The welding of Q500NH weathering steel requires special considerations to maintain its corrosion resistance and mechanical properties. Below are the key requirements and recommendations for the welding process:
1. Pre-Welding Preparation
Material Certification: Ensure the base metal and filler metal comply with GB/T 4171 (Chinese weathering steel standard) or equivalent.
Preheat Treatment:
Preheat to 100–150°C (depending on thickness) to avoid hydrogen-induced cracking (HIC).
For thick plates (>25mm), higher preheat (150–200°C) may be needed.
Joint Design: Use groove welds (V or U-type) for full penetration, avoiding sharp notches that could trap moisture and accelerate corrosion.
2. Welding Methods & Filler Metals
Recommended Processes:
Shielded Metal Arc Welding (SMAW) – E5018-W (low-hydrogen electrodes).
Gas Metal Arc Welding (GMAW/MIG) – ER70S-6 wire with 75% Ar + 25% CO₂ shielding gas.
Flux-Cored Arc Welding (FCAW) – E71T-1C JH4 (weathering-grade flux-core wire).
Filler Metal Selection:
Match or overmatch the strength (e.g., E550NH filler for Q500NH base metal).
Ensure filler metals contain Cu, Cr, Ni to maintain weathering resistance.
3. Welding Parameters & Control
Heat Input: Limit to 15–25 kJ/cm to avoid excessive softening in the heat-affected zone (HAZ).
Interpass Temperature: Keep below 250°C to prevent grain coarsening.
Post-Weld Slow Cooling: Allow gradual cooling to minimize residual stresses.
4. Post-Weld Treatment
Cleaning: Remove slag and spatter to prevent crevice corrosion.
Corrosion Protection:
Apply weathering-resistant primers on weld seams if needed.
Avoid paint systems that trap moisture (use breathable coatings).
Inspection:
Dye penetrant (PT) or ultrasonic (UT) testing for critical welds.
Check for lack of fusion, cracks, or porosity.
5. Special Considerations for Weathering Performance
Weld Seam Corrosion: The HAZ and weld metal may initially rust faster than the base metal. A uniform patina will form over time, but sealing welds with weathering-resistant coatings can help.
Avoid Contamination: Prevent oil, grease, or chloride residues (common in coastal areas) during welding.
