Stainless Steel Welding

1. Classification and Composition

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2. Schaeffler Diagram and Microstructure Prediction

The Schaeffler diagram predicts the weld metal microstructure from the chromium equivalent (Creq) and nickel equivalent (Nieq):

$$Cr_{eq} = Cr + Mo + 1{.}5 Si + 0{.}5 Nb$$

$$Ni_{eq} = Ni + 30C + 0{.}5 Mn$$

For austenitic grades, a delta ferrite (δ) content of 3–8 FN (Ferrite Number) in the weld metal is recommended to prevent hot cracking without compromising corrosion resistance. Ferrite is measured with a ferritescope (magnetic method, calibrated per EN ISO 8249) or estimated from the WRC-1992 diagram.

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3. Sensitisation — The Main Risk in Austenitic Grades

Sensitisation occurs when austenitic steel is exposed to the 425–850 °C range, where chromium precipitates as chromium carbides (Cr₂₃C₆) at grain boundaries:

$$Cr_{23}C_6 \rightarrow \text{Cr depletion in zones adjacent to grain boundaries}$$

The depleted zone (< 10.5 % Cr) loses passivation and becomes susceptible to intergranular corrosion. In the HAZ, this precipitation typically occurs 5–15 mm from the fusion line.

Preventive measures:

1. Low-carbon grades — Use L grades (e.g. 304L, 316L; C ≤ 0.03 %).
2. Stabilised grades — Use Ti-stabilised (321) or Nb-stabilised (347) grades.
3. Heat input control — Minimise time in the sensitisation range. Interpass temperature ≤ 150 °C.
4. Post-weld solution annealing — 1050–1100 °C followed by rapid water quench.

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4. Ferritic Steels — Embrittlement and Grain Growth

Ferritic grades present two specific risks:

• Irreversible grain growth in the HAZ near the fusion line (T > 1100 °C): ferritic grain grows rapidly and does not refine on cooling.
• 475 °C embrittlement: prolonged exposure causes precipitation of a Cr-rich phase.

Recommendations: minimise heat input; use Ti- or Nb-stabilised consumables; PWHT at 750–800 °C.

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5. Martensitic Steels — Mandatory Preheat

Typical protocol (EN 1011-3):

• Preheat: 200–300 °C
• Interpass temperature: maintain above preheat
• PWHT: temper at 650–750 °C immediately after welding, before full cooling

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6. Duplex Steels — Phase Balance

Critical parameters (EN ISO 15614-1 for duplex):

• Heat input: 0.5–2.5 kJ/mm
• Interpass temperature: ≤ 150 °C
• Consumable: 2–4 % excess Ni over base metal to promote austenite reprecipitation
• Purge gas: pure Argon at root to prevent Cr oxidation

Excessively fast cooling results in insufficient austenite. Excessive heat input causes intermetallic phase precipitation (sigma σ, chi χ phases at 700–950 °C), embrittling the steel.

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7. Contamination and Ferroxyl Test

Iron particle contamination causes localised galvanic corrosion. The ferroxyl test (potassium ferricyanide + nitric acid solution) detects free iron: intense blue colouration confirms contamination. Use dedicated stainless steel tools only.

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8. Consumable Selection — EN ISO 3581 and EN ISO 14343

For dissimilar joints between stainless and carbon steel, ER309L is commonly used — its higher Cr and Ni content accommodates dilution while maintaining an austenitic weld deposit.

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References

1. EN 1011-3:2000 — *Welding. Recommendations for welding of metallic materials. Part 3: Arc welding of stainless steels.* CEN, Brussels.
2. EN ISO 3581:2003 — *Welding consumables. Covered electrodes for manual metal arc welding of stainless and heat resisting steels.* ISO/CEN.
3. EN ISO 14343:2009 — *Welding consumables. Wire electrodes for arc welding of stainless and heat resisting steels.* ISO/CEN.
4. Lancaster, J.F. — *Metallurgy of Welding.* 6th ed. Abington Publishing, Cambridge, 1999.
5. Chiaverini, V. — *Aços e Ferros Fundidos.* 7ª ed. ABM, São Paulo, 2002.