316 vs 317L – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Grade 316 and grade 317L are austenitic stainless steels commonly specified where corrosion resistance outweighs the need for high strength. Engineers, procurement managers, and manufacturing planners routinely weigh trade-offs between corrosion performance, weldability, and cost when selecting between them. Typical decision contexts include chemical process piping, marine components, and equipment exposed to chloride-bearing environments where avoiding pitting and crevice corrosion is critical.
The primary practical distinction is that 317L is formulated for enhanced resistance to localized corrosion through higher molybdenum and chromium with a low-carbon limit to reduce sensitization during welding. This makes 317L a preferred choice where resistance to pitting and crevice corrosion is a design driver, while 316 is often selected where good general corrosion resistance and lower cost are prioritized.
1. Standards and Designations
- ASTM/ASME: Both grades appear in ASTM/ASME specifications for stainless plate, sheet, pipe and forgings (examples: ASTM A240 for plate/sheet).
- UNS: 316 is commonly referenced as UNS S31600; 317L is commonly referenced as UNS S31703.
- EN (European): 316 is represented in EN lists (commonly mapped to X5CrNiMo17-12-2 / 1.4401 for 316 and low-carbon variants to 1.4404); 317L maps to higher alloy EN designations (ranges vary by country and standard edition).
- JIS/GB: Japanese and Chinese national standards include equivalent compositions and mechanical requirements for these austenitic grades.
- Classification: Both 316 and 317L are stainless steels (austenitic family), not carbon, alloy, tool, or HSLA steels.
Note: exact standard numbers and equivalences vary with product form (plate, pipe, bar) and edition year; always check the current standard and UNS mapping when specifying.
2. Chemical Composition and Alloying Strategy
Typical chemical composition ranges (wt%). Values are indicative; confirm per applicable standard or mill test certificate.
| Element | 316 (typical range, wt%) | 317L (typical range, wt%) |
|---|---|---|
| C | ≤ 0.08 | ≤ 0.03 |
| Mn | ≤ 2.0 | ≤ 2.0 |
| Si | ≤ 1.0 | ≤ 1.0 |
| P | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | 16–18 | 18–20 |
| Ni | 10–14 | 11–15 |
| Mo | 2–3 | 3–4 |
| V | ≤ 0.04 (not intentionally added) | ≤ 0.04 |
| Nb (Cb) | Usually not added | Usually not added |
| Ti | Usually not added | Usually not added |
| B | Trace | Trace |
| N | ≤ 0.10 | ≤ 0.11 |
How the alloying strategy works: - Chromium (Cr) establishes the passive oxide film and general corrosion resistance. Higher Cr improves resistance to oxidizing and some reducing environments. - Molybdenum (Mo) markedly increases resistance to pitting and crevice corrosion in chloride-bearing media; 317L’s elevated Mo is the key to its superior localized-corrosion resistance. - Nickel (Ni) stabilizes the austenitic phase, improving toughness and formability. - Carbon (C) affects sensitization: higher C increases the risk of chromium carbide precipitation at grain boundaries during welding or slow cooling; the “L” (low-carbon) version minimizes this by keeping C ≤ 0.03 wt%. - Nitrogen (N) is a strong austenite stabilizer and increases strength and pitting resistance (captured in PREN), but nitrogen levels are generally low and controlled.
3. Microstructure and Heat Treatment Response
Microstructure: - Both 316 and 317L are fully austenitic (face-centered cubic) in typical industrial conditions, with a microstructure that is generally single-phase austenite plus possible low-volume carbide or nitride precipitates depending on composition and thermal history.
Response to processing: - Annealing (solution treatment around 1,040–1,120 °C followed by rapid cooling) restores an austenitic matrix and dissolves carbides, maximizing corrosion resistance and ductility. - Normalizing is not a standard treatment for austenitic stainless steels because the high-temperature austenite range and stability make conventional ferritic/pearlitic transformations inapplicable. - Quenching and tempering are not relevant for austenitic grades because they do not transform to martensite on cooling; cold work and aging can affect precipitation behavior. - Welding and slow cooling: 316 with higher carbon is more susceptible to sensitization—chromium carbide precipitation at grain boundaries—if welded without controls. 317L, with its low carbon, minimizes carbide precipitation and therefore is less susceptible to intergranular corrosion after welding. - Thermo-mechanical processing (cold work, anneal cycles) affects dislocation density, yield/strength, and can influence susceptibility to strain-induced martensite in certain austenitic formulations (less of a concern with stabilized or nitrogen-alloyed variants).
4. Mechanical Properties
Typical mechanically annealed condition values; exact values depend on product form, thickness, and specific standard.
| Property (annealed) | 316 (typical range) | 317L (typical range) |
|---|---|---|
| Tensile Strength (MPa) | ~480–620 | ~480–620 |
| Yield Strength 0.2% (MPa) | ~170–310 | ~170–300 |
| Elongation (%) | ~40–60 | ~40–60 |
| Impact Toughness (Charpy, J) | High, retains toughness at low temp | High, retains toughness at low temp |
| Hardness (HB or HRC) | Low to moderate (annealed) | Low to moderate (annealed) |
Interpretation: - In practice, 316 and 317L have broadly similar mechanical properties in the annealed condition because both are austenitic stainless steels. Differences from the low-carbon limit in 317L are minor for tensile properties; 316 can show marginally higher strength if its carbon is at the upper limit, but this comes at the cost of higher sensitization risk. - Both grades are ductile and tough at ambient and sub-zero temperatures (austenitic stainless steels are notable for excellent impact toughness).
5. Weldability
Austenitic stainless steels are generally excellent to very good in weldability due to a stable austenitic structure and absence of brittle phases when proper procedure is used. Key points: - Carbon content: The low-carbon limit in 317L reduces the risk of chromium carbide precipitation and intergranular corrosion after welding. 316 is weldable but may require low-carbon variants (316L) or post-weld solution annealing in critical applications. - Hardenability: Austenitic grades have low hardenability in the sense of forming martensite; hydrogen-induced cracking is not the typical weld failure mode, but care with heat input and interpass temperatures can control grain growth. - Microalloying: Elements like Nb or Ti, when present (stabilized grades), also reduce sensitization by tying up carbon as stable carbides; those are not typical in 316/317L.
Useful empirical weldability indices (for qualitative interpretation): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Both formulas show carbon and alloying raise hardenability and welding risk. Because 317L is low-carbon, its calculated indices will generally predict easier welding with lower risk of intergranular corrosion relative to higher-carbon 316. - Practically, use low heat input, recommended filler metals (matching or overmatching Ni-Cr-Mo alloys), and consider post-weld solution treatment for critical service when using higher-carbon variants.
6. Corrosion and Surface Protection
Stainless behavior: - For stainless grades, localized corrosion resistance is quantified with indices such as PREN: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - Using typical composition ranges, 317L usually has a higher PREN than 316 due to higher Mo (and often comparable N), indicating superior resistance to pitting and crevice corrosion in chloride-containing environments. - 316 provides good general corrosion resistance (oxidizing and many reducing environments) and resists stress corrosion cracking and crevice corrosion reasonably well but is generally less resistant to pitting in aggressive chloride environments than 317L.
Non-stainless steels: - Not applicable here; galvanizing and painting are standard protections for carbon and alloy steels but are not used for stainless where the passive film is the protection mechanism.
When PREN is not applicable: - PREN applies to austenitic and duplex stainless steels where Mo and N substantially affect localized corrosion. It’s not meaningful for plain carbon steels or for environments dominated by uniform corrosion mechanisms unattended by localized attack.
7. Fabrication, Machinability, and Formability
- Formability: Both grades are highly formable in the annealed condition (deep drawing, bending) due to austenitic ductility. Springback is higher than for ferritic steels and should be accounted for in tooling design.
- Machinability: Austenitic stainless steels are work-hardening and have poorer machinability than carbon steels. Higher molybdenum (as in 317L) can marginally reduce machinability and accelerate tool wear. Use positive rake tooling, rigid setups, and appropriate cutting speeds and feeds.
- Surface finishing: Both take standard stainless polishing, passivation, and electropolishing treatments. 317L may demand more careful passivation control where highest pitting resistance is required.
- Joining and forming: 317L’s low carbon improves weld-fabrication outcomes; for heavy cold work operations, anneal as necessary to restore ductility.
8. Typical Applications
| 316 – Typical Uses | 317L – Typical Uses |
|---|---|
| Chemical process equipment (less aggressive chemistries) | Chemical process equipment in chloride-rich or more aggressive environments |
| Marine fittings and seawater-related components (many general uses) | Heat exchangers, piping, and equipment handling chloride-bearing brines and acids where enhanced pitting resistance is needed |
| Food processing equipment and storage vessels | Pharmaceutical and high-purity environments with welding sensitivity to carbide precipitation |
| Architectural elements, fasteners | Desalination and offshore process systems where localized corrosion is a major concern |
Selection rationale: - Choose 316 when good general corrosion resistance, availability, and lower material cost are primary; it suits many marine and chemical environments that are not severely aggressive. - Choose 317L when service involves aggressive chloride environments, higher concentrations of oxidizing anions, or where welded assemblies must retain resistance to pitting/crevice corrosion without post-weld heat treatment.
9. Cost and Availability
- Cost: 317L is typically more expensive than 316 due to higher molybdenum and slightly higher nickel content. The premium increases with Mo market fluctuations.
- Availability: 316 is more widely stocked in a broad range of product forms (sheet, plate, pipe, bar, fittings, fasteners). 317L is widely available but less ubiquitous; long lead times or minimum order quantities are more likely for specialty product forms or finishes.
- Procurement: For bulk projects, cost differences can be significant; balance material premium against lifecycle costs and potential replacement or maintenance in corrosive service.
10. Summary and Recommendation
Summary table (qualitative)
| Attribute | 316 | 317L |
|---|---|---|
| Weldability | Good (requires care to avoid sensitization in thicker sections) | Excellent (low C reduces sensitization risk) |
| Strength–Toughness | Good ductility and toughness; similar in annealed condition | Similar ductility and toughness; mechanical properties comparable |
| Corrosion resistance (pitting/crevice) | Good general resistance; moderate to good localized resistance | Better localized (pitting/crevice) resistance due to higher Mo and Cr |
| Cost | Lower (more commodity) | Higher (premium alloying) |
Recommendations: - Choose 316 if you need a cost-effective, broadly available austenitic stainless for general corrosion service, where chloride exposure is moderate, and fabrication/welding can be controlled or post-weld treatment is feasible. - Choose 317L if the application demands superior resistance to pitting and crevice corrosion in aggressive chloride environments, or where welded structures must avoid sensitization without extensive post-weld heat treatment—accepting a higher material cost for improved service life.
Concluding note: Always specify the exact grade, product form, surface finish, and applicable standard in procurement documents; request mill test certificates and consider corrosion testing or engineering assessment for critical applications, since service environment and fabrication practice strongly influence long-term performance.