Type 347
UNS S34700
Niobium-stabilized chromium-nickel austenitic stainless steel · ASTM A240/A240M-26 — plate, sheet and strip · ASTM A276/A276M-25 — general-purpose stainless steel bars and shapes · ASTM A479/A479M-25 — bars and shapes for boilers and other pressure vessels
Type 347 is an 18Cr-10Ni austenitic stainless steel stabilized with niobium to resist sensitization and intergranular corrosion after welding or elevated-temperature exposure. Its general corrosion behavior is broadly comparable with Type 304, while its principal advantage is improved stability in the chromium-carbide precipitation range. The applicable product specification is essential because A240 flat product and A276/A479 bar product have different chemistry, mechanical-property and heat-treatment requirements.
- Nb ≥ 10 × carbon contentDefining addition · The applicable maximum is 1.00% under ASTM A240 and 1.10% under ASTM A276/A479.
- Resistance to sensitization after welding or thermal exposurePrimary selection reason · Niobium preferentially forms stable carbides, helping retain chromium at grain boundaries.
- 205 MPa yield; 515 MPa tensileFlat-product minimum strength · ASTM A240 annealed plate, sheet and strip; elongation is 40% minimum.
- Generally comparable with Type 304Corrosion class · The niobium addition does not provide the chloride-pitting resistance associated with molybdenum-bearing grades such as 316.
- Solution anneal and rapid coolHeat treatment · Type 347 cannot be strengthened by conventional quench-and-temper heat treatment; strength increases mainly through cold work.
- Type 347 is not Type 347HElevated-temperature caution · S34709 has controlled higher carbon and additional elevated-temperature requirements; substitution requires design-code review.
Overview
- Designation system
- AISI type designation with UNS identifier
- Product forms
- Plate, Sheet, Strip, Hot-finished bar, Cold-finished bar, Hot-rolled or extruded shapes
- Condition
- Annealed or solution-annealed, ASTM A276 Condition A, Cold-finished and annealed, where applicable
- Density
- 7.96 g/cm³ (Representative room-temperature value; not an ASTM acceptance requirement.)
What the designation means
Type 347 identifies the alloy family, while UNS S34700 identifies its chemical composition. Neither designation by itself defines product form, dimensions, finish, heat treatment, testing, tolerances or certification. Those requirements come from the invoked product specification.
| Specification | Applicable product | Technical significance |
|---|---|---|
| ASTM A240/A240M | Chromium and chromium-nickel stainless plate, sheet and strip | Controls flat-product chemistry, mechanical properties, heat treatment and other product requirements. A480/A480M supplies general flat-product requirements. |
| ASTM A276/A276M | Hot- or cold-finished bars and hot-rolled or extruded shapes | General-purpose bar and shape specification. Strength and ductility depend on finish and section size. A484/A484M supplies general requirements. |
| ASTM A479/A479M | Hot- or cold-finished bars and shapes for boilers and other pressure vessels | Includes mandatory heat-treatment and pressure-purpose provisions not established merely by ordering A276 material. A484/A484M also applies. |
A240 plate cannot be certified as A276 or A479 bar merely because its chemistry is similar, and A276 bar is not automatically acceptable where A479 or an ASME pressure-material specification is required.
Chemical definition and niobium stabilization
| Element | ASTM A240 flat product | ASTM A276/A479 bar and shapes |
|---|---|---|
| Carbon | 0.08 max | 0.08 max |
| Manganese | 2.00 max | 2.00 max |
| Phosphorus | 0.045 max | 0.045 max |
| Sulfur | 0.030 max | 0.030 max |
| Silicon | 0.75 max | 1.00 max |
| Chromium | 17.0–19.0 | 17.0–19.0 |
| Nickel | 9.0–13.0 | 9.0–12.0 |
| Niobium | 10 × C min; 1.00 max | 10 × C min; 1.10 max |
| Iron | Balance | Balance |
The applicable ASTM edition and product-analysis tolerances govern acceptance. Historical documents may use “columbium” or the symbol Cb for niobium.
In an unstabilized 18Cr-8Ni steel, exposure in the sensitizing temperature range can precipitate chromium-rich carbides at grain boundaries. The adjacent chromium-depleted regions can then become vulnerable to intergranular attack. Type 347 contains sufficient niobium to preferentially combine with carbon, reducing chromium-carbide precipitation during welding and subsequent elevated-temperature service.
The different nickel, silicon and niobium limits demonstrate why chemistry should be checked against the exact product specification rather than against a generic “347” composition table.
Mechanical requirements depend on product form and finish
| Product specification and condition | Tensile strength | 0.2% yield strength | Elongation | Reduction of area |
|---|---|---|---|---|
| ASTM A240 plate, sheet and strip | 515 MPa (75 ksi) | 205 MPa (30 ksi) | 40% | Not specified in the principal A240 requirement |
| ASTM A276 Condition A, hot-finished bar | 515 MPa (75 ksi) | 205 MPa (30 ksi) | 40% | 50% |
| ASTM A276 Condition A, cold-finished bar ≤12.7 mm | 620 MPa (90 ksi) | 310 MPa (45 ksi) | 30% | 40% |
| ASTM A276 Condition A, cold-finished bar >12.7 mm | 515 MPa (75 ksi) | 205 MPa (30 ksi) | 30% | 40% |
| ASTM A479 annealed bar or shape | 515 MPa (75 ksi) | 205 MPa (30 ksi) | 30% | 40% |
Values apply to the listed product and condition, not universally to every Type 347 item. Gauge length, specimen geometry and specification exceptions remain applicable. Extruded shapes may have lower permitted minima under the relevant standard.
The higher minimum strength for small cold-finished A276 bar is a product-condition effect, not a different alloy grade. Actual mill values can exceed the specified minima, but they should not be treated as guaranteed design properties unless the purchase order or governing design code explicitly establishes them.
A240 Type 347 flat product is limited to 201 HBW or 92 HRB maximum in the specified annealed condition. Do not transfer this hardness limit to bar product without checking the applicable bar specification and edition.
Heat treatment and elevated-temperature use
Type 347 is normally supplied solution annealed and rapidly cooled. A minimum solution-treatment temperature of approximately 1040°C (1900°F) is used in the pressure-product specifications. Rapid cooling minimizes reprecipitation during passage through the sensitizing range. ASTM A479 does not permit the alternative direct-from-hot-working solution treatment route for niobium- or titanium-stabilized grades because dissolution of stabilizing carbides without a suitable subsequent treatment can leave the material inadequately stabilized.
Solution annealing restores ductility and corrosion resistance after substantial cold work but does not harden the steel. A separate stabilization treatment is sometimes specified for particular high-temperature, corrosive duties; it is not an automatic requirement of the three cited product standards and should only be imposed through the relevant product, fabrication or project specification.
| Feature | Type 347 / S34700 | Type 347H / S34709 |
|---|---|---|
| Carbon | 0.08% maximum | 0.04–0.10% |
| Niobium relationship | At least 10 × C | At least 8 × C |
| Primary emphasis | Welded corrosion resistance and general elevated-temperature service | Controlled carbon, grain size and heat treatment for improved creep/rupture performance |
| Interchangeability | Not automatically acceptable as 347H | Separate UNS grade; design code and certification must identify it where required |
For ASME construction or service above code-defined temperature thresholds, allowable stresses, grain-size requirements and the permitted material specification must be checked directly.
Corrosion behavior and selection limits
In properly annealed material, general aqueous corrosion resistance is broadly similar to Type 304. The major improvement is resistance to intergranular corrosion after welding or exposure within the sensitizing range, not a general increase in resistance to every corrosive medium.
| Exposure or mechanism | Type 347 behavior | Selection implication |
|---|---|---|
| Intergranular attack after welding | Improved resistance due to niobium stabilization | A principal reason to select 347 where postweld solution annealing is impractical. |
| Atmospheric and mildly oxidizing environments | Generally similar to 304 | Surface finish, contamination and fabrication quality can be as important as the nominal grade. |
| Chloride pitting and crevice corrosion | No molybdenum addition; resistance remains broadly in the 304 class | Do not substitute 347 for 316 solely because 347 is considered an elevated-temperature grade. |
| Chloride stress-corrosion cracking | Austenitic structure remains susceptible under adverse combinations of chloride, tensile stress and temperature | Stabilization against intergranular corrosion does not eliminate chloride SCC. |
| High-temperature oxidation | Useful resistance within the limits established by environment, section thickness and design code | Oxidation resistance alone does not establish allowable load-bearing temperature or creep life. |
Where intergranular-corrosion testing is contractually required, the purchase order should state the test practice, sensitization treatment, acceptance criterion and whether testing applies to the base material, weld procedure, production weld or all three.
Fabrication and welding
Type 347 has the ductility and forming characteristics expected of an austenitic stainless steel, but it work-hardens substantially. Forming equipment must allow for higher forming loads and springback than carbon steel. Severe cold work can alter magnetic response, dimensional stability and corrosion performance; intermediate or final annealing may be appropriate where the governing fabrication specification permits it.
Machining requires rigid tooling, sharp cutting edges and positive feeds to avoid dwelling on a work-hardened surface. Niobium carbides can increase tool wear relative to ordinary 304. Actual cutting parameters should be based on product condition, hardness, section size and tool system rather than on a single generic machinability rating.
| Topic | Practical guidance |
|---|---|
| Weldability | Suitable for conventional fusion-welding processes when procedures appropriate to austenitic stainless steel are used. |
| Matching consumables | AWS ER347 bare wire or E347 covered electrodes are commonly used for matching-grade joints. The governing welding code, service temperature and required weld-metal properties control final selection. |
| Preheat and PWHT | Preheat is not normally required for the alloy itself, and routine postweld heat treatment is not generally required for sensitization control. Code, thickness, restraint and dissimilar-material considerations may impose other requirements. |
| Contamination control | Use stainless-dedicated tools and remove iron contamination, slag, oxide and heat tint where corrosion performance requires a clean passive surface. |
| Heat input | Control heat input, interpass temperature and joint restraint through the qualified welding procedure to manage distortion and solidification cracking. |
| Dissimilar joints | Filler selection cannot be based only on the 347 base metal; dilution, the other alloy, service environment and elevated-temperature requirements must be evaluated. |
Specifying and accepting Type 347
| Item to specify or verify | Why it matters |
|---|---|
| Exact product specification and edition | Determines product form, chemistry, properties, heat treatment and testing. |
| Type 347 and UNS S34700 | Avoids confusion with 347H, 347LN, 348, cast CF8C or a generic commercial description. |
| Dimensions, tolerances and finish | A480/A480M or A484/A484M general requirements apply through the product specification, but optional tolerances or finishes may need to be stated. |
| Condition and manufacturing route | Hot-finished, cold-finished and annealed products can have different mechanical requirements and residual stresses. |
| Heat-treatment record | Particularly important for A479 pressure-purpose material and components subsequently hot formed or heat treated. |
| Required tests and supplementary requirements | Examples include product analysis, intergranular-corrosion testing, grain-size reporting, nondestructive examination or special surface requirements. |
| Material test report traceability | Confirm heat number, specification, grade, dimensions, chemistry, mechanical results, heat treatment and required test results. |
| Pressure-code acceptance | ASTM compliance alone does not establish acceptance under ASME or another construction code; the permitted specification, edition, allowable stress and service limits must also be checked. |
Do not approve a substitution solely from an equivalence table. Compare the complete product standard, chemistry, heat treatment, mechanical requirements, dimensions, testing, certification and design-code status.
Sources
- ASTM A240/A240M-26 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General ApplicationsASTM International
- ASTM A276/A276M-25 — Standard Specification for Stainless Steel Bars and ShapesASTM International
- ASTM A479/A479M-25 — Standard Specification for Stainless Steel Bars and Shapes for Use in Boilers and Other Pressure VesselsASTM International
- Practical Guidelines for the Fabrication of High Performance Austenitic Stainless SteelsNickel Institute
- Guidelines for the Welded Fabrication of Nickel-Containing Stainless Steels for Corrosion-Resistant ServicesNickel Institute
- ATI 347 Austenitic Stainless SteelATI
- The Role of Niobium in Austenitic and Duplex Stainless SteelsCBMM Niobium Technology
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