316L vs 317L – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Austenitic stainless steels 316L and 317L are common selections where corrosion resistance, formability, and weldability are required. Engineers, procurement managers, and manufacturing planners frequently weigh the trade-offs between corrosion performance and material cost, as well as considerations for fabrication and long‑term service in chloride‑bearing or acidic environments. The principal practical distinction between 316L and 317L is the higher molybdenum and slightly different chromium content of 317L, which increases resistance to localized corrosion at the expense of higher material cost; this is why these grades are often compared when specifying piping, vessels, heat exchangers, or sanitary equipment for aggressive service.
1. Standards and Designations
Common standards and designations where 316L and 317L appear:
- ASTM / ASME: A240 (plate), A276 (bars), A182 (forgings), etc.
- EN: EN 10088-2 (stainless steels) and related product standards.
- JIS: SUS316L, SUS317L (Japanese Industrial Standard equivalents).
- GB/T: Chinese standards include similar compositions and product forms.
Both 316L and 317L are austenitic stainless steels (stainless, not carbon, alloy, tool, or HSLA steels). The “L” suffix indicates low carbon (enhanced resistance to sensitization during welding).
2. Chemical Composition and Alloying Strategy
The following table gives typical composition ranges (wt%) commonly cited for commercial 316L and 317L. These are representative ranges from common product specifications; exact limits depend on the specific standard and product form.
| Element | 316L (typical wt%) | 317L (typical wt%) |
|---|---|---|
| C | ≤ 0.03 | ≤ 0.03 |
| Mn | ≤ 2.0 | ≤ 2.0 |
| Si | ≤ 1.0 | ≤ 1.0 |
| P | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | 16.0–18.0 | 18.0–20.0 |
| Ni | 10.0–14.0 | 11.0–15.0 |
| Mo | 2.0–3.0 | 3.0–4.5 |
| V | — (trace) | — (trace) |
| Nb (Cb) | — (optionally stabilized grades) | — |
| Ti | — (optionally stabilized grades) | — |
| B | Trace | Trace |
| N | ≤ 0.10 (trace to small) | ≤ 0.10 (trace to small) |
Alloying strategy and effects: - Chromium provides the passive film and bulk corrosion resistance. - Nickel stabilizes the austenitic structure and improves toughness and formability. - Molybdenum is the primary element that boosts resistance to pitting and crevice corrosion in chloride-containing environments; 317L typically contains more Mo than 316L. - Low carbon (L) reduces the risk of chromium carbide precipitation (sensitization) in the heat‑affected zone during welding.
3. Microstructure and Heat Treatment Response
Microstructure: - Both 316L and 317L are fully austenitic (face-centred cubic) in the annealed condition across typical service temperatures. - They do not transform to martensite on cooling from annealing temperatures and so do not respond to quench‑and‑temper cycles used for ferritic or martensitic steels.
Heat treatment response: - Solution anneal (commonly 1010–1150°C followed by rapid quench) restores ductility, dissolves precipitates, and returns the alloy to a fully austenitic, corrosion-resistant condition. - There is no strengthening by conventional heat treatment (they are non‑hardening by thermal treatment); strength is increased only by cold work (strain hardening). - Sensitization (chromium carbide precipitation) can occur in the range of roughly 450–850°C if carbon is present; low carbon (L) grades mitigate this risk. - Stabilizing additions (Nb or Ti, not typical for standard 316L/317L) are used only where repeated exposure to sensitizing temperatures is expected; otherwise solution annealing plus low carbon is the usual approach.
4. Mechanical Properties
Typical, annealed mechanical properties for commercial 316L and 317L are broadly similar; 317L may show slightly higher tensile strength in some product forms due to composition. Values below are indicative for annealed material (actuals depend on product form and standard).
| Property (annealed) | 316L (typical) | 317L (typical) |
|---|---|---|
| Tensile strength (MPa) | 480–620 | 490–640 |
| 0.2% Proof / Yield (MPa) | 170–310 | 170–320 |
| Elongation (A%) | ≥ 40% (depends on thickness) | ≥ 40% (depends on thickness) |
| Impact toughness (Charpy) | Good, retains toughness at low T | Good, similar to 316L |
| Hardness (HB or HRB) | Moderate (annealed) | Moderate (annealed) |
Interpretation: - Both grades are ductile and tough in the annealed condition. Differences in mechanical properties are small for common product forms; any modest increase in strength for 317L comes mainly from higher alloy content (Mo and sometimes Cr/Ni), not from heat‑treatable mechanisms. - For applications where higher yield strength is required, cold work or alternative alloys should be considered rather than expecting large differences between 316L and 317L.
5. Weldability
Weldability considerations for both grades are favorable but require good practice: - Low carbon minimizes sensitization in welded joints, making both grades suitable for fusion welding without post‑weld solution anneal in many cases. - Nitrogen and nickel content help maintain austenite and ductility in weld metal. - Molybdenum increases corrosion resistance but will not prevent solidification or hot cracking; filler selection and heat input control are important.
Useful weldability indices:
- Carbon equivalent (IIW formula):
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
- Preventive cracking factor ($P_{cm}$):
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both 316L and 317L calculate to low carbon equivalents compared with higher‑carbon steels, indicating low hardenability and low risk of cold cracking. - 317L’s higher Mo has little adverse effect on fusion weldability when appropriate filler (matching 317L or 316L/317L‑compatible consumables) and welding parameters are used. However, specification of compatible filler metal to preserve corrosion resistance in the weld zone is essential (i.e., match molybdenum content when pitting resistance is critical). - Post‑weld solution anneal is not routinely required for these low‑carbon grades, but in highly corrosive service or for thick sections, solution annealing can restore full corrosion resistance.
6. Corrosion and Surface Protection
For stainless alloys, the Pitting Resistance Equivalent Number (PREN) is a useful index for chloride pitting resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Because 317L typically contains more molybdenum than 316L, its PREN is higher and therefore its resistance to pitting and crevice corrosion in chloride-containing environments is superior.
- PREN is an empirical index and most useful for comparing austenitic and duplex stainless steels with similar microstructures; it should not be used alone to predict general corrosion in all environments.
- For non‑stainless steels, protection strategies include galvanizing, painting, or polymer linings; those methods are not relevant for 316L/317L as the alloys rely on passive film protection.
Practical guidance: - Use 316L for general chemical and sanitary applications where chloride levels are moderate. - Use 317L where chloride pitting potential, crevice corrosion, or some acidic environments (e.g., sulfuric or phosphoric acid in certain concentrations) require enhanced localized corrosion resistance.
7. Fabrication, Machinability, and Formability
- Formability: Both grades form and deep‑draw well in the annealed condition due to stable austenitic structure and good ductility.
- Machinability: Austenitic stainless steels are work‑hardening; machinability is moderate to poor compared with carbon steels. 317L is slightly harder to machine in some cases because of higher alloy content; use sharp tooling, rigid setups, and appropriate cutting speeds.
- Surface finish and polishing: Both polish to a good finish; 317L’s higher alloy content may require slightly different polishing steps to achieve mirror finish.
- Cold working: Both respond well to cold work for strengthening but cold working increases susceptibility to strain‑induced martensite in lower‑Ni austenitics—less of a concern with these high‑Ni grades.
8. Typical Applications
| 316L — Typical Uses | 317L — Typical Uses |
|---|---|
| Food and beverage processing equipment (tanks, piping) | Chemical process equipment handling more aggressive chloride or acid streams |
| Pharmaceutical and medical devices (surgical instruments, implants—where specified) | Pollution control scrubbers, flue gas desulfurization where higher pitting resistance is needed |
| Marine fittings, seawater condensers (moderate chloride exposure) | Heat exchangers and piping in chloride‑containing process streams |
| Heat exchangers, condensers, and architectural trim | Components where crevice/pitting resistance is a priority (e.g., brine systems) |
Selection rationale: - Choose 316L for broad corrosion resistance at lower cost and easier procurement for general service. - Choose 317L when the service environment includes higher chloride concentration, crevices, or localized corrosion risk that justify the incremental cost.
9. Cost and Availability
- Cost: 317L is typically more expensive than 316L because of the higher molybdenum content and slightly higher nickel/cr chromium makeup; pricing varies with commodity metal markets and form (sheet, plate, bar, tubing).
- Availability: 316L is one of the most widely available austenitic stainless grades in a full range of product forms and finishes. 317L is commonly available but less ubiquitous in specialty forms and may have longer lead times for certain product shapes or finishes.
- Procurement advice: For high-volume standard items, 316L will usually be the economical choice; for engineered-process equipment where localized corrosion is a failure mode, specify 317L and plan for longer procurement time and higher material budget.
10. Summary and Recommendation
| Criterion | 316L | 317L |
|---|---|---|
| Weldability | Excellent (low C) | Excellent (low C), filler metal selection recommended |
| Strength – Toughness | Similar, both ductile and tough | Similar, slightly higher strength possible |
| Localized corrosion resistance (pitting/crevice) | Good | Better (higher Mo → higher PREN) |
| Cost | Lower | Higher |
| Availability | Very high | High, but slightly less ubiquitous |
Recommendation: - Choose 316L if you need a robust, economical, highly weldable austenitic stainless steel for general corrosion resistance, sanitary applications, or marine components where chloride exposure is moderate. - Choose 317L if the design must resist pitting or crevice corrosion in more aggressive chloride‑bearing or acidic services, or where the marginal increase in material cost is justified by reduced maintenance and longer service life.
Final note: Specify actual composition limits and mechanical requirements by referencing the applicable standard (ASTM, EN, JIS, GB) and the product form (plate, tube, bar). Where localized corrosion is a critical failure mode, perform application‑specific corrosion testing or consult corrosion engineers to validate the grade choice, surface finish, and weld procedure.