# Type 409 stainless steel

Selecting ferritic 409 by exact UNS variant, stabilization, product form and exposure, with bounded properties and fabrication guidance.

Reviewed: 2026-11-01

Tags: materials, stainless-steel, ferritic

## Identity and designation boundaries

ATI's **S40930** uses titanium and niobium, also called columbium, to improve resistance to weld sensitization and intergranular attack. The same bulletin distinguishes **409Cb/S40945** and discusses brazing differences. Carry the exact UNS and thermal process on the exhaust stock order. [ATI 409HP stabilization and fabrication bulletin](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/ferritic/ati_409_cb_hp_tds_en1_v1.pdf)

**Type 409** is a ferritic stainless-steel family commonly used in exhaust equipment. ATI's 409HP bulletin distinguishes **S40910, S40920 and S40930**, with different stabilization requirements, from its historical S40900 description. The short number therefore does not establish one complete chemistry or producer product. [ATI 409HP technical bulletin](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/ferritic/ati_409_cb_hp_tds_en1_v1.pdf)

SSINA's composition reference lists the three modern variants separately, each with **carbon ≤0.03%** and **chromium 10.5–11.7%**, and different titanium/niobium requirements. It explicitly recommends specifying the governing ASTM standard and exact UNS number, because composition requirements can depend on the product. [SSINA ferritic composition table](https://www.ssina.com/education/technical-resources/composition-properties/)

Outokumpu markets **Moda 409/4512 (EN 1.4512)**. Its live product range associates this offering with S40910, while its range PDF uses the EN designation without a UNS entry in that row. Preserve the actual producer and certificate rather than infer universal cross-certification from a catalogue association. [Outokumpu Moda product range](https://www.outokumpu.com/en/products/product-ranges/moda), [Moda range data sheet](https://www.outokumpu.com/-/media/files/products/moda/outokumpu-moda-range-datasheet.pdf)

## Stabilization and product selection

ATI explains that its S40930 product balances titanium and columbium, another name for niobium, to improve resistance to sensitization and intergranular attack associated with welding. Its bulletin also distinguishes ATI 409Cb/S40945 and discusses brazing differences. Those are product-selection distinctions, not permission to substitute one stabilized variant for another solely because both are called 409. [ATI stabilization and variant discussion](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/ferritic/ati_409_cb_hp_tds_en1_v1.pdf)

Keep actual heat analysis separate from specified limits and from the producer's typical composition. If welding, brazing or another thermal operation matters, confirm that the ordered variant and process qualification address it. A generic chromium-content match cannot establish inclusion behavior, surface quality or the required joint properties.

## A bounded flat-product example

Outokumpu publishes these **20°C** entries under **EN 10088-2** for its Moda 409/4512 product:

| Product form | Rp0.2 | Rm | Elongation A |
| --- | ---: | ---: | ---: |
| Cold-rolled coil/sheet | 220 MPa | 380–560 MPa | 25% |
| Hot-rolled coil/sheet | 220 MPa | 380–560 MPa | 25% |

Retain the governing specification's dimensions, specimen and sampling requirements when using the table. These flat-product entries are not generic properties of weld metal, castings or every supplied 409 variant. [Outokumpu mechanical properties, Table 4](https://www.outokumpu.com/-/media/files/products/moda/outokumpu-moda-range-datasheet.pdf)

For a material record, distinguish specification minima, producer typical data and delivered results. A tensile range is not an allowable-stress range, and an upper tensile bound must not become a minimum. Select the accepted design method before using these inputs for a loaded part.

Outokumpu's physical table gives **density 7.7 kg/dm³**, **Young's modulus 220 GPa at 20°C** and a magnetizable response for this offering. An ideal solid 100 × 100 × 10 mm blank has volume **0.1 dm³** and estimated mass **0.77 kg**. This original calculation excludes tolerances, holes, coatings and hardware. [Outokumpu physical-property table](https://www.outokumpu.com/-/media/files/products/moda/outokumpu-moda-range-datasheet.pdf)

A magnetic response cannot identify a specific grade, stabilization route or strength. Likewise, nominal mass and modulus do not establish the stiffness, fatigue life or vibration behavior of an assembled exhaust system.

## Exposure and service boundaries

Outokumpu positions Moda 409/4512 for mildly corrosive service and describes oxidation resistance in dry air. It also states that its corrosion resistance is limited where corrosiveness increases. The word “stainless” does not establish an unchanged cosmetic surface or compatibility with an arbitrary condensate. [Outokumpu 409 product description](https://www.outokumpu.com/en/products/product-ranges/moda)

For an exhaust or furnace-adjacent component, document actual metal temperature, gas chemistry, deposits, heating/cooling cycles and time under load. Assess wet shutdown and cleaning exposure separately from dry hot operation. A furnace setpoint or general oxidation statement cannot qualify a support, joint or complete assembly.

ATI's elevated-temperature discussion shows that tensile strength, ductility and rupture behavior change with temperature. It does not make oxidation resistance equivalent to load-carrying ability. Preserve exposure duration and the particular failure criterion when selecting elevated-temperature evidence. [ATI mechanical and rupture-property discussion](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/ferritic/ati_409_cb_hp_tds_en1_v1.pdf)

Specify acceptable section loss, deformation, leakage and appearance independently. Include joints, crevices, dissimilar materials and attachment details. Where the proposed service differs from published evidence, obtain producer advice or representative testing rather than assign one universal maximum operating temperature.

## Welding, forming and process history

ATI's welding section warns that filler chemistry must be balanced to avoid martensite that could impair weld corrosion performance in acidic environments. It also emphasizes avoiding carbon and nitrogen contamination. These controls belong to the actual base-material/filler/process combination. [ATI welding guidance](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/ferritic/ati_409_cb_hp_tds_en1_v1.pdf)

Qualify the joint, preparation, filler, shielding, heat input and inspection for the ordered material and service. Review brazing and dissimilar-metal joints separately. A satisfactory base-metal certificate cannot establish weld chemistry, soundness or finished joint life.

Plan forming and machining around supplied condition, thickness, rolling direction, tooling and final geometry. Use representative trials for critical bends, flanges and sealing surfaces. Define which operations create the final dimensions and which require renewed inspection.

Do not borrow an austenitic-stainless heat-treatment or finishing recipe without confirming its applicability. Record thermal and repair history where it affects acceptance. Obtain current handling information and the facility procedure appropriate to the operation and generated material.

## Release and source versions

Retain the exact variant, producer, specification, heat/lot, certificate, delivered/final condition, drawing and fabrication record. Establish dimensional, fatigue, leak or corrosion acceptance where needed. Resolve substitutions against the full record rather than accept a similar trade name or one favorable property result.

ATI's sheet prints **Version 1, 18 February 2014** and contains historical specification discussion; it does not prove the requirements of a later standard edition. The inspected Outokumpu and SSINA references are used for the stated product/composition guidance without assigning a technical edition from URL timestamps or website copyright years. Obtain controlled order-specific documents for acceptance. Review date: **2026-11-01, America/Los_Angeles**.

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