Key Features: High chromium and high nickel content, combining excellent oxidation resistance and some corrosion resistance, specifically designed for high-temperature environments.

Chemical Composition and Design Logic:

High Chromium: Content approximately 22-24%. This is the foundation of its oxidation resistance. Chromium forms a dense, stable chromium oxide protective film on the surface at high temperatures, preventing further oxidation (peeling) of the internal metal.

High Nickel: Content approximately 12-15%. High nickel ensures the stability of the austenitic structure, maintaining excellent strength and creep resistance at high temperatures, and enhancing resistance to reducing atmospheres.

Low Carbon: "S" represents low carbon (C ≤ 0.08%). The low-carbon design minimizes the precipitation of chromium carbide at high temperatures, thus maintaining the material's resistance to intergranular corrosion and high-temperature toughness, and also making it less prone to sensitization after welding.

Molybdenum-Free: Unlike 316/317, 309S typically does not contain molybdenum, therefore its resistance to chloride ion pitting at room temperature is not outstanding. Its primary battleground is high temperature, not strong corrosive solutions.

Key Performance Characteristics:

Excellent High-Temperature Oxidation Resistance:

Can withstand high temperatures below 980°C while maintaining good oxidation resistance. Intermittent use temperature can reach up to 1150°C.

Far superior heat resistance limits compared to 304 (870°C) and 316 (870°C).

Good High-Temperature Strength:

Maintains high strength and resistance to creep and fatigue at high temperatures.

Decent Corrosion Resistance:

At room temperature, its corrosion resistance is better than 304 and close to that of 316 (but not resistant to chloride ion pitting). Resistant to corrosion from various acidic media at high temperatures, especially in sulfiding environments and carburizing atmospheres.

Good Weldability and Formability:

The austenitic structure gives it good cold and hot working properties and weldability.