SUS321 Stainless Steel vs Aluminum Alloys: High-Temperature Performance

SUS321: A Titanium-Stabilised Austenitic Grade

SUS321 is the Japanese designation of a titanium-stabilised austenitic stainless steel (AISI 321 / UNS S32100, EN 1.4541, GB/T 06Cr18Ni11Ti). Chromium at 17-19% and nickel at 9-12% produce a stable austenitic matrix, and titanium is held at a minimum of five times the carbon content. That titanium addition is the defining feature: carbon is precipitated as titanium carbide instead of chromium carbide, so chromium stays available to rebuild the passive film. The resulting resistance to intergranular corrosion after exposure in the 425-815 C sensitisation band is the main reason the grade is specified for hot-section work. Typical density is 7.9 g/cm3, and the material is stocked as plate, sheet, bar, seamless tube and welded pipe to ASTM A240, ASTM A276 and ASTM A312.

Continuous Service Temperature and Oxidation Performance

SUS321 is designed for continuous service up to approximately 900 C in oxidising atmospheres, where it forms a self-repairing chromium oxide scale that slows further attack. Oxidation resistance is good to about 900 C under continuous exposure; intermittent cycling is normally held somewhat lower because repeated thermal shock can spall the scale. Between 500 C and 850 C the grade still carries useful creep and rupture strength and remains structurally stable, without the ductile-to-brittle transition that can affect ferritic grades. Because the steel is titanium stabilised, welded assemblies can be used in this band without the post-weld solution annealing that unstabilised grades may require.

Continuous service limit: about 900 C (1652 F) in air

Oxidation resistance: excellent, protective Cr₂O₃-rich scale

Intergranular corrosion: resistant, titanium stabilised

Typical sections: heat-exchanger tubes, furnace parts, exhaust hardware, high-temperature piping

Where Aluminum Alloys Reach Their Ceiling

Aluminum is attractive for its low density, but its high-temperature envelope is much narrower. Wrought alloys such as 6061-T6 (UNS A96061) and 7075-T6 (UNS A97075) are normally limited to service below about 150-200 C. Above that range, over-ageing coarsens the strengthening precipitates; 6061-T6, for example, is specified with a room-temperature tensile strength near 310 MPa and a 0.2% proof strength near 276 MPa, and both values fall away quickly once the metal is held hot. Density is roughly 2.70 g/cm3 for 6061 and 2.81 g/cm3 for 7075, so the weight saving against austenitic stainless steel approaches a factor of three. Specialised dispersion-strengthened aluminum products can extend service to roughly 260-427 C, but they are far less common, harder to weld and more expensive than conventional mill grades.

Property Comparison

Property	SUS321 Stainless Steel	Aluminum Alloys
Continuous service temperature	Up to about 900 C	About 150-200 C; specialty grades to 427 C
Oxidation resistance	Excellent, protective chromium oxide scale	Moderate, film thickens and degrades
Structural stability when hot	High, austenitic matrix	Moderate, over-ageing reduces strength
Density	About 7.9 g/cm3	About 2.70-2.81 g/cm3
Thermal conductivity	About 16 W/m.K	About 150-170 W/m.K
Best fit	High temperature plus corrosion resistance	Mid temperature, weight-critical structures

Selection, Fabrication and Delivery Guidance

Choose SUS321 when metal temperature exceeds roughly 500 C, when oxidation or intergranular corrosion is a risk, or when welded joints must survive repeated thermal cycling.

Choose aluminum alloys when duty temperature stays below about 400 C and mass reduction, thermal conductivity or a non-magnetic response governs the design.

Between 400 C and 500 C the decision turns on load, cycle count and environment; a thicker aluminum section that runs cool may still be the lighter answer.

Heat transfer is roughly ten times faster through aluminum, so a support frame may be aluminum while the hot gas path remains stainless steel.

SUS321 works readily: it can be cold formed, welded by TIG, MIG or resistance methods and machined with carbide tooling at conservative feeds. Plate and sheet are supplied pickled and annealed, or with a No.1, 2B or brushed finish, cut to size by plasma, waterjet or laser. Hot-finished and cold-drawn tube is produced to ASTM A312 and ASTM A213 where pressure service applies. Plates are shipped on wooden pallets with interleaving paper, edge protection and moisture-barrier wrapping to prevent transit staining, and each bundle carries a mill test certificate quoting heat number, chemical analysis and mechanical results.

Frequently Asked Questions

Q: What is the maximum continuous service temperature of SUS321?
About 900 C in oxidising air. The limit is set by scale growth and creep rather than by melting point, and it should be reduced for cyclic duty or reducing atmospheres.

Q: Why is titanium added to SUS321?
Titanium combines with carbon to form stable titanium carbides at a minimum ratio of five times the carbon content, which prevents chromium carbide precipitation at grain boundaries and preserves intergranular corrosion resistance after hot exposure or welding.

Q: Can aluminum replace SUS321 in a heat exchanger?
Only when the working temperature stays below roughly 150-200 C for conventional alloys, or below 400 C for specialty grades. Above that, aluminum loses strength while SUS321 keeps it, so the temperature margin favours stainless steel.

Q: Is SUS321 magnetic?
In the annealed condition it is essentially non-magnetic austenitic steel. Cold work can create a slight magnetic response, but it remains far less magnetic than ferritic or martensitic grades.

Q: How should SUS321 be welded?
Use a stabilised matching filler such as ER347 or an equivalent titanium- or niobium-stabilised electrode, keep interpass temperature low and avoid excessive heat input. Post-weld annealing is normally unnecessary because the grade is already stabilised.

Q: Which product forms and standards are available?
Plate and sheet to ASTM A240, bar to ASTM A276, and seamless or welded tube to ASTM A213 and ASTM A312, with dimensional tolerances and certification agreed at order stage.