If you need a stainless steel that excels in high-temperature and corrosive environments, AISI 347 stainless steel is a top choice. Its niobium stabilization sets it apart from standard alloys, making it ideal for aircraft exhausts, chemical reactors, and power plant piping. This guide covers its properties, performance, and real-world uses.
What are the key specifications of AISI 347?
Understanding AISI 347’s core composition and standards is the first step to using it effectively. Its unique niobium stabilization gives it advantages over unstabilized grades.
Chemical composition and standards
AISI 347 includes 17–19% chromium, 9–13% nickel, and 0.80–1.50% niobium plus tantalum. The niobium binds with carbon to prevent corrosion. It adheres to global standards for consistency.
| Standard | Designation |
|---|---|
| UNS | S34700 |
| ASTM | A240 (plates, sheets, strips) |
| ASME | SA-240 |
| EN | 1.4550 equivalent |
Physical and mechanical properties
AISI 347 delivers reliable strength and stability across applications.
| Property | Value |
|---|---|
| Density | 8.0 g/cm³ |
| Melting point | 1,398 °C |
| Minimum yield strength | 205 MPa |
| Minimum tensile strength | 515 MPa |
| Maximum Brinell hardness | 201 HB |
A U.S. aircraft manufacturer uses ASTM A240 sheets for engine exhaust components. They rely on the 205 MPa yield strength to handle vibration and 800 °C heat during flight.
What high-temperature properties does AISI 347 offer?
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: Resists rust and scaling up to 1,000 °C. This makes it ideal for furnace parts and flare stacks.
- Creep strength: The 100,000-hour creep strength is about 105 MPa at 650 °C. It can operate for decades without deforming.
- Allowable stress: Per ASME standards, it’s approved for pressure vessels at temperatures up to 870 °C.
- Thermal cycling resistance: Handles repeated heating and cooling in expansion bellows without cracking.
A European refinery installed AISI 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.
How does AISI 347 resist corrosion?
The niobium stabilization isn’t just for high heat. It also makes the alloy highly resistant to corrosion, especially intergranular corrosion.
Key corrosion-resistant traits
- Intergranular corrosion immunity: Niobium binds with carbon to form niobium carbide, preventing chromium depletion at grain boundaries. This gives it weld decay resistance—a major upgrade over unstabilized alloys like 304.
- Pitting and stress corrosion: AISI 347 resists pitting in salty or acidic environments better than 304L. It also has strong chloride stress corrosion cracking resistance for marine parts.
- Chemical performance: Handles dilute nitric acid up to 60 °C. The caustic corrosion rate is ≤0.02 mm/year in 10% sodium hydroxide.
A food processing plant in Asia switched from 304L to AISI 347 for acid tanks. The intergranular corrosion immunity eliminated leaks, cutting maintenance costs by 40%.
What heat treatment and microstructure control are needed?
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
| Process | Temperature | Purpose |
|---|---|---|
| Solution annealing | 950–1,100 °C, water quench | Dissolves unwanted carbides, restores austenitic structure |
| Stabilizing anneal | 870–900 °C | Ensures niobium fully reacts with carbon, prevents sensitization |
| Stress relief | 450–600 °C | Reduces stresses from welding or forming |
Other considerations
- Hot working range: 1,150–900 °C for forging or rolling. This keeps the material ductile and avoids cracking.
- Grain size: ASTM 5–8 (finer grains = higher strength) is required by standards like ASME SA-240.
- Sensitization avoidance: Avoid heating to 425–815 °C for long periods to prevent chromium depletion.
How should AISI 347 be welded and fabricated?
Welding and machining AISI 347 is straightforward, but following best practices preserves its properties.
Welding tips
| Parameter | Recommendation |
|---|---|
| Filler metal | ER347 to match base metal’s niobium content |
| Preheat | No preheat required for thicknesses up to 25 mm |
| Post-weld heat treatment | Stabilizing anneal at 870–900 °C optional for thick parts |
| TIG welding parameters | 120–160 amps, 10–14 volts, argon shielding gas |
Machining and forming
- Machining speeds: Use 80–100 m/min with coated carbide tools. TiAlN coatings last twice as long as uncoated tools.
- Formability: Deep drawing works well for parts like expansion bellows. Use oil lubrication to avoid scratches.
- Distortion control: Use clamping during welding and slow cooling for pressure vessel applications.
What product forms are available?
AISI 347 is available in a wide range of forms to fit different projects.
| Form | Typical Sizes |
|---|---|
| Plates | 3 mm to 200 mm thickness |
| Sheets | 16 gauge (1.5 mm) to 1/2 inch (12.7 mm) |
| Seamless pipe | ASTM A312 for high-pressure piping |
| Round bar | 10 mm to 300 mm diameter |
| Angle iron | 20×20 mm to 100×100 mm |
| Flat bar | ±0.1 mm tolerance for precision |
Where is AISI 347 used?
AISI 347’s versatility makes it a top choice across industries.
- Aerospace: Aircraft exhaust systems handle 800+ °C heat and resist jet fuel corrosion.
- Refining: Hydrocracker tubes stand up to high pressure and 850 °C temperatures.
- Chemical processing: Reactors resist acids and high heat for making fertilizers and plastics.
- Power generation: Steam piping and heat exchanger tubes operate reliably in steam and high-pressure environments.
- Specialized uses: Nuclear fuel cladding and flare stack tips handle 1,000 °C intermittent heat.
A U.S. power plant replaced 304 pipes with AISI 347 seamless pipe. The 347 pipes lasted 12 years compared to 5 years for 304, reducing downtime by 60%.
Conclusion
AISI 347 stainless steel delivers exceptional high-temperature strength and corrosion resistance. Its niobium stabilization prevents intergranular corrosion and provides creep strength up to 650 °C. For aerospace exhausts, chemical reactors, and power plant piping where both heat and corrosion are concerns, it offers reliable performance. While it costs more than standard 304, its longer service life and reduced maintenance often justify the investment.
FAQ
What’s the difference between AISI 347 and 321?
347 uses niobium for stabilization, giving better high-temperature strength up to 1,000 °C. 321 uses titanium, offering superior corrosion resistance in mild acids. 347 is better for furnace parts and nuclear applications. 321 excels in aircraft exhausts and food processing.
Do I need to preheat AISI 347 before welding?
No. For most thicknesses up to 25 mm, no preheat is required. 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.
Can AISI 347 be used in seawater?
Yes for parts that also need high heat resistance. Its chloride stress corrosion cracking resistance makes it suitable for marine parts like exhaust elbows. For fully submerged parts, 316L or 317L are better options, as their higher molybdenum content resists pitting.
What filler metal should I use for welding AISI 347?
Use ER347 filler metal for TIG or MIG welding. It matches the base metal’s niobium content, ensuring the weld has the same corrosion and heat resistance as the parent material.
How does AISI 347 perform in nitric acid service?
It handles dilute nitric acid well up to 60 °C. The niobium stabilization prevents intergranular corrosion, making it suitable for chemical processing equipment handling nitric acid.
Discuss Your Projects with Yigu Rapid Prototyping
At Yigu Rapid Prototyping, we supply AISI 347 stainless steel for aerospace, refining, and power generation projects. We source ASTM A240 plates and seamless pipe from certified mills, ensuring compliance with global standards. Our team advises on heat treatment and welding procedures to maximize performance. Contact us to discuss your next high-temperature application.