Comprehensive Analysis of Welded Pipe Technology: From Process Principles to Practical Applications

1. Overview of Welded Pipes

Welded pipes are manufactured by rolling steel plates or strips into cylindrical shapes and welding the seams. Widely used in construction, energy, transportation, and mechanical manufacturing, they offer advantages such as high production efficiency, low cost, and flexible specifications compared to seamless steel pipes. The core technologies involve material selection, forming processes, welding techniques, and quality control, making them indispensable basic materials in modern manufacturing.

2. Classification and Characteristics of Welded Pipes

(1) Classification by Forming Process

1. Straight Seam Welded Pipe

· Formed by longitudinally rolling steel plates/strips, with straight welds along the pipe axis.

· Common processes: High-Frequency Resistance Welding (HFRW), Submerged Arc Welding (SAW), Tungsten Inert Gas Welding (TIG).

· Features: Fast production speed (up to 200m/min), suitable for medium-small diameters (Φ10-Φ1200mm), widely used in construction scaffolding and fluid pipelines.

1. Spiral Welded Pipe

· Rolled with a spiral angle (18°-36°), resulting in helical welds.

· Main process: Spiral Submerged Arc Welding (SSAW), capable of producing large diameters (Φ219-Φ3620mm) and thick walls (6-25mm).

· Features: Uniform stress distribution in welds, strong impact resistance, commonly used in long-distance oil/gas pipelines (e.g., West-East Gas Pipeline Project).

(2) Classification by Welding Method

· Electric Resistance Welding (ERW): Utilizes high-frequency current to generate resistive heat for melting metal without filler material. Narrow welds (0.5-1mm) and high efficiency make it suitable for thin-walled pipes (wall thickness ≤12mm).

· Arc Welding: Includes Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), and SAW. Requires filler wire, offering high weld strength for thick-walled pipes and complex conditions.

3. Core Production Process Analysis

(1) Forming Process

1. Roll Forming

· Gradually bends flat plates into pipes using multiple roller sets. Key parameters: roll design (pass system), forming speed (5-200m/min), edge alignment accuracy (≤0.2mm).

· Technical challenges: Controlling roundness (ovality ≤1%) and weld gap (0.5-1.5mm) to avoid cracking during forming.

1. Pre-Bending and Straightening

· Pre-bending: Pre-shapes steel strip edges to reduce weld stress concentration;

· Straightening: Eliminates longitudinal bending via tension straighteners, with straightness ≤1.5mm/m.

(2) Welding Process Control

1. HFRW Key Parameters

· Frequency: 100-400kHz (high frequency for thin walls, low frequency for thick walls);

· Welding pressure: 5-20kN (ensures weld fusion);

· Upset speed: 10-30mm/s (removes flash).

1. SAW Technical Points

· Uses granular flux to cover the arc, with deep penetration (3-8mm). Controls welding current (600-1500A), voltage (28-40V), and speed (30-80cm/min) to prevent porosity and slag inclusions.

(3) Post-Processing

· Weld Grinding: Removes internal/external flash, achieving surface roughness Ra≤12.5μm;

· Non-Destructive Testing (NDT): 100% ultrasonic testing (UT) for internal defects, hydrostatic testing (pressure ≥1.5×design pressure) for tightness;

· Anti-Corrosion Treatment: Galvanizing (coating thickness ≥85μm), epoxy powder spraying (300-500μm), enhancing corrosion resistance.

4. Quality Control and Standards

(1) Key Testing Indicators

1. Mechanical Properties: Tensile strength (≥300MPa), yield strength (≥235MPa), elongation after fracture (≥25%);

1. Geometric Dimensions: Outer diameter tolerance (±1%), wall thickness tolerance (±10%), length tolerance (±5mm);

1. Surface Quality: No cracks, scabs, or folds allowed; scratch depth ≤0.5mm.

(2) International Standards

· API Spec 5L: Pipeline steel for oil/gas industry, divided into PSL1/PSL2 grades;

· EN 10219: European standard for welded steel tubes in structures, specifying bend, flattening, and flaring test requirements;

· GB/T 3091: Chinese standard for galvanized welded pipes for low-pressure fluid transport, defining zinc coating adhesion test methods.

5. Application Fields and Technological Innovations

(1) Typical Applications

· Construction: Rectangular/square steel tubes for steel structures (e.g., Q345B material, section size 50×50-600×600mm), bridge support pipes;

· Energy: Electrofusion welded steel pipes for shale gas extraction (H₂S-resistant, complying with NACE MR0175), stainless steel welded pipes for nuclear plants (316L, cleanliness Class 1);

· Automotive: High-precision welded pipes for chassis (wall thickness tolerance ±0.05mm, roundness ≤0.1mm), reducing weight by 20%-30%.

(2) Cutting-Edge Technologies

1. Intelligent Production: AI visual inspection systems with ≥99% defect recognition accuracy, combined with industrial robots for fully automatic grinding;

1. High-Performance Materials: Development of high-strength low-alloy steel (e.g., X80, X100) pipes with yield strength 550-690MPa for high-pressure gas transmission;

1. Green Processes: Promotion of chromium-free passivation and water-based coatings, with welding fume purification efficiency ≥95% and energy consumption reduced by ≥15%.

6. Future Development Trends

Driven by global infrastructure and high-end manufacturing demands, welded pipe technology will evolve toward "high precision, high strength, intelligence, and greenness." Through optimized die design (using CAE simulation), new welding power sources (e.g., pulsed plasma welding), and full-process digital management systems, the industry will enhance product quality stability and production efficiency, providing critical material support for advanced equipment manufacturing and clean energy development.