I. Welding Characteristics and Difficulties Analysis

• Large-diameter (DN300+) bellows often feature multi-layer stainless steel structures (3-10 layers, wall thickness 0.5-2mm), requiring prevention of interlayer fusion defects during welding;
• Corrugated structures create narrow welding spaces, especially at crests/valleys where stress concentration occurs (stress concentration factor up to 3-5 times).

• Common materials like 304/316L stainless steel have low thermal conductivity (16W/m·K), causing rapid local temperature rise and susceptibility to intergranular corrosion (must control dwell time in sensitization range 450-850℃);
• Thin-walled structures (δ≤2mm) are prone to significant thermal deformation, with ordinary arc welding causing deformation rates of 0.5-1%, requiring special process control.

• Sealing weld leakage rate ≤1×10⁻⁹Pa·m³/s (helium mass spectrometry standard);
• Weld joint tensile strength ≥90% of base metal, elongation ≥30% (GB/T 13452.2 standard).

II. Welding Material and Equipment Selection

III. Core Steps of Welding Process

1. Pre-welding Pretreatment

• Groove machining:
  • Machined by mechanical cutting (flame cutting prohibited), V-groove angle 60±5°, root face 1±0.5mm, gap 2-3mm (DN500 example);
  • Groove surface roughness Ra≤12.5μm, cleaned with acetone, sanded to metallic luster.
• Preheating requirements:
  • When ambient temperature <0℃, preheat to 50-80℃ (monitored by infrared thermometer), interlayer temperature ≤150℃ for multi-layer welding.

2. Welding Implementation Process

• Multi-layer welding strategy:

  Layer	Welding method	Current (A)	Voltage (V)	Speed (mm/min)	Key points
  Base layer	TIG pulse	100-120	10-12	80-100	Backside argon protection, penetration rate ≥95%
  Filler layer	TIG DC	140-160	12-14	60-80	Each layer thickness ≤1.5mm, interpass slag cleaning
  Cap layer	TIG pulse	110-130	10-12	90-110	Pulse frequency 20Hz, reduces undercut and reinforcement

• Anti-deformation process:
  • Adopt symmetric segmental welding (each segment 50-100mm), welding sequence like 'valley first, crest later' to reduce radial shrinkage stress;
  • Rigid fixing: use special tooling clamps (stainless steel) to fix bellows, deformation controlled ≤0.3mm/m.

3. Post-welding Treatment

• Stress relief:
  • Low-temperature annealing (300-350℃×2h), furnace cooling to 100℃ then air cooling, eliminating >90% welding stress;
• Surface treatment:
  • Pickling and passivation (nitric acid + hydrofluoric acid mixture), passivation film thickness 2-3μm, salt spray test ≥1000h without corrosion;
• Nondestructive testing:
  • 100% X-ray flaw detection (GB/T 3323-2019, Grade II qualified), supplemented by dye penetrant testing (MT) for surface crack inspection.

IV. Welding Technology for Special Working Conditions

V. Common Welding Defects and Prevention Measures

VI. Key Points of Welding Quality Control

1. Process qualification:
   • Conduct welding procedure qualification (PQR) according to ASME IX or GB/T 19866, covering parameters like wall thickness, diameter, and material combinations;
2. Welder qualification:
   • Certified welders must pass 45° fixed pipe all-position welding assessment (test piece δ=2mm, DN300), with X-ray qualification rate ≥98%;
3. Process monitoring:
   • Real-time record welding current, voltage, interlayer temperature (e.g., using data loggers), adjust immediately if deviation exceeds ±5%;

VII. Application of Automated Welding Technology

• Laser welding:
  • Suitable for thin-walled bellows (δ≤1mm), welding speed 1-2m/min, heat-affected zone width ≤0.1mm, deformation ≤0.1mm/m;
• Robotic TIG welding:
  • Adopt visual tracking system (accuracy ±0.1mm), suitable for complex corrugated trajectories, welding efficiency 3x higher than manual welding, defect rate <0.5%;

VIII. Industry Standards and Case References

• Execution standards:
  • National standard GB/T 12777-2019 General Technical Conditions for Metal Bellows Expansion Joints;
  • American standard EJMA-9 Expansion Joint Manufacturers Association Standard.
• Typical case:
  • A DN1000 bellows compensator (316L, -196℃) for an LNG receiving station, using pulse TIG welding + backside argon purging, passed 1.5x design pressure (1.6MPa) hydrostatic test without leakage, low-temperature impact test (-196℃) impact energy ≥27J.