310S vs 309 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers, procurement managers, and manufacturing planners commonly face a trade-off when specifying high-temperature stainless steels: corrosion resistance, high-temperature strength and creep resistance, weldability, and material cost. Grades 310S and 309 (and their “S” low‑carbon variants) are both austenitic stainless steels selected for elevated-temperature service, furnace components, and dissimilar-metal joining—but they are optimized slightly differently.
The principal technical distinction between the two is their alloying balance: 310S contains significantly more chromium and nickel than 309, which shifts its performance toward higher temperature oxidation resistance and creep strength, while 309 is often chosen where a balance of good high‑temperature strength, resistance to scaling, and economy (or compatibility for dissimilar welds) is required. Because both are austenitic stainless steels with similar room‑temperature properties, selection is most often driven by expected service temperature, oxidation environment, and budget.
1. Standards and Designations
- Common standards:
- ASTM / ASME: A240 (plate/sheet), A182 (for forgings/pipe fittings in some variants), other product‑specific standards.
- EN: EN 10088 series (designation varies by national numbering).
- JIS and GB: national equivalents with similar compositions and suffixes (e.g., JIS SUS, GB/T).
- UNS: 309/309S (e.g., UNS S30900 / S30908), 310S (e.g., UNS S31008).
- Classification: Both 309 and 310S are austenitic stainless steels (stainless class) — not carbon steels, tool steels, or HSLA. The “S” suffix denotes the low‑carbon variant intended to reduce sensitization during welding.
2. Chemical Composition and Alloying Strategy
The following table shows typical composition ranges for commercial 309 (and 309S) and 310S (low‑carbon 310S). Values are representative ranges used by most mill specifications and standards; product certificates should always be consulted for project procurement.
| Element | 309 / 309S (typical, wt%) | 310S (typical, wt%) |
|---|---|---|
| C | ≤ 0.08 (309S) / up to ~0.20 (309) | ≤ 0.08 (low‑carbon 310S) |
| Mn | 1.0 – 2.0 | 1.0 – 2.0 |
| Si | 0.5 – 1.0 | 0.5 – 1.5 |
| P | ≤ 0.04 – 0.045 | ≤ 0.04 – 0.045 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | ~22 – 24 | ~24 – 26 |
| Ni | ~12 – 15 | ~19 – 22 |
| Mo | ≤ 0.6 (usually nil) | ≤ 0.6 (usually nil) |
| Nb / Ti / V | usually not added | usually not added |
| B | trace only | trace only |
| N | trace – 0.12 | trace – 0.12 |
How alloying affects performance: - Chromium is the primary contributor to high‑temperature oxidation resistance and passive corrosion resistance. Higher Cr improves scale formation at elevated temperatures. - Nickel stabilizes the austenitic matrix, enhances ductility, and improves creep and high‑temperature strength. Higher Ni in 310S is a major reason for its superior performance at very high temperatures. - Carbon content affects sensitization (chromium carbide precipitation) during welding or slow cooling. Low‑carbon “S” grades reduce the risk of intergranular corrosion after welding. - Manganese and silicon assist with hot‑workability and deoxidation; neither grade is intended for pitting or crevice corrosion resistance since both contain little or no molybdenum.
3. Microstructure and Heat Treatment Response
- Typical microstructure: Both 309 and 310S are fully austenitic in the annealed condition. Grain size and precipitates depend on thermomechanical history and cooling rate.
- Heat treatment:
- Austenitic stainless steels are not hardened by conventional quench-and-temper cycles—mechanical properties are set by cold work and grain structure. Solution annealing (e.g., 1,040–1,100 °C followed by rapid cooling) dissolves carbides and restores ductility.
- For “S” grades, solution annealing and rapid cooling minimize carbide precipitation and preserve corrosion resistance.
- Effects of thermo‑mechanical processing:
- Cold working increases strength (work hardening) and can reduce ductility; both grades respond similarly to cold work.
- At elevated service temperatures, long exposures can cause grain growth and, with higher carbon grades, carbide precipitation at grain boundaries. 310S’s higher Ni and Cr content tends to slow detrimental microstructural changes at very high temperatures compared with 309.
- Normalizing/quenching: Not applicable in the same way as for ferritic or martensitic steels; control of thermal cycles and post‑weld heat treatment (PWHT) considerations differ because these austenitic grades retain toughness and resist hardening.
4. Mechanical Properties
The mechanical properties of annealed austenitic stainless steels are influenced by product form (sheet, plate, pipe), cold work, and exact chemistry. The table below summarizes representative, typical ranges for annealed material (values are indicative; consult supplier-certified data for design calculations).
| Property (annealed, typical) | 309 / 309S | 310S |
|---|---|---|
| Tensile strength (MPa) | ~510 – 750 | ~520 – 750 |
| Yield strength, 0.2% (MPa) | ~200 – 320 | ~200 – 320 |
| Elongation (%, in 50 mm) | ~35 – 55 | ~35 – 55 |
| Impact toughness (Charpy V‑notch, J) | Ductile at RT; retains toughness | Ductile at RT; retains toughness |
| Hardness (HRB) | ~80 – 95 | ~80 – 95 |
Interpretation: - At room temperature, both grades exhibit similar strength and ductility ranges; differences are modest because both are austenitic. - At elevated temperatures, 310S generally maintains scale resistance and creep strength better than 309, attributable to its higher combined Cr and Ni content. - Impact toughness at room temperature is usually good for both; neither grade is chosen for high‑hardness applications.
5. Weldability
Austenitic stainless steels are generally weldable, and the low‑carbon “S” variants improve resistance to sensitization. Weldability assessment often uses empirical indices such as the carbon equivalent:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
and the more detailed Pcm formula:
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Low carbon content in 309S and 310S reduces the risk of chromium carbide precipitation and intergranular attack after welding; this makes them easier to weld than corresponding high‑carbon grades. - Both grades have low hardenability (austenitic stainless steels do not form martensite on cooling), minimizing cold cracking risk. Welding cracking risks are dominated by solidification cracking, hot tearing, and contamination rather than martensitic transformation. - 309 is frequently used as a filler metal to join stainless steels to carbon steels (because its composition bridges the two), whereas 310S is selected when the weldment must retain superior high‑temperature oxidation resistance. - Preheat and post‑weld heat treatment are generally not required to avoid martensitic transformation, but attention to dilution, interpass temperatures, and filler selection is critical for high‑temperature service and carburizing/oxidizing atmospheres.
6. Corrosion and Surface Protection
- As stainless austenitic grades, both 309 and 310S form a chromium oxide passive film that provides general corrosion resistance in non‑aggressive environments.
- PREN (Pitting Resistance Equivalent Number) is commonly used for assessing resistance to localized pitting when molybdenum and nitrogen are present:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Applicability:
- For 309 and 310S, PREN is of limited utility because both typically contain little or no molybdenum and only low nitrogen levels; their resistance to pitting and crevice corrosion in chloride‑bearing environments is therefore limited compared with Mo‑bearing stainless alloys (e.g., 316, duplex grades).
- High‑temperature oxidation:
- 310S, with its higher Cr and Ni, has superior scaling resistance and retains strength better at continuous service temperatures higher than 309.
- 309 offers good oxidation resistance up to moderately high temperatures and can be more economical in many furnace and heat‑treatment applications.
- Surface protection for non‑stainless parts: Not applicable for these stainless grades; if used in aggressive chloride or acidic environments, consider coatings, cladding, or different alloys.
7. Fabrication, Machinability, and Formability
- Formability and bending: Both grades are highly ductile and exhibit excellent formability in the annealed condition. Typical sheet forming operations are straightforward, but springback should be accounted for because of high work hardening.
- Machinability: Austenitic stainless steels are more difficult to machine than carbon steels. 309 and 310S work‑harden rapidly and have lower thermal conductivity—this demands rigid setups, sharp tooling, and controlled feed/rate. 310S may be marginally more ductile and slightly easier to machine in some conditions, but neither is classified as easy‑machining.
- Surface finishing: Both take standard stainless finishes and polishing well. Care must be taken to avoid contamination (iron pickup) during fabrication, which can impair corrosion resistance.
- Welding fabrication: Use appropriate filler metallurgy for service conditions; select 309 filler for dissimilar joints and 310/310S filler when high oxidation resistance is required.
8. Typical Applications
| 309 / 309S (Common uses) | 310S (Common uses) |
|---|---|
| Furnace parts and linings exposed to cyclic heating | Furnace elements and continuous high‑temperature service (heaters, muffles) |
| Heat‑treatment fixtures and retorts | High‑temperature process vessels and ducting (oxidizing atmospheres) |
| Welding filler for joining stainless to carbon steel | High‑temperature corrosion/oxidation resistant components up to highest practical temperatures |
| Exhaust systems and industrial furnace hardware | Heat exchangers and combustion equipment where high Ni/Cr needed |
| Expansion joints and flue liners | Glass and petrochemical process equipment at elevated temperatures |
Selection rationale: - Choose 309 when cost and good high‑temperature performance are needed but the maximum service temperature is moderate and when frequently joining to carbon steels. - Choose 310S when the application requires superior long‑term scaling resistance and creep performance at higher sustained temperatures, and the budget allows for the higher alloy cost.
9. Cost and Availability
- Relative cost: 310S is generally more expensive than 309 due to higher nickel and chromium contents. Nickel price volatility can significantly affect 310S pricing.
- Availability: Both grades are widely produced and available in sheet, plate, pipe, tube, and bar. Certain specialized product forms or very large sizes of 310S may have longer lead times than 309, depending on mill inventories and market conditions.
- Procurement tip: For projects sensitive to material cost, evaluate whether the higher performance of 310S at operating temperature yields lifecycle savings (longer life, fewer replacements) that offset the higher initial material cost.
10. Summary and Recommendation
| Property | 309 / 309S | 310S |
|---|---|---|
| Weldability | Excellent for stainless welding; preferred filler for dissimilar joints | Excellent; low‑carbon S grade reduces sensitization |
| Strength–Toughness (RT) | Good, similar to other austenitics | Good, similar at RT; superior high‑temperature creep/scale resistance |
| Cost | Lower | Higher |
Final recommendations: - Choose 310S if your design requires the best available austenitic high‑temperature oxidation resistance and creep performance for continuous or long‑duration service at elevated temperatures, or when oxidation scale resistance at the upper end of austenitic temperature ranges is critical. - Choose 309 (or 309S) if you need a cost‑effective austenitic stainless that offers good high‑temperature strength, frequent compatibility for dissimilar‑metal welding (e.g., joining to carbon or low‑alloy steels), and adequate scaling resistance for moderate high‑temperature service.
Always confirm the exact alloy specification and mill test certificates for critical components, and consider lab or vendor data for creep rupture limits, oxidation test results, and weld filler compatibility for the intended environment and service temperature.