AISI 347H Stainless Steel: Properties, Fabrication & Industrial Applications Guide

AISI 347H stainless steel is a high-carbon, niobium-stabilized austenitic alloy designed for extreme high-temperature and corrosive environments. Its higher carbon content (vs. standard 347) boosts creep resistance, making it a top choice for power plants, refineries, and chemical facilities. This guide covers everything from its core specs to real-world uses, helping you make informed decisions for high-heat projects.

1. Material Overview & Key Specifications

Understanding AISI 347H’s composition and standards is essential to leveraging its full potential. Below is a clear breakdown of its essential properties.

Chemical Composition & Standards

The AISI 347H composition includes 17–19% chromium, 9–13% nickel, 0.80–1.50% niobium (plus tantalum), and a critical 347H carbon 0.04–0.10 % —this higher carbon content enhances high-temperature strength. There’s no single 347H chemical formula; instead, it adheres to global standards for consistency:

• 347H UNS S34709 (Unified Numbering System)
• 347H ASTM A240 / 347H ASME SA-240 (for plates, sheets, and strips)
• 347H EN 1.4912 equivalent (European standard for matching performance)

Physical & Mechanical Properties

AISI 347H delivers reliable strength and stability across demanding applications. Key metrics are organized in the table below:

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.

2. High-Temperature Properties & Creep Resistance

AISI 347H’s greatest strength is its performance under sustained high heat. Its higher carbon content and niobium stabilization make it far more creep-resistant than standard 347.

Critical High-Temperature Traits

• Creep Strength: The 347H 100,000-hour creep strength (a key industry benchmark) is ~115 MPa at 650 °C—meaning it can operate for decades without deforming. For shorter tasks, 347H short-time tensile at 700 °C is ~300 MPa.
• Oxidation Resistance: It resists rust and scaling up to 1050 °C—347H oxidation resistance up to 1050 °C makes it ideal for furnace radiant tubes and flare stacks. The 347H steam oxidation rate is also low (≤0.15 mm/year at 800 °C in pure steam).
• Allowable Stress: Per 347H ASME Section II allowable stresses, it’s approved for pressure vessels at temperatures up to 870 °C.
• Service Limit: The 347H service temperature limit is 870 °C for continuous use and 980 °C for intermittent exposure.

A case study: A European refinery installed 347H refinery hydrocracker tubes (operating at 850 °C). After 12 years, the tubes showed no creep or oxidation, saving the refinery $200,000 in replacement costs.

3. Corrosion Resistance & Stabilization Benefits

While AISI 347H is known for high heat resistance, its niobium stabilization also delivers exceptional corrosion protection—even in harsh chemicals.

Key Corrosion-Resistant Traits

• Intergranular Corrosion Immunity: Niobium binds with carbon to form NbC (niobium carbide), preventing chromium depletion at grain boundaries. This gives 347H intergranular corrosion immunity and 347H weld decay resistance—a major upgrade over unstabillized alloys like 304.
• Pitting & Stress Corrosion: 347H pitting corrosion vs 316H is competitive—347H resists small holes (pitting) in acidic environments, while 316H excels in chlorides. It also has strong 347H chloride stress corrosion cracking resistance for marine or chemical applications.
• Chemical Performance: It handles 347H nitric acid service (resists dilute nitric acid up to 60 °C) and has a low 347H caustic corrosion rate (≤0.02 mm/year in 10% sodium hydroxide). It also resists 347H polythionic acid cracking (a risk in refineries after shutdowns).

A U.S. chemical plant switched from 316H to 347H for acid reactor vessels—347H intergranular corrosion immunity eliminated leaks, and the higher carbon handled 750 °C operating temperatures.

4. Heat Treatment & Microstructure Control

Proper heat treatment is critical to unlocking AISI 347H’s full potential. It ensures the alloy maintains strength, resists corrosion, and avoids harmful phases like sigma.

Key Heat Treatment Processes

• Solution Annealing: Heat to 1050–1100 °C, hold for 30–60 minutes, then water-quench. This dissolves unwanted carbides and restores a uniform austenitic structure—critical for 347H carbide solution treatment.
• Stabilizing Anneal: Heat to 870–900 °C to ensure niobium fully reacts with carbon. This step enhances corrosion resistance and prevents sensitization.
• Grain Size Control: The 347H grain size control ASTM 7 min (coarser grains) is required by ASME standards—coarser grains improve creep resistance at high temperatures.
• Hot Working: Use 1150–900 °C as the 347H hot working range for forging or rolling. This keeps the material ductile and avoids cracking.

Other Considerations

• Sensitization Avoidance: Avoid heating to 347H sensitization temperature 425–815 °C for long periods—this prevents chromium depletion.
• Residual Stress Relief: Heat to 450–600 °C to reduce stresses from welding or forming. Even after this, 347H intergranular corrosion after PWHT remains minimal.

5. Welding, Fabrication & Machining Guidelines

Welding and machining AISI 347H require simple adjustments to preserve its high-temperature and corrosion properties.

Welding Tips

• Filler Metal: Use 347H filler metal ER347H (for TIG/MIG welding) to match the base metal’s carbon and niobium content. 347H carbon matching filler is critical—using standard 347 filler reduces creep resistance.
• Preheat & PWHT: 347H preheat temperature is typically not required for thicknesses up to 25 mm. For thicker parts, 347H post-weld heat treatment (PWHT) (600–650 °C) helps restore 347H HAZ creep strength (heat-affected zone) and relieve stress.
• Weldability: 347H weldability rating is “excellent”—it has low 347H hot cracking susceptibility when using proper parameters.

Machining & Forming

• Speeds & Feeds: 347H machining speeds and feeds should be 10–15% lower than carbon steel. For example, use 80–100 m/min speed with 347H coated carbide tool life (TiAlN coatings last 2x longer than uncoated tools).
• Formability & Distortion: 347H cold working limits allow moderate bending and forming. For 347H distortion control techniques, use clamping during welding and slow cooling. For pressure vessels, follow 347H joint design for pressure vessels (thicker welds for strength).

6. Product Forms, Sizes & Supply Chain

AISI 347H is available in a wide range of forms to fit nearly any high-temperature project.

Common Product Forms

• Plates: 347H stainless steel plate thickness chart ranges from 3 mm to 200 mm, including 347H pressure vessel plate (certified to ASME SA-240).
• Pipes & Tubes: 347H pipe schedule dimensions include Schedule 40 to Schedule 160; 347H seamless tube ASTM A312 is used for boiler and superheater tubes.
• Bars & Fittings: 347H round bar stockists offer 10–300 mm diameter bars; 347H forged fittings (elbows, tees) suit high-pressure piping.
• Specialty Forms: 347H coil slit widths (10–1250 mm), 347H flat bar tolerances (±0.1 mm), and 347H hollow bar suppliers (for lightweight parts).

Supply Chain Tips

Work with 347H round bar stockists and global distributors for quick access to common sizes. For unique parts, choose suppliers offering 347H custom forgings (e.g., furnace components).

7. Industry Applications & Use Cases

AISI 347H’s versatility makes it a top choice for high-heat, high-corrosion industries. Here are key applications:

• Power Generation: 347H boiler tubes, 347H superheater headers, and 347H power plant headers handle 700–850 °C steam and pressure.
• Refining: 347H refinery hydrocracker tubes and 347H flare stack tips resist high heat and chemical exposure.
• Chemical Processing: 347H heat exchanger shells and 347H ethylene cracking coils operate reliably in acidic, high-temperature environments.
• Other Uses: 347H furnace radiant tubes (1000 °C intermittent heat) and 347H steam distribution manifolds (corrosion-resistant steam flow).

A real example: A U.S. power plant replaced 321H boiler tubes with 347H boiler tubes—the 347H tubes showed 50% less creep after 8 years, extending maintenance intervals.

Yigu Technology’s Perspective

At Yigu Technology, we recommend AISI 347H for clients needing high-temperature creep resistance and corrosion protection. We source 347H ASTM A240 plates and 347H seamless tube ASTM A312 from certified mills, ensuring compliance with ASME standards. For power and refinery clients, we prioritize 347H 100,000-hour creep strength verification. Our team advises on welding (using 347H filler metal ER347H) and heat treatment to maximize performance. For long-term high-heat projects, AISI 347H is a cost-effective, reliable choice.

FAQ

1. What’s the difference between AISI 347 and 347H?
   347H has a higher carbon content (0.04–0.10% vs. 347’s ≤0.08%), boosting 347H high-temperature strength and creep resistance. Both use niobium for corrosion protection, but 347H is better for long-term high-heat service (e.g., boiler tubes), while 347 suits lower-heat applications.
2. Do I need preheat for welding AISI 347H?
   No—347H preheat temperature is not required for thicknesses up to 25 mm. For thicker parts (>25 mm), a 100–150 °C preheat reduces cracking risk. Always use 347H filler metal ER347H and optional PWHT to restore HAZ strength.
3. Can AISI 347H resist sulfuric acid?
   Yes—347H sulfuric acid performance is strong in dilute solutions (≤20% concentration, ≤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.