Shunda Road, Lincheng Town Science and Technology Industrial Park, Xinghua City, Jiangsu Province
English
русский
Español
عربى
309S stainless steel is the low-carbon version of grade 309, an austenitic chromium-nickel alloy with 22.0–24.0% chromium and 12.0–15.0% nickel. Designated UNS S30908 in the United States and 1.4833 in the European system, it is specified mainly for components operating in oxidizing atmospheres at temperatures up to roughly 1000 °C in continuous service.
The defining specification limit is carbon: 309S is capped at 0.08% C, while straight 309 allows up to 0.20%. Lower carbon suppresses chromium carbide precipitation at grain boundaries when the material is welded or held in the 430–900 °C range, so 309S is preferred for welded fabrications and thermally cycled equipment.
Do not confuse product form with grade. 309S is a wrought specification used for sheet, plate, bar, and pipe. When the same Cr-Ni alloy family is needed for a cast part such as a furnace roller, kiln plate, or heat-treatment fixture, foundries work to cast grades such as ASTM A297 Grade HH rather than the wrought 309S specification.
Content
Chemical Composition of 309S (UNS S30908)
The chemistry of 309S is set by standards such as ASTM A240 and equivalent European specifications; every high-temperature property follows from the balance below.
| Element | 309S (S30908) | Why it matters |
|---|---|---|
| Carbon (C) | 0.08 max | Limits grain-boundary carbide precipitation |
| Chromium (Cr) | 22.0–24.0 | Forms the protective Cr2O3 scale against oxidation |
| Nickel (Ni) | 12.0–15.0 | Stabilizes austenite and maintains strength at temperature |
| Manganese (Mn) | 2.00 max | Deoxidation and minor austenite contribution |
| Silicon (Si) | 1.00 max | Improves oxidation and carburization resistance |
| Phosphorus (P) | 0.045 max | Residual element controlled for ductility |
| Sulfur (S) | 0.030 max | Residual element controlled for hot workability |
Compared with the higher-alloy 310S (24–26% Cr, 19–22% Ni), 309S offers a lower-cost base while still delivering practical oxidation resistance around 1000 °C. The extra nickel in 310S buys a visible margin in creep strength and resistance to severe thermal cycling, which is why 310S tends to replace 309S once the operating temperature climbs above roughly 1000 °C.
Mechanical and Physical Properties
In the annealed condition 309S is soft, ductile, and non-magnetic; the values below describe the fabricated condition, not the hot-service condition.
| Property | Annealed value |
|---|---|
| Minimum tensile strength | 515 MPa (75 ksi) |
| Minimum yield strength | 205 MPa (30 ksi) |
| Minimum elongation | 40% |
| Hardness | 217 HB maximum, typically 160–180 HB |
| Density | 7.98 g/cm³ |
| Modulus of elasticity | ≈200 GPa |
| Melting range | ≈1400–1450 °C |
| Mean coefficient of thermal expansion, 20–100 °C | ≈15 µm/(m·K) |
| Thermal conductivity, 100 °C | ≈15–16 W/(m·K) |
Do not select 309S on the basis of its 515 MPa tensile minimum if the service temperature is 900 °C. Allowable stress falls sharply above 600 °C, and the practical limit is set by oxidation rate and creep lifespan for the exact section thickness. When you send out an RFQ, state the atmosphere, peak temperature, and load; the supplier can then decide between 309S, a cast HH/HK grade, or a 310S upgrade.
309S vs. 310S and Cast Heat-Resistant Grades
The closest cast relative of 309S is ASTM A297 Grade HH, a 25Cr-12Ni alloy whose nominal composition shares about 96% of its elements with the wrought grade but carries more carbon to give castings high-temperature creep strength.
This distinction is the most common source of confusion in procurement. If you send a drawing for a furnace roller and write “309S” on it, a casting foundry will normally propose HH-type chemistry (24–28% Cr, 11–14% Ni, 0.20–0.50% C), because a low-carbon wrought chemistry will not develop the cast microstructure and rupture strength needed in a thick load-bearing section. Select the product form first: wrought 309S where you can cut, form, and weld sheet or pipe; cast A297 grades where the part is shaped, heavy, and loaded at temperature.
| Grade | Form | Carbon | Chromium | Nickel | Typical role |
|---|---|---|---|---|---|
| 309 / 309S | Wrought sheet, plate, bar, pipe | ≤0.20 / ≤0.08% | 22–24% | 12–15% | Fabricated liners, ducts, welded tube assemblies |
| 310 / 310S | Wrought sheet, plate, bar, pipe | ≤0.25 / ≤0.08% | 24–26% | 19–22% | Higher-temperature fabricated components |
| HH (A297) | Static or centrifugal casting | 0.20–0.50% | 24–28% | 11–14% | Thick castings around 1000 °C |
| HK (A297) | Static or centrifugal casting | 0.20–0.60% | 24–28% | 18–22% | Heavier castings above 1000 °C |
For round components that carry load in a furnace, centrifugal casting is usually the right route because it produces a dense, directionally solidified structure. A rolled and welded 309S pipe is acceptable for gas ducting; a tube that must hold its own weight at 1000 °C should be a centrifugally cast product with controlled chemistry.
Centrifugally Cast Heat Resistant Stainless Steel PipeThis pipe is produced by centrifugal casting for dense, directionally solidified structure, making it suitable for load-bearing furnace components at high temperatures, such as gas ducting and tubes that must hold their own weight at 1000 °C.View Product →Fabrication and Heat Treatment
309S can be cut, formed, machined, and welded by standard austenitic stainless procedures; the one critical rule is to finish hot working with a full solution anneal and a fast cool.
Solution annealing is performed at 1038–1120 °C with a sufficient hold to dissolve chromium carbides, followed by water quenching or vigorous air cooling. The grade cannot be hardened by heat treatment; stiffness gained through cold work is simply work hardening and disappears on annealing.
Welding is straightforward. Use matching 309 or 309L filler, avoid preheat except in very rigid sections, and keep interpass temperatures moderate. The 0.08% carbon cap greatly reduces chromium carbide precipitation in the heat-affected zone compared with straight 309, which is why weldments in 309S can enter furnace service without a separate post-weld treatment.
The same reasoning extends to the fixtures that carry workpieces through heat-treatment cycles. Tray design and tray material jointly determine temperature uniformity, and our review of optimizing heat-treatment tray design explains the structural factors a foundry checks before casting tooling. For repeated thermal cycling, cast fixtures with reinforced ribs outperform thin sheet-metal versions, which is why most operators move from welded 309S frameworks to cast HH-type baskets and trays for continuous heat-treatment lines.
Stackable Heat Treatment Basket for Well-Type FurnacesDesigned for organizing parts during heat treatment, this stackable basket improves furnace capacity and airflow, ensuring uniform heating and consistent results, especially suited for continuous heat-treatment lines where cast HH-type baskets outperform welded sheet-metal versions.View Product →Typical High-Temperature Applications
309S is the default economical stainless for clean, oxidizing atmospheres up to about 1000 °C and for components that can be manufactured by welding or cold forming.
- Heat-treatment fixtures: trays, baskets, retorts, and muffles operating at 850–1000 °C.
- Furnace internals: radiant tubes, burner nozzles, combustion chambers, and refractory anchors.
- Kiln hardware for rotary kilns: kiln furniture, kiln mouth and tail protection plates, and heat-resistant sealing strips.
- Waste-to-energy and power equipment: boiler components, air preheater sections, and sootblower lances.
- Petrochemical plant components: thermowells, air manifolds, catalyst support grids, and reactor internals.
Two application limits are worth remembering. In sulfur-containing atmospheres above roughly 500 °C, the nickel in 309S is vulnerable to sulfidation, so a different alloy family may be required. Under strong thermal cycling near 1000 °C, a thin wrought liner will distort before a cast component with designed-in ribs, which is why furnace rollers and protection plates for kilns are almost always cast rather than fabricated from sheet.
Heat Resistant Centrifugal Casting Furnace RollerThis roller is engineered from heat-resistant alloys to withstand high temperatures and rotational stress, providing reliable movement in centrifugal casting furnaces. Its cast design with ribs offers better thermal shock resistance than thin wrought liners, extending service life in demanding applications.View Product →How to Specify 309S Components: Procurement Considerations
The most useful step is to state the product form and governing standard on every RFQ; a request for a “309S casting” is technically ambiguous and will produce non-comparable quotes.
- Specify ASTM A240 for wrought plate and sheet, ASTM A312 or A358 for welded pipe, and ASTM A297 for castings.
- Request the actual heat analysis and verify chromium, nickel, and carbon values on the mill test certificate.
- For castings, confirm the charge chemistry, the casting route, and the post-cast heat-treatment procedure used by the foundry.
- Send the duty cycle: peak temperature, atmosphere composition, dwell time, and cyclic frequency.
- For critical load-bearing parts above 900 °C, require creep-rupture data or documented service history rather than room-temperature tensile values.
A supplier that works with both wrought fabrications and castings in the same Cr-Ni alloy family can translate your duty cycle into the right product form without guesswork. Our heat-resistant alloy steel foundry has produced the cast grades described here for kiln, furnace, and heat-treatment lines and can recommend a cast chemistry when a wrought 309S component is not the right solution for the load case.
Frequently Asked Questions about 309S
What is the difference between 309 and 309S?
Only the carbon limit. 309 allows 0.20% max carbon; 309S is capped at 0.08%. The lower carbon restricts chromium carbide precipitation during welding and service in the 430–900 °C range, which is why 309S is preferred for welded and thermally cycled equipment.
Can 309S be used for castings?
309S is formally a wrought designation. For cast parts, foundries use ASTM A297 grades with the same Cr-Ni base, usually Grade HH (25Cr-12Ni with 0.20–0.50% C) around 1000 °C. If you submit a “309S casting” drawing, expect the foundry to propose HH-type chemistry and explain the substitution.
Which is better at high temperature: 309S or 310S?
310S. Its higher chromium (24–26%) and nickel (19–22%) give a more stable oxide scale and better creep strength, so it is specified above about 1000 °C or under severe thermal cycling. 309S remains the value choice in clean oxidizing air within its rating.
Does 309S need to be annealed after welding?
For most high-temperature service, no. The low carbon content keeps the heat-affected zone from sensitizing. If the weldment also faces aqueous corrosive media, a full solution anneal at 1038–1120 °C followed by rapid cooling can be specified.
Is 309S magnetic?
In the annealed condition it is essentially non-magnetic, as expected for an austenitic grade. Cold work can induce slight magnetism, but it is rarely significant for furnace and kiln applications.
