310S Stainless Steel: Properties and Limits at High Temperature

310S (UNS S31008) is the low-carbon version of the 25/20 austenitic stainless steel family, and it exists for one reason: to survive oxidation and creep at temperatures where ordinary heat-resistant grades fail. With 24.0-26.0% chromium and 19.0-22.0% nickel, the grade forms a tenacious chromium oxide scale that stays intact in air well above 1000 °C, making it a standard choice for furnace internals, radiant tubes and high-temperature process equipment.

Chemical Composition of 310S

ASTM A240 defines the plate, sheet and strip composition for UNS S31008. The deliberately low carbon ceiling is what distinguishes 310S from the higher-carbon 310 grade (UNS S31000, 0.25% max carbon), which offers more elevated-temperature strength but is harder to weld without sensitisation.

Element (wt.%)	310S / UNS S31008	310 / UNS S31000
Carbon	0.08 max	0.25 max
Silicon	1.50 max	1.50 max
Manganese	2.00 max	2.00 max
Phosphorus	0.045 max	0.045 max
Sulfur	0.030 max	0.030 max
Chromium	24.0-26.0	24.0-26.0
Nickel	19.0-22.0	19.0-22.0

European and Japanese equivalents are EN 1.4845 (X8CrNi25-21) and JIS SUS310S, and tubing is commonly ordered to ASTM A213 or ASTM A312 depending on whether the duty is heat exchanger tube or pressure piping. The high nickel content stabilises the austenitic structure so that it does not transform on cooling; it is also the main reason the alloy stays ductile after long furnace exposure.

Mechanical Properties at Room Temperature

Property	Requirement (ASTM A240, S31008)
Tensile strength, min	515 MPa
0.2% offset yield strength, min	205 MPa
Elongation in 50 mm, min	40%
Hardness, max	217 HBW
Structure	fully austenitic

These room-temperature minima are only a quality check. Design at temperature must use the creep and stress rupture data of the applicable pressure equipment code, because above roughly 600 °C the allowable stress of an austenitic steel is governed by creep rather than by yield. 310S retains useful creep strength to about 1000 °C and oxidation resistance in air beyond that, with published guidance commonly placing continuous service near 1150 °C and intermittent service near 1035 °C. Thick sections should be derated further, since thermal gradients and restraint drive cracking in real furnace hardware.

Oxidation, Scaling and Phase Stability

The chromium oxide scale that protects the alloy is self-healing in oxidising atmospheres: as long as oxygen is available and the surface is not mechanically removed, the scale reforms. Practical limits arise from the environment, not from the alloy alone. Cyclic atmospheres cause spalling when the scale and the metal expand at different rates, and 310S performs far better than 18/8 grades in this respect because of its higher chromium and nickel.

Two degradation mechanisms dominate service experience. The first is sigma phase: prolonged exposure in the 800-1000 °C range in this alloy family can precipitate a hard, brittle intermetallic phase that sharply reduces impact toughness. Furnace components should avoid unnecessary soaking in that window, and if embrittlement is suspected, a solution anneal near 1100 °C followed by rapid cooling restores toughness. The second is carburisation or sulfidation in process atmospheres; in strongly reducing or sulfur-bearing gases, the protective oxide is not stable and high-alloy or nickel-based materials are required instead.

Fabrication and Welding

310S is weldable by the usual arc processes. Its high nickel content makes the weld pool more viscous than that of a 304 or 316 grade, so joints need adequate current, correct joint preparation and sufficient interpass control to achieve full penetration without lack of fusion. Matching filler metals designed for 25/20 chemistry are used for elevated-temperature joints, and dissimilar joints require a filler selected for the service temperature rather than for room-temperature strength alone.

Forming is good in the annealed condition, but the grade work-hardens rapidly, so heavy cold forming requires intermediate annealing. Machining demands rigid tooling and positive feeds for the same reason. Hot forming should be finished above 900 °C and followed by annealing, and any heat tint from welding or thermal cutting must be removed by pickling or grinding before the part enters high-temperature service, since the tinted layer has reduced oxidation resistance and can act as an initiation site for scale failure.

Typical Applications

Application	Why 310S is specified
Furnace radiant tubes and muffles	oxidation resistance with creep strength at 900-1100 °C
Heat treatment fixtures and baskets	resists repeated thermal cycling and scale spalling
High-temperature heat exchangers	retains strength where 304 and 309 grade out
Waste incineration linings	tolerates hot gas and ash deposits in the oxidising zone
Molten salt and thermal storage hardware	stable austenitic structure at elevated temperature
Petrochemical reformer internals	cost-effective between 18/8 grades and nickel alloys

Where a nickel-based alloy would be the alternative, 310S is normally the first choice if the atmosphere is oxidising and the temperature stays within the limits above, because it costs substantially less and is easier to weld. Nickel-based grades become necessary above those limits or when the gas chemistry is reducing.

Frequently Asked Questions

Q: What is the maximum service temperature of 310S?
A: In oxidising air, published guidance places continuous service near 1150 °C and intermittent service near 1035 °C. In load-bearing duty the practical limit is set by creep rather than oxidation and is typically lower, so the design must be checked against the allowable stress data for the actual temperature.

Q: What is the difference between 310 and 310S?
A: Carbon content. 310 (UNS S31000) permits up to 0.25% carbon, which raises high-temperature strength, while 310S (UNS S31008) is limited to 0.08% carbon, which improves weldability and resistance to sensitisation at the cost of some creep strength.

Q: Can 310S be used in sulfur-bearing furnace atmospheres?
A: Not safely. Sulfur-bearing or strongly reducing gases destabilise the protective chromium oxide scale and lead to rapid sulfidation attack. In those conditions a higher-nickel alloy designed for reducing service is required rather than an austenitic stainless steel.

Q: What causes sigma phase in 310S and how is it reversed?
A: Long exposure in the 800-1000 °C range allows chromium-rich intermetallic sigma phase to precipitate, lowering impact toughness. Avoiding extended soaking in that window prevents the problem, and a solution anneal near 1100 °C with rapid cooling restores a single austenitic structure.

Q: How should 310S be welded for high-temperature service?
A: Use a matching 25/20 filler metal, prepare joints for full penetration, and control heat input because the viscous weld pool makes lack of fusion the most common defect. Mechanical cleaning of the completed joint removes heat tint, which otherwise reduces oxidation resistance.

Q: Is 310S magnetic?
A: No. It is fully austenitic at room temperature and remains essentially non-magnetic even after welding. This is one reason it is used for components where magnetic interference must be avoided.