Metal Materials
AISI 347 Stainless Steel: Properties, Fabrication & Industrial Applications Guide
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.
Key takeaways
- Understanding AISI 347’s core composition and standards is the first step to using it effectively.
- The niobium stabilization isn’t just for high heat. It also makes the alloy highly resistant to corrosion, especially intergranular corrosion.
- Proper heat treatment ensures AISI 347 keeps its strength and corrosion resistance.
- Welding and machining AISI 347 is straightforward, but following best practices preserves its properties.
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 |
How to write a spec that gets priced correctly
A drawing is a contract. If it leaves a number open, the shop fills that number in with its own default. The default is rarely the one you had in mind. Put these five lines on every RFQ and most of the back-and-forth disappears before it starts.
- Alloy designation and the standard behind it
- Temper or heat treatment condition
- Mechanical properties you actually need
- Mill certificate and traceability requirement
- Finish and the faces it applies to
The point is not paperwork. Each line removes a decision the shop would otherwise make for you. A complete RFQ comes back priced. An incomplete one comes back guessed.
What actually drives the price
Buyers usually push on material because it is the line they can see. In practice the material is rarely the biggest number on a quote. These five lines decide most of it:
- alloy grade and mill availability
- stock size versus finished size
- heat treatment and testing
- machinability
- certification
Ask for the quote split by line. A supplier who cannot break it down is guessing, and you will pay for the guess. The line you can usually renegotiate is the tolerance band, not the raw material.
Problems you will meet, and the fix
Every process has a failure mode it prefers. The same four show up again and again on this kind of work, and none of them are mysteries once you know what to look for.
- Substituting a similar grade without telling anyone
- Hardness that varies across a single bar
- Corrosion in a weld heat-affected zone
- Distortion after heat treatment on a thin part
Catch the fault on the first part, not the hundredth. A first-article check costs an hour. A bad batch costs the whole order. Write the check into the purchase order so nobody has to remember it.
What to check on the first article
The first article is the cheapest inspection you will ever buy. Use it properly and the rest of the run becomes boring, which is exactly what you want.
- Every dimension on the drawing, with the measured value written beside it.
- The material certificate, matched to the heat or lot number on the parts.
- The finish against the sample you approved, under the same light.
- Fit against the mating part, assembled rather than measured.
- Packaging, because parts that arrive damaged were never in tolerance.
Check in this order: datums, then mating features, then cosmetic faces. If the datums are wrong, every number after them is noise. Keep the report on file. When a problem shows up in month six, that document is the only thing that tells you what changed.
How to compare two quotes fairly
Two numbers are not comparable unless the scope matches. Line them up before you decide, because a low number with three gaps is not a low number.
Same drawing, same material, same finish, same inspection. Anything else is a different job wearing the same part number. Then look at what each quote leaves out. Freight, packaging, certificates and tooling are the usual gaps.
Ask each supplier to state their assumptions in writing. The one who does is usually the one who has run the process before.
Scaling from one part to a batch
The first part teaches you the process. The hundredth teaches you the process cost.
Watch three things as volume grows: setup amortisation, tool wear, and how much hand work the finish still needs. If the unit price barely moves between ten and a hundred, the cost sits in material or in hand work, not in setup. Ask which, because only one of them is negotiable.
Lead time behaves the same way. A one-off is scheduled around other jobs. A repeat order is scheduled around your forecast. Give the shop a forecast and the lead time gets shorter, not longer.
How to read a supplier's quote
A quote is a short document with a long shadow. Four things in it decide whether the job runs smoothly, and only one of them is the price.
- The scope line. It should name the process, the material and the finish, not just the part.
- The tolerance basis. General tolerance or the drawing, and which standard it follows.
- The exclusions. Tooling, freight, certificates and inspection are the usual omissions.
- The validity date. Material prices move, and an open-ended quote is a promise nobody can keep.
Read the exclusions before you read the total. A quote that leaves out tooling is not cheaper. It is a quote for a different job.
What to send with the first order
The first order sets the pattern for every order after it. Send a complete package once and the shop files it, and the second order takes a fraction of the time.
- A STEP or native model plus a PDF drawing carrying the same revision letter.
- The material callout by standard number, not by trade name.
- The finish and the exact faces it applies to.
- Any test the part must pass: fit, seal, drop, salt spray, dielectric.
- A contact who can answer a question inside one working day.
Then write down what was agreed. An email that confirms the material, the finish and the date is worth more than a longer purchase order that leaves them open.
When this is the wrong choice
A good supplier tells you when the process does not fit. These are the cases where we say so.
- The volume is so low that setup costs more than the parts are worth.
- The tolerance is tighter than the process can hold without a second operation.
- The geometry cannot be made without a tool feature that will not pay back.
- The material you need is not available in the form the process requires.
- The schedule is shorter than the tooling or setup time, and nobody has said so yet.
None of these are refusals. They are reasons to look at a different route, and the earlier that conversation happens the cheaper it is. The most expensive mistake in manufacturing is a process chosen before the question was understood.
Planning the ramp from prototype to production
The gap between one good part and a thousand identical ones is a plan, not a machine. Three decisions close it.
First, freeze the design. Every change after tooling starts is a re-cut, and re-cuts are where schedules go. Second, decide the inspection level. A prototype is checked by eye; a production part is checked against a report. Third, agree the change process in writing, including who pays for it.
Write the ramp down as dates and owners, not as intentions. A ramp plan with three names and three dates is worth more than a page of good intentions.
What documentation to expect
Paperwork is not bureaucracy when a part fails at the customer. It is the only record of what was agreed. A normal shipment should carry four documents.
- A dimensional report covering every callout on the drawing.
- A material certificate traceable to the heat or lot number.
- A finish or coating certificate where the specification requires one.
- A packing list that matches the carton labels.
If a supplier cannot produce these on request, they are not running a controlled process. Ask for one sample set before you place the first order and you will know exactly what you are buying.
How to handle a change after the order
Changes happen. What matters is whether the change is priced, scheduled and recorded, or absorbed quietly and discovered later.
Send the change as a new revision, not as a note in an email. Ask for three answers in writing: what it costs, what it does to the date, and what happens to material or tooling already committed. Then decide.
A change without a revision number is a rumour. The shop will build what it has on file, and both sides will be right and unhappy at the same time.
How the work actually runs, step by step
Most buyers see two numbers: a price and a date. Between them sits a sequence that decides both. On metal work work it looks like this.
- File review. The model and the drawing are checked against each other. Where they disagree, the shop stops and asks, because guessing is how scrap gets made.
- DFM feedback. The features that add cost and the features that will not hold tolerance are listed. This is the cheapest hour in the whole project.
- Setup or tooling. Fixtures, programs or a tool are built. Everything downstream inherits the quality of this step.
- First article. One part is made and measured against the drawing before anything else runs. Nothing is signed until it passes.
- Production. The run goes ahead with the parameters that produced the approved first article, not with new ones.
- Inspection and packing. The callouts are measured, the certificate is matched, and the parts are packed so they arrive in the condition they left in.
Ask any supplier to walk you through these six steps on your part. The answers tell you more about the shop than the price does.
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%.
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.
Our solution for AISI 347 Stainless Steel: Properties
YiguProto runs metal sourcing and machining in house, from a single prototype to a production batch. We quote the process, the material and the finish as separate lines, so you can see where the money goes instead of arguing with one number.
Send a STEP file or a drawing and you get a DFM report in one to two business days. It flags the features that add cost and the features that will not hold tolerance on the process you picked.
Every order ships with an inspection report on the features you call out. If a part misses, we make it again. Ready to price a part? Start with a free quote and DFM review, or read more about our metal sourcing and machining. You can also talk to an engineer before you commit to a drawing.
FAQ about AISI 347 Stainless Steel: Properties
What should I check before approving the parts?
Datums first, then the features that mate, then the cosmetic faces. Ask for a dimensional report, a material certificate and a finish sample. A part can be inside every tolerance and still not fit, so assemble one before you sign.
How do I compare two quotes fairly?
Match the scope first: same drawing, same material, same finish, same inspection. Then read the exclusions. Freight, tooling and certificates are the usual gaps, and a low number with three gaps is not a low number.
Can AISI 347 Stainless Steel: Properties be made in small quantities?
Yes, and small quantities are where a good supplier earns the next order. Setup dominates the price at low volume, so the cheapest route is usually the one with the fewest operations, not the cheapest material.
What happens if a part is out of specification?
We measure it against the same drawing you approved and, if it misses, we make it again. What matters is that the report exists, because a disagreement without data is just an opinion.
Does the process work for prototypes as well as production?
Yes, and that is usually the point. Running the prototype on the production process removes a whole class of surprises, because the part you test is made the same way as the part you sell.
What material should I specify?
Specify by standard, not by trade name. A trade name can change under you between two orders; a standard number cannot. Then add the one property that actually decides the part, whether that is strength, heat resistance or chemical exposure.
The short version
AISI 347 Stainless Steel: Properties rewards the buyer who reads the data sheet and the drawing together. Pick the option for the property that matters, not for the price per kilo.
Tighten only what mates. Test a sample before the run. Then let the process do the rest.