Type 347 stainless steel

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

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

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

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

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; RathGibson, product range

Bounded mechanical example

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

The table is a supplier reference; confirm the controlling product specification, dimensions, sampling and required acceptance values on the order. RathGibson, annealed-property table

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

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

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

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

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.