Straight Seam Welded Pipe Manufacturing Process

Straight seam welded pipes are steel pipes formed by bending steel plates or steel strips and then welding along the longitudinal axis. They are mainly divided into two production routes: ERW/HFW high-frequency resistance welding (for medium and small diameters) and LSAW straight seam submerged arc welding (for large diameters and thick walls). Here is a complete and detailed analysis of the entire production process.
 

1. Raw material acceptance and inspection
- Raw materials: Hot rolled coil (ERW), medium-thick steel plates (LSAW), materials such as Q235, Q355, X42 to X80 pipeline steel, etc.

- Inspection:

• - Verify the quality certificate (chemical composition, mechanical properties)
• - Full plate ultrasonic testing (plate inspection): Check internal layering, inclusions, and cracks in the steel plates.
• - Visual inspection: No rust, dents, scratches, or wave-like deformation.

2. Pre-treatment of steel plates/steel strips
- ERW process (steel strips):

1. Uncoiling: Uncoiling of steel coils onto the machine
2. Straightening: Multi-roller straightening machine to eliminate roll stress
3. Edge cutting/milling: Shearing or milling of the sides to ensure consistent width and straight edges
4. Edge-to-edge welding: Flash welding of the head and tail of the steel strip to achieve continuous production
5. Live coil storage: Buffering for continuous production

- LSAW Process (Steel Plate):

1. Cutting to Fixed Length: Cut the steel plate according to the length of a single pipe.
2. Edge Milling: Mill both sides, processing standard bevels (such as V-shaped), controlling burrs and angles.
3. Pre-bending: Use a pre-bending machine to pre-bend the edges of the plate to ensure proper joint alignment and prevent "pear-shaped" deformation.
 

1. ERW/HFW High Frequency Welding - Continuous Roll Forming
- Spindle arrangement / Multi-stand continuous forming:

• The steel strip undergoes 20 to 36 forming rolls, gradually transforming from sheet → open pipe blank → jointed circular pipe
• Forming section: Rough forming → Fine forming → Extrusion section
• Key controls: Edge alignment, edge deviation ≤ 0.15t, ellipticity

2. LSAW Submerged Arc Welding - JCOE/UOE Forming (Large Diameter)
- JCO Forming (Commonly Used):
1. J Forming: Half of the steel plate is stepped stamped into a J shape
2. C Forming: The other half is stamped into a C shape
3. O Forming: The entire piece is joined together to form an open-ended circular tube
4. Expansion (E): Full-length mechanical expansion to increase the size accuracy and eliminate internal stress

1. ERW/HFW High-frequency Resistance Welding (without filler metal)
- Principle: Utilizing high-frequency current, skin effect + proximity effect
- Edge is rapidly heated to 1300-1500℃ to a molten state
- Compression roller applies pressure, forging and fusing to form a metallurgical bond between base materials
- Process:
1. High-frequency welding: Induction/contact type high-frequency welding machine
2. Deburring: Scrapers on the inside and outside of the weld seam remove weld spurs
3. Welding heat treatment (optional): Medium-frequency annealing to eliminate welding stress

2. LSAW Straight Seam Submerged Arc Welding (Double-sided Multi-layer)
1. Pre-welding (Fixing Welding): MAG gas shielded welding for continuous seam alignment
2. Inner Welding: Multi-wire submerged arc welding inside the pipe (1-4 wires), with automatic welding gun
3. Outer Welding: Multi-wire submerged arc welding outside the pipe (1-4 wires)
- Parameters: Current 500-1000A, Voltage 30-38V, Speed 0.5-1.2m/min

1. Cooling and Cutting
- Air-cooling / Water-cooling to room temperature
- Cutting to the specified length (by flying saw / sawing)
2. Dimensional Finishing
- ERW:
- Roundening/Constant Diameter Unit: Ensures outer diameter accuracy
- Straightening: 2-4 roll straightening machine
- LSAW:
- Mechanical Expansion (E Process): Full-length expansion, ellipticity ≤ ±0.5%D
- Pipe End Roundening, Flat Head Chamfering
3. Pipe end processing
- Chamfering/Beveling: Create welding bevels (30° - 37.5°) for ease of on-site welding
 

1. Ultrasonic Testing (UT)
- 100% inspection of plate inspection and weld seams: cracks, incomplete penetration, slag inclusion
2. X-ray/γ-ray Testing (RT)
- 100% industrial television/photography of weld seams, according to API/GB standards
3. Hydrostatic Test
- Sequential pressure testing: P = 2St/D (S = yield strength, t = wall thickness, D = outer diameter)
- Maintain pressure for 5 to 10 seconds, no leakage, no deformation
4. Non-Destructive Testing (NDT)
- Magnetic Particle/Phonon (MT/PT): surface cracks
5. Physical and Chemical Tests (Batch)
- Tensile, bending, impact, metallographic, hardness
 

1. Surface treatment
- Rust removal: Shot blasting / Sandblasting (Sa2.5 grade)
- Corrosion protection: Painting, Galvanizing, 3PE anti-corrosion layer (Oil and gas pipelines)
2. Identification and packaging
- Marking: Specifications, material, furnace number, standard, trademark
- Packaging, weighing, storage

- Forming: Offset dimension, ellipticity, edge alignment
- Welding: Temperature, extrusion force, speed, weld height
- Heat Treatment: Annealing temperature, holding time (stress relief)
- Non-destructive Testing: 100% UT/RT, no leakage under water pressure
- Expansion: Expansion rate 0.5% - 1.5% (LSAW)