Metal Materials
What Is AISI 347H Stainless Steel and When Should You Use It?
When your equipment operates at extreme temperatures, ordinary stainless steel will not last. Creep deformation, oxidation, and weld decay become constant problems. AISI 347H stainless steel solves these issues. It is a high-carbon, niobium-stabilized austenitic alloy designed for sustained high-heat and corrosive environments.
What Are the Key Specifications of AISI 347H?
What Is Its Chemical Composition?
The performance of AISI 347H starts with its precise chemistry. Niobium stabilization and higher carbon content are the defining features.
| Element | Content Range | Role |
|---|---|---|
| Chromium (Cr) | 17–19% | Provides oxidation and corrosion resistance. |
| Nickel (Ni) | 9–13% | Stabilizes the austenitic structure. |
| Niobium (Nb) + Tantalum | 0.80–1.50% | Binds with carbon to prevent weld decay. |
| Carbon (C) | 0.04–0.10% | Higher than standard 347. Boosts creep strength. |
AISI 347H adheres to several global standards:
- UNS S34709: Unified Numbering System code.
- ASTM A240 / ASME SA-240: For plates, sheets, and strips.
- EN 1.4912: European equivalent.
What Are Its Mechanical Properties?
The numbers below show why 347H is trusted for high-temperature applications.
| Property | Value | Why It Matters |
|---|---|---|
| Density | 8.0 g/cm³ | Standard for stainless steels. |
| Melting Point | 1398°C | Withstands extreme heat. |
| Tensile Strength (min) | 515 MPa | Handles high stress without failure. |
| Yield Strength (min) | 205 MPa | Resists permanent deformation. |
| Elongation (min) | 35% | Retains ductility for forming. |
| Grain Size | ASTM 7 or coarser | Coarser grains improve creep resistance. |
Real Example: 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.
How Does AISI 347H Perform at High Temperatures?
What Makes Its Creep Resistance So Strong?
The higher carbon content and niobium stabilization give 347H exceptional creep resistance. Creep is the slow, permanent deformation that occurs under sustained stress at high temperatures.
| Property | Value | Significance |
|---|---|---|
| 100,000-Hour Creep Strength | ~115 MPa at 650°C | Can operate for decades without deforming. |
| Short-Time Tensile at 700°C | ~300 MPa | Maintains strength under brief high-heat spikes. |
| Oxidation Resistance | Up to 1050°C | Resists scaling and rust formation. |
| Steam Oxidation Rate | ≤0.15 mm/year at 800°C | Low corrosion in steam environments. |
| Service Temperature Limit | 870°C continuous; 980°C intermittent | Suitable for furnace components and flare stacks. |
Case Study: A European refinery installed 347H refinery hydrocracker tubes operating at 850°C. After 12 years, the tubes showed no creep or oxidation. This saved the refinery $200,000 in replacement costs.
How Does Niobium Stabilization Improve Corrosion Resistance?
What Corrosion Types Does It Resist?
Niobium binds with carbon to form niobium carbide (NbC). This prevents chromium depletion at grain boundaries, a common problem in unstabilized alloys like 304.
| Corrosion Type | Resistance | Why It Matters |
|---|---|---|
| Intergranular Corrosion | Immune | Niobium prevents chromium carbide formation at grain boundaries. |
| Weld Decay | Excellent | No loss of corrosion resistance near welds. |
| Pitting Corrosion | Competitive with 316H | Resists small holes in acidic environments. |
| Chloride Stress Corrosion | Strong | Suitable for marine or chemical applications. |
| Polythionic Acid Cracking | Resistant | Critical for refineries after shutdowns. |
| Caustic Corrosion Rate | ≤0.02 mm/year | Handles sodium hydroxide well. |
Real Example: A U.S. chemical plant switched from 316H to 347H for acid reactor vessels. The 347H intergranular corrosion immunity eliminated leaks. The higher carbon content also handled 750°C operating temperatures.
How Do You Heat Treat and Fabricate AISI 347H?
What Are the Critical Heat Treatment Steps?
Proper heat treatment is essential to unlock 347H’s full potential. It ensures strength, corrosion resistance, and avoids harmful phases.
| Process | Temperature | Purpose |
|---|---|---|
| Solution Annealing | 1050–1100°C | Dissolves unwanted carbides; restores uniform structure. |
| Stabilizing Anneal | 870–900°C | Ensures niobium fully reacts with carbon. |
| Stress Relief | 450–600°C | Reduces residual stress from welding or forming. |
| Hot Working Range | 1150–900°C | For forging or rolling without cracking. |
Important: Avoid prolonged heating in the sensitization range of 425–815°C. This can cause chromium depletion and reduce corrosion resistance.
What Are the Welding and Machining Guidelines?
Welding
- Filler Metal: Use ER347H to match carbon and niobium content. Standard 347 filler reduces creep resistance.
- Preheat: Not required for thicknesses up to 25mm.
- Post-Weld Heat Treatment: For thicker parts, treat at 600–650°C to restore heat-affected zone strength.
- Weldability: Excellent. Low hot cracking susceptibility with proper parameters.
Machining
- Speeds: 10–15% lower than carbon steel. Use 80–100 m/min with coated carbide tools.
- Tool Life: TiAlN coatings last twice as long as uncoated tools.
- Formability: Moderate cold working limits. Use clamping during welding to control distortion.
What Product Forms Are Available?
How Do You Source AISI 347H?
AISI 347H is available in a wide range of forms to fit nearly any high-temperature project.
| Form | Typical Sizes | Applications |
|---|---|---|
| Plates | 3–200 mm thick | Pressure vessels, boiler headers. |
| Seamless Tubes | Schedule 40–160 | Boiler and superheater tubes. |
| Round Bars | 10–300 mm diameter | Forged fittings, shafts. |
| Coils | 10–1250 mm width | Heat exchanger shells. |
Supply Chain Tips
- Work with certified stockists for common sizes.
- For unique parts, choose suppliers offering custom forgings.
- Always request mill test reports confirming composition and properties.
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.
Where Is AISI 347H Used in Industry?
What Are the Critical Applications?
The combination of high-temperature strength and corrosion resistance makes 347H indispensable in several industries.
| Industry | Applications | Why It Works |
|---|---|---|
| Power Generation | Boiler tubes, superheater headers | Handles 700–850°C steam and pressure. |
| Refining | Hydrocracker tubes, flare stack tips | Resists high heat and chemical exposure. |
| Chemical Processing | Heat exchanger shells, ethylene cracking coils | Performs in acidic, high-temperature environments. |
| Other | Furnace radiant tubes, steam manifolds | Withstands intermittent heat up to 1000°C. |
Real Example: A U.S. power plant replaced 321H boiler tubes with 347H. The 347H tubes showed 50% less creep after 8 years, extending maintenance intervals significantly.
AISI 347H stainless steel delivers exceptional performance where high temperatures and corrosion meet. Its higher carbon content provides superior creep resistance, while niobium stabilization ensures immunity to intergranular corrosion and weld decay. For power plants, refineries, and chemical facilities operating at sustained high heat, 347H is a proven, reliable choice. While it requires careful heat treatment and specific welding procedures, its long-term durability makes it a cost-effective solution for critical high-temperature applications.
Our solution for AISI 347H Stainless Steel
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 347H Stainless Steel
How much does AISI 347H Stainless Steel cost?
Price comes from five lines: material, machine or tool time, setup, finishing and inspection. The material itself is usually the smallest of them. Send a drawing and we will split the quote so you can see which line is doing the damage.
How long does a first article take?
Usually a few business days once the drawing is frozen, and longer if tooling is involved. The date we give you is the date we confirm in writing, not the date we hope for.
What is the lead time for AISI 347H Stainless Steel?
Expect 5 to 12 business days depending on mill stock. Tooling, material stock and the finishing queue all move that number, so we confirm the date in writing before the run starts rather than after.
What files do you need in order to quote?
A STEP file plus a PDF drawing that carries the same revision letter. The model carries the geometry and the drawing carries the intent. When the two disagree, the shop picks one, and it is never the one you meant.
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.
Can AISI 347H Stainless Steel 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.
The short version
AISI 347H Stainless Steel 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.