Introduction
When your equipment operates at extreme temperatures, ordinary stainless steel will not last. Creep deformation, oxidation, and weld decay become constant problems. AISI 347H stainless steel solves these issues. It is a high-carbon, niobium-stabilized austenitic alloy designed for sustained high-heat and corrosive environments. The higher carbon content boosts creep resistance, making it the top choice for power plants, refineries, and chemical facilities. This guide covers its properties, how to work with it, and where it performs best.
What Are the Key Specifications of AISI 347H?
What Is Its Chemical Composition?
The performance of AISI 347H starts with its precise chemistry. Niobium stabilization and higher carbon content are the defining features.
| Element | Content Range | Role |
|---|---|---|
| Chromium (Cr) | 17–19% | Provides oxidation and corrosion resistance. |
| Nickel (Ni) | 9–13% | Stabilizes the austenitic structure. |
| Niobium (Nb) + Tantalum | 0.80–1.50% | Binds with carbon to prevent weld decay. |
| Carbon (C) | 0.04–0.10% | Higher than standard 347. Boosts creep strength. |
AISI 347H adheres to several global standards:
- UNS S34709: Unified Numbering System code.
- ASTM A240 / ASME SA-240: For plates, sheets, and strips.
- EN 1.4912: European equivalent.
What Are Its Mechanical Properties?
The numbers below show why 347H is trusted for high-temperature applications.
| Property | Value | Why It Matters |
|---|---|---|
| Density | 8.0 g/cm³ | Standard for stainless steels. |
| Melting Point | 1398°C | Withstands extreme heat. |
| Tensile Strength (min) | 515 MPa | Handles high stress without failure. |
| Yield Strength (min) | 205 MPa | Resists permanent deformation. |
| Elongation (min) | 35% | Retains ductility for forming. |
| Grain Size | ASTM 7 or coarser | Coarser grains improve creep resistance. |
Real Example: A Japanese power plant uses 347H ASTM A240 plates for boiler headers. They rely on the 515 MPa tensile strength to handle 700°C temperatures and high pressure.
How Does AISI 347H Perform at High Temperatures?
What Makes Its Creep Resistance So Strong?
The higher carbon content and niobium stabilization give 347H exceptional creep resistance. Creep is the slow, permanent deformation that occurs under sustained stress at high temperatures.
| Property | Value | Significance |
|---|---|---|
| 100,000-Hour Creep Strength | ~115 MPa at 650°C | Can operate for decades without deforming. |
| Short-Time Tensile at 700°C | ~300 MPa | Maintains strength under brief high-heat spikes. |
| Oxidation Resistance | Up to 1050°C | Resists scaling and rust formation. |
| Steam Oxidation Rate | ≤0.15 mm/year at 800°C | Low corrosion in steam environments. |
| Service Temperature Limit | 870°C continuous; 980°C intermittent | Suitable for furnace components and flare stacks. |
Case Study: A European refinery installed 347H refinery hydrocracker tubes operating at 850°C. After 12 years, the tubes showed no creep or oxidation. This saved the refinery $200,000 in replacement costs.
How Does Niobium Stabilization Improve Corrosion Resistance?
What Corrosion Types Does It Resist?
Niobium binds with carbon to form niobium carbide (NbC). This prevents chromium depletion at grain boundaries, a common problem in unstabilized alloys like 304.
| Corrosion Type | Resistance | Why It Matters |
|---|---|---|
| Intergranular Corrosion | Immune | Niobium prevents chromium carbide formation at grain boundaries. |
| Weld Decay | Excellent | No loss of corrosion resistance near welds. |
| Pitting Corrosion | Competitive with 316H | Resists small holes in acidic environments. |
| Chloride Stress Corrosion | Strong | Suitable for marine or chemical applications. |
| Polythionic Acid Cracking | Resistant | Critical for refineries after shutdowns. |
| Caustic Corrosion Rate | ≤0.02 mm/year | Handles sodium hydroxide well. |
Real Example: A U.S. chemical plant switched from 316H to 347H for acid reactor vessels. The 347H intergranular corrosion immunity eliminated leaks. The higher carbon content also handled 750°C operating temperatures.
How Do You Heat Treat and Fabricate AISI 347H?
What Are the Critical Heat Treatment Steps?
Proper heat treatment is essential to unlock 347H’s full potential. It ensures strength, corrosion resistance, and avoids harmful phases.
| Process | Temperature | Purpose |
|---|---|---|
| Solution Annealing | 1050–1100°C | Dissolves unwanted carbides; restores uniform structure. |
| Stabilizing Anneal | 870–900°C | Ensures niobium fully reacts with carbon. |
| Stress Relief | 450–600°C | Reduces residual stress from welding or forming. |
| Hot Working Range | 1150–900°C | For forging or rolling without cracking. |
Important: Avoid prolonged heating in the sensitization range of 425–815°C. This can cause chromium depletion and reduce corrosion resistance.
What Are the Welding and Machining Guidelines?
Welding
- Filler Metal: Use ER347H to match carbon and niobium content. Standard 347 filler reduces creep resistance.
- Preheat: Not required for thicknesses up to 25mm.
- Post-Weld Heat Treatment: For thicker parts, treat at 600–650°C to restore heat-affected zone strength.
- Weldability: Excellent. Low hot cracking susceptibility with proper parameters.
Machining
- Speeds: 10–15% lower than carbon steel. Use 80–100 m/min with coated carbide tools.
- Tool Life: TiAlN coatings last twice as long as uncoated tools.
- Formability: Moderate cold working limits. Use clamping during welding to control distortion.
What Product Forms Are Available?
How Do You Source AISI 347H?
AISI 347H is available in a wide range of forms to fit nearly any high-temperature project.
| Form | Typical Sizes | Applications |
|---|---|---|
| Plates | 3–200 mm thick | Pressure vessels, boiler headers. |
| Seamless Tubes | Schedule 40–160 | Boiler and superheater tubes. |
| Round Bars | 10–300 mm diameter | Forged fittings, shafts. |
| Coils | 10–1250 mm width | Heat exchanger shells. |
Supply Chain Tips
- Work with certified stockists for common sizes.
- For unique parts, choose suppliers offering custom forgings.
- Always request mill test reports confirming composition and properties.
Where Is AISI 347H Used in Industry?
What Are the Critical Applications?
The combination of high-temperature strength and corrosion resistance makes 347H indispensable in several industries.
| Industry | Applications | Why It Works |
|---|---|---|
| Power Generation | Boiler tubes, superheater headers | Handles 700–850°C steam and pressure. |
| Refining | Hydrocracker tubes, flare stack tips | Resists high heat and chemical exposure. |
| Chemical Processing | Heat exchanger shells, ethylene cracking coils | Performs in acidic, high-temperature environments. |
| Other | Furnace radiant tubes, steam manifolds | Withstands intermittent heat up to 1000°C. |
Real Example: A U.S. power plant replaced 321H boiler tubes with 347H. The 347H tubes showed 50% less creep after 8 years, extending maintenance intervals significantly.
Conclusion
AISI 347H stainless steel delivers exceptional performance where high temperatures and corrosion meet. Its higher carbon content provides superior creep resistance, while niobium stabilization ensures immunity to intergranular corrosion and weld decay. For power plants, refineries, and chemical facilities operating at sustained high heat, 347H is a proven, reliable choice. While it requires careful heat treatment and specific welding procedures, its long-term durability makes it a cost-effective solution for critical high-temperature applications.
FAQ
What is the difference between AISI 347 and 347H?
347H has a higher carbon content (0.04–0.10% vs. 347’s ≤0.08%). This boosts high-temperature strength and creep resistance. Both use niobium for corrosion protection, but 347H is better for long-term high-heat service like boiler tubes. 347 suits lower-heat applications.
Do I need preheat for welding AISI 347H?
No. Preheat is not required for thicknesses up to 25mm. For thicker parts above 25mm, a 100–150°C preheat reduces cracking risk. Always use ER347H filler metal. Optional post-weld heat treatment restores heat-affected zone strength.
Can AISI 347H resist sulfuric acid?
Yes, in dilute solutions. It performs well in sulfuric acid up to 20% concentration at temperatures up to 60°C. It resists pitting and intergranular corrosion, making it suitable for chemical tanks and piping handling mild sulfuric acid. For concentrated acid, consider 317L.
Discuss Your Projects with Yigu Rapid Prototyping
At Yigu Rapid Prototyping, we recommend AISI 347H for clients needing high-temperature creep resistance and corrosion protection. We source 347H ASTM A240 plates and 347H seamless tubes from certified mills, ensuring compliance with ASME standards. For power and refinery clients, we prioritize verification of 100,000-hour creep strength. Our team advises on welding with ER347H filler and heat treatment to maximize performance. If your next project demands long-term reliability at extreme temperatures, let’s discuss how AISI 347H can work for you.