Comparison of SUS304 and SUS309S: General-Purpose vs High-Chromium High-Temperature Oxidation-Resistant Austenitic Stainless Steel

Overview: Two Austenitic Workhorses

SUS304 (AISI 304, UNS S30400, EN 1.4301) is the most widely used austenitic stainless steel in the world. SUS309S (AISI 309S, UNS S30908, closest European designation EN 1.4833) is a higher-alloy austenitic grade with roughly 22-24% chromium and 12-15% nickel. The extra chromium and nickel give SUS309S substantially better resistance to high-temperature oxidation and scaling, which is the single most important reason to choose it over SUS304. For general-purpose, room-temperature and mildly corrosive applications, SUS304 remains the economical default.

Chemical Composition Comparison

The guaranteed composition ranges below follow JIS G4304 for hot-rolled stainless plate, sheet and strip; ASTM A240 sets essentially the same limits for the UNS equivalents.

Element	SUS304 (JIS G4304)	SUS309S (JIS G4304)
Carbon (C)	≤ 0.08%	≤ 0.08%
Silicon (Si)	≤ 1.00%	≤ 1.00%
Manganese (Mn)	≤ 2.00%	≤ 2.00%
Phosphorus (P)	≤ 0.045%	≤ 0.045%
Sulfur (S)	≤ 0.030%	≤ 0.030%
Chromium (Cr)	18.00-20.00%	22.00-24.00%
Nickel (Ni)	8.00-10.50%	12.00-15.00%

Mechanical Properties Comparison

In the annealed condition both grades have similar room-temperature strength, and both remain fully austenitic and non-magnetic after solution annealing. The table lists the minimum guaranteed values for plate per ASTM A240.

Property (annealed)	UNS S30400	UNS S30908
0.2% proof stress Rp0.2	≥ 205 MPa	≥ 205 MPa
Tensile strength Rm	≥ 515 MPa	≥ 515 MPa
Elongation (2 in)	≥ 40%	≥ 40%
Hardness	≤ 201 HBW	≤ 217 HBW

High-Temperature Performance

Oxidation resistance is where the two grades diverge. SUS304 can be used for continuous service up to roughly 870 °C and intermittent service up to about 925 °C, but repeated exposure in the 425-860 °C range can cause chromium-carbide precipitation at grain boundaries, which lowers corrosion resistance. SUS309S pushes continuous service to approximately 1050 °C and intermittent exposure to about 1095 °C, because its higher chromium content forms a more stable protective oxide scale. For furnace parts, radiant tubes, heat-treatment fixtures, annealing boxes and exhaust components, SUS309S is therefore the safer choice.

Selection Guidance

Choose SUS304 when the operating temperature stays below 400 °C and the environment is mildly corrosive: food processing equipment, kitchen appliances, architectural trim, tanks, piping and general fabrication. Choose SUS309S when parts see sustained temperatures above 500 °C, cyclic heating, or the risk of scaling in air: furnace components, boiler parts, kiln liners, thermocouple protection tubes and welding consumables for dissimilar joints. If both high-temperature strength and creep resistance are required, or if the atmosphere is sulphidising, a fully austenitic heat-resistant grade with higher nickel such as UNS S31008 should also be evaluated.

FAQ

Is SUS309S stronger than SUS304?

At room temperature the guaranteed strength of the two grades is nearly identical. The real difference appears at elevated temperature, where SUS309S retains its oxide scale and mechanical integrity far longer than SUS304.

What is the EN equivalent of SUS309S?

The closest European designation is EN 1.4833 (X12CrNi23-13), which matches the 22-24% chromium and 12-15% nickel range of AISI 309S. SUS309S is the JIS name; UNS S30908 is the ASTM name.

Can SUS304 be used for furnace parts?

Only below roughly 870 °C continuous, and with awareness of sensitisation risk between 425 °C and 860 °C. For dependable furnace service, SUS309S or a higher heat-resistant grade is recommended.

Are SUS304 and SUS309S magnetic?

Both are austenitic and essentially non-magnetic after solution annealing. Light magnetism can appear locally after heavy cold working or welding, but this does not affect performance.

Is SUS309S more expensive than SUS304?

Yes. The higher chromium and nickel content makes SUS309S noticeably more expensive, so it should be specified only where its temperature advantage is actually needed.

Can SUS309S replace SUS310S?

Not at the highest service temperatures. SUS310S (UNS S31008) contains about 25% chromium and 20% nickel and handles even higher temperatures. SUS309S is a middle step between SUS304 and SUS310S.