316 vs 316L – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
Type 316 and 316L austenitic stainless steels are two of the most widely specified grades in industry, from piping and pressure vessels to chemical-processing equipment and food-contact surfaces. Engineers, procurement managers, and manufacturing planners often weigh trade-offs between slightly higher strength, fabrication behavior, corrosion resistance, and price when choosing between them. Typical decision contexts include welded assemblies where intergranular corrosion or sensitization is a concern, versus applications where marginally higher yield strength or lower material cost is preferred.
The fundamental distinction is the controlled carbon content: the “L” variant is produced with a lower maximum carbon level to reduce the risk of chromium carbide precipitation at grain boundaries after welding or exposure to sensitizing temperatures. That difference directly affects susceptibility to intergranular corrosion and informs material selection in welded or high-temperature-exposed components.
1. Standards and Designations
Common international standards and designations for these grades include:
- ASTM/ASME: ASTM A240 / ASME SA-240 (plate, sheet) — UNS S31600 (316), UNS S31603 (316L)
- EN: EN 10088-2 / EN 10088-3 (stainless steels) — X5CrNiMo17-12-2 (316), X2CrNiMo17-12-2 (316L)
- JIS: SUS316, SUS316L
- GB (China): 0Cr17Ni12Mo2 and 00Cr17Ni12Mo2 (roughly corresponding)
Classification: Both 316 and 316L are austenitic stainless steels (stainless). They are not carbon, alloy, tool, or HSLA steels.
2. Chemical Composition and Alloying Strategy
The two grades share essentially the same principal alloying elements (Cr, Ni, Mo) with the principal controlled variable being carbon. Below is a concise composition table showing common limits or typical ranges as specified by widely used standards. Values are given as weight percent (wt%); where a standard sets a maximum, that is shown.
| Element | 316 (typical limits) | 316L (typical limits) |
|---|---|---|
| C | ≤ 0.08 wt% (max) | ≤ 0.03–0.035 wt% (max) |
| Mn | ≤ 2.0 wt% (max) | ≤ 2.0 wt% (max) |
| Si | ≤ 1.0 wt% (max) | ≤ 1.0 wt% (max) |
| P | ≤ 0.045 wt% (max) | ≤ 0.045 wt% (max) |
| S | ≤ 0.030 wt% (max) | ≤ 0.030 wt% (max) |
| Cr | 16.0–18.0 wt% (typical) | 16.0–18.0 wt% (typical) |
| Ni | 10.0–14.0 wt% (typical) | 10.0–14.0 wt% (typical) |
| Mo | 2.0–3.0 wt% (typical) | 2.0–3.0 wt% (typical) |
| V | Not specified / trace | Not specified / trace |
| Nb (Cb) | Not normally present (unless stabilized) | Not normally present (unless stabilized) |
| Ti | Not normally present (unless stabilized as 316Ti) | Not normally present |
| B | Not normally present / trace | Not normally present / trace |
| N | Controlled to low levels (trace) | Controlled to low levels (trace) |
Alloying strategy and effects: - Chromium (Cr) provides a passive oxide film that gives stainless steels their corrosion resistance. - Nickel (Ni) stabilizes the austenitic microstructure and improves toughness and formability. - Molybdenum (Mo) increases resistance to pitting and crevice corrosion in chloride-containing environments. - Carbon increases strength through solid-solution strengthening and can contribute to carbide formation at grain boundaries when combined with chromium and thermal exposure. Lowering carbon in 316L minimizes chromium carbide precipitation and improves resistance to intergranular corrosion after welding or exposure to sensitizing temperatures.
3. Microstructure and Heat Treatment Response
Microstructure: - Both 316 and 316L are fully austenitic after solution annealing; the microstructure is face-centered cubic (FCC) austenite with possible small amounts of delta ferrite in some cast or welded microstructures. - Carbide precipitation: At temperatures between roughly 425–870°C (the sensitization range), carbon and chromium can form chromium-rich carbides ($\text{Cr}_{23}\text{C}_6$) at grain boundaries. This depletes chromium adjacent to grain boundaries and increases susceptibility to intergranular corrosion.
Heat treatment response and processing: - Solution annealing (typical for final processing): Heating to a solutionizing temperature (e.g., 1,000–1,100°C) followed by rapid cooling restores a single-phase austenitic structure and dissolves precipitates for both grades. - Normalizing and quenching are not commonly used for austenitic stainless steels because they do not transform to martensite; thermo-mechanical processing (cold working followed by solution annealing) is more typical. - 316L is less susceptible to carbide precipitation during slow cooling or post-weld thermal cycles due to the lower carbon content; this improves resistance to intergranular corrosion without the need for post-weld solution annealing in many cases. - Stabilized variants (e.g., 316Ti or 316Cb/Nb) intentionally add Ti or Nb to tie up carbon as stable carbides and therefore prevent chromium carbide formation—useful where high-temperature service precludes solution anneal.
4. Mechanical Properties
Quantitative mechanical properties depend on product form (plate, bar, tube), cold work, and heat treatment. Rather than fixed numbers, the practical comparison is:
| Property | 316 | 316L |
|---|---|---|
| Tensile Strength | Similar (comparable ultimate tensile strength) | Similar |
| Yield Strength | Slightly higher (due to higher C) | Slightly lower (reduced yield) |
| Elongation / Ductility | Comparable, good ductility | Comparable, often slightly higher ductility |
| Impact Toughness | Comparable and generally good at ambient temps | Comparable and generally good |
| Hardness | Similar / dependent on cold work | Similar; slightly lower in annealed condition |
Why these differences: - Carbon contributes to yield strength through solid-solution and potential carbide presence; 316 typically shows marginally higher yield strength than 316L in annealed conditions. - Lower carbon in 316L can improve ductility and toughness marginally and is preferred where ductility after welding is critical.
5. Weldability
Weldability of both grades is excellent relative to many steels; austenitic stainless steels are widely used in welded fabrications. Key weldability considerations:
- Carbon effect: Lower carbon reduces the driving force for chromium carbide precipitation during post-weld cooling; 316L therefore has superior resistance to weld-sensitization compared with 316.
- Hardenability: Austenitic stainless steels do not transform to martensite on cooling, so hydrogen cracking is less of a concern than in ferritic or martensitic steels. However, hot cracking and sigma-phase formation in certain thermal cycles can be relevant.
- Use of filler metals: Matching or overmatching filler alloys (e.g., ER316L) are common to preserve corrosion resistance.
Relevant equations used by welding engineers (qualitative interpretation only): - Chromium equivalent / carbon equivalent for hardenability or weldability assessment: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - A more complex predictive index: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation: - Lower $C$ reduces both $CE_{IIW}$ and $P_{cm}$ contributions and therefore indicates a lower tendency to form deleterious phases on slower cooling or extended exposures. Thus 316L scores better in these indices for minimizing sensitization risk. - Practical implication: For welded structures with long isothermal holds or slow cooling in the sensitization range, 316L or stabilized grades are recommended; for short weld cycles and where strength is prioritized, 316 may be acceptable with appropriate procedures.
6. Corrosion and Surface Protection
Stainless context: - For stainless steels the pitting resistance equivalent number (PREN) is commonly used to compare localized corrosion resistance in chloride environments: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - Because 316 and 316L have similar Cr and Mo, their intrinsic pitting resistance is essentially equivalent (assuming nitrogen levels are similar). The critical difference lies in post-weld or heat-exposed conditions: 316L’s lower carbon reduces grain-boundary chromium depletion and therefore reduces susceptibility to intergranular corrosion.
Non-stainless context: - (Not applicable here; for non-stainless steels, protective systems such as galvanizing or coatings are discussed.)
When indices are not applicable: - PREN is useful for ranking alloys specifically for pitting in chloride-containing media. It does not capture general corrosion resistance, mechanical behavior, or susceptibility to intergranular corrosion due to carbide precipitation.
7. Fabrication, Machinability, and Formability
- Formability: Both 316 and 316L exhibit excellent formability (deep drawing, bending) thanks to austenitic ductility. 316L can be marginally easier to form in annealed condition due to slightly lower yield.
- Machinability: Austenitic stainless steels are work-hardening; machinability is generally moderate to poor compared with carbon steels. 316 and 316L machine similarly, though process parameters and tooling determine practical chip control and surface finish.
- Surface finishing: Both take common stainless finishes (polish, bead blast, passivation). Passivation after fabrication is recommended to restore chromium-rich passive film, particularly after welding or pickling.
- Welding and post-weld treatment: 316L reduces need for post-weld solution annealing in many situations; however in highly corrosive applications or where maximum resistance is required, solution anneal may still be specified.
8. Typical Applications
| 316 | 316L |
|---|---|
| Heat exchangers, pumps and valves in marine and chemical service (where slightly higher strength or standard 316 material is available) | Chemical process piping, pharmaceutical equipment, and medical devices where weld integrity and resistance to intergranular corrosion after welding are critical |
| Fasteners, fittings, and hardware for marine and architectural use | Large welded tanks, reactor vessels, and piping where slow cooling or post-weld thermal exposure is expected |
| General-purpose food processing equipment | Cryogenic tanks and piping where low-carbon minimizes risk of carbide precipitation and where forming/welding after fabrication is common |
| Components where standard 316 stock is more economical and fabrication methods do not induce sensitization | Any application requiring minimization of sensitization risk without stabilization or solution anneal |
Selection rationale: - Choose 316 when slightly higher yield strength without special welding sensitivity is acceptable and cost/availability favors it. - Choose 316L when fabrication involves extensive welding, post-weld heat exposure, or when code/industry practice requires low-carbon grades to avoid intergranular corrosion.
9. Cost and Availability
- Cost: 316L is typically priced slightly higher than 316 because of tighter control on carbon and sometimes additional processing; however market premiums are small and vary with region and supply conditions.
- Availability: Both grades are widely available in sheet, plate, bar, tube, and pipe. 316 is often more common in commodity stock; 316L is readily available in welded and seamless pipe, sheet, and fittings due to its broad demand in pharmaceuticals, petrochemical, and food industries.
- Long-lead or specialty product forms (large forgings, heavy plate) may have lead times; specify grade early in procurement to secure supply and avoid substitution.
10. Summary and Recommendation
| Criteria | 316 | 316L |
|---|---|---|
| Weldability | Good; increased sensitization risk vs 316L | Better for welded assemblies; lower sensitization risk |
| Strength–Toughness | Slightly higher yield strength; similar UTS and toughness | Slightly lower yield strength; comparable toughness and ductility |
| Cost | Slightly lower (often) | Slightly higher (often) |
Recommendation: - Choose 316 if your design favors marginally higher yield strength, the fabrication process minimizes time in the sensitization range (rapid cooling or post-weld solution anneal is performed), or when cost/stock availability favors 316. - Choose 316L if your assembly includes extensive welding, slow cooling, or service exposures that could cause sensitization; if codes or qualification standards require low-carbon material; or when maximized resistance to post-weld intergranular corrosion is important.
Final practical note: For critical high-temperature or highly corrosive services, consider alternative approaches—stabilized grades (316Ti/316Cb), higher-alloy stainless families (e.g., duplex, superaustenitic), post-weld solution annealing, or specifying filler metals appropriately. Always review applicable codes and environmental exposures with corrosion and welding engineers before finalizing material selection.