Density, g/cm³
0.288 lb/in³ at 20 °C. One source gives a wider band of 7.7 to 8.03 g/cm³.
Grade description
AISI 347H, unified number S34709, is a chromium-nickel austenitic stainless steel stabilised with niobium and tantalum. Niobium ties up carbon as niobium carbide instead of allowing chromium carbide to form at the grain boundaries, so the chromium stays in solution and the steel keeps its corrosion resistance after exposure to 800–1500 °F (427–816 °C).
The H suffix means carbon is held to a range of 0.04 to 0.10 % rather than a 0.08 % ceiling. That deliberate carbon addition was made to improve creep resistance and to raise strength at temperatures above 1000 °F (537 °C). In most heats the carbon level allows dual certification to both 347 and 347H.
The steel has a face centred cubic structure, is non-magnetic in the annealed condition, and cannot be hardened by heat treatment. Strength is raised only by cold work. It is ductile enough to be stamped, blanked, spun and drawn.
Technical data on this page was last checked on 1 October 2026.
347 against 347H
The two grades share a chromium and nickel base and differ mainly in carbon and in the niobium stabilisation ratio. The consequence is a trade: 347H carries more load at temperature, 347 is the lower-carbon choice where creep is not the governing condition.
| Item | Type 347 | Type 347H |
|---|---|---|
| UNS number | S34700 | S34709 |
| Carbon, % | 0.08 max | 0.04–0.10 |
| Nb + Ta stabilisation | 10 × (C + N) min, 1.00 max | 8 × (C + N) min, 1.00 max |
| Creep strength above 537 °C | Lower | Higher, the reason the grade exists |
| Toughness and durability | Good | Tougher and generally more durable |
| Dual certification | Usually possible, since the carbon ranges overlap | |
The 10 × and 8 × ratios above are the ASTM A240 plate limits. Name the governing specification on the order, because the stabilisation ratio and the silicon ceiling are written differently in the plate, forging and tube standards.
Standards and equivalents
| Standard or system | Designation |
|---|---|
| UNS | S34709 |
| AISI / ASTM type | 347H |
| Forgings | ASTM A182 / A182M Grade F347H · ASTM A336 · ASTM A403 |
| Bar | ASTM A479 |
| Plate, sheet, strip | ASTM A240 · ASME SA 240 · ASTM A480 |
| Tube and pipe | ASTM A213 · A249 · A271 · A312 · A376 · A430 · A813 · A814 |
| DIN / EN | 1.4961 · X8CrNiNb16-13 |
| SAE | SAE 30347H |
Note on 1.4550. DIN 1.4550 / X6CrNiNb18-10 is the European equivalent of Type 347, not of 347H. The 347H counterpart is 1.4961 / X8CrNiNb16-13, which is classified as a creep resisting steel under EN 10302. The nickel and chromium bands of 1.4961 are not identical to S34709, so a drawing calling for one should not be filled with the other without written agreement.
Chemical composition
For forgings the governing document is normally ASTM A182 / A182M Grade F347H. The plate standard ASTM A240 is written to a tighter chromium band and a lower silicon ceiling, so both are given below.
| Element | ASTM A182 F347H, forgings | ASTM A240 347H, plate |
|---|---|---|
| Carbon, C | 0.04–0.10 | 0.04–0.10 |
| Chromium, Cr | 17.00–20.00 | 17.00–19.00 |
| Nickel, Ni | 9.00–13.00 | 9.00–13.00 |
| Niobium + tantalum, Nb + Ta | Stabilising addition | 8 × (C + N) min, 1.00 max |
| Manganese, Mn | 2.00 max | 2.00 max |
| Silicon, Si | 1.00 max | 0.75 max |
| Phosphorus, P | 0.045 max | 0.045 max |
| Sulfur, S | 0.030 max | 0.030 max |
| Iron, Fe | Balance, roughly 63 to 74 % | |
On the niobium figure. The stabilisation requirement is a ratio, not a fixed band, so the minimum niobium moves with the carbon and nitrogen of the heat. Published datasheets that quote a flat 0.32 to 1.00 % niobium are reporting the range that the ratio produces in practice at typical carbon levels. We work to the ratio stated in the governing standard and report the measured niobium on the certificate.
Mechanical properties
Specification minima and typical measured values are two different things and are kept apart here. Order acceptance is against the minima.
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength, min | 515 MPa | 75 ksi |
| Yield strength, 0.2 % offset, min | 205 MPa | 30 ksi |
| Elongation in 50 mm or 4D, min | 30 % | 30 % |
| Reduction of area, min | 50 % | 50 % |
| Brinell hardness | Not specified in A182 for this grade | |
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength | 515 MPa | 75,000 psi |
| Yield strength, 0.2 % offset | 205 MPa | 30,000 psi |
| Elongation in 50 mm | 40 % | 40 % |
| Hardness, Brinell | 201 max | 201 max |
| Hardness, Rockwell B | 95 | 95 |
| Modulus of elasticity | 193 GPa | 28.0 × 106 psi |
| Poisson's ratio | 0.27–0.30 | 0.27–0.30 |
| Rupture strength, 750 °C, 100,000 h | 38–39 MPa | 5,510–5,660 psi |
Conflicting published figures. Some grade datasheets list 480 MPa tensile and 29 % elongation as typical for 347H. Those numbers fall below the 515 MPa and 30 % minima of ASTM A182 F347H and should not be used for design or acceptance. Where a published value sits below the governing specification, the specification governs. What a heavy forged section actually achieves depends on section size, reduction and the position of the test coupon, all of which are agreed before production starts.
Physical properties
7.96
16.3
Thermal conductivity, W/m·K
At 100 °C, equal to 13.3 BTU/hr/ft²/ft/°F at 200 °F.
500
Specific heat, J/kg·K
0 to 100 °C, equal to 0.12 BTU/lb·°F over 32 to 212 °F.
| Property | Value | Condition |
|---|---|---|
| Melting range | 1398–1446 °C | 2550–2635 °F |
| Electrical resistivity | 72 microhm·cm | As published, at 20 ° |
| Thermal expansion | 16.0 × 10-6 /°C | 20–100 °C · 9.2 × 10-6 in/in·°F over 68–212 °F |
| Thermal expansion | 18.9 × 10-6 /°C | 20–600 °C · 10.5 × 10-6 in/in·°F |
| Thermal expansion | 20.5 × 10-6 /°C | 20–1000 °C · 11.4 × 10-6 in/in·°F |
| Magnetic response | Non-magnetic | Annealed. May turn slightly magnetic after cold work |
The expansion coefficient is high compared with ferritic and martensitic grades and rises further with temperature. Allow for it when 347H forgings are bolted or welded to lower-expansion materials in hot service.
Service temperature
Three temperatures matter when specifying 347H, and they are often confused with one another.
427–816 °C800 to 1500 °F, the chromium carbide precipitation range. Unstabilised grades such as 304 are sensitised here. 347H is stabilised specifically to survive it.
above 537 °CAbove 1000 °F, where the added carbon of 347H gives higher strength and creep resistance than plain 347.
816 °C1500 °F, the figure published for good oxidation resistance and creep strength, and for performance in strongly oxidising conditions.
The safe limit for a particular part is not a single number. It depends on the atmosphere, the stress, the design life and the operating cycle, and it must be taken from the design code that governs the equipment rather than from a grade datasheet. The 100,000 hour rupture strength of 38 to 39 MPa at 750 °C gives an idea of how little load the grade will carry indefinitely at the top of its range.
The steel also keeps good toughness at low temperature, which is why the grade appears in equipment that cycles between cryogenic and hot service.
Corrosion behaviour
General corrosion resistance is comparable to Type 304. The advantage of the stabilised grade shows after exposure to elevated temperature, where it resists intergranular attack that would damage an unstabilised steel, and it performs better than Type 321 in the carbide precipitation range and somewhat better in strongly oxidising environments up to 1500 °F (816 °C).
Suited to
Nitric solutions. Most dilute organic acids at moderate temperature. Pure phosphoric acid at lower temperature, and dilute solutions to about 10 % at elevated temperature. Chloride-free and fluoride-free caustic solutions at moderate temperature. Polythionic acid stress corrosion cracking in hydrocarbon service.
Not suited to
Chloride solutions, including low concentrations. Sulfuric acid. For chloride-bearing or marine duty, select a duplex or high-molybdenum austenitic grade instead.
Heat treatment
347H is unresponsive to hardening heat treatment. There is no quench and temper route. The only thermal treatment in normal use is solution annealing, which dissolves precipitates, restores the austenitic structure and relieves the stresses left by forging.
| Stage | Practice |
|---|---|
| Temperature | 1010–1193 °C (1850–2000 °F) |
| Soak | By section thickness, long enough to bring the whole section to temperature |
| Cooling | Water quench |
| Hardening | Not applicable. Strength is raised only by cold work |
Upper end of the range. ASTM A182 carries a caution that a solution annealing temperature above 1950 °F may impair properties in the stabilised grades, F347, F347H, F348 and F348H. Where the certificate has to satisfy A182, agree the annealing temperature in writing before the charge goes in, rather than working to the top of the wider 2000 °F figure quoted in general datasheets.
Forging and hot working
Forging, upsetting and other hot work is carried out at 1149 to 1232 °C (2100 to 2250 °F). The grade does not air harden, so there is no risk of untempered martensite on cooling, but the work does need to finish hot enough to avoid tearing and then be solution annealed to restore the structure.
| Stage | Temperature or action |
|---|---|
| Hot working range | 1149–1232 °C (2100–2250 °F) |
| After forging | Solution anneal and water quench |
| Cold forming | Readily stamped, blanked, spun and drawn. Work hardens, so interstage annealing may be needed |
In-house capability
Open-die forging
Bars, blocks, solid and hollow shafts and stepped shafts, made to customer drawings.
Ring rolling
Seamless rolled rings, sleeves, flange blanks and casings with circumferential grain flow.
Heat treatment
Solution annealing with recorded furnace charts and quench records.
Testing
Chemical analysis, tensile, impact and hardness testing, ultrasonic and liquid penetrant inspection.
Machining and welding
Machining
347H is slightly tougher to machine than Type 304 and the cold work hardening rate makes it less machinable than Type 410. Hold the speed down, keep the feed constant and positive, and do not allow the tool to dwell, because the surface will work harden under a rubbing edge.
| Tool | Lubrication | Depth of cut | Feed | Speed |
|---|---|---|---|---|
| High speed steel | Cutting oil | 6 mm | 0.5 mm/t | 12–16 m/min |
| High speed steel | Cutting oil | 3 mm | 0.4 mm/t | 18–23 m/min |
| High speed steel | Cutting oil | 1 mm | 0.2 mm/t | 23–28 m/min |
| Carbide | Dry or cutting oil | 6 mm | 0.5 mm/t | 67–76 m/min |
| Carbide | Dry or cutting oil | 3 mm | 0.4 mm/t | 81–90 m/min |
| Carbide | Dry or cutting oil | 1 mm | 0.2 mm/t | 99–108 m/min |
Welding
The grade welds readily by most standard fusion and resistance processes and post-weld heat treatment is not required. Oxyacetylene welding is not preferred. Use a matching filler and clean the weld area afterwards to get the best corrosion resistance. Because the stabilising element is in the parent metal, a filler that is not itself stabilised will give a weld deposit with less resistance to intergranular attack than the material either side of it.
Products supplied
| Form | Items |
|---|---|
| Bars | Forged round bars, square bars, flat and rectangular bars, stepped bars |
| Hollow sections | Hollow bars, sleeves, bushings, barrels, casings, shells, cylinders, hubs, housings |
| Rings | Forged rings, seamless rolled rings, flange blanks |
| Discs and blocks | Forged discs, blocks, plates, tubesheet blanks |
| Pressure parts | Forged valve bodies and body blanks, fitting blanks, header and nozzle forgings |
| Fasteners | Double-ended studs, bolts, nuts, high-temperature bolting stock |
Where the grade is used
Power generation
Superheater and reheater components, boiler tubes and casing, radiant superheaters, high pressure steam piping and the forged parts that connect them.
Refining and chemical
Equipment for severe corrosive duty in chemical processing and petroleum refining, heat exchangers and tube, waste heat recovery, pharmaceutical plant.
Aerospace and process
Aircraft exhaust stacks and collector rings, cabin heaters, heavy wall welded equipment, food processing equipment and storage vessels.
Common questions
What is the difference between 347 and 347H?
Carbon. Type 347 is capped at 0.08 % carbon while 347H is held to a range of 0.04 to 0.10 %. The higher carbon was specified to improve creep resistance and to give higher strength above 1000 °F (537 °C), and it makes the steel tougher and generally more durable. The niobium stabilisation ratio also differs, 10 × (C + N) minimum for 347 against 8 × (C + N) minimum for 347H in ASTM A240. Because the carbon ranges overlap, many heats can be dual certified to both.
Can 347H be hardened by heat treatment?
No. It is austenitic and unresponsive to hardening heat treatment. Hardness and strength can only be increased by cold working. The solution anneal at 1010 to 1193 °C followed by a water quench is used to dissolve precipitates and relieve stress, not to harden the steel.
What is the maximum service temperature?
Published data gives good oxidation resistance and creep strength to 1500 °F (816 °C), and the grade is specifically intended for the 800 to 1500 °F (427 to 816 °C) carbide precipitation range where unstabilised grades sensitise. The usable limit for a given part is set by the design code, the atmosphere, the stress and the required life, not by the grade alone. For reference, the 100,000 hour rupture strength at 750 °C is only 38 to 39 MPa.
Is 347H the same as DIN 1.4961?
1.4961, X8CrNiNb16-13, is the European creep resisting steel normally cross-referenced to 347H under EN 10302. It is not the same as 1.4550, X6CrNiNb18-10, which corresponds to plain Type 347. The composition bands of 1.4961 and S34709 are not identical, so the order must name which standard governs and we certify against that one.
Does 347H need post-weld heat treatment?
Not as a rule. The grade can be readily welded by most standard processes and a post-weld heat treatment is not necessary, which is one of the reasons it is chosen over unstabilised grades for hot service. Oxyacetylene welding is not preferred. Use a matching filler and clean the weld area afterwards. Where the fabrication code or the client specification calls for a solution anneal after welding, we carry it out and record it.
Is 347H magnetic?
It is non-magnetic in the annealed condition because the structure is austenitic. Cold working can make it slightly magnetic. A weak magnetic response in a machined or cold worked surface is normal and is not on its own evidence of a wrong grade.
Will 347H work in seawater or chlorides?
No. It does not perform well in chloride solutions even at small concentrations, and it is unsuitable for sulfuric acid. Its strengths are nitric solutions, dilute organic acids, phosphoric acid within limits, chloride-free caustics and polythionic acid service in hydrocarbons. For chloride-bearing or marine duty, ask us about duplex or high-molybdenum austenitic grades.
Can you supply one-off or small quantity 347H forgings?
Yes. Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory, so single pieces, prototypes and production batches are all made to drawing. Send the drawing or the dimensions, the quantity, the governing standard and any test requirement to sales@steelforgepieces.com.
What do you need for a quotation?
Grade, dimensions or drawing, quantity, the governing standard, the delivery condition and any inspection requirement. A quotation normally follows within one working day.
About the supplier
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. We make forged bars, seamless rolled rings, forged flanges, discs, shafts and custom forged components in carbon steel, alloy steel, tool steel, stainless steel and nickel alloys.
The data on this page is drawn from published grade standards and from our own production experience with the 347 group. The figures are guidelines. The values that apply to a given order are those stated on the mill test certificate issued with it.
Enquiries for AISI 347H forgings
Send a drawing or a description and we will come back with a price, a lead time and the test package.
- Grade and the standard that governs it
- Dimensions, rough or finished, and the machining allowance
- Quantity and required delivery date
- Delivery condition, as forged or solution annealed
- Testing and inspection requirements
Jiangyin Jiangnan Metal Co., Ltd.
No.1 Chengxiqiao Road, Zhouzhuang Town,
Jiangyin City, Jiangsu Province, China
Tel, WhatsApp and WeChat +86 189-2135-9659
Email sales@steelforgepieces.com
Jiangyin City, Jiangsu Province, China
Tel, WhatsApp and WeChat +86 189-2135-9659
Email sales@steelforgepieces.com