If you need stainless steel that thrives in extreme heat—think boiler tubes, refinery furnaces, or power plant piping—AISI 304H stainless steel is the solution. The “H” stands for “high carbon,” a key tweak that gives it far better high-temperature strength than standard 304 or low-carbon 304L. This guide breaks down everything you need to know to select, fabricate, and source AISI 304H for your high-heat projects.
1. AISI 304H Stainless Steel: Material Overview & Specifications
AISI 304H’s ability to handle high temperatures starts with its precise chemistry and adherence to strict industry standards. Let’s dive into its core details:
Key Chemical Composition
| Element | Content Range | Role in AISI 304H |
|---|---|---|
| Chromium (Cr) | 18.0–20.0% | Forms a protective oxide layer (resists high-temp oxidation) |
| Nickel (Ni) | 8.0–12.0% | Stabilizes the austenitic structure (prevents brittle phases at high heat) |
| Carbon (C) | 0.04–0.10% | The “H” advantage—strengthens grain boundaries for creep resistance (critical for long-term high-heat use) |
| Manganese (Mn) | ≤2.0% | Boosts formability and prevents cracking during hot working |
| Silicon (Si) | ≤1.0% | Enhances oxidation resistance at temperatures above 800°C |
Critical Specifications & Physical/Mechanical Traits
| Specification/Property | Value | Why It Matters for High-Heat Use |
|---|---|---|
| UNS Designation | S30409 | Global identifier for sourcing high-carbon 304 |
| ASTM/ASME Standards | ASTM A240 (sheets/plates), ASME SA-240 | Ensures quality for pressure vessels and high-temp equipment |
| Density | 7.93 g/cm³ | Consistent with other 304 grades—easy to calculate weight for structural designs |
| Melting Point | 1,400–1,450°C (2,550–2,650°F) | Withstands extreme heat (e.g., furnace interiors) |
| Tensile Strength (Room Temp) | 515 MPa (74,700 psi) | Strong enough for structural high-heat parts (e.g., headers) |
| Yield Strength (Room Temp) | 170 MPa (24,700 psi) | Resists deformation under initial heat stress |
| Hardness | 95 HRB (Rockwell B) / 210 BHN | Tough enough for wear, but machinable with proper tools |
Example: A power plant uses ASME SA-240 304H plates for boiler headers—meeting tensile strength specs ensures the parts don’t fail at 900°C.
2. AISI 304H Stainless Steel: High-Temperature Properties & Creep Resistance
AISI 304H’s biggest strength is its performance at extreme temperatures—especially its resistance to creep (slow deformation under long-term heat and pressure). Here’s how it stacks up:
High-Temperature Performance Breakdown
| Temperature | Key Property | Value | Real-World Relevance |
|---|---|---|---|
| 550°C (1,020°F) | Short-Time Tensile Strength | 310 MPa (45,000 psi) | Maintains strength for refinery reformer tubes |
| 700°C (1,290°F) | 100,000-Hour Creep Strength | 20 MPa (2,900 psi) | Resists slow deformation in long-running boilers |
| 870°C (1,600°F) | Oxidation Resistance | No scaling (up to 1,000°C) | Safe for furnace components and flare stacks |
| 900°C (1,650°F) | Service Temperature Limit | Maximum continuous use temp | Avoids sigma phase embrittlement (brittleness from high-heat phase changes) |
Critical High-Heat Advantages (vs. 304L)
| Trait | AISI 304H | AISI 304L | Why It Matters |
|---|---|---|---|
| Creep Resistance (700°C) | 20 MPa (100,000 hours) | 12 MPa (100,000 hours) | 304H lasts 2x longer in high-pressure boilers |
| Oxidation Resistance | Up to 1,000°C | Up to 870°C | 304H works in hotter furnace environments |
| Sensitization Risk | Low (controlled grain size) | Very Low (low carbon) | 304H avoids weld decay and handles high heat |
Case Study: A refinery replaced 304L reformer tubes with 304H. The 304L tubes failed after 2 years (creep deformation), while 304H tubes lasted 5 years—saving $300,000 in replacement costs.
3. AISI 304H Stainless Steel: Heat Treatment & Processing
Proper heat treatment is critical to unlock AISI 304H’s high-temperature potential. Here’s what you need to know:
Key Heat Treatment Processes
- Solution Annealing: Heat to 1,040–1,100°C (1,900–2,010°F), hold for 30–60 minutes, then quench in water. This:
- Dissolves excess carbides (prevents sensitization).
- Controls grain size to ASTM 7 or coarser (coarse grains improve creep resistance—fine grains weaken at high heat).
- Softens the metal for subsequent forming.
- Stabilizing Anneal (Optional): For thick parts (>25mm), heat to 850–900°C (1,560–1,650°F) after solution annealing. This reduces residual stress without sacrificing creep strength.
- Residual Stress Relief: For welded parts, heat to 800–850°C (1,470–1,560°F) to relieve stress—critical for pressure vessels.
Processing Guidelines
- Hot Working: Use temperatures of 1,100–1,260°C (2,010–2,300°F) for forging, rolling, or bending. Avoid working below 900°C—this causes brittleness.
- Cold Working: Limit cold work to 20–30% (e.g., bending or stamping). Excess cold work increases strain hardening, which reduces creep resistance. Anneal after cold working to reset properties.
- Grain Size Control: Never use fine-grain 304H (ASTM 8 or finer). Coarse grains (ASTM 7 or coarser) are mandatory for high-heat applications—they resist creep better.
Pro Tip: A manufacturer once solution-annealed 304H at 980°C (too low)—the grain size was ASTM 9 (fine), and the parts failed in a boiler after 6 months. Re-annealing at 1,050°C fixed the grain size and performance.
4. AISI 304H Stainless Steel: Welding, Fabrication & Machining Guidelines
Welding AISI 304H requires care to maintain its high-temperature strength—using the right consumables and techniques is key.
Welding Best Practices
| Aspect | Recommendation | Why It Works for High-Heat Use |
|---|---|---|
| Filler Metal | ER308H (TIG/MIG) or E308H (stick welding) | Matches 304H’s carbon content—ensures weld metal has the same creep resistance as the base metal |
| Preheat Requirements | 150–200°C (300–390°F) for parts >25mm thick | Prevents cold cracking and reduces HAZ (Heat-Affected Zone) stress |
| Post-Weld Heat Treatment (PWHT) | 800–850°C (1,470–1,560°F), hold 1 hour per 25mm thickness | Relieves HAZ stress and stabilizes carbides—critical for creep resistance |
| HAZ Sensitization Mitigation | Use low-heat input welding (TIG > MIG) | Minimizes time spent in the sensitization range (450–850°C), avoiding weld decay |
Machining & Forming Tips
- Machining Speeds/Feeds: Use carbide tools (TiAlN-coated) to handle 304H’s hardness:
- Turning: 80–120 m/min speed, 0.1–0.15 mm/rev feed.
- Milling: 60–100 m/min speed, 0.05–0.1 mm/tooth feed.
- Cutting Fluid: Use heavy-duty soluble oil to reduce friction—304H’s high carbon can cause tool wear if not lubricated.
- Formability: For high-heat parts (e.g., tubes), use hot forming (1,100–1,200°C) instead of cold forming. Hot forming maintains grain size and creep resistance.
Example: A boiler manufacturer uses TIG welding with ER308H for 304H tubes. PWHT at 820°C ensures the welds resist creep—their boilers run continuously at 850°C without issues.
5. AISI 304H Stainless Steel: Product Forms, Sizes & Supply Chain
AISI 304H is available in forms tailored to high-heat applications. Here’s how to source it:
Common Product Forms & Sizes
| Form | Typical Sizes | Key High-Heat Uses |
|---|---|---|
| Plates | 6–100mm thickness; 1x2m to 3x6m | Boiler headers, pressure vessel shells |
| Seamless Pipes/Tubes | 10–300mm OD; 1–10mm wall thickness | Superheater tubes, refinery reformer tubes |
| Round Bars | 10–200mm diameter; 1–6m length | Forged fittings, furnace bolts |
| Forged Fittings | 1/2″–24″ sizes (elbows, tees) | High-pressure steam piping connections |
| Coil Stock | 1–5mm thickness; 1219mm width | High-temperature ducting for furnaces |
Supply Chain Tips
- Price: 304H costs $4.00–$5.00 per kg (2024 estimates)—slightly more than 304 ($3.00–$4.00/kg) but worth it for high-heat durability.
- Lead Time: 3–4 weeks for stock sizes (e.g., 20mm OD tubes); 6–8 weeks for custom sizes (e.g., thick pressure vessel plates).
- Suppliers: Choose ISO 9001-certified suppliers who provide mill test reports (MTRs) confirming:
- Carbon content (0.04–0.10%).
- Grain size (ASTM 7 or coarser).
- Creep test data (for critical applications like boilers).
Pro Tip: A power plant ordered 304H superheater tubes from a supplier with ASME SA-240 certification—this ensured the tubes met pressure vessel code requirements.
6. AISI 304H Stainless Steel: Industry Applications & Case Studies
AISI 304H is the go-to grade for industries where long-term high-heat performance is non-negotiable. Here are its top uses:
1. Power Generation
- Boiler Tubes & Superheater Headers: Carries high-pressure steam (800–900°C) in coal, gas, or nuclear power plants. A 500MW power plant uses 304H tubes—they’ve operated for 8 years without creep deformation.
- Steam Distribution Manifolds: Distributes high-temp steam to turbines—304H’s oxidation resistance prevents rust buildup.
2. Oil & Gas Refining
- Hydrogen Reformer Tubes: Converts natural gas to hydrogen (900–950°C)—304H’s creep resistance avoids tube failure. A refinery reported 304H tubes last 3x longer than 304L in this application.
- Flare Stack Tips: Burns off excess gas at 1,000°C—304H resists scaling and corrosion from fuel byproducts.
3. Chemical & Petrochemical
- Ethylene Cracking Coils: Breaks down hydrocarbons into ethylene (850–950°C)—304H’s high-temp strength maintains coil shape.
- Furnace Components: Liners, doors, and ducting for chemical furnaces—304H withstands continuous heat without brittleness.
4. Heavy Industry
- Heat Exchanger Shells: Handles high-temp fluids (e.g., molten salts)—304H’s pressure resistance avoids leaks.
- High-Temperature Ducting: Moves hot gases in steel mills—304H resists wear from dust and heat.
Yigu Technology’s Perspective
At Yigu Technology, AISI 304H is our specialty for power, refinery, and chemical clients needing high-heat reliability. We supply 304H plates, seamless tubes, and bars (UNS S30409, ASTM A240/ASME SA-240) with MTRs confirming carbon content (0.04–0.10%) and grain size (ASTM 7+). For a refinery project, we provided custom 304H reformer tubes—our strict PWHT (820°C) ensured welds matched base metal creep strength, and the tubes have run for 4 years without issues. We also offer technical support, sharing heat treatment and welding tips to maximize part life. While 304H costs more upfront, its long service life delivers ROI by cutting downtime and replacements.
FAQ
- Can AISI 304H be used in marine environments?
No—304H’s high carbon doesn’t improve chloride resistance. It will rust in saltwater within 1–2 years. Use 316H (adds molybdenum for chloride resistance) for marine high-heat applications (e.g., shipboard boilers). - What’s the difference between AISI 304H and 304L for high-heat use?
304H (0.04–0.10% carbon) has far better creep resistance (lasts longer at high heat) and a higher service temp (1,000°C vs. 304L’s 870°C). 304L (≤0.03% carbon) is better for low-heat corrosion-prone areas (e.g., food processing) but fails quickly in high-heat equipment. - Do I need to control grain size for AISI 304H?
Yes—grain size is critical. AISI 304H must have a coarse grain size (ASTM 7 or coarser) to resist creep. Fine grain sizes (ASTM 8+) weaken at high temperatures and cause premature failure in boilers or furnaces. Always confirm grain size via MTR before use.
