X2CrNiMo18-14-3 · Ni 12.5–15% · Delta Ferrite ≤1 FN · PRE ~26
316L with elevated nickel (12.5–15%) to suppress delta ferrite to ≤1 FN — the defining requirement for nuclear PWR primary circuit piping, pharmaceutical WFI systems, and fatigue-critical pressure boundaries. Standard 316L (1.4404) can contain 3–8 FN delta ferrite; 1.4435 eliminates this by design. EN 10204 3.2 (TÜV) mill certificates typically required.
Standard 316L austenitic steel contains 3–8 FN delta ferrite as a deliberate design feature — delta ferrite prevents hot cracking during welding and solidification. However, in certain service conditions, residual delta ferrite is harmful:
Delta ferrite transforms to brittle sigma phase under neutron irradiation over decades. PWR primary circuit fittings require ≤1 FN to prevent irradiation embrittlement of joints.
Delta ferrite has different electrochemical potential to austenite — creates micro-galvanic corrosion pits that harbour contamination. WFI systems specify ≤0.5 FN or ≤1 FN for cleanability.
Delta ferrite — austenite interfaces act as fatigue crack initiation sites under cyclic loading. Safety-critical pressure boundaries in fatigue service specify ≤1 FN to maximise fatigue life.
| System | Designation |
|---|---|
| EN Werkstoffnummer | 1.4435 |
| EN Grade Name | X2CrNiMo18-14-3 |
| UNS Number | S31603 (shared with 316L — W.Nr. distinguishes) |
| ASTM Fittings | A403 WP316L (must specify 1.4435 explicitly) |
| Delta Ferrite | ≤1 FN (ferrite number) — measured per AWS A4.2 |
| Certificate | EN 10204 3.2 (TÜV/Lloyd's) typically required |
| Filler (GTAW) | ER316L — select low-ferrite heat |
| Element | 1.4404 (316L) | 1.4435 (Nuclear) | Effect |
|---|---|---|---|
| Ni | 10.0–13.0% | 12.5–15.0% | Higher Ni → fully austenitic → delta ferrite suppressed to ≤1 FN |
| Cr | 16.5–18.5% | 16.5–18.5% | Same |
| Mo | 2.0–2.5% | 2.5–3.0% | Slightly higher Mo — PRE ~26 |
| C | ≤0.030% | ≤0.030% | Both low-carbon |
| PRE | ~24 | ~26 | Marginal chloride improvement vs 316L |
| Delta ferrite (FN) | 3–8 FN typical | ≤1 FN | The defining difference |
