310S vs 321 – 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 choice between 310S and 321 stainless steels when specifying parts that must balance high-temperature performance, corrosion resistance, weldability, and cost. Typical decision contexts include high-temperature furnace components, heat exchangers, exhaust systems, and welded assemblies that may be exposed to sensitization conditions.
The central practical distinction between the two grades is their alloying strategy for high-temperature and post-weld stability: 310S is a high-chromium, high-nickel austenitic alloy optimized for oxidation and creep resistance at elevated temperatures, while 321 is a titanium-stabilized austenitic alloy engineered to resist intergranular corrosion after welding by preventing chromium carbide precipitation. Because of that difference, designers compare them when the application places simultaneous demands on temperature capability, weld performance, and long-term corrosion resistance.
1. Standards and Designations
Major standards and designations commonly used for these grades include: - ASTM/ASME: 310S — ASTM A240/A240M (heat-resisting, stainless steel), 321 — ASTM A240/A240M (stabilized austenitic stainless steel). - EN (Europe): 310S approximately EN 1.4845 / X10CrNi25-21; 321 approximately EN 1.4541 / X6CrNiTi18-10. - JIS (Japan): equivalents exist (e.g., SUS310S, SUS321). - GB (China): corresponding GB/T designations are commonly used for sheet and plate.
Classification: both 310S and 321 are austenitic stainless steels (stainless alloy class), not carbon steels, tool steels, or HSLA.
2. Chemical Composition and Alloying Strategy
| Element (wt%) | 310S (typical range) | 321 (typical range) |
|---|---|---|
| C | ≤ 0.08 | ≤ 0.08 |
| Mn | ≤ 2.0 | ≤ 2.0 |
| Si | ≤ 1.5 | ≤ 0.75 |
| P | ≤ 0.045 | ≤ 0.045 |
| S | ≤ 0.03 | ≤ 0.03 |
| Cr | 24 – 26 | 17 – 19 |
| Ni | 19 – 22 | 9 – 12 |
| Mo | — (trace if any) | — (trace if any) |
| V | — | — |
| Nb | — | — |
| Ti | — | ~0.4 – 0.7 (stabilizer; typically ≥ 5×C) |
| B | — | — |
| N | ≤ 0.10 | ≤ 0.10 |
Notes on alloying strategy - 310S relies on high chromium and nickel contents to stabilize an austenitic matrix at elevated temperatures, improving oxidation resistance and high-temperature strength. - 321 contains titanium in amounts sufficient to tie up carbon as stable carbides (TiC) and so prevent chromium carbide precipitation (sensitization) in the 425–870°C range. Titanium does not substantially increase base corrosion resistance, but it preserves it after welding or thermal exposure.
How alloying affects properties - Chromium increases oxidation resistance and passive-film stability. - Nickel stabilizes the austenitic matrix, improves toughness and ductility, and enhances strength at elevated temperature. - Titanium in 321 improves post-weld intergranular corrosion resistance by forming stable carbides instead of chromium carbides. - Elevated carbon (not typical here) increases strength but raises susceptibility to sensitization; both grades are low-carbon variants to limit that effect.
3. Microstructure and Heat Treatment Response
Typical microstructures - Both 310S and 321 are fully austenitic at room temperature in standard annealed conditions. Grain structure is equiaxed austenite after anneal. - 321 in as-welded or exposed conditions contains TiC/Ti(C,N) precipitates that tie up carbon; these precipitates are typically fine and distributed at grain boundaries and within grains. - 310S does not contain titanium: in thermal excursions within the sensitization range, chromium carbides (Cr23C6) may precipitate near grain boundaries unless care is taken with heat input and cooling.
Heat treatment response and processing - Annealing: both are annealed to restore ductility after cold work (typical anneal followed by controlled cooling). Solutions for stainless austenitics are usually in the region recommended by standards (follow supplier datasheets). - Normalizing and quench/temper: not applicable in the same sense as for martensitic steels—these austenitic alloys are not hardened by quenching. Their response to conventional quench-and-temper is minimal because they are non-transforming austenitic alloys. - Thermo-mechanical processing: cold work increases strength by strain hardening in both grades; however, cold work plus subsequent heating in 321 does not produce sensitization to the same extent as 310S because titanium stabilizes carbon.
Practical implication - For components subject to repeated thermal cycles or welding, 321 offers more predictable post-weld microstructure and resistance to intergranular attack. For continuous high-temperature oxidation-resistant service, 310S’s higher Cr/Ni content gives an advantage.
4. Mechanical Properties
| Property (typical, annealed, room temperature) | 310S | 321 |
|---|---|---|
| Tensile strength | Comparable; both are moderate (austenitic) | Comparable; similar range |
| Yield strength (0.2% offset) | Comparable; moderate | Comparable; moderate |
| Elongation (ductility) | High (good formability) | High (good formability) |
| Impact toughness | Good, notch-tough at RT; retains toughness at elevated T | Good; retains toughness after weld exposures due to stabilization |
| Hardness (annealed) | Low-to-moderate (easily cold-worked) | Low-to-moderate (easily cold-worked) |
Interpretation - Both grades exhibit similar room-temperature mechanical behavior because both are austenitic stainless steels in the annealed condition. Differences are subtle: 310S’s higher Ni and Cr provide somewhat better high-temperature strength and creep resistance; 321’s titanium content helps maintain toughness and corrosion resistance after welding and thermal exposure. For design-critical load-bearing parts, designers should use supplier-certified mechanical test data for the product form and heat treatment specified.
5. Weldability
Weldability considerations focus on carbon content, stabilizing elements, and hardenability. For stainless steels, carbon equivalents and Pcm can be useful qualitative indicators.
Common equations used to estimate weld cracking/hardenability tendencies: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation - Both 310S and 321 are relatively easy to weld compared with ferritic or martensitic steels because austenitic microstructure does not undergo martensitic transformation and has low hardenability. - 321 has an advantage for welded components subjected to post-weld thermal cycles because titanium prevents chromium carbide formation and thus reduces the risk of intergranular corrosion in the heat-affected zone (HAZ). - 310S, although weldable, requires careful control of welding parameters and post-weld procedures if the assembly will see sensitizing temperatures or corrosive environments; filler selection and proper post-weld cooling are important. - Preheat and post-weld heat treatment are generally not required for these austenitic grades, but good welding practice (appropriate consumables, control of heat input, and cleaning) is important to avoid contamination and nitrogen loss.
6. Corrosion and Surface Protection
Stainless behavior - Both 310S and 321 form protective chromium-rich passive films in oxidizing environments; their resistance to general corrosion is similar in many aqueous environments. - 310S has higher Cr and Ni, so it offers superior high-temperature oxidation resistance and better performance in oxidizing atmospheres at elevated temperatures. - 321’s titanium stabilization is specifically targeted to prevent intergranular corrosion after exposure to the sensitization temperature range.
PREN (pitting resistance equivalent number) is often used for comparing pitting resistance in chloride-containing environments: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ - PREN is most useful for duplex and austenitic stainless steels that contain molybdenum and nitrogen. For 310S and 321, which typically contain little or no Mo, PREN is not a primary selection criterion: both have modest pitting resistance relative to Mo-bearing grades.
Non-stainless alternatives and surface protection - Where stainless grades are not required, common surface protections include galvanizing, painting, and cladding. These are not substitutes for stainless performance at high temperature or in aggressive oxidizing environments; selection depends on expected service conditions.
7. Fabrication, Machinability, and Formability
- Machinability: As austenitic stainless steels, both are tougher to machine than carbon steels; they work-harden readily and require sharp tooling, rigid setups, and appropriate feeds/speeds. 310S may be somewhat more difficult to machine because of higher alloy content, but differences are modest.
- Formability: Both exhibit excellent ductility and formability in the annealed condition. Springback and work-hardening are common considerations; intermediate anneals may be required for heavy forming.
- Surface finish and polishing: 310S’s higher corrosion resistance generally takes polish well and withstands oxidizing high-temperature exposure; 321 polishes well too.
- Cold working: Both can be cold worked to increase strength, but cold work reduces ductility and may change corrosion behavior unless re-annealed.
8. Typical Applications
| 310S — Typical Uses | 321 — Typical Uses |
|---|---|
| Furnace parts, muffles, and radiant tubes (high-temperature oxidation service) | Aircraft exhaust components, expansion joints, and welded chemical processing equipment (stabilized against intergranular corrosion) |
| Heat-exchanger components operating at elevated temperature | Bolting, flanges, and welded fabrications where post-weld corrosion resistance is critical |
| Burner liners, annealing baskets | Petrochemical and food processing equipment exposed to cyclic thermal loads and welding |
Selection rationale - Choose 310S where continuous or cyclic high-temperature oxidation resistance and high-temperature strength are primary requirements. - Choose 321 where welded joints must retain corrosion resistance in the sensitization range or where the component will see repeated thermal cycling and welding is extensive.
9. Cost and Availability
- Cost: 310S is typically more expensive than 321 on a per-kilogram basis because of its higher nickel and chromium content. Nickel is the major cost driver.
- Availability: Both are widely available worldwide in sheet, plate, tubing, and bar forms, but certain product forms (e.g., large-diameter seamless tube in 310S) can be less common and command longer lead times. Local market conditions and nickel price volatility affect relative cost and lead times.
10. Summary and Recommendation
| Attribute | 310S | 321 |
|---|---|---|
| Weldability | Good, but watch HAZ sensitization in some conditions | Very good for welded structures (titanium-stabilized) |
| Strength–Toughness (room temp) | Comparable to 321; better high-temperature strength | Comparable to 310S; better retained corrosion resistance after welding |
| Cost | Higher (higher Ni/Cr) | Lower (moderate Ni/Cr) |
Recommendations - Choose 310S if: - The application requires superior high-temperature oxidation resistance or creep resistance. - Continuous service at elevated temperatures or oxidizing atmospheres is the dominant design driver. - The extra material cost is justified by performance at temperature.
- Choose 321 if:
- The part will be welded frequently or will be exposed to transient thermal cycles that could cause sensitization.
- Long-term resistance to intergranular corrosion in welded structures is important.
- You want a cost-effective austenitic grade with good general corrosion resistance and stable post-weld performance.
Final note Always confirm material selection with supplier mill certificates and consider component geometry, expected thermal cycles, environment (oxidizing vs. reducing, chloride presence), and fabrication process. For critical applications, request full mechanical test data for the specific product form and, when in doubt, perform representative mock-up welding and corrosion testing to validate the chosen grade in the intended service conditions.