SMAW (stick welding): Choose E7016‑G (AWS standard) or J507CrNiCu (GB standard) low-hydrogen electrodes. Dry the electrodes at 350–400°C for 1–2 hours before use, and keep them in a thermal insulation barrel during welding to prevent moisture absorption (a key factor for cold cracking).
GMAW (MIG) / FCAW: Use ER50‑6NiCu solid wire or E81T1‑W2C flux-cored wire, with an Ar+20–25% CO₂ shielding gas mixture to ensure arc stability and reduce porosity.
GTAW (TIG): Adopt ER50‑6NiCu wire for root pass welding of thin plates (≤6mm) to ensure precise penetration and clean welds.
Surface Cleaning: Remove rust, oil, mill scale, and moisture within 20mm of the weld joint using a wire brush, grinder, or acetone degreaser. Contaminants can cause porosity, incomplete fusion, and hydrogen-induced cracking.
Joint Design Optimization:
For plates ≤6mm: Use square grooves with a 1–2mm root gap for butt joints.
For plates >6mm: Adopt 60–70° V-grooves with a 1–2mm root face to ensure full penetration.
Tack welds: Use the same filler metal as the main weld; tack length 10–15mm, spacing 150–200mm. Remove defective tacks immediately to avoid propagating cracks.
Controlled Preheating:
Plates ≤16mm and ambient temperature ≥5°C: No preheating required.
Plates >16mm or ambient temperature ≤0°C: Preheat the joint area (50mm on both sides) to 80–150°C using a pyrometer to monitor temperature uniformity-this reduces cooling rate and prevents cold cracking in the heat-affected zone (HAZ).
Heat Input Control: Maintain heat input at 15–25 kJ/cm (max ≤30 kJ/cm). Excessive heat input coarsens HAZ grains and reduces toughness; insufficient heat input causes incomplete fusion.
Example parameters: 3.2mm SMAW electrode → 100–140 A, 22–26 V, travel speed 150–200 mm/min; 1.0mm GMAW wire → 180–220 A, 24–28 V, shielding gas flow 20–25 L/min.
Interpass Temperature Management: Keep interpass temperature at 80–150°C (max ≤200°C). Cool the weld zone with forced air if overheated to avoid HAZ overaging.
Welding Operation Norms: Use short arcs to reduce spatter and porosity; adopt multi-pass welding for thick plates, and thoroughly clean slag between passes. Grind any defects (undercut, slag inclusion) before the next pass.
Immediate Post-Weld Cleaning: Remove slag, spatter, and weld beads; grind smooth any undercut deeper than 0.5mm. Back-gouge single-sided welds to remove root defects, then re-weld the back side for full penetration.
Stress Relief Treatment (if needed):
For thick plates (>20mm) or high-stress structures: Perform post-weld heat treatment (PWHT) at 550–620°C for 1–2 hours per 25mm thickness, followed by furnace cooling to ≤150°C before air cooling-this reduces residual stress and cracking risk.
For thin plates (≤16mm): Skip PWHT; rely on low-hydrogen welding and preheating to control stress.
Corrosion Resistance Synchronization: Do not fully paint the weld. Instead, apply a weathering steel passivator or patina accelerator to the weld and HAZ to promote uniform rust layer formation, ensuring the weld's corrosion resistance matches the base metal.
Visual Inspection (VT): Check for surface defects (cracks, porosity, undercut, incomplete fusion). Require no visible cracks, undercut depth ≤0.5mm, and no continuous undercut longer than 100mm.
Non-Destructive Testing (NDT):
Ultrasonic Testing (UT): For plates ≥8mm; meet Grade II per GB/T 11345 to detect internal defects (e.g., lack of penetration, slag inclusion).
Radiographic Testing (RT): For critical load-bearing joints; meet Grade II per GB/T 3323 to identify tiny internal cracks or porosity.
Magnetic Particle Testing (MT): For surface cracks on welds and HAZ, especially for thick plates or low-temperature welding.
Mechanical Property Testing (for WPS qualification): Ensure weld tensile strength ≥490 MPa, impact toughness (at -40°C, V-notch) ≥27 J, and 180° bend test with no cracks.
