Low-Carbon Stainless Steel 309S

1. What Is 309S Stainless Steel?

309S is the low-carbon grade of type 309 austenitic stainless steel, designated UNS S30908 and covered by ASTM A240 for plate, sheet and strip, and by EN 10088-2 as material number 1.4833. Its chromium content of 22.00-24.00% and nickel content of 12.00-15.00% place it well above the 304/316 families, which is why 309S is selected when components must resist oxidation and scaling at temperatures up to about 980 C.

The letter S indicates a maximum carbon content of 0.08%, compared with 0.20% for the standard grade 309. The lower carbon limit reduces the risk of chromium carbide precipitation at grain boundaries when the steel is heated in the sensitizing range of approximately 425-900 C, for example during welding or during service at intermediate temperatures.

2. Chemical Composition (ASTM A240)

The table below gives the specified limits for Type 309S in ASTM A240. Iron makes up the balance.

Element	Limit (wt %)
Carbon (C)	0.08 max
Manganese (Mn)	2.00 max
Phosphorus (P)	0.045 max
Sulfur (S)	0.030 max
Silicon (Si)	0.75 max
Chromium (Cr)	22.00-24.00
Nickel (Ni)	12.00-15.00
Nitrogen (N)	0.10 max

3. Mechanical Properties (Annealed)

In the annealed condition, Type 309S plate, sheet and strip are required by ASTM A240 to meet the following minimum values.

Property	Minimum Value
Tensile strength	515 MPa
Yield strength (0.2 % offset)	205 MPa
Elongation in 50 mm	40 %

4. Heat Resistance

309S resists scaling in air during continuous service up to about 980 C and can tolerate somewhat higher temperatures for short, intermittent exposures. The high chromium content forms a stable, adherent oxide film that slows further oxidation. For long-term load-bearing service above roughly 550 C, designers should evaluate creep behavior; where maximum creep strength is required, higher-carbon H grades are often specified instead.

5. Fabrication and Typical Applications

309S can be formed by conventional rolling, bending and deep drawing. It is weldable by common processes such as TIG, MIG and submerged arc welding; preheating is not normally required, and the low carbon content helps maintain intergranular corrosion resistance in the as-welded condition. Typical applications include:

Furnace parts, radiant tubes and muffles

Heat exchangers, recuperators and air heaters

Boiler components and superheater supports

Kiln and oven linings and fixtures

Chemical processing equipment exposed to elevated temperatures

Frequently Asked Questions

What is the difference between 309 and 309S? The maximum carbon content is 0.20% for grade 309 and 0.08% for 309S. The lower-carbon version is preferred where welding is involved and where intergranular corrosion resistance in the sensitizing temperature range matters.

What is the maximum service temperature of 309S? For oxidation resistance in air, continuous service up to about 980 C is commonly cited. Where the component carries mechanical stress, the maximum design temperature is lower and depends on creep properties.

Is 309S magnetic? No. 309S is fully austenitic in the annealed condition and is essentially non-magnetic, although heavy cold working can induce a slight magnetic response.

Which standards cover 309S? ASTM A240 covers plate, sheet and strip; ASTM A312 and A358 cover pipe; ASTM A213 covers small-diameter tubes; EN 10088-2 lists the equivalent material 1.4833.

Can 309S be welded to 304 or 316 stainless steel? Yes. 309S is often used as a transition material because its higher alloy content tolerates dilution from lower-alloyed base metals, but the filler metal should be qualified by a welding procedure, typically a 309-type filler.