309S High-Chromium-Nickel Austenitic Stainless Steel

309S is a high-chromium-nickel austenitic stainless steel, designed for high-temperature oxidation-resistant applications. Its high chromium content forms a dense Cr₂O₃ oxide film at high temperatures, providing excellent resistance to high-temperature oxidation, suitable for components in high-temperature oxidizing environments.

Chemical Composition (wt%): C≤0.08, Cr=22.00-24.00, Ni=12.00-15.00, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Fe=Balance

Mechanical Properties (Annealed): Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤217HB

Performance Advantages: Excellent high-temperature oxidation resistance (continuous service temperature up to 1050℃); good high-temperature strength and creep resistance; excellent weldability; stable performance in high-temperature oxidizing atmospheres.

Applications: Industrial furnace liners, heat treatment furnace fixtures, high-temperature furnace door seals, high-temperature flue gas pipelines in power generation industry, boiler combustion chamber components.

Equivalent Grades: UNS S30908, JIS SUS309S, EN 1.4828, GB 06Cr23Ni13

Q&A

Q1: Why can 309S be used at 1050℃ while 304 cannot? A1: 309S can be used at 1050℃ mainly due to its high chromium content (22.00-24.00wt%), which is 20% higher than 304's chromium content (18.00-20.00wt%). At high temperatures, chromium in 309S reacts with oxygen to form a dense and stable chromium oxide (Cr₂O₃) film on the surface, which can effectively isolate the base metal from high-temperature oxidizing atmospheres and prevent further oxidation. In contrast, 304's lower chromium content results in a thinner and less stable oxide film; when the temperature exceeds 870℃, this oxide film will become unstable, peel off easily, and the base metal will undergo rapid oxidation and corrosion. 

 

Q2: What is the difference in high-temperature oxidation resistance between 309S and 316? A2: The high-temperature oxidation resistance of 309S is significantly superior to that of 316, mainly due to the higher chromium content of 309S. 309S contains 22.00-24.00wt% chromium, while 316 only contains 16.00-18.00wt% chromium. At high temperatures (above 900℃), 309S forms a denser and more stable Cr₂O₃ oxide film, which has excellent resistance to high-temperature oxidation and spalling, and can maintain effective protection for a long time in oxidizing atmospheres.

Q3: What welding materials are suitable for 309S stainless steel? A3: The suitable welding materials for 309S stainless steel are mainly ER309L welding wire and E309L electrodes. ER309L welding wire is preferred for gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW) because it has a matching chemical composition with 309S, especially the same high chromium-nickel content, ensuring that the weld has the same high-temperature oxidation resistance and mechanical properties as the base metal. The "L" in ER309L indicates low carbon content (≤0.03wt%), which can avoid intergranular corrosion in the weld heat-affected zone and improve the welding performance of the weld. During welding, it is necessary to control the heat input to ≤200J/mm to avoid excessive grain growth in the weld area, which would reduce the high-temperature creep strength. 

 

Q4: Can 309S be used in reducing atmospheres at high temperatures? A4: No, 309S is not suitable for long-term service in high-temperature reducing atmospheres. 309S is designed for high-temperature oxidizing environments, and its excellent performance mainly relies on the protective effect of the surface Cr₂O₃ oxide film. In reducing atmospheres (such as hydrogen-rich, carbon monoxide-rich or methane-rich gases) at high temperatures, the Cr₂O₃ oxide film on the surface of 309S will be reduced to metallic chromium, losing its protective effect, and the base metal will undergo rapid corrosion. 

 

Q5: What are the key factors affecting the service life of 309S in high-temperature oxidizing environments? A5: Several key factors affect the service life of 309S in high-temperature oxidizing environments. The operating temperature: exceeding the maximum continuous service temperature (1050℃) will accelerate the aging and spalling of the surface oxide film, leading to rapid oxidation of the base metal and a sharp reduction in service life. To extend the service life, it is necessary to strictly control the operating temperature, avoid corrosive media, ensure welding quality, eliminate residual stress, and perform good surface cleaning before use.