304L vs 321 – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
304L and 321 are two widely used austenitic stainless steels whose selection is a frequent engineering dilemma. Engineers, procurement managers, and manufacturing planners commonly weigh corrosion resistance, fabrication and welding behavior, elevated-temperature stability, and cost when choosing between them. The principal practical contrast is how each alloy handles carbides during welding and high-temperature service: 304L relies on low carbon to avoid sensitization, while 321 relies on titanium stabilization to tie up carbon and prevent chromium carbide precipitation.
Because both grades are austenitic stainless steels with similar chromium and nickel contents, they are compared often in piping, pressure vessels, heat exchangers, and fabricated components where weldability and high-temperature corrosion resistance determine the choice.
1. Standards and Designations
- Common international standards and designations:
- ASTM/ASME: 304L — ASTM A240 / ASME SA-240 (UNS S30403), 321 — ASTM A240 / ASME SA-240 (UNS S32100).
- EN: 304L roughly corresponds to EN 1.4307; 321 corresponds to EN 1.4541 (or 1.4541/1.4550 variants depending on titanium content).
- JIS, GB: national equivalents exist with similar chemistries and properties (consult specific standards for exact limits).
- Classification: Both 304L and 321 are austenitic stainless steels (stainless family). They are not carbon steels, alloy steels, tool steels, or HSLA grades.
2. Chemical Composition and Alloying Strategy
The following table lists the key elements and typical ranges or maximums according to common specifications (ranges are typical and depend on standard/specification and product form).
| Element | 304L (typical spec ranges) | 321 (typical spec ranges) |
|---|---|---|
| C (max, wt%) | ≤ 0.03 | ≤ 0.08 |
| Mn (wt%) | ≤ 2.00 | ≤ 2.00 |
| Si (wt%) | ≤ 0.75 | ≤ 0.75 |
| P (wt%) | ≤ 0.045 | ≤ 0.045 |
| S (wt%) | ≤ 0.03 | ≤ 0.03 |
| Cr (wt%) | 17.5–20.0 | 17.0–19.0 |
| Ni (wt%) | 8.0–12.0 | 9.0–12.0 |
| Mo (wt%) | — (usually ≤0.10) | — (usually ≤0.10) |
| V (wt%) | — | — |
| Nb (wt%) | — | — |
| Ti (wt%) | — | typically 0.15–0.70 (stabilizer) |
| B (wt%) | — | — |
| N (wt%) | ≤ 0.10 | ≤ 0.10 |
Alloying strategy and effects: - Chromium (Cr) provides general corrosion resistance by forming a passive oxide film. - Nickel (Ni) stabilizes the austenitic structure, improving toughness and ductility. - Low carbon in 304L reduces the tendency for chromium carbide precipitation (sensitization) during slow cooling after welding. - Titanium in 321 forms stable titanium carbides/nitrides, preventing chromium carbide formation during exposure to the sensitization range (~425–850°C). This gives 321 an advantage for elevated-temperature service and applications involving cyclic high-temperature exposure. - Absence of Mo means neither grade is optimized for high-chloride pitting resistance; Mo-bearing grades (e.g., 316) are preferred for chlorides.
3. Microstructure and Heat Treatment Response
Microstructure: - Both 304L and 321 are fully austenitic (face-centered cubic) in the annealed condition. They exhibit good toughness and ductility down to cryogenic temperatures. - 304L: austenite matrix with minimal carbide precipitation when properly heat treated or when carbon content is kept low. - 321: austenite matrix with dispersed Ti(C,N) precipitates that act as stabilizers and reduce chromium carbide formation at grain boundaries.
Heat treatment response: - Austenitic stainless steels are not heat-treatable to increase strength by quenching and tempering like ferritic/ martensitic steels. Mechanical properties are obtained by cold work or by stabilizing/solution annealing. - Solution anneal: heating to ~1010–1120°C followed by rapid cooling restores the ductile, corrosion-resistant microstructure for both grades. - 304L: because of low carbon, it is less susceptible to intergranular corrosion after welding and does not require stabilization. - 321: titanium additions make it more tolerant of slow cooling from welding or stress-relief temperatures; Ti must be present in an amount sufficient to combine with available carbon (commonly at least 5×C by wt).
Thermo-mechanical processing: - Cold working raises strength and hardness for both grades by strain hardening; recrystallization occurs only after solution annealing. - Elevated-temperature exposure: 321 performs better than unstabilized grades in the 400–900°C range because Ti prevents chromium carbide precipitation that causes sensitization.
4. Mechanical Properties
Typical mechanical property ranges (annealed condition) depend on product form (sheet, plate, bar) and standard—values below are representative ranges for engineering comparison.
| Property | 304L (annealed, typical) | 321 (annealed, typical) |
|---|---|---|
| Tensile strength (UTS) | ~480–700 MPa | ~480–700 MPa |
| Yield strength (0.2% offset) | ~170–300 MPa | ~170–300 MPa |
| Elongation (in 50 mm) | ≥40% (typical) | ≥40% (typical) |
| Impact toughness (qualitative) | Good, retains toughness at low temps | Good, similar to 304L |
| Hardness (HRB/HV) | Moderate (annealed) | Moderate (annealed) |
Interpretation: - In the annealed condition, 304L and 321 have very similar strength, ductility, and toughness. - Differences in mechanical performance are typically small at ambient temperature; the main advantage of 321 shows in high-temperature stability and creep/oxidation resistance, where titanium stabilization helps maintain properties after prolonged exposure.
5. Weldability
Weldability depends on carbon, alloying elements, and susceptibility to solidification cracking or sensitization.
Relevant weldability indices: - Carbon equivalent (IIW) is a widely used index to assess weldability and hardenability influence: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - The Pcm index is another measure related to the propensity for cold cracking and weldability: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - 304L: The deliberately low carbon content reduces $CE_{IIW}$ and $P_{cm}$ contributions from carbon, lowering the risk of intergranular corrosion (sensitization) after welding. As a result, 304L is widely regarded as easy to weld with conventional filler metals (matching composition or 308L filler), and many fabrication shops prefer it for welded constructions that will not see severe high-temperature service. - 321: Titanium stabilization reduces sensitivity to carbide precipitation in the heat-affected zone; therefore, 321 can be welded without the same low-carbon restriction and still resist intergranular corrosion on slow cooling. However, welding practice should still control dilution and filler selection; matching 321 filler or a stabilized filler is often recommended for critical high-temperature applications. - Solidification cracking and hot cracking are generally not problematic for these austenitic stainless steels in normal fabrication. Preheat and post-weld heat treatment are not typically required for structural thicknesses, but parameters depend on joint design and service.
Practical guidance: - Choose low-carbon filler (e.g., 308L) for 304L base metal to maintain low carbon in weld metal and avoid sensitization. - For 321, matching stabilized filler or conventional austenitic filler is acceptable when the weld and service temperatures are considered.
6. Corrosion and Surface Protection
- Both 304L and 321 are stainless and rely on a passive Cr-rich oxide for corrosion resistance. Neither has significant Mo; therefore, neither is optimal for chloride-rich, pitting-prone environments (316/316L or duplex grades would be preferred).
- Use of indices:
- Pitting Resistance Equivalent Number (PREN) is normally used to compare pitting resistance in Mo-containing stainless steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- For 304L and 321, Mo ≈ 0 so PREN reduces to approximately $\text{Cr} + 16\times\text{N}$; however, the PREN concept is more relevant where Mo and higher nitrogen produce measurable differences.
- Sensitization:
- 304L: low carbon minimizes chromium carbide precipitation during welding — good resistance to intergranular corrosion after welding.
- 321: Ti ties up carbon, providing resistance to sensitization even if carbon is higher, which is beneficial for sustained high-temperature applications.
- Non-stainless protection methods (for non-stainless steels) like galvanizing or painting do not apply to these stainless grades for general corrosion control but can be used for aesthetic or additional protection when appropriate.
7. Fabrication, Machinability, and Formability
- Formability: Both 304L and 321 excel in cold forming and deep drawing due to austenitic ductility. 304L is slightly more popular for complex forming because of widespread availability and consistent low-carbon chemistry.
- Machinability: Austenitic stainless steels have poor machinability compared with carbon steels due to high work hardening; 321 may show similar machinability to 304L, with slight differences depending on final microstructure and inclusion content. Use sharp tooling, rigid setups, and appropriate cutting speeds and feeds.
- Surface finishing: Both respond well to polishing and passivation treatments. Electropolishing improves corrosion resistance and surface finish.
- Welding fabrication: 304L commonly requires L-grade fillers for weld metal low carbon; 321 may use stabilized fillers especially when the structure will remain at elevated temperatures.
8. Typical Applications
| 304L — Typical uses | 321 — Typical uses |
|---|---|
| Chemical process equipment for moderately corrosive environments (no heavy chlorides) | Aircraft exhaust systems and high-temperature manifolds |
| Food processing equipment, dairy, brewing tanks, and kitchen equipment | Heat exchangers and furnace components exposed to cyclic high-temperature service |
| Piping, tanks, and welded assemblies where post-weld corrosion resistance is important | Automotive and petrochemical components operating in 400–900°C range |
| Architectural and structural applications where weldability and formability are priorities | Components requiring stabilization against sensitization during prolonged high-temp exposure |
Selection rationale: - Use 304L where fabrication economy, weldability with minimal special handling, and general corrosion resistance are the priorities. - Use 321 where service includes repeated exposure to elevated temperatures, thermal cycling, or where stabilization against carbide precipitation is necessary.
9. Cost and Availability
- Cost: 304L is generally more cost-effective than 321 because it is produced in higher volumes and lacks the expensive stabilization element inventory and processing considerations. Market pricing varies with nickel and chromium market conditions.
- Availability: 304L is more commonly stocked in a wide variety of forms (sheet, plate, tube, bar, wire) and surface finishes. 321 is widely available but may be less common in some specialty product forms or thick sections.
- Procurement note: For large projects, confirm mill certifications and availability lead time; stabilized grades like 321 can have longer lead times for certain product forms.
10. Summary and Recommendation
Summary table (qualitative)
| Attribute | 304L | 321 |
|---|---|---|
| Weldability | Excellent for general fabrication (low-C reduces sensitization) | Very good; superior for high-temp post-weld stability due to Ti stabilization |
| Strength–Toughness (ambient) | Similar, good ductility and toughness | Similar, good ductility and toughness |
| High-temperature stability | Moderate (can sensitize if carbon not controlled) | Superior for cyclic/high-temp exposure (Ti stabilization) |
| Cost | Generally lower | Generally higher |
| Availability | Very high | High, but sometimes less in specialty forms |
Recommendation: - Choose 304L if: your application requires excellent general-purpose corrosion resistance, frequent welding with normal fabrication practice, good formability, and lower material cost. 304L is the default for many food, pharmaceutical, architectural, and general chemical processing components where chloride exposure is limited. - Choose 321 if: components will experience prolonged or cyclic exposure to elevated temperatures (typically in the 400–900°C range), or where post-weld high-temperature stability and resistance to carbide precipitation are critical. 321 is preferred for exhaust, furnace, and certain heat-exchanger applications where titanium stabilization prevents sensitization without strict low-carbon control.
Final note: Both grades are mature, broadly specified austenitic stainless steels. The optimal selection depends on the balance among fabrication practices (especially welding procedures), operating temperature profile, corrosion exposure (chlorides vs. general), and lifecycle cost. For critical welded structures exposed to high temperatures, consult material standards and welding engineers to specify the appropriate filler metal, pre/post-weld treatments, and quality control tests.