309S and 310S Stainless Steel: High-Temperature Grade Guide

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

1. What Are 309S and 310S Stainless Steel?

309S and 310S are austenitic heat-resistant stainless steels engineered for elevated-temperature service where oxidation, thermal cycling, and carburization — rather than aqueous corrosion — determine service life. Their performance comes from deliberately high chromium and nickel content, not from hardness or mechanical strength at temperature.

These grades serve a fundamentally different function than 304 or 316. 316's value is measured by chloride pitting resistance (molybdenum); 309S and 310S are measured by how well they resist oxidation at temperature (chromium and nickel). A buyer who selects 310S expecting the chloride resistance of 316 has made a specification error — these grades are optimized for heat, not chlorides.

2. Chemical Composition of 309S and 310S

Reference: ASTM A240 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip.

3. Why 309S and 310S Resist High-Temperature Degradation

High-temperature failure is fundamentally different from aqueous corrosion. Three mechanisms dominate service life in furnace and combustion environments. Each is addressed by specific alloy design features in 309S and 310S.

3.1 Oxidation Resistance — The Chromium Oxide Barrier

At elevated temperature, the metal surface reacts with oxygen to form an oxide scale. The difference between acceptable and unacceptable performance depends on whether this scale is protective (thin, adherent, slow-growing) or breakaway (thick, spalling, rapid metal loss).

Chromium forms Cr₂O₃ — a dense, adherent oxide that acts as a diffusion barrier, dramatically reducing further oxidation. Higher chromium content maintains this protective scale to higher temperatures. Above approximately 900–1000°C, Cr₂O₃ can convert to volatile CrO₃ in flowing air; higher-Cr grades like 310S resist this transition longer because a thicker, more stable Cr₂O₃ scale forms initially. This is the fundamental reason 310S (25% Cr) operates ~115°C higher than 309S (22–24% Cr) in oxidizing air.

3.2 Thermal Cycling Resistance — The Nickel Contribution

Furnace components experience repeated heating and cooling. Each cycle subjects the material to expansion and contraction. If the oxide scale has a different expansion coefficient than the base metal, or if the base metal lacks ductility at temperature, the scale spalls — exposing fresh metal to rapid oxidation.

Nickel stabilizes the austenitic FCC structure, which maintains ductility to higher temperatures. This allows the material to accommodate thermal strains without cracking and helps the oxide scale remain adherent through cycles. 310S (20% Ni) significantly outperforms 309S (12–15% Ni) under cyclic conditions — not because the oxide is chemically different, but because it stays attached longer.

3.3 Carburization and Sulfidation Resistance

Furnace atmospheres contain carbon monoxide, hydrocarbons, and sulfur compounds from combustion. Carburization — carbon diffusing into the metal and forming internal chromium carbides — embrittles the material and locally depletes chromium, reducing oxidation resistance. The dense Cr₂O₃ scale provides a physical barrier, and high nickel reduces carbon solubility and diffusion rate in the matrix. Sulfidation — attack by sulfur-containing gases — can cause catastrophic grain boundary damage. No standard stainless steel is fully immune to sulfidation, but the high-chromium scale and high-nickel matrix of 310S provide the best available resistance before moving to nickel-based alloys.

4. 309S Stainless Steel Properties

309S is an austenitic grade with approximately 22–24% Cr and 12–15% Ni. It is non-magnetic in the annealed condition, cannot be hardened by heat treatment, and is readily weldable by all common processes (TIG, MIG, SMAW) using ER309 or ER309L filler metal.

Typical applications: furnace components and burner parts operating at moderate temperatures, heat treatment baskets and fixtures, kiln liners and supports, combustion equipment where the temperature does not warrant the 310S premium, and dissimilar metal welding (309 filler is the industry standard for joining carbon steel to stainless steel — its high alloy content compensates for dilution from the carbon steel side).

5. 310S Stainless Steel Properties

310S contains approximately 25% chromium and 20% nickel — the highest Cr and Ni content among common wrought austenitic grades. It is fully austenitic, non-magnetic in the annealed condition, and weldable with ER310 filler metal.

Typical applications: furnace tubes and radiant tubes in petrochemical heaters, heat treatment baskets operating above 1000°C, combustion chamber components, cement kiln internals, thermal oxidizers, and high-temperature flue gas handling equipment. The common thread: sustained high temperature combined with carburizing or mildly sulfur-containing atmospheres that would prematurely degrade lower-alloy grades.

6. 309S vs 310S: Understanding the Difference

The distinction between these grades is driven entirely by alloy content — and the performance gap they create at elevated temperature.

The fundamental selection logic: Choose 309S when the temperature and atmosphere are moderate and cost efficiency matters. Choose 310S when maximum oxidation resistance among wrought austenitic grades is required — higher temperature ceiling, carburizing atmosphere, cyclic thermal conditions, or extended service life expectations that justify the nickel premium.

7. Temperature Capability and Application Limits

Temperature ratings are guidelines, not guarantees. Actual performance at a given temperature depends on multiple factors operating simultaneously:

8. Common Applications of 309S and 310S

9. Limitations and Common Selection Mistakes

Heat-resistant grade selection errors are expensive because they manifest as in-service failures — not at incoming inspection.

Mistake 1 — Using 304 or 316 for high-temperature oxidation service. These are corrosion-resistant grades, not heat-resistant grades. At ~18% Cr, 304 oxidizes rapidly above approximately 870°C intermittent. In a furnace operating at 950°C, 304 will lose section thickness and fail — not because the material is defective, but because it was specified for the wrong degradation mechanism. Select the grade for the failure mode: chlorides → 316; high-temperature oxidation → 309S/310S.

Mistake 2 — Assuming 310S solves all high-temperature problems. 310S has limits: it cannot withstand molten salts, severe sulfidation (H₂S above certain concentrations), or temperatures near 1150°C under mechanical load where creep becomes limiting. Above 310S's capability, nickel-based alloys (Alloy 600, 601, 800H/HT) or cast heat-resistant grades are required.

Mistake 3 — Ignoring atmosphere and fabrication condition. Weld heat tint (chromium-depleted oxide) initiates premature oxidation. Surface contamination with carbon steel creates localized non-protective scale. Pickling and passivation after fabrication, proper filler metal selection, and clean surface condition before first heat-up are essential — a correctly specified grade can fail if fabrication undermines its oxidation resistance.

10. Procurement Guidance for 309S and 310S

11. How Shangyou Supports 309S and 310S Procurement

• ASTM Specification Verification: Every order confirmed against the applicable ASTM standard — Cr and Ni content verified against the specified grade range before dispatch
• Chromium and Nickel Content Confirmation: The defining elements verified from MTC heat analysis: Cr 22–24% (309S) or 24–26% (310S); Ni 12–15% (309S) or 19–22% (310S). Out-of-spec values trigger rejection
• Carbon Content Verification: Confirmed ≤ 0.08% for S-grade — carbon directly affects weldability and long-term elevated-temperature performance
• MTC 3.1 Documentation: Full chemical composition and mechanical properties traceable to heat number — actual data, not nominal values
• PMI Testing Coordination: XRF or OES confirms Cr and Ni content, providing independent verification of the alloy elements that determine oxidation resistance
• Dimensional and Surface Inspection: Visual inspection for surface defects and dimensional conformance — surface condition directly affects oxide scale formation and adherence at temperature
• Third-Party Inspection: Pre-shipment inspection with SGS, Bureau Veritas, TÜV, or Intertek — MTC review, PMI, dimensional check, and surface assessment
• Technical Grade Selection Support: Our engineering team reviews your temperature, atmosphere, and loading conditions to confirm that 309S or 310S is appropriate — or recommend an alternative if service conditions exceed grade capabilities

12. Frequently Asked Questions

Q1: What is 309S stainless steel?
309S (UNS S30908, EN 1.4833) is an austenitic heat-resistant stainless steel with approximately 22–24% chromium and 12–15% nickel. It is designed for elevated-temperature oxidation resistance — not aqueous chloride corrosion. The "S" suffix indicates low carbon (≤ 0.08%), which reduces sensitization during welding and minimizes carbide precipitation at temperature. Typical applications include furnace components, burner parts, kiln liners, and heat treatment fixtures operating at temperatures up to approximately 1035°C continuous in clean oxidizing air. 309 filler metal is also the standard choice for dissimilar welding of carbon steel to stainless steel.

Q2: What is 310S stainless steel?
310S (UNS S31008, EN 1.4845) is an austenitic heat-resistant stainless steel with approximately 24–26% chromium and 19–22% nickel — the highest alloy content among common wrought austenitic grades. This composition provides maximum oxidation resistance before moving to nickel-based alloys, with a continuous service capability of approximately 1150°C in clean oxidizing air. The high nickel provides superior thermal cycling resistance and improved carburization resistance. Key applications include furnace tubes, radiant tubes, heat treatment baskets above 1000°C, and petrochemical furnace components exposed to carburizing atmospheres.

Q3: What is the difference between 309S and 310S stainless steel?
The difference is driven by alloy content: 309S has ~22–24% Cr and ~12–15% Ni; 310S has ~24–26% Cr and ~19–22% Ni. The extra ~2% chromium raises the oxidation ceiling by approximately 115°C. The additional ~7–8% nickel significantly improves thermal cycling resistance (oxide stays adherent through more cycles), carburization resistance (slower carbon diffusion into the matrix), and long-term microstructural stability at temperature. 310S also permits higher silicon (≤ 1.50% vs. ≤ 0.75%), further enhancing very-high-temperature oxidation. The cost difference is driven primarily by nickel — 310S carries a meaningful alloy premium justified only when service conditions exceed 309S capabilities.

Q4: Is 310S stainless steel corrosion resistant?
310S provides ordinary aqueous corrosion resistance consistent with its chromium and nickel content, but it is not optimized for chloride pitting resistance. It contains no molybdenum — its room-temperature corrosion resistance is general, not chloride-specific. A buyer selecting 310S for an application involving chlorides at ambient temperature has made a specification error; 316L (2–3% Mo, PREN ~23–28) or duplex grades would be more appropriate. 310S is designed for high-temperature oxidation degradation — match the grade to the degradation mechanism, not to a generalized notion of "corrosion resistant."

Q5: Can 310S stainless steel be used at 1000°C?
In clean, still, oxidizing air with light or no mechanical loading, yes — 310S can typically operate at 1000°C and well above (continuous limit ~1150°C in air). However, this capability is reduced by: carburizing or reducing atmospheres, flowing gas (can volatilize Cr₂O₃ as CrO₃ at very high temperatures), mechanical loading (creep strength decreases with temperature), cyclic heating/cooling (oxide spalling), and surface contamination. Temperature alone is insufficient to confirm suitability — always provide the complete atmosphere, loading, and cycling conditions.

Q6: Is 309S better than 304 stainless steel for high-temperature applications?
Yes — for oxidation resistance at elevated temperature. 309S contains ~22–24% Cr versus 304's ~18%, directly translating to better Cr₂O₃ protective scale formation. In clean air, 304 is limited to approximately 870°C intermittent / 925°C continuous, while 309S extends this to approximately 980°C intermittent / 1035°C continuous. For applications consistently above 900°C, 309S is the correct minimum specification — 304 will oxidize at a rate that may be unacceptable for the required service life. However, if the environment involves chlorides rather than high temperature, the selection logic reverses — this is why defining the degradation mechanism is the first step in grade selection.

Q7: Can 309S and 310S be welded?
Yes, both are readily weldable by TIG, MIG, and SMAW. Use matching filler metals: ER309 or ER309L for 309S; ER310 for 310S. The low carbon of S-grades reduces sensitization risk compared to the original higher-carbon versions. For thick-section or critical welds, low-carbon filler (ER309L, ER310L with ≤ 0.03% C) provides additional margin against intergranular corrosion. Post-weld pickling and passivation to remove heat tint is essential — the chromium-depleted oxide left by welding is a preferential site for oxidation initiation at elevated temperature, regardless of the base metal grade.

Q8: What stainless steel is best for furnace applications?
There is no single "best" — furnace applications span wide ranges of temperature, atmosphere, and loading. For moderate temperatures (below ~1000°C in clean oxidizing conditions, lightly loaded), 309S provides a cost-effective balance. For higher temperatures or aggressive atmospheres (above ~1000°C, carburizing, cyclic), 310S provides greater oxidation resistance and thermal stability. For the most severe conditions exceeding wrought austenitic grade capabilities, nickel-based alloys (Alloy 600, 601, 800H/HT) or cast heat-resistant grades are required. The correct grade depends on the specific combination of temperature, atmosphere, cycling, and loading — not on a universal ranking.

Q9: Does higher nickel always mean better heat resistance?
Not universally. Nickel improves thermal cycling resistance, carburization resistance, and microstructural stability — but it is not sufficient alone. Chromium is the primary element for oxidation resistance, which is the most common high-temperature degradation mechanism. A hypothetical grade with 30% Ni and 15% Cr would have worse oxidation resistance than 310S (25% Cr, 20% Ni) despite higher nickel. Both elements are critical: chromium forms the protective scale; nickel stabilizes the structure and provides ductility. A balanced Cr-Ni ratio appropriate to the service conditions is more important than maximizing either element individually.

Q10: What information should I provide when ordering heat-resistant stainless steel?
Provide as a minimum: (1) Grade + UNS (309S/S30908 or 310S/S31008); (2) Product standard and form with dimensions; (3) Service temperature — continuous and peak values; (4) Atmosphere composition — air, combustion gases, carburizing, sulfur-containing; (5) Thermal cycling — continuous or cyclic, with approximate frequency; (6) Mechanical loading — lightly loaded or structural; (7) Required MTC type — EN 10204 3.1 minimum; (8) Supplementary testing — PMI, IGC per ASTM A262 if welded. The more complete the service description, the more accurately the supplier can confirm grade suitability — or flag a potential mismatch before the material is shipped.

Related Reading

• [What Is Stainless Steel? Types, Properties, Manufacturing and Uses]
• [Stainless Steel Grades Chart: AISI, UNS, EN and JIS Cross-Reference Guide]
• [304 and 304L Stainless Steel: Properties, Composition and Uses]
• [316 and 316L Stainless Steel: Properties, Corrosion and Uses]
• [L, H and Dual-Certified Stainless Steel Grades Explained]
• [Does Stainless Steel Rust? Causes, Corrosion Types and Prevention]

Technical References

• ASTM A240 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip
• ASTM A312 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Pipe
• ASTM A262 — Standard Practices for Detecting Susceptibility to Intergranular Attack
• Outokumpu Stainless Steel Handbook — Heat-Resistant Grades
• Nickel Institute — High-Temperature Characteristics of Stainless Steels
• ASM Handbook Volume 1 — Properties and Selection: Irons, Steels, and High-Performance Alloys
• British Stainless Steel Association — Selection of Stainless Steels for High-Temperature Service