1. Raw Material Preparation
Material source: Stainless steel coils or plates (grades 304, 316L, 321, duplex, etc.).
Slitting: Wide coils are slit into strips matching the required pipe circumference.
Edge trimming: Edges are precisely sheared to ensure good weld seam quality.
Inspection: Check chemical composition, surface quality (free of scratches, pits, or oil).
2. Forming
Equipment: Continuous forming mill with multiple roller stands.
Process:
The flat strip passes through rollers that gradually bend it into a tubular shape.
For round pipe → rolls progressively form the strip into a circle.
For square/rectangular pipe → formed from round pipe then reshaped in sizing rolls.
Control: Ensure perfect alignment of strip edges for welding.
3. Welding
Welding methods:
TIG (GTAW): Widely used for thin-wall precision tubes, especially in food/pharma.
Plasma arc welding: Higher penetration, suitable for thicker walls.
High-Frequency Induction Welding (HFIW): High-speed production of industrial piping.
Key points:
Shielding gas (argon, argon + hydrogen) prevents oxidation.
Welding parameters (current, voltage, speed) carefully controlled for full penetration.
Weld seam must fuse completely without porosity or inclusions.
4. Weld Bead Treatment
External weld bead: Ground or polished off if smooth appearance is required.
Internal weld bead: For sanitary pipes, orbital grinders smooth the inner weld to eliminate crevices where bacteria could accumulate.
5. Heat Treatment (Solution Annealing)
Reason: Welding creates a heat-affected zone (HAZ) with stress and reduced corrosion resistance.
Process:
Temperature: 1050–1150 °C.
Furnace: Continuous bright annealing furnace with hydrogen/nitrogen protective atmosphere.
Cooling: Rapid water quench or gas cooling to avoid carbide precipitation.
Effect: Restores uniform microstructure, ductility, and corrosion resistance.
6. Sizing and Straightening
Sizing: Pipes pass through sizing rolls to achieve precise OD and wall thickness.
Tolerance: ±0.3% typical on OD.
Straightening: Multi-roll straighteners ensure straightness deviation <1 mm/m.
7. Cutting
Equipment: Flying saws, abrasive cutters, or laser cutting machines.
Lengths: Standard 6 m, 12 m, or custom.
Ends:
Plain ends.
Beveled ends (for butt welding in pipelines).
Threaded ends (for mechanical fittings).
8. Surface Treatment
Pickling: Acid cleaning (HF + HNO₃) removes oxide scale from welding and annealing.
Polishing:
Mechanical polishing → decorative/structural pipes.
Electro-polishing → high-purity, smooth inner surface for pharma/semiconductor.
Passivation: Forms protective chromium oxide film to maximize corrosion resistance.
9. Inspection and Testing
Dimensional checks: OD, wall thickness, length, ovality.
Visual inspection: Weld appearance, surface defects.
Non-Destructive Testing (NDT):
Eddy Current Test → detects surface flaws in weld zone.
Ultrasonic Test → checks weld penetration and internal soundness.
Hydrostatic Test: Pipes pressurized with water (up to 10–15 MPa) to verify leak-tightness.
Standards: ASTM A269, A312, A778, EN 10217-7, JIS G3448.
10. Finishing and Packaging
End finishing: Deburring, chamfering, plastic end caps.
Marking: Grade, size, standard, heat number.
Packaging: Bundled with steel straps, plastic wrapping, wooden cases for export.
11. Applications of Welded Stainless Steel Pipes
Construction & architecture: Railings, handrails, decorative tubing.
Fluid transport: Water, gas, low/medium pressure liquids.
Food & pharmaceutical: Sanitary piping, dairy, brewery, clean-in-place (CIP) systems.
Chemical industry: Process lines not requiring seamless strength.
Mechanical structures: Furniture, frames, automotive exhaust systems.
