Stainless Steels: Types and Weldability
Introduction
Stainless steels are iron-chromium alloys with a minimum of 10.5% Cr, which forms a passive layer of chromium oxide (Cr₂O₃) responsible for corrosion resistance. Welding these steels requires specific precautions that differ significantly from carbon steels.
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Stainless Steel Families
1. Austenitic (300 Series)
Typical composition: 16–26% Cr, 6–22% Ni, C ≤ 0.08% Examples: 304 (1.4301), 316 (1.4401), 321 (1.4541)
Main problem — Sensitisation: When slowly cooled through 450–850°C, carbon precipitates as Cr₂₃C₆ at grain boundaries, depleting chromium → intergranular corrosion. Solutions:
- Low-carbon grades: 304L, 316L (C ≤ 0.03%)
- Stabilised grades: 321 (Ti), 347 (Nb)
2. Ferritic (400 Series)
Typical composition: 10.5–30% Cr, no Ni Examples: 430 (1.4016), 444 (1.4521)
Problems: irreversible grain growth in the HAZ (>900°C) → embrittlement; 475°C embrittlement in high-Cr alloys. No phase transformation → no grain refinement by heat treatment.
Measures: low heat input, preheat 150–250°C for C > 0.06%.
3. Martensitic (400 Series)
Examples: 410 (1.4006), 420 (1.4021), 431 (1.4057)
Problems: high tendency for hard martensite in HAZ; cold cracking without preheat. Mandatory preheat: 200–300°C. Recommended PWHT: temper at 650–750°C.
4. Duplex (Austenitic-Ferritic)
Examples: 2205 (1.4462), 2507 (1.4410 — super duplex)
Problems: sigma phase (σ) precipitation between 600–1000°C → severe embrittlement; loss of ferrite/austenite balance in HAZ. Controlled heat input: 0.5–2.5 kJ/mm (2205). Interpass temperature: max. 150°C. Over-alloyed Ni consumables to restore austenite.
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Shielding Gases
| Process | Recommended gas | Notes |
|---|---|---|
| MIG/MAG | Ar + 2–3% CO₂ or Ar + 2% O₂ | Avoid CO₂ > 5% (sensitisation) |
| TIG | Pure Ar (99.99%) | No CO₂ |
| Plasma | Pure Ar | — |
| Purge (root) | Pure Ar or N₂ | Always protect root face |
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Contamination and Weld Hygiene
- Never share tools with carbon steel
- Dedicated stainless steel wire brushes only
- Clean with acetone before and after welding
- Passivation after welding (citric acid or passivation paste)
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Reference Standards
- EN 1011-3:2000 — Welding. Recommendations for welding of metallic materials. Part 3: Arc welding of stainless steels
- EN 10088-1 — Stainless steels. Part 1: List of stainless steels
- EN ISO 3581 — Welding consumables. Covered electrodes for manual metal arc welding of stainless steels
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