316L vs 317L Stainless Steel: Key Differences Explained
Two Low-Carbon Molybdenum Austenitic Grades
316L and 317L are both low-carbon versions of the molybdenum-bearing austenitic family. The L means a maximum carbon content of 0.03%, which prevents chromium carbide precipitation during welding and preserves corrosion resistance in the heat-affected zone. The two grades share the same basic structure but differ in alloying level: 317L is the higher-alloyed, more corrosion-resistant and more expensive option.
Chemical Composition Comparison
| Element (wt %) | 316L (S31603 / 1.4404) | 317L (S31703 / 1.4438) |
|---|---|---|
| Carbon (C) | ≤ 0.03 | ≤ 0.03 |
| Chromium (Cr) | 16.0 – 18.0 | 18.0 – 20.0 |
| Nickel (Ni) | 10.0 – 14.0 | 11.0 – 15.0 |
| Molybdenum (Mo) | 2.0 – 3.0 | 3.0 – 4.0 |
| Manganese (Mn) | ≤ 2.00 | ≤ 2.00 |
| Silicon (Si) | ≤ 0.75 | ≤ 0.75 |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 | ≤ 0.030 |
| Nitrogen (N) | ≤ 0.10 | ≤ 0.10 |
Mechanical and Physical Properties
| Property | 316L | 317L |
|---|---|---|
| Tensile strength (min) | 485 MPa | 595 MPa |
| Yield strength (min) | 170 MPa | 260 MPa |
| Elongation (min) | 40% | 35% |
| Hardness (max) | 217 HB | 217 HB |
| Density | 7.99 g/cm³ | 7.98 g/cm³ |
| Melting range | 1370 – 1400°C | 1370 – 1400°C |
Corrosion Resistance: Where 317L Earns Its Premium
The extra molybdenum and chromium raise 317L's pitting resistance equivalent number and its performance in reducing media. 317L is the conventional choice for flue-gas desulfurization (FGD) systems handling hot, chloride-laden scrubber slurries, for pulp and paper bleaching circuits, and for processes holding hot organic and fatty acids where 316L shows pitting. For chloride-containing environments with high demands, the nitrogen-alloyed variant 317LMN (UNS S31726) with molybdenum raised to 4.0-5.0% offers even higher resistance. In marine, food and pharmaceutical service, 316L remains the cost-effective standard.
Equivalent Designations
| System | 316L | 317L |
|---|---|---|
| UNS | S31603 | S31703 |
| EN | 1.4404 / X2CrNiMo17-12-2 | 1.4438 / X2CrNiMo18-15-4 |
| JIS | SUS316L | SUS317L |
| GB | 022Cr17Ni12Mo2 | 022Cr19Ni13Mo3 |
Application Guidance for Buyers
Use 316L for general chemical and petrochemical process equipment, pressure vessels, heat exchangers, marine and offshore components, food and beverage lines, pharmaceutical systems and water treatment.
Specify 317L when the process stream contains hot chlorides, sulfuric or sulfurous acids, organic and fatty acids, or when previous 316L installations have shown pitting or crevice corrosion.
Compare lifecycle cost, not just material price: 317L can eliminate unscheduled shutdowns in aggressive service, while 316L remains the economical choice for benign environments.
Confirm the product form standard (ASTM A240 plate, A312 pipe, A269 tubing) and require EN 10204 3.1 certificates.
Common Misconceptions
"316L and 317L are interchangeable." They are not: the molybdenum gap changes performance in chlorides and reducing acids.
"Higher strength means better corrosion resistance." The strength difference is a consequence of alloying; corrosion resistance is decided by chemistry and surface condition.
"317L is only for flue-gas plants." It also serves pulp and paper, chemical tankers, dyeing and textile equipment, and pharmaceutical crystallizers.
Frequently Asked Questions
What is the main difference between 316L and 317L?
Molybdenum content: 2.0-3.0% for 316L versus 3.0-4.0% for 317L, plus higher chromium and nickel in 317L, giving better pitting and crevice corrosion resistance.
Is 317L stronger than 316L?
Yes, per standard minimums: 595 MPa vs 485 MPa tensile, 260 MPa vs 170 MPa yield.
When should I choose 317L over 316L?
In hot chloride environments, sulfuric/sulfurous acid service, pulp bleaching and flue-gas desulfurization, or where 316L has already pitted.
Are 316L and 317L weldable?
Both weld easily; the low carbon prevents sensitization. Use matching filler (ER316L / ER317L) and keep heat input moderate.
Is 317L magnetic?
Essentially non-magnetic when annealed, like all austenitic grades; heavy cold work may induce slight magnetism.
What is 317LMN?
A higher-alloyed version (UNS S31726) with 4.0-5.0% molybdenum and added nitrogen for the most demanding chloride service.
