ASTM F15 / UNS K94610 Forging Parts Forged rings, seamless rolled rings, flanges, discs, bars, sleeves, bushings, tube sheets and shafts in 29Ni-17Co-Fe controlled-expansion alloy

01 What is ASTM F15?

ASTM F15 is the ASTM International specification covering an iron-nickel-cobalt sealing alloy containing nominally 29% nickel and 17% cobalt with the balance iron. The Unified Numbering System designation for this chemistry is UNS K94610. Unlike a structural alloy, ASTM F15 is bought for the shape of its thermal expansion curve rather than for its strength. Everything else about the alloy is subordinate to that one property.

The mechanism is magnetic rather than purely thermal. In the ferromagnetic state the iron-nickel-cobalt lattice undergoes magnetostrictive contraction that partially cancels ordinary thermal expansion, giving an unusually low and unusually flat expansion coefficient. That cancellation disappears at the Curie temperature of 435 °C, and above it ASTM F15 expands like an ordinary alloy. The knee in the curve at the Curie point is called the inflection point, and it defines the entire working envelope of the material: a glass-to-metal seal made with ASTM F15 is only dimensionally trustworthy below roughly 450 °C.

The 17% cobalt addition is what separates ASTM F15 from the simpler iron-nickel expansion alloys. Cobalt raises the Curie point from around 280 °C for Invar 36 and around 360 °C for Alloy 42 up to 435 °C, and it flattens the curve so that expansion tracks hard borosilicate glass across the full sealing and annealing range rather than only near room temperature. That is why hermetic hard-glass seals are made with ASTM F15 rather than Invar.

What ASTM F15 is used for

ASTM F15 is the standard metal partner in hermetic glass-to-metal seals: power tubes, microwave tubes and travelling-wave tubes, X-ray tube envelopes, vacuum interrupters, transistor headers and TO packages, integrated-circuit lead frames, diode and laser-diode housings, optoelectronic butterfly packages, hybrid microelectronic packages, and vacuum feedthroughs for scientific instruments and accelerators. Secondary uses take advantage of the low expansion alone: bimetal thermostat elements, precision instrument frames, microwave cavity resonators, and low-expansion tooling.

Forged product forms Jiangyin Jiangnan Metal supplies in ASTM F15

• Forged rings
• Seamless rolled rings
• Contoured rolled rings
• Forged flanges
• Forged discs & blanks
• Forged round bar
• Forged flat bar & blocks
• Forged sleeves
• Forged bushings
• Tube sheets
• Forged shafts & spindles
• Forged tube & pipe sections
• Near-net-shape seal housings
• Machined-to-print parts

02 ASTM F15 equivalents: Kovar, UNS K94610, 4J29, 1.3981, Nilo K

Engineers meet this alloy under a dozen names, and purchasing departments often reject a quotation because the name on the drawing does not match the name on the offer. Every designation in the table below describes the same 29% nickel, 17% cobalt, balance iron chemistry. Jiangyin Jiangnan Metal Co., Ltd. accepts purchase orders written against any of them and issues a multi-designation material test certificate that cross-references the rest.

03 ASTM F15 chemical composition

The nominal composition of ASTM F15 / UNS K94610 is 29.0% nickel, 17.0% cobalt and the balance iron at approximately 53.5%. Nickel and cobalt are held to narrow bands because the coefficient of thermal expansion responds sharply to both: a nickel shift of only 0.5% moves the expansion coefficient by roughly 0.2 × 10⁻⁶/°C, which is enough to take a matched seal out of tolerance. Every other element in the specification is a controlled residual, limited because it either shifts the Curie point, hardens the alloy, or damages the glass seal.

Note on limits. The values above are the working limits Jiangyin Jiangnan Metal Co., Ltd. procures and produces to. ASTM revises its specifications periodically; the revision of ASTM F15 in force at your contract date governs. State the revision on the purchase order if your quality system requires it, and we will certify against that revision.

Melting route

For sealing-grade ASTM F15 the primary melt route is vacuum induction melting (VIM), optionally followed by a VAR or ESR remelt where dissolved gas content and inclusion cleanliness are critical. Vacuum-tube envelopes, aerospace feedthroughs and semiconductor packaging all normally require the remelt step. Vacuum melting matters here for a specific reason: nitrogen and oxygen dissolved in the melt come out as bubbles at the glass interface during sealing. For non-sealing structural applications where the alloy is used only for its low expansion (instrument frames, tooling, thermostat components), an EAF + VOD + ESR route is fully acceptable and appreciably cheaper. Tell us which case applies and we will quote the correct route rather than the expensive one by default.

04 Physical properties of ASTM F15

05 Thermal expansion of ASTM F15, the property you actually buy

The mean coefficient of thermal expansion of ASTM F15 is approximately 5.5 × 10⁻⁶/°C over 30–200 °C, falls to about 5.1 × 10⁻⁶/°C over 30–400 °C, and is approximately 5.3 × 10⁻⁶/°C over 30–450 °C. Above the Curie point of 435 °C the magnetostrictive contraction that suppressed expansion disappears, and the mean coefficient climbs steeply, reaching roughly 11.5 × 10⁻⁶/°C for the 30–900 °C interval. This is why the curve shape, not a single number, is what a seal designer needs.

06 Glass and ceramic sealing with ASTM F15

A glass-to-metal seal is a chemical bond, not a mechanical fit. Molten glass wets and dissolves a thin, adherent oxide layer that has been deliberately grown on the ASTM F15 surface, and on cooling the two materials must shrink at nearly the same rate or the joint tears itself apart. Both halves of that sentence carry a manufacturing requirement, and both are routinely missed.

Which glasses match ASTM F15

Surface preparation: the part of the process that decides whether the seal leaks

ASTM F15 will not bond to glass in its as-machined state. Three preparation steps are mandatory, and skipping any of them produces a seal that passes a visual inspection and fails a helium leak test.

Jiangyin Jiangnan Metal Co., Ltd. supplies ASTM F15 forgings in any of these states: as-forged with machining stock, bright hydrogen annealed, wet-hydrogen decarburised, or pre-oxidised ready for sealing. State the required delivery condition on the enquiry; it changes both price and packing.

07 Mechanical properties of ASTM F15 by temper

ASTM F15 is not a structural alloy and is rarely selected for strength. Its tensile properties are specified mainly so that a forming or machining shop knows what it is dealing with, and so that a sealing part is soft enough not to load the glass. Almost all sealing components are supplied in the annealed temper.

08 Heat treatment of ASTM F15

Annealing

ASTM F15 is annealed by heating to 850–1000 °C (1560–1830 °F), preferably in dry hydrogen or cracked ammonia, and cooling under control. A protective reducing atmosphere is used rather than vacuum or inert gas because the goal is not only to prevent scale but to actively reduce existing surface oxides. Hold time is roughly one hour per 25 mm of section. Cooling rate matters more than most operators expect: cooling too quickly leaves residual stress that later relaxes and shifts dimensions during sealing, so a controlled cool of about 200 °C per hour down to 200 °C is standard practice for precision parts.

Wet-hydrogen decarburising anneal

Any part destined for a glass seal receives an additional anneal in wet hydrogen, meaning hydrogen bubbled through water at room temperature, typically at 1050–1100 °C. The water vapour oxidises surface carbon to CO and carries it away. Without this step the residual carbon reacts with the pre-oxidation layer during sealing and generates carbon monoxide gas at the glass interface, producing bubbles and a seal that fails helium leak testing at the 10⁻⁹ mbar·L/s level even though it looks perfect.

Pre-oxidation before sealing

ASTM F15 is deliberately oxidised by heating in air at 600–1000 °C, with the temperature and time chosen to give the oxide film thickness the sealing process requires. The resulting iron-nickel-cobalt oxide is the chemical bridge into the glass. Thin films seal cleanly; over-thick films become the weakest layer in the joint and shear off inside the glass. Because oxide thickness is set by a time-temperature product, it should be qualified once for each part geometry and then locked into the traveller.

Stress relief

Machined parts that will not be fully annealed can be stress relieved at 800–900 °C in a reducing atmosphere. This removes machining stresses without a full recrystallisation and without significantly coarsening the grain.

09 Forging, machining and welding ASTM F15

Forging practice

ASTM F15 is forged from about 1150 °C down to a finishing temperature not below 900 °C. The alloy is single-phase austenitic across the hot-working range and forges readily, but it work-hardens quickly and is intolerant of finishing cold: below roughly 900 °C the flow stress climbs and edge cracking becomes likely. Practice at Jiangyin Jiangnan Metal Co., Ltd. for this grade is a soak at 1150–1180 °C, multi-step reduction with reheats rather than heavy single strokes, a forging ratio of at least 4:1 to break down the cast structure, and a slow furnace cool afterwards. Sulfur-bearing lubricants are prohibited on the dies for any part that will later be sealed.

Machining

Machinability of ASTM F15 is roughly 50% of free-machining B1112 steel. The alloy is gummy, work-hardens under a rubbing edge, and produces long stringy chips. Practical parameters: sharp positive-rake carbide, rigid setups with minimum overhang, turning speeds of roughly 25–45 m/min, a positive constant feed of 0.10–0.25 mm/rev, flood coolant, and no dwelling. A tool that stops cutting while still in contact will glaze the surface and the next pass will chatter. Use chlorine- and sulfur-free cutting fluids on any sealing part, and degrease thoroughly afterwards.

Welding and joining

ASTM F15 welds readily by GTAW, electron-beam, laser and resistance methods, using matching filler wire. The controlling principle is contamination rather than metallurgy: keep heat input low, maintain full argon shielding including a back purge, and keep the joint free of carbon, sulfur and oxygen. All three spoil the glass seal later, even when the weld itself is sound. Brazing to alumina ceramic is done with molybdenum-manganese metallisation plus nickel plating, or with an active braze alloy. After any joining operation, re-anneal in wet hydrogen before pre-oxidation.

10 ASTM F15 vs Invar 36, Alloy 42, Alloy 46, Alloy 48 and Alloy 52

The controlled-expansion family is a ladder: as nickel content rises from 29% to 52%, expansion rises with it, and each rung is matched to a different sealing partner. Choosing the right rung is entirely a question of what you are sealing to, not of quality or price.

The short version

• Sealing to hard borosilicate glass, or building a hermetic vacuum or semiconductor package? ASTM F15. Nothing else on the ladder combines a 435 °C Curie point with a flat curve.
• Need the absolute minimum expansion, cryogenic service, or dimensional stability with no sealing? Invar 36.
• Making integrated-circuit lead frames, or want most of the F15 behaviour without paying for cobalt? Alloy 42, but check that your service temperature stays well under its lower Curie point.
• Sealing to soda-lime or other soft glass? Alloy 48 or Alloy 52. Using F15 here guarantees a cracked seal.

11 Where ASTM F15 forgings are used

ASTM F15 appears wherever a metal must stay dimensionally locked to a glass or ceramic across a temperature cycle. The list below groups the applications by the property being exploited, which is more useful for material selection than an alphabetical list of parts.

Hermetic glass-to-metal seals, the primary application

• Vacuum electronics: power tubes, microwave tubes, travelling-wave tubes, magnetrons, klystrons, X-ray tube envelopes and anode assemblies.
• Vacuum interrupters and switchgear: forged end rings, seal flanges and shield housings for medium-voltage interrupters.
• Semiconductor packaging: TO headers, transistor leads and headers, diode packages, DIP and flat-pack lids, hybrid microelectronic packages, integrated-circuit lead frames.
• Optoelectronics and photonics: butterfly packages, laser-diode housings, fibre-optic feedthrough ferrules, detector windows.
• Scientific and accelerator hardware: electrical feedthroughs, viewport frames, ion-gauge and pressure-transducer bodies, mass-spectrometer components.
• Lighting: lamp lead-ins, photographic flash-bulb assemblies, discharge-lamp seal components.

Low expansion used on its own, without a glass seal

• Bimetal thermostat elements. ASTM F15 forms the low-expansion side of the strip.
• Precision instrument frames and optical benches where dimensional stability matters more than absolute minimum expansion.
• Microwave cavity resonators and precision condenser blades, where a cavity dimension sets the operating frequency.
• Metrology fixtures, gauge bodies and clock components including balance wheels and pendulum rods.
• Low-expansion tooling for aerospace composite layup, though Invar 36 is more usual where the lowest possible expansion is wanted.

Industries served

• Vacuum electronics
• Semiconductor & microelectronics
• Photonics & optoelectronics
• Medical imaging (X-ray tubes)
• Aerospace & defence
• Power switchgear
• Scientific instruments
• Particle accelerators & fusion research
• Sensors & transducers
• Precision instrumentation
• Lighting

12 ASTM F15 production capability at Jiangyin Jiangnan Metal

Jiangyin Jiangnan Metal Co., Ltd. operates an open-die forging works at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. The plant runs 1-, 3-, 5- and 9-tonne open-die forging hammers, a 5,000-tonne hydraulic press, and 3-metre and 6-metre seamless ring-rolling mills, with in-house heat treatment, machining, and a mechanical and metallurgical test laboratory. ASTM F15 is produced as a specialty controlled-expansion grade on the same line, with additional atmosphere and cleanliness controls specific to sealing alloys.

Process flow for an ASTM F15 forging order

Equipment

13 Quality, testing and certification

ASTM F15 is bought by industries that qualify a supplier once and then audit the paperwork forever, so the certificate matters as much as the metal. Every ASTM F15 forging leaves Jiangyin Jiangnan Metal Co., Ltd. with an EN 10204 3.1 material test certificate as standard, and EN 10204 3.2 with a client-nominated third-party witness (Lloyd's Register, DNV, Bureau Veritas, ABS or TÜV) is available on request.

What appears on the certificate

• Heat number and full traceability from melt to shipment.
• Complete chemical analysis against the ASTM F15 limits, with the specification revision stated.
• Measured mean coefficient of thermal expansion, with the temperature interval and the ASTM E228 test conditions.
• Cross-reference of equivalent designations (UNS K94610, 4J29, W.Nr. 1.3981), so the certificate satisfies multi-region project documentation.
• Heat-treatment record including atmosphere, temperature, hold and cooling rate.
• Mechanical test results and delivery temper.
• NDE results with the applied standard and acceptance class.
• Delivery surface condition: as-forged, bright annealed, decarburised or pre-oxidised.

14 ASTM F15 forging weight calculator

Use the finished weight to populate a request for quotation. ASTM F15 is dense at 8.36 g/cm³, about 7% heavier than carbon steel, so an estimate carried over from a steel drawing will understate the weight and the price.

15 How to specify an ASTM F15 forging order

Seven lines on a purchase order remove almost every source of dispute on this grade. The second one is the line most buyers omit, and it is the one that decides whether the part works.

Drawing callout you can copy

Copy this block into the material callout box of your drawing and adjust the expansion window to your seal design. Sending it with an enquiry usually removes one full round of technical clarification.

16 Ten mistakes engineers make when ordering ASTM F15

Collected from enquiries, rejected first articles and post-delivery reviews. Each of these costs weeks when caught at goods-in and costs a product qualification when caught later.

1. Specifying "Kovar" on the purchase order. Kovar® is a Carpenter Technology trademark; a PO naming it can technically only be filled by that producer. Fix: write ASTM F15 / UNS K94610 and list the trade names as "or equivalent" if you want to keep them on the drawing.

2. Giving a CTE number with no temperature interval. "CTE 5.3" is meaningless on its own, because this alloy's coefficient ranges from 5.1 to 11.5 depending on the interval. Fix: always write the interval, and the tolerance you can live with.

3. Assuming ASTM F15 seals to Corning 7740 Pyrex. It does not; the mismatch is about 2.0 × 10⁻⁶/°C. Fix: use the 7052 / 7056 family, or a graded seal, or tungsten.

4. Designing for service above 450 °C. Above the 435 °C Curie point the controlled expansion simply stops existing. Fix: keep matched seals below 450 °C, or move to a ceramic-to-metal brazed design.

5. Skipping the wet-hydrogen decarburising anneal. The seal looks perfect and fails helium leak testing because of CO bubbles. Fix: make the anneal atmosphere an explicit line on the drawing, not an assumption.

6. Allowing sulfur-bearing cutting fluid at the machining stage. Sulfur surfaces during pre-oxidation and creates leak paths that no later cleaning can undo. Fix: prohibit it on the drawing and audit the machine shop, not just the forge.

7. Over-oxidising before sealing. A thick oxide becomes the weakest layer and the seal shears inside the oxide rather than bonding. Fix: qualify the time-temperature product once per geometry and lock it into the traveller.

8. Ignoring grain size. Coarse grain gives orange peel on drawn surfaces and an uneven oxide. Fix: specify ASTM 5 or finer per ASTM E112.

9. Confusing ASTM F15 with Invar 36 for cryogenic work. They are different alloys with roughly a factor of four between their expansion coefficients. Fix: for LNG, metrology and cryogenics specify Invar 36 / ASTM F1684.

10. Carrying a weight estimate over from a steel drawing. At 8.36 g/cm³ ASTM F15 is about 7% denser than carbon steel, and cobalt makes it many times more expensive per kilogram, so the weight error goes straight into the budget. Fix: use the weight calculator above.

17 Glossary

ASTM F15

The ASTM International specification for an iron-nickel-cobalt sealing alloy of nominally 29% nickel, 17% cobalt, balance iron, used for hermetic seals to hard borosilicate glass.

UNS K94610

The Unified Numbering System designation for the ASTM F15 chemistry.

4J29

The Chinese GB designation for the same alloy. The most common name for it in Asian supply chains.

W.Nr. 1.3981

The European Werkstoff material number for the same alloy, also written FeNi29Co17.

Controlled-expansion alloy

An iron-nickel or iron-nickel-cobalt alloy whose composition is tuned so that thermal expansion follows a target curve, usually that of a glass or ceramic it must seal to. Also called a low-expansion or sealing alloy.

Curie temperature

The temperature at which a ferromagnetic material becomes paramagnetic. For ASTM F15 that temperature is 435 °C. It marks the end of the controlled-expansion range, because the low expansion depends on magnetic ordering.

Inflection point

The knee in the expansion curve near the Curie temperature, above which the coefficient of thermal expansion rises steeply toward ordinary metallic values.

Matched seal

A glass-to-metal seal in which the two materials have nearly identical expansion curves, so residual stress after cooling is small. ASTM F15 with Corning 7052 is the classic example.

Compression seal

A seal in which the outer metal body contracts more than the glass on cooling, holding the glass in compression. Used deliberately where a matched pair is unavailable, because glass is far stronger in compression than in tension.

Wet-hydrogen anneal

An anneal in hydrogen bubbled through water, used to decarburise the surface of ASTM F15 so residual carbon cannot reduce the oxide layer and generate carbon monoxide bubbles inside the glass.

Pre-oxidation

Deliberate growth of a thin adherent iron-nickel-cobalt oxide film by heating in air at 600–1000 °C. The oxide dissolves into the molten glass and forms the chemical bond of the seal.

Glass set point

The temperature on cooling below which the glass behaves elastically rather than viscously, and from which residual seal stress starts to accumulate. Roughly the annealing point of the glass.

ASTM E228

The standard test method for linear thermal expansion of solid materials with a push-rod dilatometer. This is the normal way to verify that an ASTM F15 heat meets its expansion requirement.

Seamless rolled ring

A ring made by piercing a forged billet and rolling it on a radial-axial ring mill, giving continuous circumferential grain flow with no weld.

Forging ratio

The ratio of starting cross-section to finished cross-section. A ratio of at least 4:1 is used on ASTM F15 to break down the cast structure and give consistent expansion behaviour.

EN 10204 3.1 / 3.2

Certificate types. 3.1 is issued by the manufacturer's own independent quality department; 3.2 adds a third-party or customer witness.

Helium leak rate

The standard measure of hermeticity for a sealed package, usually required below 1 × 10⁻⁹ mbar·L/s for vacuum electronics and microelectronic packaging.

18 Frequently asked questions about ASTM F15

19 Standards and technical references

Chemistry, expansion, heat-treatment and sealing data on this page are drawn from the published standards and engineering references below. Test results printed on our certificates are independent measurements traceable to calibrated instruments in our own laboratory.

1. ASTM F15, Standard Specification for Iron-Nickel-Cobalt Sealing Alloy, ASTM International, West Conshohocken, PA.
2. ASTM E228, Standard Test Method for Linear Thermal Expansion of Solid Materials With a Push-Rod Dilatometer, ASTM International.
3. ASTM E831, Standard Test Method for Linear Thermal Expansion of Solid Materials by Thermomechanical Analysis, ASTM International.
4. ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications, ASTM International.
5. ASTM A388/A388M, Standard Practice for Ultrasonic Examination of Steel Forgings, ASTM International.
6. ASTM E8/E8M, Standard Test Methods for Tension Testing of Metallic Materials, ASTM International.
7. ASTM E112, Standard Test Methods for Determining Average Grain Size, ASTM International.
8. ASTM E165/E165M, Standard Practice for Liquid Penetrant Testing for General Industry, ASTM International.
9. AMS 7726, AMS 7727 and AMS 7728, iron-nickel-cobalt controlled-expansion alloy specifications, SAE International.
10. MIL-I-23011, Iron-Nickel Alloys for Magnetic Applications, Class 1, US Department of Defense.
11. EN 10204, Metallic products: types of inspection documents, CEN, Brussels.
12. EN 10228-3, Non-destructive testing of steel forgings, part 3: ultrasonic testing of ferritic or martensitic steel forgings, CEN.
13. SEP 1921, Ultrasonic testing of steel bars and forgings, Stahl-Eisen-Prüfblatt, VDEh, Düsseldorf.
14. ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, ASM International, Materials Park, OH. See the section on low-expansion and controlled-expansion alloys.
15. ASM Handbook, Volume 4: Heat Treating, ASM International. Atmosphere heat treatment of controlled-expansion alloys.
16. Carpenter Technology Corporation, technical datasheet for the iron-nickel-cobalt sealing alloy manufactured to ASTM F15.
17. Special Metals Corporation, Nilo and Nilomag Alloys technical publication.
18. Corning Incorporated, Glass Sealing Data. Expansion characteristics of Corning code 7040, 7052, 7056, 7720 and 7740 glasses.
19. Schott AG, technical information on 8245, 8250 and related sealing glasses.
20. Espe, W., Materials of High Vacuum Technology, Pergamon Press. Glass-to-metal seal practice and surface preparation.

Standards are cited by designation without a revision number because revisions are periodic; the revision in force at your contract date governs. All trademarks named on this page belong to their respective owners.

20 Cite this page

If you are quoting these figures in a specification, report, datasheet or answer, please cite the source so readers can reach the original data and ask us follow-up questions.

21 Request a quotation for ASTM F15 forgings

Send the drawing, the grade written as ASTM F15 / UNS K94610, the expansion window with its temperature interval, the required delivery condition and the certificate level. If you would rather have the enquiry written for you, use the generator below.

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