Duplex and Super Duplex Welding: Ferrite Control and Why It Matters
Duplex and super duplex stainless steels derive their properties from a balanced 50/50 austenite-ferrite microstructure. Welding disrupts this balance — too much ferrite causes brittleness and reduces corrosion resistance; too little means the SCC benefits of duplex disappear. Ferrite Number (FN) control is not optional — it is a contractual requirement on PED Category III–IV and NACE MR0175 qualified orders.
Why Duplex Microstructure Is Unstable During Welding
The duplex structure (α-ferrite + γ-austenite) is produced by solution annealing at 1020–1100°C and rapid quenching. During welding, the heat-affected zone (HAZ) passes through a fully ferritic temperature range above ~1200°C before austenite re-precipitates on cooling. The rate of austenite re-formation depends on heat input, interpass temperature, and the nitrogen content of the filler. If the weld cools too fast, austenite cannot nucleate sufficiently and the weld metal remains excessively ferritic (FN >80). If heat input is too high or interpass temperature too elevated, secondary phases (sigma phase, chi phase) precipitate — destroying toughness and pitting resistance.
Target Ferrite Number Range
| Grade | Target FN (weld deposit) | NACE ISO 15156-3 limit | Action if out of range |
|---|---|---|---|
| Duplex 2205 (1.4462) | FN 30–65 | FN 30–70 | Reject weld — re-weld or solution anneal |
| Super Duplex 2507 (1.4410) | FN 35–65 | FN 30–70 | Reject weld — re-weld or solution anneal |
FN is measured using a calibrated ferritescope on the weld deposit and HAZ. It is not the same as volume-% ferrite, but correlates closely at the typical ranges for duplex. The measurement is non-destructive and should be performed on every production weld in critical service — not just on WPS qualification coupons.
Filler Selection
Duplex 2205: ER2209 (AWS A5.9) — over-alloyed with 9%Ni to compensate for austenite deficit in the weld. Do not substitute ER316L (wrong chemistry, wrong FN). Do not use ER2209 for super duplex — it is under-alloyed for 2507.
Super Duplex 2507: ER2594 (AWS A5.9) — 25Cr-9Ni-4Mo-0.25N. This is the only acceptable filler for 2507 in critical service. ER2209 is NOT acceptable for 2507 buttweld joints. The filler chemistry difference is substantial and using ER2209 on 2507 will produce a weld with insufficient PRE and likely excessive ferrite.
Heat Input and Interpass Temperature
For duplex 2205: heat input 0.5–2.5 kJ/mm, interpass temperature ≤250°C. For super duplex 2507: tighter — heat input 0.5–1.5 kJ/mm, interpass temperature ≤100°C. The narrower window for 2507 reflects its higher alloy content and greater sensitivity to sigma phase precipitation, which forms rapidly at 700–900°C in 2507.
These limits must be stated in the Welding Procedure Specification (WPS) and monitored during production. A single interpass overshoot to 120°C on 2507 does not automatically fail the weld — but it should trigger a ferrite check and corrosion test on the relevant weld.
Impact of Sigma Phase on Corrosion Resistance
Sigma phase (σ) precipitates at grain boundaries in the temperature range 650–950°C — the range that a poorly controlled interpass temperature maintains the metal in during multi-pass welding. Sigma phase is chromium- and molybdenum-rich, so its precipitation depletes the surrounding matrix of these elements, sharply reducing PRE and creating sites for preferential pitting. A weld with sigma phase can look perfectly acceptable visually and have correct FN — but fail ASTM G48 Method A pitting corrosion test. G48 is the standard qualification test for duplex and super duplex welds on corrosion-critical projects.
What to Specify on the Purchase Order
- State the filler grade explicitly: ER2209 for 2205; ER2594 for 2507
- Require ferritescope measurement report, FN per weld joint, target FN 30–70
- Specify ASTM G48 Method A corrosion testing on WPS qualification coupons (minimum 24 hours at the specified temperature — 22°C for 2205, 40°C for 2507)
- State interpass temperature limit in the PO technical supplement: ≤250°C (2205) or ≤100°C (2507)
- For NACE MR0175 qualification: reference ISO 15156-3 and require FN certificate per weld joint