After cutting into individual pipes, each batch undergoes a stringent first‑piece inspection, evaluating weld mechanical properties, chemical composition, fusion quality, and surface condition, followed by nondestructive testing. Only after confirming compliance with manufacturing standards is the batch officially released for production. (11) Areas marked by continuous ultrasonic inspection along the welds are re‑examined manually using ultrasonic and X‑ray techniques; if defects are confirmed, they are repaired and then subjected to further nondestructive testing until the defects are fully eliminated. (12) Welds at butt joints of the steel strip and T‑joints where they intersect with the spiral welds are all inspected via X‑ray fluoroscopy or radiography. (13) Each steel pipe undergoes a hydrostatic pressure test, with pressure applied radially sealed. Test pressure and duration are strictly controlled by a microcomputer‑based hydraulic testing system, and test parameters are automatically printed and recorded. (14) Pipe ends are machined to precisely control end‑face perpendicularity, bevel angle, and root face.

There are two primary ways to specify spiral steel pipe dimensions: one uses diameter × wall thickness, and the other employs the nominal diameter (DN). The former is the most common; diameter refers to the outer diameter of the spiral steel pipe, typically denoted by the symbol “Ф,” while wall thickness is indicated in millimeters (mm). For example, a spiral steel pipe with an outer diameter of 219 mm and a wall thickness of 6 mm would be expressed as 219×6.