1. Standard Composition (JIS G3125 SPA-H)
| Element | Composition Range (%) | Primary Role |
|---|---|---|
| C (Carbon) | ≤ 0.12% | Strength ↑, Weldability ↓ |
| Si (Silicon) | 0.25–0.75% | Deoxidation, Corrosion Resistance ↑ |
| Mn (Manganese) | 0.20–0.50% | Strength ↑, Sulfur Fixation |
| P (Phosphorus) | 0.07–0.15% | Corrosion Resistance ↑, Brittleness ↑ |
| S (Sulfur) | ≤ 0.035% | Machinability ↑, Toughness ↓ |
| Cu (Copper) | 0.25–0.55% | Key for Patina Formation, Corrosion Resistance ↑ |
| Cr (Chromium) | 0.30–1.25% | Oxidation Resistance ↑, Strengthening |
| Ni (Nickel) | ≤ 0.65% | Toughness ↑, Corrosion Resistance ↑ |
| Others (Al, Nb, V) | Trace | Grain Refinement, Precipitation Hardening |
2. Role of Each Element & Its Effects
(1) Carbon (C) – ≤ 0.12%
Pros:
Increases strength and hardness.
Cons:
Reduces weldability (risk of cold cracking).
Solution: Low carbon content in SPA-H ensures better welding performance.
(2) Silicon (Si) – 0.25–0.75%
Effects:
Acts as a deoxidizer (removes oxygen during steelmaking).
Enhances corrosion resistance by stabilizing the rust layer.
Increases strength but may reduce toughness if excessive.
(3) Manganese (Mn) – 0.20–0.50%
Key Roles:
Counteracts sulfur's brittleness (forms MnS inclusions).
Improves hardenability and strength.
(4) Phosphorus (P) – 0.07–0.15%
Unique in Weathering Steel:
Forms a dense, protective rust layer (patina).
Drawback: Increases brittleness at low temperatures (requires controlled levels).
(5) Sulfur (S) – ≤ 0.035%
Negative Impact:
Forms MnS inclusions, reducing toughness and ductility.
Solution: Keep sulfur levels low (modern steelmaking minimizes S).
(6) Copper (Cu) – 0.25–0.55%
Critical for Weathering Resistance:
Promotes stable rust layer formation (patina).
Disadvantage: May cause hot shortness (cracking during hot working).
(7) Chromium (Cr) – 0.30–1.25%
Benefits:
Enhances oxidation and atmospheric corrosion resistance.
Strengthens the steel by forming carbides.
(8) Nickel (Ni) – ≤ 0.65%
Key Contributions:
Improves toughness and low-temperature impact resistance.
Helps counteract copper-induced hot cracking.
(9) Trace Elements (Al, Nb, V)
Aluminum (Al):
Deoxidizer, refines grain structure.
Niobium (Nb) & Vanadium (V):
Precipitation hardening, improves strength and toughness.
3. Practical Considerations
Welding SPA-H:
Use low-hydrogen electrodes (E7018-W).
Preheat if thickness > 20mm (to avoid HAZ cracking).
Painting (if needed):
Weathering steel usually left unpainted, but if coated, use breathable primers.
