AISI 347H Stainless Steel
1. Introduction
347H (UNS S34709) is a niobium-stabilised austenitic stainless steel designed for long-term service in elevated-temperature environments where resistance to intergranular corrosion, creep, and sensitisation must be maintained simultaneously. The niobium addition — present at a minimum of ten times the carbon content — preferentially forms stable niobium carbides, preventing chromium depletion at grain boundaries and eliminating the sensitisation risk that affects unstabilised austenitic grades in prolonged high-temperature service. This stabilisation mechanism makes 347H particularly well suited for welded assemblies and components exposed to the sensitisation range (approximately 427–850°C) for extended periods.
347H is used across power generation, petrochemical, and industrial heating industries, where its combination of creep resistance, structural stability, and intergranular corrosion resistance supports reliable long-term performance. Typical applications include superheater tubes in steam power plants, fired heater tubes and pressure vessels in oil refineries, ammonia converter cooling tubes, reactor vessels, and furnace heating elements where service at 550–850°C demands both mechanical integrity and corrosion stability.
347H and 347: When to Choose
347H and 347 share the same niobium-stabilised composition but differ in carbon content. Standard 347 (UNS S34700) permits carbon up to 0.08 wt%, while 347H specifies a tighter carbon range of 0.04–0.10 wt% with a carbon minimum — ensuring sufficient carbon is present to support niobium carbide precipitation and maximise creep strength. The higher and controlled carbon range of 347H gives it superior creep rupture strength at elevated temperatures, making it the preferred choice for long-term pressure-bearing service above 550°C. Where elevated-temperature creep resistance is not the primary requirement, standard 347 offers broader availability and slightly lower material cost.
2. Chemical Composition
The composition of 347H is defined by the requirements of ASTM and EN specifications for niobium-stabilised austenitic stainless steels. The niobium addition — at a minimum of ten times the carbon content — is the defining characteristic of this grade, providing stabilisation against chromium carbide precipitation and supporting elevated-temperature structural integrity.
Table 1 — Chemical Composition of 347H (UNS S34709)
| Element | Nominal / Limits (wt%) |
|---|---|
| Carbon (C) | 0.06 (nominal) |
| Silicon (Si) | 0.4 (nominal) |
| Manganese (Mn) | 1.8 (nominal) |
| Phosphorus (P) | ≤ 0.040 |
| Sulfur (S) | ≤ 0.015 |
| Chromium (Cr) | 17.5 (nominal) |
| Nickel (Ni) | 11.0 (nominal) |
| Niobium (Nb) | ≥ 10 × C |
| Iron (Fe) | Balance |
The niobium-to-carbon ratio requirement (Nb ≥ 10 × C) is the critical specification parameter, ensuring that sufficient niobium is present to stabilise all available carbon as niobium carbides, leaving chromium in solid solution to maintain corrosion resistance at grain boundaries.
3. Mechanical and Physical Properties
347H is designed to maintain strength and structural stability under long-term elevated-temperature loading. Its controlled carbon content and niobium stabilisation combine to deliver reliable creep performance across the service temperature range of 550–850°C.
3.1 Tensile Properties at Room Temperature
Table 2 — Mechanical Properties of 347H at 20°C (Alleima 8R40)
| Property | Value |
|---|---|
| 0.2% Proof Strength (Rp0.2) | ≥ 220 MPa / ≥ 32 ksi |
| 1.0% Proof Strength (Rp1.0) | ≥ 250 MPa / ≥ 36 ksi |
| Tensile Strength (Rm) | 515–690 MPa / 75–100 ksi |
| Elongation | ≥ 40% |
| Hardness | ~155 HV |
3.2 Proof Strength at Elevated Temperature
Table 3 — Minimum Proof Strength at Temperature (Alleima 8R40)
| Temperature | Rp0.2 (MPa) | Rp1.0 (MPa) |
|---|---|---|
| 50°C | 195 | 232 |
| 100°C | 175 | 210 |
| 200°C | 155 | 185 |
| 300°C | 139 | 167 |
| 400°C | 129 | 159 |
| 500°C | 124 | 155 |
| 550°C | 118 | 152 |
3.3 Creep Rupture Strength
347H provides excellent long-term creep resistance in the 540–700°C range, making it a benchmark grade for pressure-bearing elevated-temperature service.
Table 4 — Creep Rupture Strength (Alleima 8R40)
| Temperature | 10,000 h (MPa) | 100,000 h (MPa) |
|---|---|---|
| 540°C | 253 | 186 |
| 550°C | 237 | 172 |
| 580°C | 192 | 135 |
| 600°C | 166 | 115 |
| 620°C | 142 | 97 |
| 650°C | 112 | 74 |
| 700°C | 74 | 48 |
| 800°C | 28 | 16 |
3.4 Physical Properties
Table 5 — Physical Properties of 347H
| Property | Value |
|---|---|
| Density | 7.9 g/cm³ / 0.29 lb/in³ |
| Young's Modulus (20°C) | 200 GPa / 29 × 10⁶ psi |
| Young's Modulus (400°C) | 172 GPa |
| Young's Modulus (600°C) | 155 GPa |
| Thermal Conductivity (23°C) | 14 W/m·°C |
| Thermal Conductivity (400°C) | 20 W/m·°C |
| Specific Heat (23°C) | 485 J/kg·°C |
| Specific Heat (400°C) | 540 J/kg·°C |
4. Corrosion Resistance
347H provides corrosion resistance broadly comparable to standard 304/347 in aqueous environments. Its primary corrosion advantage lies in its resistance to intergranular attack in elevated-temperature service — an area where unstabilised grades are vulnerable.
4.1 Intergranular Corrosion Resistance
Niobium stabilisation is the defining characteristic of 347H's corrosion performance. By binding available carbon as stable niobium carbides, the grade prevents chromium depletion at grain boundaries during thermal exposure, eliminating the sensitisation mechanism that produces intergranular corrosion in unstabilised austenitic grades. This makes 347H particularly reliable for welded components and for applications involving extended exposure in the sensitisation range.
4.2 Aqueous and Wet Corrosion
In aqueous service, 347H offers comparable resistance to 304 in:
- Organic acids at moderate temperatures
- Salt solutions including sulphates, sulphides, and sulphites
- Caustic environments at moderate temperatures
4.3 Gas Corrosion at Elevated Temperature
347H can be used in oxidising gas atmospheres up to its service temperature limit:
- Air: up to 850°C (1,560°F)
- Steam: up to 750°C (1,380°F)
- Synthesis gas (ammonia synthesis): up to ~550°C (1,020°F)
4.4 Molten Salt Resistance
347H has demonstrated resistance to molten nitrate salts at temperatures up to 570°C, supporting its use in thermal energy storage applications for concentrated solar power systems.
5. Applications
347H is selected for applications where long-term creep resistance, intergranular corrosion resistance, and structural stability must be maintained simultaneously at elevated temperatures.
5.1 Power Generation
- Superheater tubes and reheater tubing
- Boiler casings and pressure vessels
- Reactor vessels in steam and synthesis gas service
5.2 Oil Refinery and Petrochemical Service
- Fired heater tubes
- Furnace tubes in refineries and vinyl chloride production
- Cooling tubes in ammonia converters
5.3 Thermal Energy Storage
347H and 321H have demonstrated resistance to molten nitrate salts up to 570°C, supporting their use in thermal storage systems for concentrated solar power (CSP) plants.
5.4 Industrial Furnace and Heat-Processing Equipment
- Furnace heating elements
- Stack liners and flue components
- Tanks for storing organic chemicals at elevated temperatures
- Valves and flanges in elevated-temperature process streams
6. Welding
347H has good weldability. Its niobium stabilisation eliminates the need for post-weld annealing to restore intergranular corrosion resistance in most service conditions, making it a practical choice for welded assemblies exposed to sensitising temperatures.
6.1 Recommended Processes
Preferred welding method is GTAW/TIG; other suitable processes include:
- Gas Metal Arc Welding (GMAW/MIG)
- Shielded Metal Arc Welding (SMAW/MMA)
7. Heat Treatment
347H cannot be hardened by heat treatment. Heat treatment is used to relieve residual stresses after fabrication or to restore a fully annealed microstructure after heavy cold work.
7.1 Stress Relieving
Temperature: 850–950°C (1,560–1,740°F)
Hold time: 10–15 minutes
Cooling: air
7.2 Solution Annealing
Temperature: 1,000–1,100°C (1,830–2,010°F)
Hold time: 5–20 minutes
Cooling: rapid — air or water quench
8. Fabrication
347H offers fabricability consistent with standard austenitic stainless steels. Its properties in forming, cutting, and bending are similar to 304 and 321.
8.1 Cold Bending
Post-bend annealing is not normally required but should be considered depending on the degree of bending and service conditions.
8.2 Hot Bending
Hot bending should be carried out in the range 850–1,100°C (1,560–2,010°F) and should be followed by solution annealing to restore a carbide-free microstructure.
8.3 Machining and Cutting
347H work hardens during machining in the same manner as other austenitic stainless steels. Sharp, rigid tooling and deeper cuts at controlled speeds help avoid surface hardening. Standard stainless-steel cutting and grinding practices apply.
9. Conclusion
347H (UNS S34709) is a niobium-stabilised austenitic stainless steel offering excellent creep rupture strength, reliable resistance to intergranular corrosion, and structural stability in long-term elevated-temperature service up to 850°C. Its niobium stabilisation eliminates sensitisation risk in welded assemblies and thermally exposed components, while its controlled carbon content maximises creep performance relative to standard 347. These characteristics make 347H a dependable selection for superheater tubes, fired heater applications, pressure vessels, and elevated-temperature process equipment in power generation, petrochemical, and industrial heating industries where both mechanical reliability and corrosion integrity must be sustained over extended service lifetimes.
