1. Mandatory Use of Weathering-Grade Welding Consumables
Shielded Metal Arc Welding (SMAW, the most common method for SMA570W): Select electrodes that replicate SMA570W's alloy profile. The industry standard is AWS A5.5 E8018-G (or equivalent Japanese JIS Z3211 D5018-G). These electrodes contain 0.20–0.40% Cu and 0.30–0.60% Cr, ensuring the weld forms a uniform patina with the base metal.
Gas Metal Arc Welding (GMAW): If using GMAW, opt for AWS A5.18 ER80S-G wires. Avoid generic high-strength consumables (e.g., E7018, ER70S-6) that lack Cu/Cr-these will cause the weld to develop orange rust within months, while the SMA570W base metal retains its dark protective patina.
Pre-consumable check: Verify the consumable's material certificate to confirm Cu/Cr content; even small deviations (e.g., Cu <0.20%) can compromise long-term weathering.
2. Strict Control of Heat Input and Interpass Temperature
Heat input range: Maintain heat input between 15–35 kJ/cm (calculate as: [Voltage × Current × 60] / [Travel speed (mm/min)]). For common thicknesses:
10–15mm plates (SMAW): Use 3.2mm E8018-G electrodes, set current to 120–150A, voltage to 22–24V, and travel speed to 10–12 cm/min (heat input ≈20–25 kJ/cm).
20–30mm plates (SMAW): Switch to 4.0mm electrodes, current 160–180A, voltage 24–26V, travel speed 12–14 cm/min (heat input ≈28–32 kJ/cm).
Interpass temperature limit: Do not exceed 250°C between welding passes. Use a contactless temperature gun to monitor the HAZ-temperatures above 250°C will soften the HAZ, lowering its yield strength by 10–15% (to <500MPa) and increasing brittleness at low temperatures (e.g., impact energy at -40°C may drop below 30J).
3. Thorough Surface Preparation to Eliminate Contaminants
Cleaning scope: Remove all contaminants from a 25mm-wide area on both sides of the weld joint (extending beyond the joint ensures no residues are pulled into the weld pool).
Cleaning methods:
Use stainless steel wire brushes or sandblasting to remove mill scale and light rust. Avoid carbon steel brushes-they leave iron particles that act as "rust seeds" on the SMA570W surface.
Wipe the cleaned area with acetone or isopropyl alcohol to eliminate oil, grease, or salt residues (critical for coastal or industrial environments, where salts accelerate corrosion).
Pre-weld inspection: Ensure the cleaned surface has a uniform metallic luster with no visible stains; any remaining contaminants should be re-cleaned before welding.
4. Post-Weld Treatment to Support Patina Formation
Remove weld residues: Immediately after welding, use a stainless steel brush or needle gun to clear slag, spatter, and flux residues. These residues trap moisture and block oxygen from reacting with Cu/Cr in the weld metal-preventing patina formation.
Neutralize acidic fluxes: If using acidic electrodes (rare for SMA570W, but possible), wipe the weld area with a 5–8% baking soda (sodium bicarbonate) solution to neutralize any remaining acidity. Rinse thoroughly with clean water and air-dry-acidic residues can etch the steel surface, dissolving corrosion-resistant elements.
Avoid over-grinding: Only grind the weld to repair defects (e.g., cracks, undercut). Over-grinding removes the outer layer of the weld metal (which is rich in Cu/Cr), exposing a sub-layer with lower weathering resistance. If grinding is necessary, use a fine-grit abrasive (≥120 grit) and blend the ground area smoothly with the base metal to avoid sharp edges that trap moisture.
5. Critical Post-Weld Inspections
Visual inspection: Check the weld for no porosity, cracks, undercut (depth ≤0.5mm), or incomplete penetration-these defects act as starting points for corrosion and reduce structural integrity.
Mechanical testing (for critical structures): For load-bearing components (e.g., bridge girders), test weld coupons to confirm:
Tensile strength: Weld yield strength ≥540MPa (95% of SMA570W's base metal yield strength).
Impact toughness: ≥34J at -40°C (matching SMA570W's base metal requirement).
Patina uniformity check: After 3–6 months of outdoor exposure, inspect the weld and base metal together. A uniform dark brown/black patina across both areas indicates successful welding; uneven coloring (e.g., orange rust on the weld) means consumables or post-weld treatment were improper.
