310 vs 310S – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Type 310 and 310S are austenitic stainless steels commonly specified for high-temperature service. Engineers, procurement managers, and manufacturing planners frequently weigh the trade-offs between corrosion resistance, high-temperature strength, and weldability when choosing between them—especially where furnace components, heat exchangers, or welded assemblies will operate in elevated-temperature environments.
The primary technical distinction between the two grades is the carbon specification: 310 allows a higher maximum carbon content than 310S, while their chromium and nickel levels are essentially the same. That carbon difference drives decisions about susceptibility to carbide precipitation (sensitization), weldability, and sometimes marginal differences in strength at elevated temperature. Because they share the same austenitic chemistry otherwise, they are compared closely in design and fabrication decisions.
1. Standards and Designations
Common standards and designations for these grades include: - ASTM/ASME: Type 310 (UNS S31000), Type 310S (UNS S31008); referenced in ASTM A240 (plate, sheet, and strip), A312 (seamless and welded pipe), and other product standards. - EN: 1.4841 (310), 1.4845 (310S) in some European designation schemes. - JIS: SUS310, SUS310S (Japanese standards correspond closely). - GB (China): GB/T product standards for stainless steels often reference equivalent chemistries.
Classification: both 310 and 310S are austenitic stainless steels (stainless, high-alloy group). They are not carbon steels, tool steels, or HSLA.
2. Chemical Composition and Alloying Strategy
Table: Typical composition ranges (wt%) as commonly specified in standards such as ASTM A240. Values are representative ranges; check the specific material certificate for batch values.
| Element | 310 (typical range) | 310S (typical range) |
|---|---|---|
| C | 0.08–0.25 (max 0.25) | 0.03–0.08 (max 0.08) |
| Mn | ≤ 2.0 | ≤ 2.0 |
| Si | ≤ 1.0 | ≤ 1.0 |
| P | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | 24.0–26.0 | 24.0–26.0 |
| Ni | 19.0–22.0 | 19.0–22.0 |
| Mo | — (trace) | — (trace) |
| V | — | — |
| Nb (Cb) | — | — |
| Ti | — | — |
| B | — | — |
| N | ≤ 0.10 (trace) | ≤ 0.10 (trace) |
How alloying affects performance: - Chromium and nickel establish the austenitic matrix and provide oxidation and corrosion resistance at elevated temperature. High Cr (~25%) gives excellent scaling resistance. - Nickel stabilizes the austenitic phase and maintains toughness. - Carbon increases high-temperature strength and creep resistance to some degree but also increases risk of carbide precipitation in the sensitization temperature range (approximately 425–870°C). - Lower carbon in 310S reduces risk of intergranular carbide precipitation after welding or exposure in the sensitization band, improving corrosion resistance in welded or sensitized components.
3. Microstructure and Heat Treatment Response
Microstructure: - Both grades are fully austenitic in the annealed condition. Typical grain structures are stable austenite unless significant cold work or delta ferrite formation occurs during welding thermal cycles. - No martensitic transformation occurs on quenching (austenitic stainless steels are non-hardenable by quench and temper).
Heat treatment and thermal processing: - Solution annealing (commonly 1050–1120 °C) followed by rapid cooling restores an austenitic, corrosion-resistant microstructure and dissolves precipitates. - Because they cannot be hardened by quenching, strength adjustments rely on cold work or alloy selection. - The higher carbon in 310 increases the driving force for chromium carbide precipitation during exposure in the sensitization range, which can lead to grain boundary chromium depletion and intergranular corrosion. 310S’s lower carbon minimizes this risk. - Welding thermal cycles: both grades are weldable, but 310S is less prone to post-weld sensitization and requires less attention to post-weld heat treatments intended to avoid intergranular corrosion.
4. Mechanical Properties
Table: Representative mechanical properties for annealed material (flat-rolled/typical conditions). These are indicative; product form, thickness, and specification may change values.
| Property | 310 (annealed, typical) | 310S (annealed, typical) |
|---|---|---|
| Tensile strength (MPa) | ~500–600 (typical) | ~500–600 (typical) |
| Yield strength (0.2% offset, MPa) | ~200–260 | ~200–240 |
| Elongation (%) | ≥ 40 (good ductility) | ≥ 40 (slightly better ductility tendency) |
| Impact toughness | High, retains toughness to low T | High, similar or marginally better due to lower carbon |
| Hardness (HB / HRC) | Moderate; annealed hardness typically in austenitic stainless range | Similar or slightly lower in annealed condition |
Interpretation: - Mechanical properties in the annealed state are very similar because the austenitic matrix is the same. The slightly higher carbon in 310 can give marginally higher strength in some conditions, especially after some cold work or long-term high-temperature exposure, but at the cost of increased sensitization risk. - Both grades have excellent toughness and ductility compared with ferritic/ martensitic steels, especially at low temperatures.
5. Weldability
Weldability depends strongly on carbon equivalent and the tendency to form hard or brittle microstructures in the heat-affected zone (HAZ). Useful empirical indicators include the IIW carbon equivalent and the Pcm formula:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation for 310 vs 310S: - The primary variable in these formulas for 310/310S is $C$. 310S’s lower carbon gives a lower $CE_{IIW}$ and $P_{cm}$, indicating reduced risk of HAZ problems and better weldability in terms of avoiding sensitization and maintaining ductility after welding. - Austenitic stainlesss generally do not form hard martensite in the HAZ, but carbide precipitation and intergranular attack are concerns. For welded fabrications exposed in the sensitization range, 310S is usually preferred. Where post-weld service involves only very high temperatures (above the carbide dissolution range) or where creep strength is critical and sensitization is not an issue, 310 may be acceptable. - Preheat and PWHT are rarely used to avoid martensite (not applicable), but solution annealing may be specified post-weld where corrosion performance is critical.
6. Corrosion and Surface Protection
- Both 310 and 310S are corrosion-resistant due to high Cr and Ni. They offer excellent oxidation resistance in high-temperature oxidizing atmospheres (scaling resistance).
- For chloride stress corrosion cracking resistance, austenitic steels without molybdenum are generally susceptible under aggressive chloride environments; neither grade is specialized for chloride resistance.
- PREN (pitting resistance equivalent number) is usually applied to stainless steels containing Mo and N. For reference:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Because 310/310S typically contain negligible Mo and low N, PREN is not a meaningful discriminator for pitting resistance in these grades; their resistance depends more on surface condition, environment, and temperature.
- Surface protection: for non-stainless steels one would consider galvanizing or coatings; for 310/310S, surface finish, pickling, passivation, or aluminizing (for extreme oxidation resistance) are relevant, depending on service. 310S’s lower C improves resistance to intergranular corrosion where carbides could otherwise form.
7. Fabrication, Machinability, and Formability
- Formability: Both grades form and bend well in the annealed condition but work-harden rapidly (typical austenitic behavior). Use proper tooling and allowance for springback.
- Machinability: Austenitic stainless steels are difficult to machine compared with mild steels: they work-harden, have low thermal conductivity, and require rigid setups, sharp tooling, and appropriate feeds. 310/310S are similar in machinability; 310S may be marginally easier due to slightly lower hardness in some conditions.
- Welding and forming sequence planning: prefer forming before welding where possible to avoid localized hardening and to control distortion.
- Surface finishing: grinding, polishing, and passivation follow standard austenitic stainless practices.
8. Typical Applications
| 310 (common uses) | 310S (common uses) |
|---|---|
| Furnace parts, muffles, heat-treat baskets, industrial ovens where high-temperature oxidation resistance is primary and welding is controlled | Welded heat-exchanger components, chemical processing equipment where weld sensitization must be minimized |
| Burner and combustion hardware, radiant tubes, kiln components where high-temperature creep and scaling resistance are required | Piping, fittings, and welded vessels in high-temp but corrosive environments where post-weld corrosion resistance is needed |
| High-temperature flue gas applications where occasional fabrication can be done without extensive welding | Where frequent welding, post-fabrication machining, or service in the sensitization range requires a low-carbon alternative |
Selection rationale: - Choose 310 where maximum high-temperature strength and oxidation resistance are the priority and where fabrication can be controlled to avoid sensitization issues. - Choose 310S where welded assemblies will be placed in the sensitization temperature window, or where post-weld corrosion resistance and improved weldability are required.
9. Cost and Availability
- Cost: 310S is often priced slightly higher than 310 because of production controls required to achieve the lower carbon specification and because it is commonly specified for more critical welded applications. Actual price differences are modest and vary with market nickel and chromium prices.
- Availability: both grades are widely available in sheet, plate, coil, pipe, and tube forms. 310 is sometimes more commonly stocked for standard high-temperature components, while 310S is commonly stocked for pressure parts and welded fabrications.
- Lead times: depend on product form and size; procuring large-diameter or heavy-section product in specialty grades may increase lead time.
10. Summary and Recommendation
Table: Quick summary
| Attribute | 310 | 310S |
|---|---|---|
| Weldability | Good, but greater sensitization risk after welding | Better — lower carbon reduces sensitization and improves post-weld corrosion resistance |
| Strength – Toughness | High-temperature strength comparable; 310 may show marginally higher strength in some high-temp exposures | Similar toughness; slightly better ductility and lower risk of grain boundary carbide issues |
| Cost | Slightly lower or comparable | Slight premium typical |
Final recommendations: - Choose 310 if your priority is maximum high-temperature oxidation/corrosion resistance where the component will not be susceptible to sensitization problems (for example, as replaceable furnace internals or non-welded high-temperature components), or when marginally higher high-temperature creep strength is required and welding exposure/conditions are controlled. - Choose 310S if your design involves extensive welding, requires minimized risk of intergranular corrosion after welding, or will spend significant time in the sensitization temperature range. 310S is the safer specification for welded pressure parts and fabricated vessels where post-fabrication corrosion resistance is critical.
Closing note: Both grades are excellent choices for high-temperature service. The carbon specification is the key differentiator: assess weld procedures, intended service temperatures (particularly whether components will traverse or remain in the 425–870°C sensitization range), and cost/availability to make the final selection.