Is 308 or 309 Better for Stainless to Stainless Steel?

When it comes to welding stainless steel to stainless steel, one of the most common questions engineers and fabricators ask is:
“Should I use 308 or 309 filler metal?”

Both 308 and 309 are austenitic stainless steel filler materials widely used in the fabrication and repair of stainless steel structures. However, they serve different purposes depending on the base metal composition, welding conditions, and service environment.

In this detailed article, SAKYSTEEL, a global stainless steel manufacturer and exporter, explains the differences between 308 and 309 stainless filler metals, their chemical and metallurgical characteristics, and which one performs better for stainless-to-stainless welding applications.

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1. Introduction to 308 and 309 Stainless Steels

308 Stainless Steel

308 stainless steel (UNS S30800) is an austenitic chromium-nickel alloy designed primarily for welding 304 and 304L stainless steels. It contains around 20% chromium and 10% nickel, giving it excellent corrosion resistance and good weldability.

Because of its similar composition to 304, 308 filler is the standard choice when joining 304 to 304 or other 300-series stainless steels in non-severe environments.

309 Stainless Steel

309 stainless steel (UNS S30900) is also an austenitic chromium-nickel alloy, but it contains higher chromium (≈23%) and nickel (≈13%) than 308. This makes it more resistant to oxidation, thermal fatigue, and scaling at high temperatures.

Originally developed as a heat-resistant grade, 309 is now also used as a transition filler metal for joining dissimilar stainless steels and for welding between stainless steel and carbon steel.

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2. Chemical Composition

The chemical makeup determines how each alloy performs during and after welding.

• 308 Stainless Steel Composition:

  • Chromium: 19–21%

  • Nickel: 9–11%

  • Carbon: ≤0.08%

  • Manganese: ≤2%

  • Silicon: ≤1%

• 309 Stainless Steel Composition:

  • Chromium: 22–24%

  • Nickel: 12–15%

  • Carbon: ≤0.08%

  • Manganese: ≤2%

  • Silicon: ≤1%

Key Difference:

309 has higher alloy content, improving resistance to heat and oxidation but making it slightly more expensive than 308.

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3. Microstructure and Metallurgical Characteristics

Both 308 and 309 are austenitic stainless steels, meaning their crystal structure is face-centered cubic (FCC). This gives them excellent ductility, toughness, and corrosion resistance.

However, their ferrite content after solidification differs:

• 308 weld metal typically contains 3–10% ferrite, which reduces hot cracking.

• 309 weld metal tends to have higher ferrite content (up to 15%), improving resistance to solidification cracking under high heat.

Thus, 309 is better for welding thicker sections or materials with high heat input, where cracking is a concern.

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4. Welding Applications of 308

308 filler metal is the industry standard for welding austenitic stainless steels like 304, 304L, and 301.

Typical Uses:

• Welding 304/304L to 304/304L

• Fabrication of tanks, piping, and structural components

• Food and beverage processing equipment

• Architectural and decorative applications

308L (low carbon version) is often used to prevent carbide precipitation and maintain corrosion resistance in as-welded joints without post-weld heat treatment.

Advantages of 308:

• Excellent match for 304 and similar stainless steels

• Good weld appearance and easy slag removal

• Maintains corrosion resistance in most environments

Limitations:

• Not ideal for high-temperature service above 800°C

• Not suitable for joining stainless steel to carbon steel

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5. Welding Applications of 309

309 filler metal is designed for higher alloy content and wider compatibility than 308.

Typical Uses:

• Welding stainless steel to carbon steel or low alloy steel

• Joining dissimilar stainless steels (e.g., 304 to 310, or 316 to 321)

• Repair welding of unknown stainless grades

• High-temperature components like furnace parts, heat exchangers, and exhaust systems

309’s higher chromium and nickel content allow it to tolerate composition mismatch between different alloys without cracking or corrosion.

Advantages of 309:

• Excellent resistance to oxidation and scaling up to 1100°C

• Superior tolerance for mixed or unknown base metals

• Reduced cracking tendency during high-heat welding

Limitations:

• Slightly more expensive than 308

• Overmatching composition may reduce ductility for thin sheet welds

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6. Corrosion Resistance

Both grades provide excellent corrosion resistance, but the service environment determines which one performs better.

• 308: Outstanding in atmospheric and mild chemical conditions, similar to 304 stainless.

• 309: Superior in oxidizing and high-temperature environments, tolerating thermal cycling without scaling or carburization.

For marine or chloride-rich environments, neither 308 nor 309 performs as well as molybdenum-containing grades like 316 or 317.

However, for industrial heat equipment and transition joints, 309 clearly outperforms 308.

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7. High-Temperature Resistance

This is one of the most important differences between 308 and 309.

• 308: Performs well up to 870°C (1600°F) before losing strength and oxidation resistance.

• 309: Maintains integrity up to 1100°C (2010°F), making it ideal for furnace and exhaust systems.

So, if the welded joint is exposed to continuous or cyclic heating, 309 is the better choice.

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8. Mechanical Properties

Typical mechanical properties for both filler metals (weld deposit condition):

• 308:

  • Tensile Strength: ~550 MPa

  • Yield Strength: ~350 MPa

  • Elongation: 35%

• 309:

  • Tensile Strength: ~620 MPa

  • Yield Strength: ~420 MPa

  • Elongation: 35%

Conclusion:
309 welds are stronger and more resistant to high-temperature deformation, while 308 welds are more ductile and easier to finish for aesthetic or sanitary applications.

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9. Weldability and Fabrication

Both 308 and 309 have excellent weldability and can be used with all common processes — GTAW (TIG), GMAW (MIG), SMAW (Stick), and FCAW (Flux-Cored).

However, consider the following:

• Use 308L when welding low-carbon stainless steels to prevent intergranular corrosion.

• Use 309 when joining dissimilar metals or when the exact base composition is unknown.

To avoid cracking, control heat input and maintain interpass temperatures below 150°C.

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10. Matching Base Metals

A general rule of thumb: