Overview
Both AISI 321 and AISI 316Ti are titanium-stabilized austenitic stainless steels designed to resist intergranular corrosion after welding or high-temperature service. While they share similar base elements (chromium, nickel, titanium), their key difference lies in molybdenum content — AISI 316Ti contains 2–3% Mo, while AISI 321 does not. This distinction fundamentally shapes their corrosion resistance profiles and optimal applications.
Chemical Composition Comparison
| Element | AISI 321 (12Cr18Ni10Ti) | AISI 316Ti (08Cr17Ni13Mo2T) |
|---|---|---|
| Carbon (C) | ≤ 0.08% | ≤ 0.08% |
| Chromium (Cr) | 17.0–19.0% | 16.0–18.0% |
| Nickel (Ni) | 9.0–12.0% | 10.0–14.0% |
| Molybdenum (Mo) | — | 2.0–3.0% |
| Titanium (Ti) | 5×C ≤ Ti ≤ 0.70% | 5×C ≤ Ti ≤ 0.70% |
| Manganese (Mn) | ≤ 2.0% | ≤ 2.0% |
| Silicon (Si) | ≤ 1.0% | ≤ 1.0% |
Corrosion Resistance — The PREN Difference
The PREN (Pitting Resistance Equivalent Number) formula = %Cr + 3.3×%Mo + 16×%N quantifies resistance to pitting corrosion:
- AISI 321: PREN ≈ 18.0 (Cr ~18%, Mo = 0)
- AISI 316Ti: PREN ≈ 25.3 (Cr ~17%, Mo ~2.5%)
A PREN > 24 is generally considered suitable for seawater service. AISI 316Ti achieves this threshold, while AISI 321 does not. In chloride-containing environments such as chemical processing plants, marine equipment, and pipelines, AISI 316Ti significantly outperforms AISI 321.
Both grades resist intergranular corrosion in the heat-affected zone (HAZ) after welding due to titanium stabilization. Ti preferentially forms TiC, preventing chromium carbide precipitation at grain boundaries.
Mechanical Properties
| Property | AISI 321 | AISI 316Ti |
|---|---|---|
| Tensile Strength (Rm) | ≥ 515 MPa | ≥ 515 MPa |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa |
| Elongation (A) | ≥ 40% | ≥ 40% |
| Density | 7.93 g/cm³ | 8.00 g/cm³ |
Temperature Performance
- AISI 321 can operate continuously to 900°C (intermittently to 950°C) in oxidizing environments. The titanium stabilization prevents harmful chromium carbide formation at these temperatures.
- AISI 316Ti is typically rated to 870°C in oxidizing conditions. Above this, molybdenum can form volatile oxides, reducing protective scale stability.
For high-temperature applications without chlorides, AISI 321 is the preferred choice. For applications requiring both temperature resistance and chloride tolerance, 316Ti provides a superior balance.
International Standards
| Standard | AISI 321 | AISI 316Ti |
|---|---|---|
| GOST (Russia) | 12Cr18Ni10Ti | 08Cr17Ni13Mo2T |
| GB (China) | 06Cr18Ni11Ti (0Cr18Ni10Ti) | 06Cr17Ni12Mo2Ti |
| DIN/EN | 1.4541 (X6CrNiTi18-10) | 1.4571 (X6CrNiMoTi17-12-2) |
| UNS | S32100 | S31635 |
Weldability
Both grades are excellent for welding without post-weld heat treatment. Titanium stabilization eliminates the need for solution annealing after welding to restore intergranular corrosion resistance. Recommended filler metals:
- AISI 321: ER321 or E321-16 electrodes
- AISI 316Ti: ER316L or E316L-16 electrodes
Cost Comparison
AISI 316Ti is typically 15–30% more expensive than AISI 321 due to the molybdenum content, which is a costly alloying element. The price difference varies with market Mo prices, product form (sheet, tube, bar), and order volume. If chloride resistance is not required, AISI 321 offers a more economical solution while retaining excellent high-temperature and general corrosion performance.
Selection Guide Summary
- Need chloride/pitting resistance? → Choose AISI 316Ti
- Need maximum temperature capability in air? → Choose AISI 321 (up to 900°C continuous)
- Marine or chemical plant environments? → Choose AISI 316Ti (higher PREN)
- Budget-sensitive, no chloride exposure? → Choose AISI 321
- Furnace components, heat exchangers in oxidizing air? → Choose AISI 321