1. Welding Process Selection
2. Filler Metal Selection (Critical for Corrosion Match)
Core Rule: Use weathering‑grade fillers with Cu/P/Cr/Ni to ensure the weld's rust layer matches the base metal and avoids galvanic corrosion.
SMAW: E7016‑G (AWS) / J507CrNiCu (GB); dry at 350–400°C for 1–2 hours, hold at 100–150°C in a thermal oven.
GMAW/FCAW: ER50‑6NiCu (GMAW) or E81T1‑W2C (FCAW); store in sealed containers to prevent moisture absorption.
GTAW: ER50‑6NiCu (0.8–1.2 mm diameter) for thin sheets and root passes.
3. Pre‑Weld Preparation
Surface Cleaning: Remove mill scale, rust, oil, and moisture within 20 mm of the joint using a wire brush, grinder, or solvent (acetone/degreaser) to prevent porosity and hydrogen cracking.
Joint Design:
Butt joints: Square groove for t ≤6 mm (root gap 1–2 mm); V‑groove (60–70° included angle, 1–2 mm root face) for t >6 mm.
Fillet joints: Leg length = 0.7× plate thickness; lap joints min. overlap 25 mm.
Tack welds: Use the same filler as the main weld; length 10–15 mm, spacing 150–200 mm; remove defective tacks before main welding.
Preheating:
t ≤16 mm, temp ≥5°C: No preheat needed.
t >16 mm or ambient ≤0°C: Preheat to 80–150°C (use a pyrometer to verify); for t >25 mm, preheat to 100–150°C.
Preheat the entire joint area (≥50 mm on both sides) uniformly to avoid thermal gradients.
4. Welding Parameters (Heat Input & Interpass Control)
Heat Input: Keep at 15–25 kJ/cm (avoid >30 kJ/cm) to prevent grain coarsening and reduced toughness in the HAZ.
SMAW: 3.2 mm electrode → 100–140 A, 22–26 V, travel speed 150–200 mm/min.
GMAW: 1.0 mm wire → 180–220 A, 24–28 V, 20–25 L/min Ar+CO₂, travel speed 200–300 mm/min.
Interpass Temperature: Maintain at 80–150°C (max ≤200°C) to avoid HAZ overheating; cool with forced air if exceeding the limit.
Arc & Travel: Use short arcs to reduce spatter and porosity; maintain consistent travel speed for uniform bead shape.
Multi‑Pass Welding: Clean slag thoroughly between passes; grind any defects (e.g., undercut, incomplete fusion) before the next pass.
5. Post‑Weld Treatment
Immediate Treatment:
Remove slag, spatter, and weld spatter with a hammer, wire brush, or grinder; grind smooth any undercut >0.5 mm.
Back‑gouge single‑sided welds to remove root defects, then re‑weld the back side for full penetration.
Stress Relief:
For thick sections (t >20 mm) or high‑stress structures: Post‑weld heat treatment (PWHT) at 550–620°C for 1–2 hours per 25 mm thickness, followed by furnace cooling to ≤150°C before air cooling.
For thin sections (t ≤16 mm): No PWHT needed; use low‑hydrogen processes and proper preheat to control residual stress.
Corrosion Protection:
Do NOT paint the entire weld; instead, apply a weathering steel passivator or patina accelerator to the weld and HAZ to promote uniform rust layer formation.
In high‑salt coastal areas: Apply a thin matte transparent glaze to the weld to enhance corrosion resistance without hiding the rust aesthetic.
