1. Machining (Cutting, Drilling, Milling)
Tool Wear:
Higher alloy content (Cu, Cr) increases hardness, leading to 10–15% faster tool wear compared to Q355B.
Recommendation: Use carbide-tipped tools and reduce cutting speeds by 10%.
Surface Quality:
Produces smooth edges but may require deburring due to increased toughness.
2. Welding
Weldability:
Good with proper techniques, but prone to hydrogen-induced cracking (HIC) if procedures are not controlled.
Key Practices:
Electrodes: Use low-hydrogen electrodes (e.g., E5015-G for SMAW, ER50-6 for GMAW).
Preheating: Required for thicknesses >20 mm (preheat to 100–150°C to avoid HIC).
Post-Weld Heat Treatment (PWHT): Not mandatory but recommended for critical structures to relieve stress.
Weld Corrosion Resistance:
Weld zones may lose weathering properties; use weathering-resistant filler metals to match base metal performance.
3. Forming (Bending, Rolling)
Cold Forming:
Ductility: Adequate for most bends (elongation ≥22%), but higher yield strength requires 20% more force than Q355B.
Minimum Bend Radius: 2× thickness for thicknesses ≤16 mm (vs. 1.5× for Q355B).
Hot Forming:
Can be heated to 850–1100°C for complex shapes; avoid temperatures below 700°C to prevent embrittlement.
4. Surface Treatment
Pre-Rusting for Aesthetics:
Sandblasting (SA 2.5) followed by chemical oxidation (e.g., copper sulfate spray) accelerates patina formation.
Painting/Coating:
Generally unnecessary due to self-protecting rust layer, but required if aesthetic consistency is critical (e.g., architectural cladding).
5. Corrosion Performance During Processing
Interim Protection:
Unpainted Q355NH may develop superficial rust during storage/fabrication; this does not harm final performance.
For long-term storage, cover or apply temporary inhibitors to avoid heavy scaling.
6. Comparison with Q355B
| Process | Q355NH | Q355B |
|---|---|---|
| Tool Wear | Moderate (10–15% higher than Q355B) | Low |
| Welding | Requires low-H electrodes and preheating (>20 mm) | Standard electrodes, no preheating needed |
| Bending Force | Higher (due to ≥355 MPa yield strength) | Lower |
| Corrosion Post-Processing | Self-protecting; no painting needed | Requires painting/coating |
7. Best Practices for Processing
Storage: Keep dry to prevent excessive rust before fabrication.
Cutting: Use plasma/laser cutting for clean edges; avoid flame cutting for thin plates (<10 mm).
Welding: Dry electrodes thoroughly and maintain interpass temperature (200°C max).
Forming: For tight bends, anneal or heat to 600–800°C to reduce springback.
8. Applications and Examples
Success Cases:
Used in bridges (e.g., Beijing-Zhangjiakou High-Speed Railway) with welded connections.
Architectural facades (e.g., Shenzhen Bay Stadium) with curved panels formed via cold rolling.
Limitations: Avoid use in continuous immersion or high-chloride splash zones without additional protection.
Conclusion
Q355NH offers excellent corrosion resistance and adequate processability for most structural applications, though it demands slightly more attention during welding and forming than ordinary carbon steel. Its key advantage-eliminating lifelong painting-outweighs minor processing adjustments.
