Stainless Steel 347H
Stainless Steel 347H (UNS S34709) is the high-carbon, coarse-grained variant of niobium-stabilized 347. It is designed for long-term creep service in boilers and pressure vessels above 550°C. A mandated carbon minimum of 0.04% and ASTM grain size No. 7 or coarser set it apart from standard 347 in high-temperature design.
1. Chemical Composition
The table shows 347H elemental ranges. The carbon floor and niobium-to-carbon ratio control grain coarsening and creep performance after solution annealing.
| Grade | C | Mn | P | S | Si | Cr | Ni | Mo | Other |
| TP347H | 0.04–0.10 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | ≤ 1.00 | 17.00–19.00 | 9.00–13.00 | — | Nb: 8xC–1.0 |
Note: The carbon range 0.04-0.10% is intentional; ASTM A213 requires grain size ASTM No. 7 or coarser in 347H to validate elevated-temperature allowable stress values.
2. Mechanical Properties
These minimum mechanical requirements apply to 347H in the solution-annealed condition. ASME allowable stresses at temperatures above 550°C exceed those of standard 347.
| Grade | Condition | Tensile Strength Min, MPa (ksi) | Yield Strength Min, MPa (ksi) | Elongation Min, % | Heat Treatment Temp |
| TP347H | Annealed | 515 (75) | 205 (30) | 35 | 1040°C (1900°F) min |
Note: Elevated-temperature creep rupture data for 347H shows notably better performance than standard 347 at 600-800°C, as validated in ASME Section II Part D stress tables.
3. Equivalent Grade
This table lists international standard equivalents for 347H, so design engineers and procurement teams can identify approved substitutes across global supply chains.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| 347H | S34709 | S34779 | 07Cr18Ni11Nb | SUS347H | X6CrNiNb18-10 | 1.4961 | 08X18H12Б |
*DIN 1.4550 covers the base niobium-stabilized composition; the H-designation grain size and carbon minimum are defined under ASTM/ASME standards.
4. Key Technical Advantages
- Optimized for Creep-Controlled Design: The mandatory coarse grain structure (ASTM No. 7 or coarser), produced by the elevated carbon minimum and high-temperature solution anneal, gives superior resistance to creep deformation and stress rupture under sustained loading above 550°C.
- Higher ASME Allowable Stresses at Elevated Temperature: ASME Section I and VIII Division 1 assign higher allowable stress values to 347H than to standard 347 at operating temperatures above 550°C. This allows lighter-weight pressure component design in boiler and refinery applications.
- Niobium Stabilization with Elevated Carbon: Unlike unstabilized H-grades (such as 304H or 316H), the niobium addition in 347H prevents sensitization during fabrication while the elevated carbon produces the coarse grain needed for creep performance. Unstabilized grades cannot offer both benefits.
- Technical Note: Mill test reports for 347H must confirm both the carbon content (0.04-0.10%) and grain size (ASTM No. 7 or coarser). Acceptance of material not meeting grain size requirements for high-temperature ASME Code service is a non-conformance. Specify solution annealing at 1065°C minimum to ensure compliance.
5. Common Manufacturing Standards
ASTM A213: Standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes — TP347H with mandatory grain size per table in specification.
ASTM A312: Standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipes — TP347H designation.
ASTM A269: Standard specification for seamless and welded austenitic stainless steel tubing for general service — 347H applicable.
ASTM A249: Standard specification for welded austenitic steel boiler, superheater, heat-exchanger, and condenser tubes — TP347H applicable for elevated-temperature service.
DIN EN 10216-5: Seamless steel tubes for pressure purposes — stainless steel (1.4550 base composition; grain size per ASTM for H-designation compliance).
JIS G3463: Stainless steel boiler and heat exchanger tubes — SUS347HTB designation.
GB/T 13296: Seamless stainless steel tubes for boiler and heat exchanger use — 06Cr18Ni11Nb designation.
GOST 9940 / 9941: Seamless stainless steel tubes — 08X18H12Б equivalent applied.
Manufacturing Standards Comparison Table:
| Standard | ASTM | EN/DIN | JIS | GB/T | GOST |
| Seamless Boiler Tube | A213 TP347H | EN 10216-5 (1.4550) | G3463 SUS347HTB | GB/T 13296 | GOST 9941 |
| Seamless Pipe | A312 TP347H | EN 10216-5 (1.4550) | G3459 SUS347H | GB/T 14976 | GOST 9940 |
| Welded Boiler Tube | A249 TP347H | EN 10217-7 (1.4550) | G3463 SUS347H | GB/T 12771 | — |
6. Primary Applications
- Power Station Boiler Superheater Tubes: High-pressure steam service in supercritical and ultra-supercritical boilers where 347H's elevated-temperature allowable stresses permit thinner-wall tube design compared to standard 347.
- Petroleum Refinery Heater Coils: Radiant and convection section tubes in atmospheric distillation, vacuum distillation, and catalytic reforming furnaces operating continuously at 600-800°C.
- Heat Recovery Steam Generators (HRSG): Superheater and reheater modules in combined-cycle power plants where tube materials must endure rapid thermal transients without creep or sensitization damage.
- Hydrocarbon Processing Transfer Lines: High-temperature transfer lines between reactors and fractionators in delayed coking and visbreaking units that need niobium-stabilized material resistant to carburization.
- High-Temperature Pressure Vessels: ASME Code Section VIII vessels in sulfur recovery and thermal oxidation units where 347H's grain-size-dependent allowable stresses optimize pressure vessel wall thickness calculations.