Why Duplex Stainless Steel Welding Is Different
Duplex stainless steels (DSS) contain a roughly 50/50 mixture of austenite and ferrite phases. However, this dual-phase microstructure is sensitive to thermal cycling during welding. Incorrect welding procedures can shift the phase balance, precipitate harmful intermetallic phases, and severely degrade both corrosion resistance and mechanical properties.
The two most common families are:
- Duplex (22% Cr): UNS S31803/S32205 (SAF 2205) — the workhorse grade
- Super Duplex (25% Cr): UNS S32750/S32760 (SAF 2507) — for more demanding environments
Welding Metallurgy Fundamentals
Phase Balance
- Target ferrite content in weld metal: 30–60% (FN 30–60)
- Minimum austenite: 30% for adequate toughness and corrosion resistance
- Maximum ferrite: 70% — excessive ferrite reduces toughness
Harmful Phase Precipitation
- Sigma (σ) phase: Forms at 600–1000°C. Hard, brittle, eliminates corrosion resistance.
- Chi (χ) phase: Similar to sigma, forms at 650–950°C
- 475°C embrittlement: Ferrite phase hardens at 400–540°C
- Cr₂N (chromium nitride): Forms in HAZ when nitrogen is lost. Reduces pitting resistance.
Filler Metal Selection
| Base Metal | GMAW/GTAW Wire | SMAW Electrode |
|---|---|---|
| 2205 (S31803) | ER2209 | E2209-17 |
| 2507 (S32750) | ER2594 | E2594-17 |
| 2304 (S32304) | ER2304 | E2304-17 |
Shielding Gas Requirements
GTAW (TIG)
- Standard: Ar + 2% N₂
- Root side purge: Ar + 2% N₂ (nitrogen prevents Cr₂N formation)
GMAW (MIG)
- Standard: Ar + 2% CO₂ + 1.5% N₂ or Ar + 5% CO₂ + 1% N₂
Heat Input Control
| Grade | Thickness | Min (kJ/mm) | Max (kJ/mm) |
|---|---|---|---|
| 2205 | ≤ 10mm | 0.3 | 1.5 |
| 2205 | 10–25mm | 0.5 | 2.5 |
| 2205 | > 25mm | 0.5 | 3.0 |
| 2507 | ≤ 10mm | 0.2 | 1.0 |
| 2507 | 10–25mm | 0.3 | 2.0 |
| 2507 | > 25mm | 0.4 | 2.5 |
Interpass Temperature Control
- Maximum for 2205: 150°C (300°F)
- Maximum for 2507: 100°C (212°F)
- Use temperature-indicating sticks between passes
Welding Process Recommendations
| Process | Suitability | Notes |
|---|---|---|
| GTAW (TIG) | Excellent | Best for root pass and thin sections |
| GMAW (MIG) | Good | Higher deposition rate. Spray transfer preferred. |
| FCAW | Good | High deposition for fill/cap passes |
| SAW | Good | For thick sections only |
Post-Weld Treatment
- Solution annealing: 1,020–1,100°C followed by water quench
- Pickling: HNO₃/HF acid mixture to remove heat tint
- Passivation: Citric acid or nitric acid treatment
Welding Defect Prevention
- Porosity: Clean surfaces, ensure adequate gas coverage
- Hot cracking: Uncommon in DSS but can occur with incorrect filler selection
- Hydrogen cracking: Use low-hydrogen consumables and maintain minimum heat input
- Lack of fusion: DSS has higher viscosity. Ensure adequate heat input.
CoreMetal Steel — Duplex Stainless Steel Products
CoreMetal Steel supplies duplex (2205/S31803) and super duplex (2507/S32750) in plate, sheet, pipe, tube, bar, and fittings. All products include mill test certificates showing phase balance and PREN values.
Email: tracy@coremetalsteel.com | Phone: +86 18291910632