Comparison of SUS304 and SUS309S: General-Purpose vs High-Chromium High-Temperature Oxidation-Resistant Austenitic Stainless Steel

SUS304 and SUS309S are austenitic stainless steels with different performance orientations, with the core difference being chromium and nickel content (SUS309S: Cr=22.00-24.00%, Ni=12.00-15.00%; SUS304: Cr=18.00-20.00%, Ni=8.00-10.50%). The high chromium-nickel content of SUS309S significantly improves high-temperature oxidation resistance, making them suitable for different temperature and oxidation environment requirements.

Core Parameter Comparison

Parameter	SUS304 Stainless Steel	SUS309S Stainless Steel
Chemical Composition (wt%)	C≤0.08, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Cr=18.00-20.00, Ni=8.00-10.50, Fe=Balance	C≤0.08, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Cr=22.00-24.00, Ni=12.00-15.00, Fe=Balance
Mechanical Properties (Annealed)	Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤201HB	Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤217HB
Service Temperature	-196℃ to 870℃ (continuous service)	-196℃ to 1050℃ (continuous service)
Equivalent Grades	EN 1.4301, UNS S30400, AISI 304	EN 1.4828, UNS S30908, AISI 309S

Key Performance Differences: 1. High-temperature oxidation resistance: SUS309S's high chromium content forms a dense Cr₂O₃ oxide film at high temperatures, which can resist oxidation at 1050℃; SUS304's oxidation resistance is poor above 870℃, and the oxide film is easy to peel off. 2. Corrosion resistance: SUS309S has better corrosion resistance in high-temperature oxidizing atmospheres (such as high-temperature flue gas) than SUS304; both have similar corrosion resistance in room-temperature low-corrosion environments. 3. Strength: Both have similar room-temperature strength, but SUS309S has higher high-temperature strength. 4. Cost: SUS309S is 60-80% more expensive than SUS304. 5. Formability: Both have good formability, but SUS309S's high alloy content increases deformation resistance slightly.

Applicable Scenario Distinction: SUS304 is suitable for general medium-temperature (≤870℃) low-corrosion components, such as food processing equipment, decorative pipelines, and indoor heat exchangers. SUS309S is suitable for high-temperature oxidation-resistant components, such as industrial furnace liners, high-temperature flue gas pipelines, heat treatment furnace fixtures, and boiler combustion chamber components (870-1050℃).

Practical Q&A

Q1: Why can SUS309S be used at 1050℃ while SUS304 cannot? A1: SUS309S's chromium content (22-24%) is 20% higher than SUS304, which can form a dense and stable chromium oxide film at high temperatures, preventing internal metal oxidation; SUS304's oxide film is loose and easy to peel off above 870℃, leading to rapid corrosion.

Q2: Can SUS309S be used in high-temperature reducing atmospheres? A2: No. It is only suitable for high-temperature oxidizing atmospheres; in reducing atmospheres (such as hydrogen-rich gas), its corrosion resistance is poor; it is recommended to use nickel-based alloys for high-temperature reducing atmospheres.

Q3: What welding materials are used for SUS309S? A3: Use ER309L welding wire; control welding heat input ≤200J/mm to avoid grain coarsening; post-weld passivation treatment is recommended to improve corrosion resistance; no post-weld heat treatment is required for general scenarios.

Q4: What is the service life difference between SUS309S and SUS304 in high-temperature furnace liners? A4: In 1000℃ oxidizing atmosphere, SUS309S's service life is ≥5 years, while SUS304's service life is ≤6 months, which cannot meet long-term service requirements.

Q5: How to select between SUS304 and SUS309S? A5: Choose SUS304 if the service temperature is ≤870℃ and the environment is low-corrosion; choose SUS309S only if the service temperature is 870-1050℃ and high-temperature oxidation resistance is required, regardless of cost.