AISI 321 and AISI 316Ti are two austenitic stainless steels produced on the same principle: both are stabilised with titanium. The difference between them lies in the grade they are based on — 321 is a 304 derivative, 316Ti a 316 derivative.
What does stabilisation mean?
When austenitic stainless steels spend a long time in the 450–850 °C range, carbon combines with chromium and forms chromium carbides at the grain boundaries. Chromium is depleted in that zone and intergranular corrosion begins. Lowering the carbon (the L grades) is one solution; the second is adding titanium. Titanium ties up the carbon, chromium stays free and the protective layer is not damaged.
The difference from the L grades is this: low carbon provides protection after welding, but it is not enough if the part is going to work at high temperature continuously. Titanium-stabilised grades stay stable under those conditions too. That is why 321 and 316Ti are preferred for exhaust systems, heat exchangers and furnace components.
What makes the difference: molybdenum
There is no molybdenum in the composition of AISI 321; chromium is 17.00–19.00% and nickel 9.00–12.00%. AISI 316Ti contains 2.00–2.50% molybdenum and its nickel rises to 10.50–13.50%. Molybdenum provides resistance to pitting corrosion in chloride-bearing environments.
Comparison in numbers
| PROPERTY | AISI 321 | AISI 316Ti |
|---|---|---|
| Carbon (C) | max 0.08% | max 0.08% |
| Chromium (Cr) | 17.00 – 19.00% | 16.50 – 18.50% |
| Nickel (Ni) | 9.00 – 12.00% | 10.50 – 13.50% |
| Molybdenum (Mo) | — | 2.00 – 2.50% |
| Sulphur (S) | max 0.03% | max 0.015% |
| Tensile strength Rm | 515 MPa | 500 – 700 MPa |
| Yield strength Rp0.2 | 205 MPa | > 200 MPa |
| Elongation A50 | 40% | 40 / 30% |
| Hardness | max 217 HB | max 215 HB |
| Thermal conductivity | 16.1 W/m·K | 15 W/m·K |
| Thermal expansion | 16.6 µm/m·K | 15.9 × 10⁻⁶/K |
| UNS | S32100 | S31635 |
| W.Nr (DIN/EN) | 1.4541 | 1.4571 |
| AMS | 5510 · 5557 · 5559 · 5570 · 5576 · 5645 | 5507 · 5524 · 5573 · 5584 · 5648 · 5649 · 5653 |
| ASTM | A240 · A276 · A314 · A484 | A276 · A479 · A484 · A240 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
AISI 321 is sufficient — exhaust manifolds, heat exchanger tubes, furnace and boiler components, hot air lines in aerospace. It is the right choice for high-temperature applications without a chloride load. If the specification calls out AMS 5510 or AMS 5557, the grade being asked for is 321.
AISI 316Ti is required — where high temperature and a chloride or acid load occur together: chemical plants, the paper industry, hot lines in marine environments, process equipment.
Neighbouring grades
For the low-carbon approach instead of stabilisation, see our AISI 316 – AISI 316L comparison; for the effect of molybdenum, see our AISI 304 – AISI 316 comparison. For higher temperatures there are AISI 310 and AISI 314.
Technical datasheets
AISI 321 — chemical composition, mechanical values, standard equivalents
AISI 316Ti — 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.