# Type 347 stainless steel

Niobium-stabilized UNS S34700: grade distinctions, product-specific properties, thermal history and corrosion limits.

Reviewed: 2026-11-01

Tags: materials, stainless-steel, stabilized

## Identity and stabilization

RathGibson's **347H pipe and tubing** listing requires **grain size No. 7 or coarser**. Its corresponding **347** entry gives no grain-size value. Check the ordered form, standard edition and applicable grain-size requirement before accepting dual certification. [RathGibson 347 and 347H pipe and tubing](https://www.pccenergy.com/companies/rathgibson/rathgibson-materials/347-stainless-steel-alloys.html)

Type 347, UNS S34700, is an austenitic stainless steel stabilized with niobium, historically called columbium, and tantalum. Carpenter explains that these elements have a stronger affinity for carbon than chromium: their carbides tend to form within grains rather than as continuous grain-boundary patterns. This is the reason for choosing a stabilized grade where thermal exposure and subsequent intergranular corrosion are concerns. [Carpenter Technology, 347 alloy description](https://www.carpentertechnology.com/alloy-finder/347)

Sandmeyer's plate sheet lists chromium 17–19%, nickel 9–13% and carbon at most 0.08% by mass for 347. It identifies 347H as UNS S34709, with carbon 0.04–0.10%, for enhanced creep resistance. Grade limits, producer typical chemistry and actual heat analysis are separate information. [Sandmeyer, Alloy 347/347H specification sheet](https://www.sandmeyersteel.com/wp-content/uploads/Alloy347-SpecSheet.pdf#page=1)

## Alternatives and specification boundaries

ATI distinguishes titanium-stabilized 321 from columbium/tantalum-stabilized 347 and 348; 348 additionally restricts tantalum and cobalt for nuclear applications. These distinctions prevent a generic “stabilized stainless” description from establishing interchangeability. [ATI, 321/347/348 technical data sheet](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/austenitic/ati_321_347_348_tds_en2_v1.pdf#page=1)

Choose a product standard by form and application. Carpenter lists ASTM A479/ASME SA479 among its specifications and offers forms including bar, rounds, billet, strip, wire and rod. RathGibson separately lists A269, A/SA249 and A/SA312 for 347 tubing/pipe. A bar listing does not qualify a welded tube, and a tube listing does not qualify plate. [Carpenter, forms and specifications](https://www.carpentertechnology.com/alloy-finder/347); [RathGibson, product range](https://www.pccenergy.com/companies/rathgibson/rathgibson-materials/347-stainless-steel-alloys.html)

## Bounded mechanical example

RathGibson's table, headed **annealed condition at −20°F to +100°F**, lists these 347 pipe/tube properties:

| Property | Published entry |
| --- | ---: |
| Tensile strength | 75,000 psi / 515 MPa |
| Yield strength | 30,000 psi / 205 MPa |
| Elongation in 2 inches, minimum | 35% |

The table is a supplier reference; confirm the controlling product specification, dimensions, sampling and required acceptance values on the order. [RathGibson, annealed-property table](https://www.pccenergy.com/companies/rathgibson/rathgibson-materials/347-stainless-steel-alloys.html)

For a **purely illustrative calculation**, a uniform 100 mm² section carrying 5,000 N has nominal stress `5,000 / 100 = 50 MPa`. Its ratio to the published 205 MPa yield entry is `50 / 205 = 0.244`. This is not a component safety factor: it excludes pressure-code allowables, temperature effects, joints, stress concentrations, corrosion loss and time-dependent deformation.

For design and inspection, retain whether a value is a specification requirement, typical producer result or delivered test result. Record test temperature, direction, gauge length and condition. A room-temperature tensile figure cannot establish creep-rupture life or fatigue performance.

## Thermal history and fabrication

ATI's bulletin treats sensitization resistance as the chief benefit during exposure in the chromium-carbide precipitation range, approximately 427–816°C. These grades are not hardenable by heat treatment. Use a qualified, product-specific thermal route rather than treating an arbitrary furnace cycle as “stabilizing.” [ATI, general information and heat treatment](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/austenitic/ati_321_347_348_tds_en2_v1.pdf#page=1)

Before forming or joining, identify the supplied condition and the required final condition. Retain the sequence of cold work, welding, stress relief and any solution/stabilization treatment. Confirm that later fabrication does not invalidate the specified corrosion test or mechanical acceptance basis. For hot-service equipment, include startup/shutdown cycles and long holds in the assessment; furnace temperature capability alone does not establish allowable load.

ATI's welding guidance emphasizes preservation of stabilizing elements, cleanliness and effective shielding. It warns that columbium-stabilized steels are more prone to hot cracking than titanium-stabilized steels. Thus weldability does not remove the need to qualify filler, joint design, heat input and inspection. [ATI, weldability section](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/austenitic/ati_321_347_348_tds_en2_v1.pdf#page=1)

Avoid assigning parent-stock properties to a completed weldment without the required joint qualification. Include heat tint removal and final surface condition in the fabrication plan where corrosion performance matters; specify the approved cleaning process rather than improvising an acid recipe.

## Corrosion and service selection

Sandmeyer describes general corrosion resistance comparable to 304 and identifies applications involving thermal sensitization concerns, including refinery polythionic-acid service. Its sheet also warns against chloride solutions and sulfuric-acid service. These statements are screening guidance, not a concentration-temperature acceptance chart. [Sandmeyer, corrosion section](https://www.sandmeyersteel.com/wp-content/uploads/Alloy347-SpecSheet.pdf#page=1)

ATI likewise states that stabilized grades remain susceptible to halide stress-corrosion cracking and have pitting/crevice-corrosion resistance similar to 304/304L. Stabilization addresses one mechanism; it does not provide molybdenum-alloyed chloride resistance. [ATI, stress-corrosion and pitting sections](https://www.atimaterials.com/Products/Documents/datasheets/stainless-specialty-steel/austenitic/ati_321_347_348_tds_en2_v1.pdf#page=1)

Document fluid composition, contaminants, temperature, wet/dry transitions, crevices and residual/applied stresses. Include cleaning and shutdown conditions. Where available evidence does not match the actual environment, request a supplier assessment or representative testing; do not attach an unqualified “corrosion resistant” label to the material record.

## Ordering and source editions

Specify exact UNS grade, product standard/edition, form and dimensions, supplied/final condition, grain-size requirements where applicable, inspection certificate and service-specific tests. Keep traceability from heat identity to the finished part and document approved substitutions explicitly.

ATI's PDF is printed **version 1, February 18, 2014**. Sandmeyer's PDF carries **Alloy 347/347H 6/2014** and **rev 02/2023 PDF link** markings. No printed revision date was verified for the Carpenter or RathGibson web pages. These references do not establish the latest contractual standard requirements.

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