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 (High-Ni) | 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 |
DN15–DN600 and larger. Outside diameters follow the EN 10220 / EN ISO 1127 pipe series; wall thickness is matched to your connecting pipe.
| Size | OD (mm) |
|---|---|
| DN 15 | 21.3 |
| DN 20 | 26.9 |
| DN 25 | 33.7 |
| DN 32 | 42.4 |
| DN 40 | 48.3 |
| DN 50 | 60.3 |
| DN 65 | 76.1 |
| DN 80 | 88.9 |
| DN 100 | 114.3 |
| DN 125 | 139.7 |
| DN 150 | 168.3 |
| DN 200 | 219.1 |
| DN 250 | 273.0 |
| DN 300 | 323.9 |
| DN 350 | 355.6 |
| DN 400 | 406.4 |
| DN 450 | 457.0 |
| DN 500 | 508.0 |
| DN 600 | 610.0 |
EN 1.4435 has higher nickel (12.5–15% vs 10.0–13.0% for 1.4404). This higher Ni shifts the Schaeffler constitution diagram fully into the austenite field, limiting delta ferrite content to ≤1 ferrite number (FN). Standard 316L (1.4404) can contain 3–8 FN delta ferrite. In nuclear PWR primary circuits, delta ferrite transforms to a brittle sigma phase over decades of irradiation, causing embrittlement. In pharmaceutical WFI service, delta ferrite creates corrosion sites that can harbour contamination. In fatigue-critical applications, delta ferrite phases provide initiation sites for fatigue cracks.
1.4435 has elevated nickel (12.5–15%) to keep delta ferrite at 1 FN or less, whereas standard 316L (1.4404) can contain 3–8 FN.
X2CrNiMo18-14-3, with a PRE of about 26.
Supplying an Indian boiler or ASME B16.9 project? The ASTM equivalent is listed on our group site ibrfittings.com: ASTM A403 WP316L fittings.
