309 and 309S Stainless Steel: Medium-to-High Temperature Oxidation Resistance

What Is 309 Stainless Steel

Type 309 is an austenitic stainless steel with a high chromium and nickel content, nominally 22 percent chromium and 13 percent nickel, designed for medium-to-high temperature oxidation resistance. It performs well in the range of 650 to 980 degrees C, filling the gap between general-purpose 304 and the extreme-temperature grade 310.

Chemical Composition

Type 309 contains 22.0 to 24.0 percent chromium, 12.0 to 15.0 percent nickel, and 0.20 percent carbon maximum in the standard grade. Type 309S limits carbon to 0.08 percent maximum with the same chromium and nickel range, preventing carbide precipitation and intergranular corrosion in welded components. Common specifications include ASTM A240 for plate and sheet and GB/T 24511 for pressure equipment.

High-Temperature Performance

The high chromium and nickel content forms a stable, adherent oxide scale that resists oxidation, spalling, and thermal cycling up to about 980 degrees C in continuous service. The grade is suitable for applications that resist high-temperature oxidation but do not require the extreme temperature capability of 310.

Limitations

Corrosion resistance in chloride environments is limited compared with molybdenum-bearing grades, and the standard type is susceptible to sensitization in the 425 to 850 degrees C range, so the low-carbon 309S is required for welded construction. Cost is higher than 304 because of the increased chromium and nickel content.

Comparison with Other Grades

304, with 18 percent chromium and 8 percent nickel, is limited to long-term service temperatures around 650 degrees C. 309, with 22 percent chromium and 13 percent nickel, extends service to about 980 degrees C with significantly better oxidation resistance. 310, with 25 percent chromium and 20 percent nickel, reaches about 1150 degrees C with resistance to sulfidation as well, for smelting furnaces and petrochemical high-temperature furnaces.

Applications

309 and 309S are used for furnace components, heat treatment baskets and trays, radiant tubes, burner parts, kiln liners, and welding consumable applications such as dissimilar-metal joints where the higher nickel content improves ductility. Specify the carbon level and solution-annealed condition to match the working conditions.

Frequently Asked Questions

Q: What is the difference between 309 and 309S?

A: 309S limits carbon to 0.08 percent for welded service.

Q: What temperature range suits 309?

A: Medium-to-high temperature oxidation service from 650 to 980 degrees C.

Q: How does 309 compare with 310?

A: 310 has higher chromium and nickel, reaching about 1150 degrees C.

Q: Is 309 suitable for chloride environments?

A: No, its corrosion resistance in chlorides is limited.

Q: What standards cover 309?

A: ASTM A240 and GB/T 24511 among others.

Q: What applications use 309?

A: Furnace components, radiant tubes, heat treatment baskets, and kiln liners.

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