304 vs 309S – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners frequently face a choice between AISI 304 and 309S stainless steels when specifying components for corrosive or high-temperature service. The decision often trades corrosion resistance and cost (304 is economical and very corrosion resistant at ambient temperatures) against high-temperature stability and oxidation resistance (309S is selected for elevated-temperature applications). Typical decision contexts include selecting materials for process piping, furnace components, exhaust systems, or welded assemblies that see intermittent or sustained high temperatures.
The principal technical distinction between these two austenitic stainless grades is their alloying strategy: 309S contains substantially higher chromium and nickel than 304, and a reduced carbon specification (the “S” suffix denotes low carbon). That alloy balance gives 309S improved oxidation and high-temperature strength, while 304 remains the default for general corrosion resistance, formability, and cost-sensitive applications.
1. Standards and Designations
- Common standards:
- ASTM/ASME: A240 / ASME SA240 (plate, sheet) — Types 304 and 309S listed.
- EN/ISO: EN 10088 series (various designations depending on product form).
- JIS/GB: Japanese and Chinese standards have corresponding grades (SUS304; SUS309S equivalents).
- Classification:
- 304: Austenitic stainless steel (stainless).
- 309S: Austenitic stainless steel (stainless), high-alloy, low-carbon variant intended for high-temperature service.
2. Chemical Composition and Alloying Strategy
The following table shows typical composition limits and ranges referenced by common specifications (values are maximums or nominal ranges used in industry standards):
| Element | 304 (typical limits) | 309S (typical limits) |
|---|---|---|
| C | ≤ 0.08 wt% | ≤ 0.03 wt% (low-carbon “S”) |
| Mn | ≤ 2.0 wt% | ≤ 2.0 wt% |
| Si | ≤ 1.0 wt% | ≤ 1.0 wt% |
| P | ≤ 0.045 wt% | ≤ 0.045 wt% |
| S | ≤ 0.03 wt% | ≤ 0.03 wt% |
| Cr | 18.0–20.0 wt% | 22.0–24.0 wt% |
| Ni | 8.0–10.5 wt% | 12.0–15.0 wt% |
| Mo | typically none | typically none |
| V, Nb, Ti, B | trace/none | trace/none |
| N | ≤ ~0.10 wt% | ≤ ~0.10 wt% |
How alloying affects properties: - Chromium: primary element for oxidation resistance and passive film stability. Higher Cr in 309S improves high-temperature oxide scale adherence and resistance to aggressive oxidizing atmospheres. - Nickel: stabilizes the austenitic phase, improves high-temperature ductility and toughness; higher Ni in 309S increases thermal stability and creep resistance at elevated temperatures. - Carbon: lower carbon in 309S (“S” grade) minimizes carbide precipitation and improves resistance to sensitization during welding and high-temperature exposure. - Silicon and minor elements influence oxidation scaling behavior; Si in small amounts can improve scale adherence at high temperatures.
3. Microstructure and Heat Treatment Response
- Both 304 and 309S are fully austenitic (face-centered cubic) in the annealed condition. They do not transform to ferrite or martensite under normal thermal cycles at room temperature.
- Microstructure under standard processing:
- Annealed: equiaxed austenite with annealing twins. Grain size depends on final anneal temperature and thermomechanical history.
- Cold-worked: increased dislocation density and potential strain-induced martensite in 304 under severe cold work; 309S, with higher Ni, is less prone to strain-induced martensite.
- Heat treatment response:
- Austenitic stainless steels are not hardenable by quench-and-temper. Solution annealing (e.g., 1010–1150 °C followed by rapid cooling) restores corrosion resistance and ductility by dissolving carbides.
- Sensitization (chromium carbide precipitation at 450–850 °C) is mitigated by the low carbon 309S composition and by solution annealing; 304 can sensitise if improperly welded or held in the sensitization range.
- Thermo-mechanical processing:
- 309S’s higher alloy content yields better retention of mechanical strength at elevated temperatures and improved creep resistance; both grades rely on cold work strengthening at ambient temperatures.
4. Mechanical Properties
Because properties vary with product form and temper, the table below provides comparative, qualitative assessments rather than absolute numerical guarantees.
| Property | 304 | 309S | Comment |
|---|---|---|---|
| Tensile strength | Typical austenitic range | Slightly higher (solution-strengthened by Ni/Cr) | 309S often has modestly higher tensile strength in the annealed condition due to alloying |
| Yield strength | Comparable | Comparable to slightly higher | Yield behaviour is similar; differences depend on cold work |
| Elongation (ductility) | High (excellent formability) | Good, but typically slightly lower than 304 | 304’s lower alloy content generally permits easier forming and higher elongation |
| Impact toughness | Very good at ambient temps | Very good; retains toughness at elevated temps better | Both retain toughness at low temps; 309S shows better high-temp toughness retention |
| Hardness | Low (work-hardens) | Slightly higher as-annealed | Hardness increases with cold work for both grades |
Interpretation: 309S typically offers slightly higher strength and superior performance at elevated temperature, while 304 provides excellent ductility and formability for room-temperature applications.
5. Weldability
- Both grades are highly weldable with standard austenitic stainless steel filler metals. Because both are austenitic, weldability is generally excellent (no preheating required for hydrogen cracking mitigation in most cases).
- Carbon level and alloying:
- 309S has a lower carbon limit to reduce sensitization; its higher Ni content reduces the tendency for sigma-phase and promotes ductile weld metal structure.
- 304 can be more susceptible to sensitization in the heat-affected zone (HAZ) if cooling is slow; low-carbon 304L or post-weld solution anneal can be used to mitigate sensitization.
- Hardenability and HAZ cracking are not usually limiting for these austenitic grades.
- Use of predictive weldability indexes:
- Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
- Chromium–equivalent (Pcm): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
- Interpretation: 309S’s higher Cr and Ni raise alloying terms, but its low carbon reduces $C$ contribution. Practically, welders often use matching or slightly higher-alloyed filler (e.g., 309L filler) when joining dissimilar steels or when a weld with superior high-temperature oxidation resistance is required.
6. Corrosion and Surface Protection
- Stainless (both 304 and 309S): corrosion resistance is dominated by chromium content and the integrity of the passive film.
- For aqueous corrosion at ambient temperatures, 304 provides excellent performance in many environments (food processing, mild chemical exposure). 309S does not normally improve aqueous corrosion significantly over 304; its advantage is at elevated temperatures.
- For high-temperature oxidation and cyclic heating, 309S forms a more protective, adherent oxide scale due to higher Cr and Ni, making it preferable for furnace parts, burners, and heat exchangers.
- Use of PREN (for comparing pitting resistance where Mo and N are significant): $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- PREN is not particularly informative for 304 or 309S because neither grade contains significant Mo; nitrogen contributions are minor, so PREN numbers will not reflect their primary oxidation performance differences.
- Non-stainless steels: for reference, carbon or low-alloy steels require coatings (galvanizing, painting, thermal barrier coatings) for corrosion protection; such measures are not typically applied to stainless grades in the same way.
7. Fabrication, Machinability, and Formability
- Machinability:
- Austenitic stainless steels are generally more difficult to machine than mild steels due to work-hardening and low thermal conductivity.
- 309S can be marginally harder to machine than 304 because of higher alloy content and work-hardening tendency; tool life may be shorter and feeds/speeds adjusted.
- Formability and deep drawing:
- 304 has excellent formability and is widely used for deep drawing, stamping, and complex shapes.
- 309S is formable but less suited to extensive deep drawing due to slightly reduced ductility and higher yield/strength.
- Surface finishing:
- Both polish and pickling practices are standard; 309S sometimes requires attention to thermal surface coloration after elevated-temperature service, and oxide scales may require mechanical or chemical removal.
8. Typical Applications
| 304 — Typical Uses | 309S — Typical Uses |
|---|---|
| Food processing equipment, kitchenware, sinks, architectural trim, chemical process piping at ambient/mild temperatures | Furnace linings, furnace hardware, radiant tubes, high-temperature ductwork, burners, heat-treating fixtures |
| Heat exchangers, tanks, and vessels for potable water and many chemicals | Welding filler for joining carbon steels to stainless steels; overlay welds requiring oxidation resistance |
| Automotive trim, fasteners, and general-purpose fabricated parts | Exhaust manifolds and high-temperature flues (intermittent service) |
Selection rationale: pick 304 for cost-effective ambient-temperature corrosion resistance and forming; pick 309S when service involves sustained or cyclic high temperatures or when weld overlays/fillers must resist oxidation.
9. Cost and Availability
- Cost:
- 304 is one of the most widely used stainless grades and is generally the lowest-cost austenitic stainless due to moderate Ni content.
- 309S contains significantly more nickel (and chromium), so raw material cost and therefore finished product cost are higher.
- Availability:
- 304 is ubiquitous across product forms: sheet, plate, coil, tube, bar, wire.
- 309S is readily available in sheet, plate, bar, and welding filler forms but may be less common in some specialty product forms or smaller markets. Lead times and minimum order quantities can be larger for 309S in certain sizes.
10. Summary and Recommendation
| Criterion | 304 | 309S |
|---|---|---|
| Weldability | Excellent; sensitization risk unless controlled | Excellent; low-carbon reduces sensitization |
| Strength–Toughness | Very good toughness, excellent ductility | Slightly higher high-temp strength; good toughness |
| Cost | Lower (economical, widely available) | Higher (more alloyed, higher cost) |
Recommendation: - Choose 304 if you need a cost-effective, highly formable austenitic stainless steel for ambient- to moderately corrosive-service conditions where high-temperature oxidation resistance is not a primary requirement (e.g., food equipment, architectural elements, general process piping). - Choose 309S if the part will operate in elevated-temperature environments (furnaces, exhausts, radiant tubes), requires improved oxidation resistance or high-temperature strength, or if the application involves welding overlays for high-temperature service. Also choose 309S where low carbon is needed to avoid sensitization and to improve weld HAZ behavior in high-temperature cycles.
Closing note: final material selection should consider service temperature profiles, exposure atmospheres (oxidizing vs. reducing), mechanical load and creep requirements, fabrication processes, and life-cycle cost. For critical applications, confirm specific material certificates and perform application-specific corrosion and high-temperature testing or consult the material supplier and metallurgical engineering resources.