SUS309S Stainless Steel: Composition, High-Temperature Oxidation Behaviour and Welding

What SUS309S Is

SUS309S is the Japanese designation for a 23 % chromium, 13 % nickel austenitic heat-resisting stainless steel, standardised in JIS G4312 for heat-resisting steel plate, sheet and strip. The same alloy appears as UNS S30908 and type 309S in ASTM A240 in the United States, as 1.4828 in EN 10088, and as 06Cr23Ni13 in the Chinese system. The suffix S marks the low-carbon variant: carbon is capped at 0.08 %, which reduces carbide precipitation during welding and keeps the alloy usable in cyclic furnace service.

The grade exists for one purpose: to resist oxidation and scaling at temperatures far above the range where ordinary 304 or 316 survive. It is a workhorse material for furnace internals, not a corrosion-resistant grade for ambient chemical service.

Chemical Composition and Mechanical Properties

Element	Requirement, %
Carbon (C)	0.08 max
Silicon (Si)	1.00 max
Manganese (Mn)	2.00 max
Phosphorus (P)	0.045 max
Sulfur (S)	0.030 max
Chromium (Cr)	22.0 - 24.0
Nickel (Ni)	12.0 - 15.0

Property	Requirement
Tensile strength, min	515 MPa (75 ksi)
Yield strength 0.2 %, min	205 MPa (30 ksi)
Elongation in 2 in., min	40 %
Typical continuous service, oxidising atmosphere	up to about 980 °C

The composition is fully austenitic at room and elevated temperature. The 22 to 24 % chromium content forms a dense chromium oxide scale that slows further oxidation, and the 12 to 15 % nickel stabilises the austenite so that the alloy does not transform to brittle phases across furnace heating and cooling cycles.

Oxidation and High-Temperature Behaviour

In air, combustion products and other oxidising atmospheres, SUS309S forms and maintains a protective scale and is regularly used for continuous service at temperatures approaching 980 °C, with short-term exposure tolerated higher. Where the operating temperature must exceed that band, grade 310S with 25 % chromium and 20 % nickel is the normal step up. The comparison is a cost decision as much as a technical one: below about 980 °C, 309S does the job with less alloy content.

Performance in reducing atmospheres is far weaker. Hydrogen, carbon monoxide and other oxygen-deficient gases break down the protective oxide and allow carbon to enter or leave the metal, producing carburisation or decarburisation and eventual cracking. Where a furnace atmosphere alternates between oxidising and reducing, the grade remains serviceable only while oxidising conditions dominate. For purely reducing high-temperature duties, nickel-based alloys are required rather than any 300-series stainless steel.

Thermal shock is the second design consideration. The grade tolerates repeated heating and cooling without cracking, but local restraint, heavy section changes and rapid quench after high-temperature service all raise stress. Furnace parts are allowed to cool naturally wherever the process permits.

Fabrication, Welding and Maintenance

SUS309S is readily formed and machined with the practices used for austenitic stainless steel, and it work-hardens quickly, so light passes and adequate coolant are needed in machining. For welding, filler metal matching the base composition is used: ER309L wire or E309L covered electrodes, classified under AWS A5.9 and A5.4, are the standard choice. Low-carbon 309L filler prevents intergranular carbide precipitation in the weld deposit, and preheating is not required. Filler metals intended for 304 or 316 must not be substituted, because their lower chromium and nickel content reduces oxidation resistance in service.

Maintenance of furnace components centres on the oxide scale. Thick scale reduces heat transfer and cracks away from the surface, so scale is removed periodically by brushing or blasting. Parts are inspected for thermal-fatigue cracks at welds and at restraint points, and components are replaced once scale becomes heavy enough to spall. High-temperature coatings are sometimes applied to extend the life of parts that see the most severe cycles.

Typical Applications

Furnace linings, muffle and retort components, annealing boxes, boiler fireboxes and baffles, radiant heaters and their supports, high-temperature conveyor belts and trays, incinerator internals, and heat-exchanger parts exposed to hot flue gas. In welded assemblies of dissimilar heat-resisting grades, 309S or 309L filler is also used as a transition layer because of its intermediate chromium and nickel content.

Frequently Asked Questions

Q: Why is SUS309S chosen for furnace linings?
A: Its 22 to 24 % chromium content forms a dense oxide layer that resists scaling at high temperature, and its 12 to 15 % nickel keeps the structure austenitic and ductile through repeated heating cycles. Grade 304 with 18 % chromium oxidises heavily at comparable temperatures, so 309S survives where 304 does not.

Q: How does SUS309S behave in reducing atmospheres?
A: Poorly. Reducing gases destroy the protective scale and allow carburisation or decarburisation, so the grade is not selected for hydrogen or carbon monoxide service. Mixed atmospheres are acceptable only when oxidising conditions dominate; purely reducing duties need nickel-based alloys.

Q: Which filler metal is used to weld SUS309S?
A: ER309L wire or E309L electrodes, matched on chromium and nickel content. The low-carbon version is preferred for welded structures because it avoids intergranular carbide precipitation. Fillers intended for 304 or 316 are unsuitable because their lower alloy content reduces high-temperature oxidation resistance.

Q: What is the difference between 309S and 310S?
A: Grade 310S contains 24 to 26 % chromium and 19 to 22 % nickel against 22 to 24 % Cr and 12 to 15 % Ni in 309S. The higher alloy content of 310S extends service temperature and improves performance in more aggressive combustion atmospheres, at higher cost.

Q: What are the equivalent grades of SUS309S?
A: ASTM A240 type 309S, UNS S30908, EN 1.4828 and Chinese 06Cr23Ni13. Chemistry and mechanical requirements are aligned, so the designations are treated as interchangeable in most specifications.

Q: How are SUS309S furnace parts maintained?
A: Remove oxide scale periodically, inspect welds and restraint points for thermal-fatigue cracks, allow unforced cooling after high-temperature service, and replace components once spalling scale threatens the section. Coatings can be applied where thermal cycling is most severe.