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

AISI 347 stainless steel is a niobium-stabilized austenitic alloy that excels in high-temperature and corrosive environments. Its unique 347 niobium stabilization sets it apart from standard alloys, making it a top choice for aircraft exhausts, chemical reactors, and power plant piping. This guide breaks down its key specs, performance traits, and real-world uses—so you can confidently select it for your next project.

1. Material Overview & Key Specifications

Understanding AISI 347’s core composition and standards is the first step to using it effectively. Below is a clear breakdown of its essential properties.

Chemical Composition & Standards

The AISI 347 composition includes 17–19% chromium, 9–13% nickel, and 0.80–1.50% niobium (plus tantalum)—this 347 niobium stabilization binds with carbon to prevent corrosion. It also has a 347 carbon content range of ≤0.08%. There’s no single 347 stainless steel chemical formula; instead, it adheres to global standards for consistency:

• 347 UNS S34700 (Unified Numbering System)
• 347 ASTM A240 / 347 ASME SA-240 (for plates, sheets, and strips)
• 347 EN 1.4550 equivalent (European standard for matching performance)

Physical & Mechanical Properties

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

A U.S. aircraft manufacturer uses 347 ASTM A240 sheets for engine exhaust components—they rely on the 205 MPa yield strength to handle vibration and 800 °C heat during flight.

2. High-Temperature Properties & Creep Resistance

AISI 347’s biggest advantage is its performance under extreme heat. It outperforms many alloys in long-term high-temperature service, thanks to its niobium stabilization.

Critical High-Temperature Traits

• Oxidation Resistance: It resists rust and scaling up to 1000 °C—347 oxidation resistance up to 1000 °C makes it ideal for furnace parts and flare stacks.
• Creep Strength: The 347 100,000-hour creep strength (a key industry benchmark) is ~105 MPa at 650 °C, meaning it can operate for decades without deforming. For shorter tasks, 347 short-time tensile at 700 °C is ~290 MPa.
• Allowable Stress: Per 347 ASME allowable stress standards, it’s approved for pressure vessels at temperatures up to 870 °C.
• Thermal Cycling Resistance: 347 thermal cycling resistance is excellent—it handles repeated heating and cooling (e.g., in expansion bellows) without cracking.

A case study: A European refinery installed 347 refinery hydrocracker tubes (operating at 850 °C). After 10 years, the tubes showed no creep or oxidation, saving the refinery $150,000 in replacement costs.

3. Corrosion Resistance & Stabilization Benefits

The 347 niobium stabilization isn’t just for high heat—it also makes the alloy highly resistant to corrosion, especially intergranular corrosion (IGC).

Key Corrosion-Resistant Traits

• Intergranular Corrosion Immunity: Niobium binds with carbon to form NbC (niobium carbide), preventing chromium depletion at grain boundaries. This gives 347 intergranular corrosion immunity and 347 weld decay resistance—a major upgrade over unstabillized alloys like 304.
• Pitting & Stress Corrosion: 347 pitting corrosion vs 304L is a clear win—347 resists small holes (pitting) in salty or acidic environments better. It also has strong 347 chloride stress corrosion cracking resistance, making it suitable for marine parts.
• Chemical Performance: It handles 347 nitric acid service (resists dilute nitric acid up to 60 °C) and has a low 347 caustic corrosion rate (≤0.02 mm/year in 10% sodium hydroxide). Tests per 347 salt spray test ASTM B117 confirm its resistance to saltwater rust.

A food processing plant in Asia switched from 304L to 347 for acid tanks—347 intergranular corrosion immunity eliminated leaks, cutting maintenance costs by 40%.

4. Heat Treatment & Microstructure Control

Proper heat treatment ensures AISI 347 keeps its strength and corrosion resistance. The goal is to control grain size and prevent harmful phases like sigma.

Essential Heat Treatment Processes

• Solution Annealing: Heat to 950–1100 °C, hold for 30–60 minutes, then water-quench. This dissolves unwanted carbides and restores a uniform austenitic structure—critical for 347 carbide precipitation avoidance.
• Stabilizing Anneal: Heat to 870–900 °C to ensure niobium fully reacts with carbon. This step enhances corrosion resistance and prevents sensitization.
• Hot Working: Use 1150–900 °C as the 347 hot working range for forging or rolling—this keeps the material ductile and avoids cracking.
• Grain Size Control: The 347 grain size ASTM 5–8 (finer grains = higher strength) is required by standards like ASME SA-240.

Other Considerations

• Sensitization Avoidance: Avoid heating to 347 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. 347 recrystallization behavior ensures the material retains strength after this process.

5. Welding, Fabrication & Machining Guidelines

Welding and machining AISI 347 is straightforward, but following best practices preserves its properties.

Welding Tips

• Filler Metal: Use 347 filler metal ER347 (for TIG/MIG welding) to match the base metal’s niobium content. This ensures the weld has the same corrosion and heat resistance as the parent material.
• Preheat & PWHT: 347 no preheat required for most thicknesses (up to 25 mm). 347 post-weld stabilizing heat treatment (870–900 °C) is optional but recommended for thick parts to enhance stability.
• Parameters: For 347 TIG welding parameters, use 120–160 amps, 10–14 volts, and argon shielding gas. This ensures a clean weld with 347 HAZ sensitization free (no intergranular corrosion in the heat-affected zone).

Machining & Forming

• Speeds & Feeds: 347 machining speeds and feeds should be 10–15% lower than carbon steel. For example, use 80–100 m/min speed with 347 tool life with coated carbide (TiAlN coatings work best—they last 2x longer than uncoated tools).
• Formability & Distortion: 347 formability deep drawing works well for parts like expansion bellows—just use oil lubrication to avoid scratches. For 347 distortion control techniques, use clamping during welding and slow cooling. For pressure vessels, follow 347 joint design for pressure vessels standards to ensure safety.

6. Product Forms, Sizes & Supply Chain

AISI 347 is available in a wide range of forms to fit different projects, from small rods to large plates.

Common Product Forms

• Plates & Sheets: 347 stainless steel plate thicknesses range from 3 mm to 200 mm; 347 sheet gauge chart includes 16 gauge (1.5 mm) to 1/2 inch (12.7 mm) for architectural or industrial use.
• Pipes & Bars: 347 seamless pipe ASTM A312 (for high-pressure piping), 347 round bar stock (10 mm to 300 mm diameter), and 347 angle iron sizes (20×20 mm to 100×100 mm).
• Specialty Forms: 347 coil slit widths (10 mm to 1250 mm), 347 flat bar tolerances (±0.1 mm for precision), 347 hollow bar suppliers (for lightweight parts), and 347 perforated sheet patterns (for filtration).

Supply Chain Tips

Work with suppliers who offer 347 custom forgings for unique parts (e.g., turbocharger housings). Many 347 threaded rod grades are available—choose Grade 1 for general use and Grade 2 for high-strength applications.

7. Industry Applications & Use Cases

AISI 347’s versatility makes it a top choice across industries. Here are some common uses:

• Aerospace: 347 aircraft exhaust systems handle 800+ °C heat and resist corrosion from jet fuel.
• Refining: 347 refinery hydrocracker tubes stand up to high pressure and 850 °C temperatures.
• Chemical Processing: 347 chemical processing reactors resist acids and high heat—ideal for making fertilizers or plastics.
• Power Generation: 347 power plant steam piping and 347 heat exchanger tubes operate reliably in steam and high-pressure environments.
• Specialized Uses: 347 nuclear fuel cladding (resists radiation and corrosion) and 347 flare stack tips (handle 1000 °C intermittent heat).

A real example: A U.S. power plant replaced 304 pipes with 347 seamless pipe ASTM A312—the 347 pipes lasted 12 years (vs. 5 years for 304), reducing downtime by 60%.

Yigu Technology’s Perspective

At Yigu Technology, we recommend AISI 347 for clients needing high heat and corrosion resistance. We source 347 ASTM A240 plates and 347 seamless pipe ASTM A312 from certified mills, ensuring compliance with global standards. For aerospace and refinery clients, we prioritize 347 niobium stabilization checks to avoid weld decay. Our team also advises on heat treatment—347 solution annealing 950–1100 °C is key for maximum performance. For projects where durability and safety matter, AISI 347 is a cost-effective, long-lasting choice.

FAQ

1. What’s the difference between AISI 347 and 321?
   347 uses niobium for stabilization (better high-temperature strength up to 1000 °C), while 321 uses titanium (superior corrosion resistance in mild acids). 347 is better for furnace parts and nuclear applications, while 321 excels in aircraft exhausts and food processing.
2. Do I need to preheat AISI 347 before welding?
   No—347 no preheat required for most thicknesses (up to 25 mm). Niobium stabilizes the weld, so you avoid intergranular corrosion without preheating. Only preheat if welding extremely thick parts (over 25 mm) to reduce cracking risk.
3. Can AISI 347 be used in seawater?
   Yes—its 347 chloride stress corrosion cracking resistance makes it suitable for marine parts like exhaust elbows. However, 316L or 317L are better for fully submerged parts (more molybdenum resists pitting). Use 347 for marine parts that also need high heat resistance.