AISI 317LMN Stainless Steel
1. Introduction
317LMN (UNS S31726) is a molybdenum- and nitrogen-enhanced low-carbon austenitic stainless steel designed for service in highly corrosive environments where standard grades such as 316L offer insufficient resistance. Its elevated molybdenum content — 4.00–5.00 wt% — combined with a controlled nitrogen addition of 0.10–0.20 wt%, provides significantly improved resistance to pitting, crevice corrosion, and uniform corrosion attack compared with 316L and standard 317L. The low carbon content (≤0.030 wt%) reduces the risk of chromium carbide precipitation during welding and thermal exposure, supporting post-weld corrosion resistance without the need for post-weld heat treatment in most applications.
317LMN is used primarily in chemical processing, pollution control, pulp and paper production, and pharmaceutical manufacturing environments where aggressive acidic or chloride-bearing media demand a higher-performance austenitic grade. Its combination of pitting resistance, fabricability, and weldability makes it a reliable selection where 316L has reached its performance limits but where higher-alloy or duplex grades are not required.
317LMN and 317L: When to Choose
317LMN and 317L share a similar chromium and nickel range but differ in two important respects: molybdenum content and nitrogen addition. Standard 317L (UNS S31703) carries 3.00–4.00 wt% Mo with no mandatory nitrogen requirement, while 317LMN specifies 4.00–5.00 wt% Mo and a defined nitrogen range of 0.10–0.20 wt%. The nitrogen addition in 317LMN increases solid-solution strengthening and — critically — further improves pitting resistance, raising the pitting resistance equivalent (PRE) above that of 317L. Where chloride-bearing environments or aggressive acid service is the primary selection driver, 317LMN is the more capable choice.
2. Chemical Composition
The composition of 317LMN reflects its position as a high-molybdenum, nitrogen-strengthened variant of the standard 317L grade. Elevated molybdenum and controlled nitrogen work together to maximise pitting resistance, while the low carbon ceiling limits carbide formation risk during thermal exposure and welding.
Table 1 — Chemical Composition of 317LMN (UNS S31726)
| Element | Typical Limits (wt%) |
|---|---|
| Carbon (C) | ≤ 0.030 |
| Manganese (Mn) | ≤ 2.00 |
| Phosphorus (P) | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 |
| Silicon (Si) | ≤ 0.75 |
| Chromium (Cr) | 17.00–20.00 |
| Nickel (Ni) | 13.50–17.50 |
| Molybdenum (Mo) | 4.00–5.00 |
| Nitrogen (N) | 0.10–0.20 |
| Iron (Fe) | Balance |
The nitrogen content in 317LMN serves a dual purpose: it increases solid-solution strengthening and augments the pitting resistance contribution of molybdenum, producing a PRE value substantially higher than 316L or standard 317L.
3. Mechanical and Physical Properties
317LMN provides mechanical properties consistent with high-molybdenum austenitic stainless steels. Nitrogen addition contributes to slightly elevated yield and tensile strength compared with the standard 316L grade, while ductility remains excellent, supporting cold forming and fabrication operations.
3.1 Tensile Properties at Room Temperature
Table 2 — Mechanical Properties of 317LMN (Annealed Condition)
| Property | Value |
|---|---|
| 0.2% Yield Strength | 45 ksi / 310 MPa |
| Tensile Strength | 90 ksi / 620 MPa |
| Elongation | 40% |
| Hardness | ~95 HRB |
3.2 Physical Properties
Physical Properties
| Property | Value |
|---|---|
| Density | 0.29 lb/in³ (8.03 g/cm³) |
| Young's Modulus | 28 × 10⁶ psi (193 GPa) |
| Electrical Resistivity | ~78 µΩ·cm (20°C) |
| Magnetic Permeability | ~1.02 (annealed condition) |
4. Corrosion Resistance
The corrosion performance of 317LMN is its primary selection driver. Its elevated molybdenum and nitrogen content improve resistance across multiple corrosion mechanisms compared with standard austenitic grades.
4.1 Pitting and Crevice Corrosion
Pitting resistance in stainless steels is commonly characterised using the pitting resistance equivalent number (PREN). 317LMN achieves a significantly higher PREN than 316L and standard 317L, reflecting the combined contribution of its elevated molybdenum and defined nitrogen addition. This makes it particularly suitable for environments where chloride-induced pitting and crevice corrosion are primary failure risks.
4.2 Resistance to Uniform Corrosion
317LMN provides strong resistance to uniform corrosion in a range of acidic media, including dilute sulphuric acid, phosphoric acid, and organic acid environments encountered in chemical and pharmaceutical processing. Its higher molybdenum content extends its useful range in these environments relative to 316L and 317L.
4.3 Sensitisation Resistance
The low carbon ceiling of ≤0.030 wt% significantly reduces the risk of chromium carbide precipitation in the sensitisation range (427–899°C), making 317LMN suitable for welded assemblies without post-weld annealing in most service conditions.
5. Applications
317LMN is selected for service in process environments where aggressive corrosive media — particularly chloride-bearing or acidic streams — demand performance beyond the capability of 316L.
5.1 Chemical Processing
317LMN is widely used in chemical process equipment exposed to halide-containing environments, dilute mineral acids, and mixed acid streams. Typical components include:
- Heat exchangers and condensers in chloride-bearing service
- Reaction vessels and storage tanks for aggressive chemical media
- Piping, valves, and fittings in acid-handling systems
- Evaporators and concentrators in sulphuric acid service
6. Welding
317LMN is weldable using all standard austenitic fusion welding processes. Its low carbon content reduces sensitisation risk, and its nitrogen addition provides additional resistance to inter-pass sensitisation during multi-pass welding.
6.1 Recommended Processes
- Gas Tungsten Arc Welding (GTAW/TIG)
- Gas Metal Arc Welding (GMAW/MIG)
- Shielded Metal Arc Welding (SMAW)
- Submerged Arc Welding (SAW)
6.2 Filler Metal Selection
Matching or over-alloyed filler metals are recommended to maintain corrosion resistance in the weld deposit:
- ER317L / ER317LMN — matching composition, maintaining Mo and N levels in the weld metal
- Over-alloyed fillers (e.g. 6% Mo-type) may be considered for critical joints in highly aggressive environments
6.3 Post-Weld Treatment
Due to its low carbon content, 317LMN does not require post-weld annealing for sensitisation control in most service conditions. Weld scale and heat tint should be removed by:
- Stainless-only wire brushing
- Grinding
- Pickling in nitric/hydrofluoric acid solution, followed by thorough water rinsing
7. Heat Treatment
317LMN cannot be hardened by heat treatment. Strength may be increased through cold working but this is not typically applicable in corrosion-critical service.
7.1 Solution Annealing
Temperature: 1900–2100°F (1040–1150°C)
Cooling: Rapid quench (water or air) to prevent carbide precipitation
8. Fabrication
317LMN offers good overall fabricability consistent with high-molybdenum austenitic stainless steels. Its higher alloy content compared with 316L results in slightly greater forming forces and more pronounced work hardening, requiring appropriate equipment and tooling selection.
8.1 Cutting and Machining
- Work hardens during machining — sharp tooling and rigid setups required
- Slower cutting speeds with deeper cuts reduce surface hardening risk
- Produces long, ductile chips typical of austenitic grades
8.2 Cold Forming
317LMN can be cold-formed using standard methods including bending, roll forming, drawing, and hydroforming. Work hardening progresses more rapidly than in 316L; heavily formed sections may require intermediate annealing to restore ductility.
8.3 Hot Working
Hot working should be carried out within the range of approximately 1700–2200°F (925–1200°C), followed by rapid cooling to maintain a carbide-free microstructure.
9. Conclusion
317LMN (UNS S31726) is a high-molybdenum, nitrogen-enhanced low-carbon austenitic stainless steel offering superior pitting and crevice corrosion resistance compared with 316L and standard 317L. Its elevated molybdenum content of 4.00–5.00 wt% and defined nitrogen addition combine to produce a significantly higher PREN, supporting reliable performance in chloride-bearing, acidic, and mixed-media process environments. Good weldability, low sensitisation risk, and standard austenitic fabricability make 317LMN a practical selection for demanding chemical processing, pollution control, and pharmaceutical applications where 316L has reached its performance limits.
