Premium AISI 309S Stainless Steel: Composition, Properties and Applications

What Is AISI 309S Stainless Steel?

AISI 309S is an austenitic stainless steel with substantially higher chromium and nickel than 304: 22.0 to 24.0% chromium and 12.0 to 15.0% nickel. This composition raises its oxidation resistance and hot strength, making it a common choice for heat-treatment equipment, furnace parts and high-temperature process components where 304 and 316 reach their limits.

The S in 309S indicates the low-carbon version of Type 309, with carbon limited to 0.08%. Lower carbon reduces the risk of chromium carbide precipitation at grain boundaries, which improves weldability and resistance to intergranular corrosion in welded assemblies used at high temperature.

Chemical Composition

The following composition is representative of AISI 309S and matches the requirements of ASTM A240 for plate, sheet and strip.

Element wt%	AISI 309S
Carbon	≤ 0.08
Manganese	≤ 2.00
Silicon	≤ 1.00
Phosphorus	≤ 0.045
Sulfur	≤ 0.030
Chromium	22.0 – 24.0
Nickel	12.0 – 15.0
Nitrogen	≤ 0.10

Mechanical and Physical Properties

Property	Value
Tensile strength	≥ 515 MPa
Yield strength	≥ 205 MPa
Elongation	≥ 40%
Hardness	≤ 217 HB
Density	7.98 g/cm³
Melting range	1400 – 1450°C
Thermal conductivity at 20°C	14.2 W/m·K
Thermal conductivity at 100°C	16.8 W/m·K
Mean coefficient of thermal expansion, 20–100°C	14.4 × 10⁻⁶ /K

Applicable Standards and Product Forms

AISI 309S is covered by several national and international standards. Plate, sheet and strip are commonly supplied to ASTM A240 and EN 10088-2; bars can be supplied to ASTM A276 and A479; JIS G4305 covers cold-rolled sheets; and GB/T 24511 covers stainless steel plate for pressure equipment. Common product forms include plate, sheet, strip, round bar, flat bar, and welded or seamless tube.

Typical Applications

Furnace and kiln components, including radiant tubes, baffles and burner parts.

Heat-treatment baskets, trays and fixtures that cycle at high temperature.

Heat exchangers and recuperators in hot gas service.

Annealing boxes, boiler baffles and support structures for petrochemical process equipment.

Frequently Asked Questions

Q1: What is the difference between 309 and 309S?
A1: 309S is the low-carbon version of Type 309: its carbon is limited to 0.08%, which reduces chromium carbide precipitation and improves weldability, while Type 309 allows carbon up to 0.20%.

Q2: Is 309S better than 304 at high temperature?
A2: Yes, for oxidation resistance. The higher chromium and nickel content of 309S allows higher service temperatures than 304, which is why it is used for furnace and heat-treatment components.

Q3: Can 309S be welded?
A3: Yes. 309S welds well and the low carbon content reduces sensitization risk; austenitic filler metal of matching composition is normally used.

Q4: What are the UNS and EN designations of 309S?
A4: AISI 309S corresponds to UNS S30908 and EN 1.4833.

Q5: What product forms is 309S available in?
A5: Plate, sheet, strip, bar and tube, supplied to standards such as ASTM A240, A276, A479, EN 10088-2, JIS G4305 and GB/T 24511.

Q6: Is 309S magnetic?
A6: No. 309S is fully austenitic in the annealed condition and is essentially non-magnetic.