309S High-Chromium-Nickel Austenitic Stainless Steel

309S is a high-chromium-nickel austenitic stainless steel built for one dominant job: surviving hot oxidizing atmospheres. With chromium at 22.00-24.00% and nickel at 12.00-15.00%, it forms a dense, self-healing chromium-oxide scale that protects the base metal at continuous service temperatures up to about 1050 C, far beyond the reach of 304-type grades.

Chemical Composition

Composition limits below are per ASTM A240 (UNS S30908) and JIS G4304 (SUS309S).

Element (wt%)	309S (ASTM A240, UNS S30908)	SUS309S (JIS G4304)
C	≤0.08	≤0.08
Si	≤0.75	≤1.00
Mn	≤2.00	≤2.00
P	≤0.045	≤0.045
S	≤0.030	≤0.030
Cr	22.0-24.0	22.00-24.00
Ni	12.0-15.0	12.00-15.00

Mechanical Properties (Annealed)

ASTM A240 UNS S30908: tensile strength ≥515 MPa, yield strength ≥205 MPa, elongation ≥40%, hardness ≤217 HB. JIS G4304 SUS309S: tensile strength ≥520 MPa, yield strength ≥205 MPa, elongation ≥40%.

High-Temperature Performance

Oxidation resistance

The high chromium content is the key. At elevated temperature, chromium reacts with oxygen to form a Cr2O3 scale that is dense, adherent and slow-growing. Published engineering data rate 309S for continuous service around 1050 C in air and oxidizing flue gases; cyclic conditions shorten the practical limit because thermal cycling stresses the scale.

Limitations

309S is not suitable for long-term service in reducing atmospheres such as hydrogen-rich, carbon-monoxide-rich or methane-rich gases. In those environments the protective chromium oxide is reduced and the metal loses its protection rapidly. It also has limited resistance to molten salts and sulfur-bearing gases.

Applications

Industrial furnace liners and muffles, heat-treatment furnace fixtures and trays, furnace door seals, radiant tubes, high-temperature flue-gas ducting in power generation, boiler combustion-chamber components and annealing covers.

Welding

Matching filler of the AWS ER309L wire and E309L electrode type is standard. The low-carbon L filler avoids chromium-carbide precipitation in the heat-affected zone and keeps the weld metal ductile. Heat input should be kept moderate and interpass temperature limited to avoid excessive grain growth in the weld region.

Designations and Equivalents

309S is UNS S30908, JIS SUS309S and GB 06Cr23Ni13. The EN 10088-2 grade 1.4828 (X15CrNiSi20-12) is a silicon-bearing heat-resisting steel often compared with 309S, but its composition differs: it contains 1.50-2.50% silicon and slightly lower chromium and nickel, so it is not an exact chemical equivalent. Specify 309S by UNS or ASTM number to avoid ambiguity.

FAQ

1. Why can 309S run at 1050 C while 304 cannot?

Chromium. 309S carries 22.00-24.00% chromium versus 18.00-20.00% for 304. Above about 870 C the thinner, less stable oxide film on 304 spalls and the metal oxidizes rapidly, while 309S maintains a dense protective Cr2O3 scale.

2. How does 309S compare with 316 at high temperature?

309S has markedly better oxidation resistance above about 900 C because of its higher chromium content, 22-24% versus 16-18%. Molybdenum in 316 helps corrosion resistance but does not help in dry hot air.

3. What welding consumables suit 309S?

AWS ER309L wire and E309L electrodes are the standard choices; the low carbon content keeps the heat-affected zone free of chromium-carbide precipitation.

4. Can 309S be used in reducing atmospheres?

No. The protective chromium-oxide scale breaks down in hydrogen-rich, CO-rich or methane-rich atmospheres, and rapid corrosion follows.

5. What shortens the life of 309S in service?

Overtemperature above the continuous-service limit, thermal cycling, surface contamination, poor weld quality and residual stress. Keeping the surface clean and the temperature under control extends scale life.