GRADE PROFILE
347H — S34709 / TP347H
347H is a niobium-stabilised austenitic with carbon controlled between 0.04 and 0.10%. That carbon is not a tolerance, it is the point of the grade: it delivers creep strength. 347H is chosen for high-temperature steam, never for corrosion.
- H and stabilised grade
- PREN 17–19 †
- Cost index 125
When to choose it
On high-temperature steam circuits. Boiler superheaters and reheaters, steam piping, reforming and cracking units in refining, heat exchangers subject to creep. Niobium ties up carbon durably, including after thousands of hours at temperature, where titanium stabilisation degrades faster.
In biomass and waste incineration, stabilisation prevents intergranular corrosion after welding on assemblies that then see severe thermal cycling.
When not to choose it
In chlorinated media. Without molybdenum, 347H performs like a 304: its PREN of 18 says so, and this is the one case where the index is worth reading on this grade — to rule a use out, not to justify one.
As a substitute for 347. The two are not interchangeable: the carbon range and grain size of the H grade are precisely what deliver code-guaranteed creep strength.
In low-temperature service. Above 0.10% carbon, ASTM requires impact testing below −45 °C under ASME VIII Division 1.
And where a 321H would do. Niobium costs more than titanium, and the two stabilisations are comparable over much of the range.
Composition
| Element | % |
|---|---|
| C | 0.04–0.1 |
| Mn | ≤ 2 |
| P | ≤ 0.045 |
| S | ≤ 0.03 |
| Si | ≤ 1 |
| Cr | 17–19 |
| Ni | 9–13 |
| Nb | ≥ 8 × %C et ≤ 1,00 % |
Mechanical properties
| Standard | Form | Rm (MPa) | Rp0.2 (MPa) | A (%) | Hardness |
|---|---|---|---|---|---|
| ASTM A312/A312M, tableau 2 | tube soudé et sans soudure | ≥ 515 | ≥ 205 | ≥ 35 | — |
Designations and standards
| Reference | Designation |
|---|---|
| EN (numeric) | no exact equivalent |
| EN (symbolic) | — |
| ASTM / AISI | TP347H |
| UNS | S34709 |
| JIS | SUS347H |
| AFNOR | — |
Tube product standards :
- ASTM A312/A312M
- ASTM A213/A213M
- ASME SA312
- ASME SA213
Availability
Available in welded and seamless tube to ASTM A312 and A213. Structured dimensional data for this grade is not yet documented in our reference set: send us your sizes. EN 10204 3.1 material certificates supplied with the goods.
Related grades
- 321H — titanium stabilisation, cheaper, over much of the same range.
- 310S (1.4845) — where hot oxidation rather than creep governs.
- 316Ti (1.4571) — stabilisation with molybdenum, for intermediate temperatures with chlorides present.
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Frequently asked questions
- Why is the PREN of 347H so low?
- Because it contains no molybdenum. PREN measures pitting resistance in chlorinated media, which is not this grade's service. A 347H at PREN 18 holds conditions in which a 316L at PREN 25 would creep — the two indices are not talking about the same thing.
- What is the difference between 347 and 347H?
- The H imposes a minimum carbon of 0.04% and a controlled grain size. An ordinary 347 can run much lower in carbon, which degrades creep strength. On hot pressure equipment the substitution is a compliance failure.
- 347H or 321H?
- 321H, titanium-stabilised, covers most cases and costs less. 347H earns its place where service life at temperature is long, or where the code or specification names it explicitly.