# Type 309 stainless steel

309-series austenitic heat-resistant stainless steel: exact grade selection, product properties and atmosphere-, time- and fabrication-dependent limits.

Reviewed: 2026-11-01

Tags: materials, stainless-steel, heat-resistant-steel

## Identify the suffix and product form

Rolled Alloys reports representative **0.2% yield strength of 45 ksi at 70°F and 22 ksi at 1200°F** for its 309 stainless reference. The corresponding short-term tensile strengths are **70 and 52 ksi**, with **50% and 26%** elongation. For a loaded furnace component, use time-dependent creep evidence and the applicable design basis. [Rolled Alloys representative tensile data](https://www.rolledalloys.com/wp-content/uploads/309_stainless-steel-data-sheet-rolled-alloys.pdf)

Type 309 is an austenitic heat-resistant stainless steel. ATI's comparison distinguishes three UNS designations and carbon limits:

| Designation | UNS | Carbon, mass % |
| --- | --- | --- |
| 309 | S30900 | ≤0.20 |
| 309S | S30908 | ≤0.08 |
| 309H | S30909 | 0.04–0.10 |

The H variant provides a controlled carbon range for creep performance. A lower-carbon S designation is not automatically the best choice for every elevated-temperature load. [ATI 309/309S/310/310S technical sheet](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/austenitic/ati_309_309s_310_310s_tds_en3_2018.pdf)

ATI's Type 309 product page gives chromium 22–24% and nickel 12–15%, with iron balance and other limits. Its named product is distinct from an unidentified item described only as “309 stainless.” [ATI 309 product data](https://www.atimaterials.com/Products/ati-309)

Order the exact grade, product standard and edition, form, dimensions, delivery condition, finish and inspection documents. Require any grain-size or high-temperature qualification needed by the application. Chemistry falling in overlapping ranges does not, by itself, establish dual certification: the delivered product must meet all relevant requirements.

Rolled Alloys' “309” sheet lists S30908/S30909 and discusses dual certification. Read the certificate rather than inferring the suffix from its heading. [Rolled Alloys 309 datasheet](https://www.rolledalloys.com/wp-content/uploads/309_stainless-steel-data-sheet-rolled-alloys.pdf)

## Mechanical properties and test boundaries

ATI's Type 309 plate entry lists **tensile strength ≥515 MPa, yield strength ≥205 MPa, elongation ≥40% and hardness ≤95 HRB**. Its webpage references ASTM A167 without an edition; confirm the current purchase-standard basis, dimensions, condition and test requirements before using these as acceptance criteria. [ATI plate specifications](https://www.atimaterials.com/Products/ati-309)

ATI's technical sheet describes its short-term tensile dataset as ASTM E8 tests, averaging two to ten samples, with 0.2% offset yield and a two-inch elongation gauge length. It explicitly restricts typical data from specification or final-design use. [ATI test basis and data disclaimer](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/austenitic/ati_309_309s_310_310s_tds_en3_2018.pdf)

Keep short-term tensile strength, time-dependent creep/rupture strength and code allowable stress separate. A furnace support may satisfy a room-temperature proof-strength comparison yet fail its deformation limit over years of hot service. Define load, temperature, duration, cycles and permissible distortion before selecting the design input.

## Heat resistance is conditional

Outokumpu's Therma 309S/4833 entry gives **1000°C** maximum application temperature with the footnote **for air, guidance only, EN 10095**. This is a named-product oxidation/service-screening statement; it is not an allowable loaded-member temperature or guaranteed life. [Outokumpu Therma range, Tables 1–2](https://www.outokumpu.com/en/products/product-ranges/-/media/files/products/therma/outokumpu_therma_range_datasheet.pdf)

Rolled Alloys distinguishes constant-temperature oxidation resistance to 1900°F from approximately 1850°F under frequent thermal cycling. It also describes only moderate resistance to carbon absorption and advises against highly carburizing atmospheres. [Rolled Alloys atmosphere and cycling guidance](https://www.rolledalloys.com/wp-content/uploads/309_stainless-steel-data-sheet-rolled-alloys.pdf)

For a heat-treatment fixture, record actual metal temperature rather than only the furnace controller setting. Include gas chemistry, oxygen potential, sulfur/carbon-bearing species, deposits, flow and heating/cooling frequency. Assess scale loss, section loss and load deformation separately. A temperature number without those conditions cannot qualify the fixture.

Rolled Alloys warns that prolonged exposure at 1100–1600°F can form sigma phase and reduce room-temperature ductility and impact resistance. Maintenance handling after cooldown therefore deserves attention even when the part has carried its hot operating load successfully. [Rolled Alloys structural-stability note](https://www.rolledalloys.com/wp-content/uploads/309_stainless-steel-data-sheet-rolled-alloys.pdf)

## Wet exposure and fabrication

Outokumpu warns that high-temperature materials may perform less satisfactorily in low-temperature wet corrosion and recommends preventing or draining acid condensates. Startup, shutdown and cleaning exposure can therefore require a separate assessment from hot-gas operation. [Outokumpu aqueous-corrosion discussion](https://www.outokumpu.com/en/products/product-ranges/-/media/files/products/therma/outokumpu_therma_range_datasheet.pdf)

ATI's combined bulletin contains aqueous test tables explicitly identified for **310S**. Those tables must not be relabeled as 309 corrosion rates simply because both grades share the document. [ATI aqueous test scope](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/austenitic/ati_309_309s_310_310s_tds_en3_2018.pdf)

Define the material condition needed after forming and joining. Identify whether the fabrication sequence changes cold work, grain structure, surface finish or the required heat-treatment condition. Qualify critical bends and attachments on representative stock. Do not impose one bend radius, stress-relief cycle or cutting recipe across every thickness and product form.

**ER309L is a welding-filler classification, not a 309L base-metal certificate.** ATI discusses low-carbon filler selection separately and calls for removal of weld heat tint or scale to restore corrosion performance near the joint. [ATI welding discussion](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/austenitic/ati_309_309s_310_310s_tds_en3_2018.pdf)

Select consumables and the welding procedure for the actual base metals, dilution, service environment and mechanical requirement. Include repair welds and dissimilar attachments. Specify final cleanup and inspect the completed joint; matching a numerical family name does not demonstrate joint suitability.

## Bounded mass and temperature example

ATI lists density **8.03 g/cm³** for its 309 plate. An ideal 100 × 100 × 2 mm solid blank has volume 20 cm³ and estimated mass **160.6 g**. This original arithmetic excludes tolerances, scale, holes and coatings; it supports preliminary mass estimation rather than acceptance. [ATI physical properties](https://www.atimaterials.com/Products/ati-309)

Likewise, 1900°F converts to (1900 − 32) × 5/9 = **1037.8°C**, while 1850°F equals **1010°C**. These conversions preserve Rolled Alloys' stated conditions; they do not reconcile its guidance and Outokumpu's into a new universal temperature limit. Select the applicable producer data and qualify the real service case.

## Source versions and related reference

ATI's bulletin carries **©2018**. Outokumpu's Therma range footer identifies **1565EN:5, November 2022**; its URL timestamp is not the edition evidence. The inspected Rolled Alloys sheet and ATI product webpage provide no clearly verified technical revision date. Obtain current order-specific specifications and design-code requirements independently of this draft's review date.

[Grades, tempers and conditions](../grades-tempers-and-conditions.md)