AISI 317L Stainless Steel
1. Introduction
317L (UNS S31703) is a molybdenum-alloyed low-carbon austenitic stainless steel offering improved resistance to pitting, crevice corrosion, and uniform corrosion compared with 316L. Its elevated molybdenum content of approximately 3.1 wt% — higher than the 2.1 wt% typical of 316L — provides measurably better performance in chloride-bearing and acidic environments, resulting in a pitting resistance equivalent (PRE) of approximately 28 versus 24 for 316L. The low carbon ceiling of ≤0.030 wt% limits carbide precipitation during welding and high-temperature exposure, making 317L suitable for welded assemblies without post-weld annealing in most service conditions.
317L is used predominantly in the United States and Asia, where it is an established selection for chemical processing, oil and gas, and industrial applications requiring corrosion performance beyond the capability of 316L. Its combination of improved pitting resistance, good formability, and reliable weldability makes it a practical upgrade from 316L where moderate increases in corrosive duty are encountered.
317L and 316L: When to Choose
317L and 316L are closely related molybdenum-alloyed austenitic grades distinguished primarily by their molybdenum content. 316L carries 2.0–3.0 wt% Mo and achieves a PRE of approximately 24, while 317L specifies 3.0–4.0 wt% Mo and a PRE of approximately 28. Where service environments involve chloride concentrations, moderately aggressive acids, or mixed chemical media that push 316L towards its corrosion limits, 317L provides a meaningful and cost-effective step up in performance without requiring a change in processing approach or fabrication practice. Where an even greater performance margin is needed — particularly in severe pitting or crevice corrosion service — 317LMN or higher-alloy grades should be considered.
2. Chemical Composition
317L is defined by its elevated molybdenum content relative to 316L and the low carbon ceiling that distinguishes the L-grade variants from their standard-carbon counterparts.
Table 1 — Chemical Composition of 317L (UNS S31703)
| Element | Typical / Limits (wt%) |
|---|---|
| Carbon (C) | 0.02 typical (≤ 0.030) |
| Chromium (Cr) | 18.2 typical |
| Nickel (Ni) | 11.6 typical |
| Molybdenum (Mo) | 3.1 typical (3.0–4.0) |
| Iron (Fe) | Balance |
The molybdenum range of 3.0–4.0 wt% is the defining specification parameter for 317L, providing improved resistance to pitting and uniform corrosion compared with 316L. The absence of a nitrogen requirement distinguishes 317L from the higher-performance 317LMN variant.
3. Mechanical and Physical Properties
317L provides mechanical properties consistent with standard low-carbon austenitic stainless steels. Its performance profile is driven primarily by corrosion resistance rather than high-temperature strength, and its tensile properties are comparable to 316L.
3.1 Tensile Properties at Room Temperature
Table 2 — Mechanical Properties of 317L (Outokumpu Ultra, EN 10088-2)
| Property | Cold Rolled | Hot Rolled | Plate |
|---|---|---|---|
| Min. Yield Strength Rp0.2 | 240 MPa | 220 MPa | 220 MPa |
| Min. Yield Strength Rp1.0 | 270 MPa | 260 MPa | 260 MPa |
| Elongation | ≥ 35% | ≥ 35% | ≥ 40% |
Per ASTM A240, minimum yield strength is 205 MPa (30 ksi), minimum tensile strength 515 MPa (75 ksi), and minimum elongation 40%.
3.2 Proof Strength at Elevated Temperature
Table 3 — Minimum Proof Strength at Temperature (EN 10088-2)
| Temperature | Rp0.2 (MPa) | Rp1.0 (MPa) |
|---|---|---|
| 100°C | 172 | 206 |
| 200°C | 147 | 177 |
| 300°C | 127 | 156 |
| 400°C | 115 | 144 |
| 500°C | 110 | 138 |
3.3 Physical Properties
Table 4 — Physical Properties of 317L (Outokumpu Ultra)
| Property | Value |
|---|---|
| Density | 8.0 g/cm³ / 0.289 lb/in³ |
| Young's Modulus (20°C) | 200 GPa / 29 × 10⁶ psi |
| Coefficient of Thermal Expansion (20–100°C) | 16.0 × 10⁻⁶/K / 8.9 µin/in·°F |
| Thermal Conductivity (20°C) | 14 W/m·K / 8.1 Btu/hr·ft·°F |
| Specific Heat Capacity (20°C) | 500 J/kg·K |
| Electrical Resistivity (20°C) | 0.85 Ω·mm²/m |
| Magnetisable | No |
4. Corrosion Resistance
317L's primary advantage over 316L is its improved resistance to localised and uniform corrosion in chloride-bearing and acidic environments, directly attributable to its higher molybdenum content.
4.1 Pitting and Crevice Corrosion
Pitting resistance is characterised using the pitting resistance equivalent:
317L achieves a PRE of approximately 28 compared with 24 for 316L, reflecting the higher molybdenum contribution. This step-up in PRE translates to a measurably higher critical pitting temperature (CPT) and improved resistance to crevice corrosion initiation in chloride-containing environments. 317L offers slightly higher resistance to pitting and crevice corrosion than 316L, making it appropriate where chloride concentrations or temperatures push 316L towards its performance threshold.
4.2 Resistance to Uniform Corrosion
317L provides good resistance to uniform corrosion in dilute sulphuric acid, phosphoric acid, and organic acid environments. Its elevated molybdenum content extends its useful operating window in these media compared with 316L, particularly at moderate acid concentrations and ambient-to-moderate temperatures.
4.3 Sensitisation and Intergranular Corrosion
The low carbon content (≤0.030 wt%) significantly reduces the risk of chromium carbide precipitation in the sensitisation temperature range (427–850°C). Ultra-range high performance austenitic grades including 317L have such low carbon that the risk of intergranular corrosion from chromium carbide precipitation during welding is considered negligible, allowing welded assemblies to be used without post-weld annealing in most service conditions.
4.4 Stress Corrosion Cracking
317L, in common with all standard austenitic stainless steels, is susceptible to chloride-induced stress corrosion cracking (SCC) at elevated temperatures. Where SCC resistance in aggressive chloride media is a primary requirement, higher-alloy grades such as 317LMN, 904L, or 254 SMO should be considered.
5. Applications
317L is selected for service conditions where 316L has reached its corrosion resistance limits and a step-up in molybdenum content provides a practical, cost-effective solution.
5.1 Chemical Processing
317L is used in chemical process equipment handling moderately aggressive acidic or chloride-bearing streams, including:
- Heat exchangers in dilute acid service
- Reaction vessels and storage tanks
- Piping, valves, and fittings in chemical handling systems
- Evaporators in phosphoric and sulphuric acid service
5.2 Oil and Gas
317L is used in oil and gas processing equipment where moderate chloride exposure and sour environments require improved performance over 316L without the cost premium of higher-alloy grades.
5.3 General Industrial Service
317L is an established grade for general industrial applications in the United States and Asia where moderately corrosive conditions demand reliable performance:
- Industrial heat exchangers
- Pressure vessels and tanks
- Architectural cladding in aggressive atmospheric environments
6. Welding
317L is suitable for welding using standard austenitic stainless steel processes. Its low carbon content minimises sensitisation risk, and no post-weld annealing is required in most service conditions.
6.1 Recommended Processes
- Gas Tungsten Arc Welding (GTAW/TIG) — preferred
- Gas Metal Arc Welding (GMAW/MIG)
- Shielded Metal Arc Welding (SMAW)
6.3 Filler Metal Selection
Matching 317L filler is standard for most applications. For demanding environments where weld-metal dilution could reduce performance below that of the base metal, over-alloyed fillers of the 317LMN or 6% Mo type may be considered.
6.4 Post-Weld Treatment
Weld scale and heat tint should be removed to restore corrosion resistance:
- Stainless-only wire brushing
- Grinding
- Pickling in nitric/hydrofluoric acid solution followed by thorough water rinsing
7. Heat Treatment
317L cannot be hardened by heat treatment.
7.1 Solution Annealing
Temperature: 1,120–1,160°C (Outokumpu typical values)
Cooling: water quench — air cooling for thicknesses below 2 mm
8. Fabrication
317L offers good overall fabricability consistent with standard and high-performance austenitic stainless steels. Its slightly higher yield strength compared with 316L results in marginally greater forming forces and increased spring back, which can be compensated for in process design.
8.1 Cold Forming
317L is suitable for all standard cold forming processes including bending, roll forming, drawing, and hydroforming. Work hardening progresses in a similar manner to 316L, and intermediate annealing may be required after heavy cold work.
8.2 Hot Forming
Hot working should be carried out in the range 850–1,250°C, normally followed by solution annealing and quenching to restore a carbide-free microstructure.
8.3 Machining
317L work hardens during machining in the same manner as other austenitic stainless steels. Sharp tooling, rigid setups, and controlled cutting speeds are required to avoid surface hardening.
9. Conclusion
317L (UNS S31703) is a molybdenum-alloyed low-carbon austenitic stainless steel providing improved pitting, crevice corrosion, and uniform corrosion resistance compared with 316L. Its molybdenum content of approximately 3.1 wt% raises the PRE from ~24 (316L) to ~28, delivering a meaningful performance step-up in chloride-bearing and acidic environments where 316L has reached its limits. Good weldability, low sensitisation risk, and standard austenitic fabricability make 317L a practical and widely applied selection in chemical processing, oil and gas, and general industrial service — particularly across the United States and Asian markets where it is an established grade. Where greater corrosion resistance is required, 317LMN with its defined nitrogen addition and higher molybdenum ceiling provides the next step in performance.
