321 steel (S32100): properties, composition, equivalents

321 stainless steel (UNS S32100, EN 1.4541): titanium-stabilized 18-8 for 425–870 °C service without sensitization. 205 MPa yield, 515 MPa tensile, creep-rated as 321H. Composition, high-temperature properties, 321 vs 304/347, 1.4541 equivalents.

What is 321?

Type 321 is 18-8 stainless stabilized with titanium (≥ 5 × carbon) so that, when the steel sits in the 425–870 °C range — in an exhaust manifold, an expansion bellows, a furnace part or a refinery heater — the carbon precipitates as harmless titanium carbide instead of chromium carbide at the grain boundaries. 304 in the same service sensitizes and corrodes intergranularly; 304L runs out of strength. 321 does neither, which makes it the standard austenitic for intermediate-temperature service and for welded equipment that cannot be annealed after welding. UNS S32100; EN 1.4541.

Room-temperature properties equal 304 (205/515 MPa minimum), corrosion resistance is a shade lower in aqueous service (Ti carbonitrides), and polishing is poorer. 321H (S32109), with 0.04–0.10 % carbon and a minimum grain size, is the creep-rated version used for ASME pressure parts above 525 °C and rated to 816 °C.

The niobium-stabilized 347 is the sister grade: Nb, unlike Ti, survives the welding arc, so 347 filler (ER347) is used for both. GOST 08Kh18N10T — the most-produced Russian stainless — and EN 1.4541 are identical.

321 chemical composition

ASTM A240/A240M-22 Table 1 (S32100). EN 1.4541 (X6CrNiTi18-10): C ≤ 0.08, Cr 17–19, Ni 9–12, Ti 5×C–0.70 — identical. Niobium-stabilized 347 (S34700 / 1.4550) is the sister grade with better weld-metal stability.

321 mechanical properties

Minimums per ASTM A240/A276/A312/A213. Typical annealed sheet: yield 250–290 MPa, tensile 590–640 MPa. ASME II-D allowable stresses for 321H extend to 816 °C (1500 °F); 100 000 h creep-rupture strength ≈ 60 MPa at 650 °C for 321H.

Impact toughness

321 requires a minimum Charpy V-notch energy of 80 J at -196 °C (typical). Austenitic; suitable for cryogenic service, though 304L is more usual there.

321 equivalent grades

Identical = same composition and mechanical limits within rounding. Near = one limit differs (e.g. carbon max or impact temperature). Functional = interchangeable for most uses after an engineering check. Full cross-reference for 321 →

Old and superseded designations

Product forms and tolerance standards

Product forms: sheet, strip, plate (ASTM A240), bar and forgings (A276, A479, A182 F321), seamless and welded pipe and tube (A312 TP321, A213 TP321H for boilers, A269), fittings (A403 WP321), flanges, wire, aircraft sheet (AMS 5510).

Dimensional tolerances: sheet/plate: ASTM A480; bar: ASTM A484; tube: A213/A1016 (boiler), A312/A999 (pipe).

Key property values

321 stainless steel yield strength

Minimum 205 MPa (30 ksi) annealed; typical 250–290 MPa. At 500 °C ≈ 130 MPa, at 700 °C ≈ 110 MPa (typical).

321 tensile strength

Minimum 515 MPa (75 ksi); typical 590–640 MPa annealed; ≈ 400 MPa at 500 °C.

321 hardness

Annealed max 217 HB / 95 HRB; typical 150–190 HB. Not hardenable by heat treatment.

High-temperature and creep properties

Continuous oxidation limit ≈ 870 °C (925 °C intermittent). ASME allowable stresses: 321 to 425 °C for full strength, 321H creep-rated to 816 °C. 100 000 h rupture strength (321H, typical): ≈ 110 MPa at 600 °C, 60 MPa at 650 °C, 30 MPa at 700 °C. Resists intergranular attack after 500–800 °C exposure (ASTM A262 practice E passes).

Corrosion resistance

PREN ≈ 18 — same class as 304 in most media; slightly lower pitting resistance in chlorides because Ti(C,N) particles act as initiation sites. Good in nitric acid, organic acids, and in the polythionic-acid environments of refinery heaters where 304 fails.

Physical properties

Density 8.0 g/cm³, E = 193 GPa, expansion 16.6 × 10⁻⁶/K (0–100 °C), 18.6 (0–500 °C), conductivity 16.1 W/m·K, non-magnetic annealed.

Heat treatment

Solution anneal 955–1065 °C (1750–1950 °F), water or air quench. Stabilizing anneal 845–900 °C 1–4 h for maximum intergranular resistance in service at 425–870 °C. Stress relief at 700–900 °C is permissible (unlike 304) because the steel is stabilized.

Weldability

Excellent weldability by all arc processes; filler ER347 / E347 (Nb-stabilized) is standard because titanium does not transfer across the arc; ER308L is acceptable when the weld sees no high-temperature service. No preheat, interpass ≤ 150 °C. Post-weld heat treatment is not needed for corrosion resistance; a stabilizing anneal may be specified for critical high-temperature equipment.

Machining, forming and heat treatment

Machinability ≈ 40 % of B1112 (Ti carbonitrides increase tool wear slightly versus 304). Forming and deep drawing are very good; spinning of exhaust cones and bellows is routine. Polishing to bright finishes is poorer (streaks) — use 304 for decorative work.

Typical applications

• Aircraft and automotive exhaust manifolds, collectors, flexible bellows
• Expansion joints and bellows in piping and turbines
• Furnace parts, heat-treatment fixtures, thermocouple sheaths, radiant tubes
• Refinery heater tubes, hydrotreater and reformer piping (321H, polythionic-acid resistance)
• Boiler superheater tubes, pressure vessels 425–815 °C (321H)
• Welded pressure equipment that cannot be post-weld annealed
• Chemical process equipment with thermal cycling; jet-engine parts (AMS 5510)

Frequently asked questions

Related grades

Verified against ASTM A240/A240M-22; ASTM A240/A240M-22 Tables 1–2; ASTM A312/A312M-22; ASTM A213/A213M-22 (TP321H); ASME BPVC II-D. Last checked: September 2026.