AISI 316L is a fully austenitic stainless steel. AISI 318 (duplex 2205), as its name suggests, is two-phase: its structure contains roughly half and half austenite and ferrite. That structural difference changes both the strength and the corrosion behaviour from the ground up.
Nitrogen and low nickel
According to our datasheets, chromium rises to 21.00–23.00% in 318 while nickel falls to 4.50–6.50% — less than half the 10.0–14.0% of 316L. The high chromium forms the ferrite phase, and the reduced nickel supports it. The austenite phase is balanced not so much by nickel as by nitrogen: in 318 nitrogen is in the 0.10–0.22% range.
Nitrogen does not only balance the phases, it also strengthens the material. The result is clear in the mechanical values: where the yield strength of 316L is 170 MPa, that of 318 is 460–500 MPa — roughly three times as much. The tensile strength rises from 485 MPa to 700–900 MPa.
The ferrite phase also provides resistance to stress corrosion cracking. That is the point at which 316L is weakest in hot chloride environments; the duplex structure largely blocks that mechanism. The low nickel content also makes the material more stable against swings in the nickel price.
Comparison in numbers
| PROPERTY | AISI 316L | AISI 318 (Duplex) |
|---|---|---|
| Structure | Austenitic | Austenite + Ferrite (duplex) |
| Carbon (C) | max 0.03% | max 0.03% |
| Chromium (Cr) | 16.0 – 18.0% | 21.00 – 23.00% |
| Nickel (Ni) | 10.0 – 14.0% | 4.50 – 6.50% |
| Molybdenum (Mo) | 2.0 – 3.0% | 2.50 – 3.50% |
| Nitrogen (N) | max 0.10% | 0.10 – 0.22% |
| Tensile strength Rm | 485 MPa | 700 – 900 MPa |
| Yield strength Rp0.2 | 170 MPa | 460 – 500 MPa |
| Elongation A50 | 40% | 20% |
| Density | 8.00 g/cm³ | 7.805 g/cm³ |
| Thermal conductivity | 16.3 W/m·K | 19.0 W/m·K |
| Thermal expansion | 17.5 × 10⁻⁶/K | 13.7 × 10⁻⁶/K |
| UNS | S31603 | S31803 · S32205 |
| W.Nr (DIN/EN) | 1.4404 | 1.4462 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
AISI 316L is sufficient — food and pharmaceutical pipework, general process equipment, welded tanks and pipes, architectural applications. It is far easier to form and to weld; there is no need to watch the heat treatment window as with duplex.
AISI 318 is required — pressure vessels, seawater lines, oil and gas process piping, paper industry equipment, structures working under stress in chloride environments. Because its strength is high, a thinner wall can be used for the same pressure; that reduces the total weight and, in most cases, the cost.
One point to watch: duplex steels become brittle if they spend a long time in the 300–1000 °C range. 318 is not used in high-temperature applications; austenitic grades are preferred there.
A step up
For more aggressive chloride environments there are the super duplex grades AISI F53 and AISI F55. If you want to raise corrosion resistance on the austenitic side, see our AISI 316L – AISI 904L comparison.
Technical datasheets
AISI 316L — chemical composition, mechanical values, standard equivalents
AISI 318 — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.