Controlled-expansion Fe-Ni-Co alloy / open-die forgings
Alloy K Forgings: ASTM F-15, UNS K94610, W.Nr 1.3981, 4J29
Jiangyin Jiangnan Metal Co., Ltd. manufactures Alloy K, the generic Fe-29Ni-17Co controlled-expansion alloy specified as ASTM F-15, UNS K94610, W.Nr 1.3981 (NiCo 29 18) and GB 4J29. We supply it as open-die forgings, seamless rolled rings, forged bars, discs, flanges, sleeves, bushings, tube sheets and shafts, made to customer drawing and delivered with EN 10204 3.1 or 3.2 certification.
Alloy K at a glance
Alloy K is a controlled-expansion iron-nickel-cobalt alloy of nominal composition 29 % nickel, 17 % cobalt and 53 % iron. Its coefficient of thermal expansion is held near 5.3 × 10-6 K-1 between 30 °C and 450 °C, which matches hard borosilicate sealing glasses and alumina ceramics. That match allows a glass or ceramic to be fused directly to the metal and to stay hermetic through thermal cycling. Above the Curie point of about 435 °C the expansion coefficient roughly doubles, so seals are designed to operate below it.
- Generic designations
- ASTM F-15, UNS K94610, W.Nr 1.3981, 4J29
- Nominal chemistry
- Fe 53 %, Ni 29 %, Co 17 %
- Mean CTE 30 to 450 °C
- approx. 5.3 × 10-6 K-1
- Curie temperature
- approx. 435 °C (815 °F)
- Density
- 8.36 g/cm³ (0.302 lb/in³)
- Melting point
- approx. 1 449 °C (2 640 °F)
- Annealing
- 850 to 1 000 °C in hydrogen or cracked ammonia
- Melting route
- EAF + VOD + ESR, or VIM + VAR on request
- Certification
- EN 10204 3.1 standard, 3.2 on request
- Manufacturer
- Jiangyin Jiangnan Metal Co., Ltd., Jiangyin, Jiangsu, China
Trademark notice. Kovar® is a registered trademark of CRS Holdings, Inc. (Carpenter Technology Corporation). Nilo® is a registered trademark of Special Metals Corporation, Pernifer® of VDM Metals, Dilver® of Aperam Alloys Imphy, and Rodar®, Telcoseal®, Sealvar®, Nicoseal® and Therlo® of their respective owners. Material produced and sold by those companies under those brands is theirs. Material manufactured by Jiangyin Jiangnan Metal Co., Ltd. is correctly described as Alloy K / ASTM F-15 / UNS K94610 / W.Nr 1.3981 / 4J29, the same generic chemistry produced independently. We are not affiliated with, sponsored by or endorsed by any of the trademark holders named above.
What is Alloy K?
Alloy K is a vacuum- or ESR-refined iron-nickel-cobalt alloy whose thermal expansion has been engineered to match hard glass and ceramic rather than to maximise strength. Ordinary steels expand roughly 12 × 10-6 per kelvin. Borosilicate sealing glass expands about 5 × 10-6. Fuse the two together and the joint cracks on the first cooling cycle. Alloy K closes that gap through the Invar effect, the magnetostrictive contraction that partly cancels normal thermal expansion in ferromagnetic Fe-Ni and Fe-Ni-Co alloys below their Curie temperature.
The 29 % nickel and 17 % cobalt balance is chosen so that the cancellation is not total, as it is in Invar 36 at about 1.3 × 10-6 K-1, but leaves a residual expansion near 5 × 10-6 K-1, close to that of Schott 8250 and Corning 7052 sealing glasses. Cobalt also raises the Curie point to about 435 °C, which is what makes the alloy usable at elevated temperature where a binary Fe-Ni alloy would already have lost its low-expansion behaviour.
The second essential property is the oxide. When Alloy K is heated in air it grows an adherent, duplex Fe-Ni-Co oxide film that molten glass wets and partially dissolves, producing a chemical bond rather than a mechanical one. A hermetic glass-to-metal seal therefore depends on three controlled variables that a forging supplier has to manage: chemistry within the ASTM F-15 band, a clean annealed microstructure free of carbon and dissolved gas, and a reproducible oxide thickness. All three are controlled at Jiangyin Jiangnan Metal Co., Ltd. through EAF + VOD + ESR melting, hydrogen or cracked-ammonia annealing, and documented pre-oxidation cycles.
Alloy K is not a structural alloy. Its annealed tensile strength of about 517 MPa is modest, it has no useful corrosion resistance in seawater or acids, and it should not be chosen for load-bearing service where a stainless or nickel-base grade would do. It is chosen when dimensional behaviour under temperature governs the design.
Equivalent designations and trade names
Engineers meet this alloy under at least a dozen names. Every designation in the table below refers to the same Fe-29Ni-17Co chemistry. Jiangyin Jiangnan Metal Co., Ltd. accepts purchase orders written against any of them and issues a multi-designation material test certificate on request.
| Standard body or region | Designation | Scope and notes |
|---|---|---|
| USA, UNS | K94610 | Generic Unified Numbering System designation |
| USA, ASTM | ASTM F-15 | Iron-nickel-cobalt sealing alloy, the primary generic specification |
| USA, SAE/AMS | AMS 7727 | Bars, forgings and rings. The aerospace spec relevant to forged product |
| USA, SAE/AMS | AMS 7726 | Wire |
| USA, SAE/AMS | AMS 7728 | Sheet, strip and plate |
| USA, military | MIL-I-23011 Class 1 | Magnetic materials, class 1 controlled expansion |
| Germany, Werkstoff | 1.3981 | DIN 17745 / SEW 385, designated NiCo 29 18 or X3NiCo29-18 |
| France, AFNOR | NF A54-301 | French national equivalent, Dilver P family |
| China, GB | 4J29 | GB/T 15018 expansion alloy, the domestic Chinese designation |
| Common technical name | Alloy 29-17 / NiCo 29 18 | Named for the nominal Ni and Co percentages |
| Registered trade names | Kovar®, Nilo K®, Pernifer® 29-18, Dilver® P, Rodar®, Telcoseal®, Sealvar®, Nicoseal®, Nicosel®, Therlo®, Vacon 12 | Property of their respective owners. We do not supply under these brands |
| Always reference the specification revision in force at the contract date. Where a drawing quotes a trade name, we cross-reference it to ASTM F-15 / UNS K94610 on the certificate. | ||
Chemical composition of Alloy K
The chemistry below is the production specification used by Jiangyin Jiangnan Metal Co., Ltd. for Alloy K forgings, and it conforms to ASTM F-15. The three intentional elements are nickel, cobalt and iron. Everything else is held low because residual elements, particularly carbon, aluminium, titanium, zirconium and magnesium, form stable oxides or gas pockets that cause pinhole leaks in a hermetic seal.
| Element | Ni | Fe | Co | C | Mn | Si | Al | Cr | Mg | Zr | Ti | Cu | Mo |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Alloy K | 29.0 | 53.0 | 17.0 | 0.04 max | 0.50 max | 0.20 | 0.10 max | 0.20 max | 0.10 max | 0.10 max | 0.10 max | 0.20 max | 0.20 max |
| Nickel and cobalt are the controlling elements. A 0.5 % shift in nickel moves the expansion coefficient by roughly 0.3 × 10-6 K-1, which is enough to break a glass seal. We therefore analyse Ni and Co by optical emission spectrometry on every heat and report the actual values on the certificate rather than the nominal ones. | |||||||||||||
Melting route. Alloy K for sealing service is melted at Jiangyin Jiangnan Metal by EAF + VOD followed by ESR (electroslag remelting) to control carbon, sulphur and non-metallic inclusions. VIM + VAR double-vacuum melting is available where dissolved hydrogen, oxygen and nitrogen have to be minimised, which is the usual requirement for high-reliability hermetic packages, vacuum-tube and microwave components.
Thermal expansion and the Curie point
The expansion curve below is the single most important engineering datum for this alloy. Read it as follows: the plotted value is the mean coefficient of thermal expansion from 30 °C up to the temperature on the horizontal axis, which is how sealing glasses are specified and therefore how the two have to be compared.
| Upper temperature | Mean CTE (x10-6 per K) | Engineering significance |
|---|---|---|
| 30 to 200 °C | approx. 5.5 | Normal service band for hermetic electronic packages |
| 30 to 300 °C | approx. 5.1 | Minimum of the curve, best match to Schott 8250 and Corning 7052 |
| 30 to 400 °C | approx. 4.9 | Still within the glass band, approaching the Curie inflection |
| 30 to 450 °C | approx. 5.3 | The headline published value for ASTM F-15 |
| 30 to 500 °C | approx. 6.2 | Above Curie, the alloy no longer behaves as a low-expansion material |
| 30 to 600 °C | approx. 7.6 | Brazing and sealing range, not a service range |
| 30 to 700 °C | approx. 8.7 | Glass-sealing soak temperature |
| 30 to 800 °C | approx. 9.7 | Pre-oxidation and hydrogen-anneal range |
| 30 to 900 °C | approx. 11.5 | Approaching plain-steel expansion, full anneal range |
| Typical published values for the ASTM F-15 chemistry, given for design screening. Exact values vary with melt chemistry, prior thermal history and degree of cold work. Where the expansion curve is contractually critical, order a certified dilatometry test to ASTM E228 on the actual heat. We can include it on the certificate. | ||
The most common Alloy K design error is comparing a room-temperature expansion figure for the metal against a 0 to 300 °C figure for the glass. The two have to be evaluated over the same interval, and that interval has to span the glass set point down to the lowest service temperature, because the residual stress in a completed seal is set on cooling rather than on heating. The calculator below does this comparison correctly.
Glass and ceramic seal-match checker
Pick the glass or ceramic your Alloy K part seals to, state the temperature interval, and see whether the pair produces a matched, compression or mismatched seal, together with the residual strain.
Screening tool only. Partner expansion coefficients are nominal published values over roughly 20 to 300 C and vary by manufacturer and batch. Real seal integrity also depends on oxide thickness, glass viscosity at set point, cooling rate, joint geometry and whether the design is a matched, compression or graded seal. Confirm with the glass or ceramic supplier's data sheet and a qualification build before production.
Alloy K thermal expansion calculator
Enter a dimension and a temperature interval to get the mean expansion coefficient over that interval, the total change in length, and the diametral change for fits and clearances.
Values derive from the published mean-CTE curve in Table 3 for the ASTM F-15 chemistry. Below the Curie point the curve is flat and predictions are reliable. Across 435 C the instantaneous coefficient changes rapidly, so treat results spanning that point as indicative. For contractual work, order certified dilatometry to ASTM E228 on the delivered heat.
Mechanical properties of Alloy K
Alloy K is supplied annealed. In that condition it is soft, very ductile and easy to deep-draw, spin, machine and form. That is intentional, because sealing components are usually formed after annealing and any residual cold work would alter the expansion curve. The values below are typical for annealed open-die forged product supplied by Jiangyin Jiangnan Metal Co., Ltd.
| Property | ksi | MPa | Note |
|---|---|---|---|
| Yield strength, 0.2 % offset | 50 | 345 | Low. Do not design Alloy K as a structural member |
| Tensile strength | 75 | 517 | Rises substantially with cold work. Anneal before sealing |
| Elongation in 2 in (50 mm) | 30 % | Good formability for drawn cups, eyelets and headers | |
| Hardness | approx. 68 HRB | Machines cleanly with sharp, positive-rake tooling | |
| Modulus of elasticity | 20 x 106 | approx. 138 | Typical published value for the F-15 chemistry |
| Typical values for annealed forged product, reported for design screening. Actual tested values for the delivered heat appear on the EN 10204 3.1 or 3.2 certificate. Cold-worked Alloy K can exceed 800 MPa tensile but is re-annealed to restore the expansion characteristic. | |||
Physical properties of Alloy K
| Property | Value | Unit | Condition or note |
|---|---|---|---|
| Density | 8.36 (0.302) | g/cm3 (lb/in3) | Room temperature. Use this for forging weight |
| Melting point | approx. 1 449 (2 640) | C (F) | Solidus |
| Curie temperature | approx. 435 (815) | C (F) | Ferromagnetic below, paramagnetic above |
| Mean CTE 30 to 450 C | approx. 5.3 | x10-6 per K | The design value for glass sealing |
| Mean CTE 30 to 900 C | approx. 11.5 | x10-6 per K | Above Curie, approaching plain steel |
| Thermal conductivity | approx. 17.3 | W/m.K | Low. Allow for it in seal-heating cycles |
| Specific gravity | 8.36 | n/a | |
| Magnetic behaviour | Ferromagnetic | n/a | Below 435 C. Not suitable where a non-magnetic part is required |
| Corrosion resistance | Low | n/a | Comparable to plain steel. Plate or seal-coat for humid service |
Annealing and pre-oxidation
Annealing
Alloy K is supplied in the annealed condition, produced by heating to 850 to 1 000 °C (1 560 to 1 830 °F), preferably in dry hydrogen or cracked ammonia. The protective atmosphere does two jobs at once. It prevents scale, and it decarburises and degasses the surface. Both matter, because carbon left in solution and hydrogen or oxygen trapped in the lattice will come out as bubbles when the part is later heated under molten glass, producing pinhole leaks in a package that has to hold a vacuum for twenty years.
Annealing also erases the cold work introduced by forging, machining or drawing. That is not cosmetic. Cold work shifts the expansion curve, and a part that was formed after its final anneal will not expand the way the certificate says it does. Anneal after forming, not before. Where geometry forces forming after annealing, a stress-relief cycle at 760 to 870 °C in hydrogen restores most of the characteristic.
Pre-oxidation for glass sealing
Alloy K is deliberately oxidised by heating in air at 600 to 1 000 °C (1 110 to 1 830 °F), with the exact temperature and time chosen to give the oxide thickness the sealing glass requires. Too thin and the glass will not wet the surface, producing a weak mechanical joint that leaks. Too thick and the oxide layer itself becomes the weak link, because the glass dissolves only the outer part and the seal fractures within the oxide. A grey-blue to blue-black film is the visual target. The quantitative target is normally specified as a weight gain per unit area.
| Cycle | Temperature | Atmosphere | Purpose |
|---|---|---|---|
| Full anneal | 850 to 1 000 °C | Dry hydrogen or cracked ammonia | Soften, recrystallise, decarburise, degas. The standard delivery condition |
| Stress relief | 760 to 870 °C | Dry hydrogen | After machining or forming, when a full anneal would distort the part |
| Pre-oxidation | 600 to 1 000 °C | Air, or wet hydrogen | Grow a controlled oxide film for glass wetting |
| Hot working and forging | 1 010 to 1 175 °C | Air (furnace) | Open-die forging and ring rolling. Finish above 950 °C |
| Glass sealing | approx. 950 to 1 050 °C | Wet hydrogen or nitrogen | Performed by the sealing house, above the Curie point |
| Cycle times scale with section thickness. Allow roughly 30 minutes per 25 mm at temperature. Furnace charts are recorded for every batch and referenced on the material test certificate. | |||
Anneal and pre-oxidation recipe generator
Pick the objective and section thickness to get a complete, printable cycle for your heat-treatment shop: temperature, soak time, atmosphere and cooling instruction.
Cycles follow the ASTM F-15 practice described above and general expansion-alloy heat-treatment practice. Verify on test coupons from the same heat before committing production parts, and confirm oxide targets with your sealing house, since oxide specifications are usually proprietary to the seal design.
Curie point and service temperature check
Enter your service temperature to see whether Alloy K still behaves as a controlled-expansion alloy, what the expansion coefficient will actually be, and what to use instead if it does not.
Forging practice for Alloy K
Alloy K forges within a 1 010 to 1 175 °C (1 850 to 2 150 °F) window. It is more forgiving than a nickel-base superalloy but has two habits that cause scrap if ignored.
First, it is hot short if sulphur is present. Fuel-fired furnaces burning sulphur-bearing fuel, or contamination from previous charges, will crack the billet along grain boundaries. Electric or clean gas-fired furnaces are used at Jiangyin Jiangnan Metal for this grade, and expansion alloys are scheduled on dedicated tooling to avoid nickel-sulphide pickup.
Second, the alloy work-hardens quickly below about 950 °C. Finishing blows delivered too cold leave residual stress that distorts the part during the subsequent anneal and shifts the expansion curve locally. We finish above 950 °C and reheat rather than chase the last reduction on a cooling billet.
After forging, parts are slow-cooled, rough-machined, then fully annealed at 850 to 1 000 °C in hydrogen. Ultrasonic examination is carried out to EN 10228-3, SEP 1921 or ASTM A388 as the order specifies. Because sealing components are usually small relative to the forged blank, near-net-shape forging is rarely economic in this grade. We generally supply generous-stock forgings or rough-machined blanks and let the customer's precision shop take the final cut after the anneal.
Forged forms and available sizes
Jiangyin Jiangnan Metal Co., Ltd. produces the following Alloy K forged products to customer drawing or dimensional specification.
| Product form | Size range | Typical delivery condition |
|---|---|---|
| Seamless rolled rings | 120 to 1 200 mm OD, wall from 20 mm | Annealed, rough machined |
| Forged discs and disks | 80 to 800 mm diameter | Annealed |
| Forged round bars | 20 to 350 mm diameter, up to 3 000 mm long | Annealed, peeled or turned |
| Forged sleeves and bushings | 60 to 600 mm diameter | Annealed, bored |
| Forged flanges and tube sheets | To drawing, up to 900 mm | Annealed |
| Forged shafts | 30 to 300 mm diameter, up to 3 000 mm | Annealed, rough machined |
| Single-piece weight | Up to approx. 1 500 kg | Larger sizes quoted on enquiry |
| Expansion alloys are normally ordered in modest sections, so this envelope reflects practical demand rather than plant capacity. Our presses and ring mill handle far larger work in other grades. If you need an Alloy K forging outside these figures, ask us. The constraint is usually ingot availability rather than the equipment. | ||
Alloy K forging weight calculator
Calculate net and rough forging weight at the Alloy K density of 8.36 g/cm3, which is the figure you need to put a realistic quantity on an enquiry.
Net weight uses a density of 8.36 g/cm3. Rough forging weight adds machining stock, which for this grade typically runs 20 to 40 % on simple shapes and more on profiled rings. Allow the higher figure, because Alloy K is normally finish-machined after annealing.
Applications of Alloy K
Every application below exists for the same reason. A metal part has to hold its dimensions relative to a glass, ceramic or optical component across a temperature range. Alloy K forgings supplied by Jiangyin Jiangnan Metal Co., Ltd. go into the following service.
Hermetic packages and lead frames
Transistor headers, diode packages, hybrid and integrated-circuit lead frames, TO-can bases and microelectronic housings, where the glass-to-metal seal has to hold a vacuum or dry-nitrogen fill for the life of the device.
Power tubes, magnetrons, microwave cavities
High-power transmitting valves, microwave tubes, radar and microwave cavity resonators, echo boxes and filters. This is the original application for the alloy and remains the most demanding.
Lamp seals, flash bulbs, feedthroughs
Electric lamp bulb seals, photographic flash bulbs, pressure and temperature sensor feedthroughs, and instrument penetrations that have to remain hermetic under cycling.
Transfer lines and containment
Containers and transfer lines for LNG tankers, and cryogenic instrumentation where differential contraction between the vessel and its instrumentation has to be controlled.
Dimensionally stable optics and metrology
Optical benches and mounts, metrology fixtures, scientific instrument frames, spacers and precision condenser blades, where thermal drift is the error budget.
Bimetal strip and thermostats
The low-expansion side of thermostatic bimetal strip, bimetal thermostats, temperature regulators and electrical circuit breakers.
Composite moulds and stable tooling
Advanced composite moulds for aerospace, where the tool has to match the cured part's expansion, and positioning devices requiring low thermal drift.
Shielding and small transformers
Magnetic shielding, small electrical transformer components, and special electronic housings that exploit the alloy's ferromagnetic behaviour below the Curie point.
Alloy K compared with other controlled-expansion alloys
Choosing an expansion alloy is a matter of matching the partner material's expansion, not of picking the strongest or cleanest alloy. The table sets Alloy K against the grades most often considered alongside it, all of which Jiangyin Jiangnan Metal also forges.
| Alloy | Nominal chemistry | Mean CTE (x10-6 per K) | Curie point | Matches |
|---|---|---|---|---|
| Invar 36 | Fe-36Ni | approx. 1.3 (20 to 100 C) | approx. 279 C | Nothing. Used where near-zero expansion itself is the requirement |
| Alloy 42 / Invar 42 | Fe-42Ni | approx. 4.5 to 5.3 | approx. 350 C | Hard glass and silicon. Common lead-frame alloy |
| Alloy K (this page) | Fe-29Ni-17Co | approx. 5.3 (30 to 450 C) | approx. 435 C | Hard borosilicate sealing glass and alumina ceramic. Highest Curie point of the group |
| Alloy 45 | Fe-45Ni | approx. 7.0 | approx. 400 C | Intermediate glasses |
| Alloy 46 | Fe-46Ni | approx. 7.5 | approx. 460 C | Alumina ceramic, intermediate glass |
| Alloy 48 | Fe-48Ni | approx. 8.5 | approx. 500 C | Sapphire, higher-expansion ceramic |
| Alloy 52 | Fe-51Ni | approx. 10.2 | approx. 520 C | Soft soda-lime glass |
| Nominal published values for screening. Alloy K is the correct choice when the partner sits in the 4.6 to 5.2 band and the assembly also sees temperatures above roughly 350 C, where Alloy 42 has already passed its Curie point and lost its low-expansion behaviour. | ||||
Multi-standard designation lookup
Type any name you have on a drawing, such as Alloy K, Kovar, K94610, F-15, 1.3981, 4J29, NiCo 29 18, Dilver or Pernifer, and see every equivalent designation at once.
Production capability
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, operating since 2008 and exporting to more than 40 countries. Raw material, forging, heat treatment, machining and inspection are all under one roof, which is what makes a tightly controlled grade like Alloy K practical, since the expansion characteristic is set by melting and annealing rather than by any single operation.
| Stage | Equipment or method | Applied to Alloy K |
|---|---|---|
| Melting | EAF + VOD + ESR, or VIM + VAR on request | Controls C, S and inclusions, which is critical for leak-free seals |
| Forging, heavy | Hydraulic press, 5 000 t class | Billet breakdown and cogging of larger blanks |
| Forging, hammers | 1 t, 3 t, 5 t and 9 t open-die hammers | Bars, discs, sleeves, shafts |
| Ring rolling | 3 m and 6 m seamless ring-rolling mills | Seamless rolled rings and gear-ring blanks |
| Heat treatment | Controlled-atmosphere furnaces, hydrogen or cracked ammonia | 850 to 1 000 C anneal, 600 to 1 000 C pre-oxidation, charts recorded |
| Machining | CNC turning, boring and milling | Rough or finish machining to drawing |
| NDT | Ultrasonic to EN 10228-3, SEP 1921 or ASTM A388, magnetic particle | Every forging, class per order |
| Laboratory | Optical emission spectrometer, universal test machine, impact tester, hardness tester, metallographic microscope | Chemistry, tensile, hardness, grain size on every heat |
Standards, testing and certification
Alloy K forgings are supplied against the following specifications and inspection documents.
ASTM F-15 and UNS K94610
The governing generic chemistry specification. AMS 7727 applies where the order calls for aerospace bars, forgings and rings. DIN 17745 / W.Nr 1.3981 and GB/T 15018 (4J29) apply to European and Chinese projects.
EN 10228-3, SEP 1921, ASTM A388
Ultrasonic examination class stated on the order. Acceptance criteria and scanning plan agreed before production for critical parts.
ASTM E228 dilatometry
Certified coefficient-of-thermal-expansion curve on the actual heat, over the customer's specified interval. Available as an addition to the standard certificate.
EN 10204 3.1 and 3.2
3.1 mill certificate is standard. 3.2 with third-party witness by TUV, BV, DNV, ABS or Lloyd's is issued on request through the client's nominated inspection body.
ISO 9001:2015
Certified quality management system covering incoming material, process control, inspection and traceability. Heat number traceability retained for ten years.
Customer hold points
Customers may witness chemistry analysis, heat-treatment cycles, mechanical testing and final NDE. Hold points are added to the travel sheet at no charge.
How to specify an Alloy K forging order
State the generic designation
Write ASTM F-15 / UNS K94610 / W.Nr 1.3981 on the purchase order rather than a trademarked brand name. A PO that demands a registered brand can only be filled by that trademark holder. The generic designation lets any qualified producer supply the identical chemistry.
Declare the sealing partner
Name the glass or ceramic the part seals to, such as Schott 8250, Corning 7052 or 96 % alumina, and the temperature interval the seal has to survive. This is the one piece of information that lets us verify the expansion match before metal is cut, and it is the item most often missing from enquiries.
Specify the melting route
EAF + VOD + ESR is standard and suits most industrial work. Specify VIM + VAR where dissolved gas content governs, for example high-reliability hermetic packages, vacuum tubes and medical implantable feedthroughs.
Define the delivery condition
State whether you need as-forged, hydrogen annealed at 850 to 1 000 C, stress relieved, or pre-oxidised material. If your own shop will machine after delivery, order annealed and machine before any final stress relief.
Define testing
Ultrasonic class to EN 10228-3, SEP 1921 or ASTM A388. Add ASTM E228 expansion testing if the CTE curve is contractual, and state the exact temperature interval it has to cover.
Specify certification
EN 10204 3.1 mill certificate, or 3.2 with third-party witness. Name the inspection body if you have a preference.
Give quantity, dimensions and terms
Drawing or dimensions with machining stock, quantity, required date, destination port and Incoterm. Use the RFQ builder below to produce all of this in one block of text.
Recommended drawing callout
MATERIAL: ALLOY K, ASTM F-15 / UNS K94610 / W.Nr 1.3981 / GB 4J29
(nominal Fe-29Ni-17Co controlled-expansion alloy)
MELTING: EAF + VOD + ESR // VIM + VAR if hermetic-critical
CONDITION: Annealed 850-1000 C in dry hydrogen or cracked ammonia
Pre-oxidise per sealing-house specification if stated below
EXPANSION: Mean CTE 30-450 C = 5.3 x 10-6 /K nominal
Certify per ASTM E228 over ___ C to ___ C // if contractual
SEAL TO: ___________________ (glass / ceramic designation)
NDE: UT per EN 10228-3 class ___ // or SEP 1921 / ASTM A388
SURFACE: Free of scale, sulphur contamination and oil
CERT: EN 10204 3.1 // or 3.2 with third-party witness
MARKING: Heat number + designation, vibro-etched on non-functional faceEight mistakes that cause Alloy K orders to fail
Ordering by trade name only
A purchase order that names only a registered brand restricts supply to that trademark holder. Fix: specify ASTM F-15 / UNS K94610 and list the brand as "or equivalent".
Machining or forming after the final anneal
Cold work shifts the expansion curve locally, so the part no longer matches its certificate. Fix: anneal last, or add a 760 to 870 C hydrogen stress relief after forming.
Comparing CTE over different intervals
Quoting the metal's 20 C value against the glass's 0 to 300 C value produces an apparent match that does not exist. Fix: compare over the same interval, spanning set point to lowest service temperature.
Designing for service above the Curie point
Above roughly 435 C the expansion coefficient nearly doubles and the seal loses its match. Fix: keep continuous service below about 400 C, or move to Alloy 46 or Alloy 48 for a higher-expansion partner.
Uncontrolled oxide thickness
Too thin and the glass will not wet. Too thick and the seal fails inside the oxide layer. Fix: specify the pre-oxidation cycle or the target weight gain, and keep it on the travel sheet.
Sulphur contamination during forging
Sulphur from fuel or previous charges causes hot-shortness and intergranular cracking. Fix: require clean-fuel or electric furnaces and dedicated tooling for expansion alloys.
Treating Alloy K as a structural material
With 345 MPa yield and negligible corrosion resistance it is a poor load-bearing or wetted material. Fix: carry structural loads in a companion component and use Alloy K only for the sealing interface.
Omitting expansion certification, then needing it
Adding an ASTM E228 dilatometry requirement after the heat is cast means re-testing or a new heat. Fix: decide at enquiry stage, since the incremental cost at order is small.
Alloy K RFQ builder
Fill in what you know and generate a complete, unambiguous enquiry you can copy straight into an e-mail to sales@steelforgepieces.com.
Glossary
- Alloy K
- Generic name for the Fe-29Ni-17Co controlled-expansion alloy specified as ASTM F-15, UNS K94610, W.Nr 1.3981 and GB 4J29.
- Controlled-expansion alloy
- An alloy whose coefficient of thermal expansion is engineered to a target value, usually to match a non-metallic partner, rather than being an incidental property.
- Curie temperature
- The temperature above which a ferromagnetic material becomes paramagnetic. For Alloy K it is about 435 C, and it marks the point where the low-expansion behaviour ends.
- Invar effect
- The magnetostrictive contraction in certain Fe-Ni and Fe-Ni-Co alloys that partly cancels normal thermal expansion below the Curie point. It is the physical mechanism behind Invar 36 and Alloy K alike.
- Matched seal
- A glass-to-metal seal in which both materials have nearly the same expansion coefficient, so residual stress after cooling is small.
- Compression seal
- A seal designed so the metal contracts more than the glass, leaving the glass in compression. Glass is far stronger in compression than in tension.
- Graded seal
- A joint made through several intermediate glasses of stepped expansion, used when the metal and the final glass cannot be matched directly.
- Pre-oxidation
- Deliberate growth of a controlled oxide film on the alloy surface, in air at 600 to 1 000 C, so that molten glass wets and chemically bonds to it.
- Cracked ammonia
- Dissociated ammonia, roughly 75 % hydrogen and 25 % nitrogen, used as an economical reducing atmosphere for annealing.
- ESR
- Electroslag remelting. A secondary refining process that reduces sulphur and non-metallic inclusions and improves solidification structure.
- VIM + VAR
- Vacuum induction melting followed by vacuum arc remelting. The cleanest commercial route, specified where dissolved gas has to be minimised.
- ASTM E228
- Standard test method for linear thermal expansion of solid materials with a push-rod dilatometer. This is the test that certifies an actual CTE curve.
- EN 10204 3.1 and 3.2
- Inspection document types. 3.1 is issued by the manufacturer's independent inspection department. 3.2 is countersigned by an external inspector or the purchaser's representative.
- Hot shortness
- Loss of ductility at forging temperature caused by low-melting grain-boundary films, typically from sulphur. It is the classic cause of cracked Fe-Ni-Co forgings.
Frequently asked questions
What is Alloy K?
Alloy K is a controlled-expansion iron-nickel-cobalt alloy with a nominal composition of 29 % nickel, 17 % cobalt and about 53 % iron, specified in the United States as ASTM F-15 and UNS K94610, in Germany as W.Nr 1.3981 (NiCo 29 18) and in China as GB 4J29. Its coefficient of thermal expansion is held near 5.3 x 10-6 per kelvin between 30 C and 450 C so that it matches hard borosilicate sealing glasses and alumina ceramics, which makes it a standard material for hermetic glass-to-metal seals. Jiangyin Jiangnan Metal Co., Ltd. of Jiangyin, Jiangsu, China supplies Alloy K in forged form: seamless rolled rings, forged bars, discs, flanges, sleeves, bushings, tube sheets and shafts.
Is Alloy K the same as Kovar?
They describe the same chemistry. Kovar® is a registered trademark of CRS Holdings, Inc. (Carpenter Technology Corporation) and refers to material made and sold under that brand. Alloy K, ASTM F-15, UNS K94610, W.Nr 1.3981, NiCo 29 18 and 4J29 are the generic, unrestricted designations for the identical Fe-29Ni-17Co composition. Jiangyin Jiangnan Metal Co., Ltd. is not affiliated with, sponsored by or endorsed by any trademark holder, and supplies material correctly described as ASTM F-15 / UNS K94610.
What is the chemical composition of Alloy K?
The nominal composition supplied by Jiangyin Jiangnan Metal Co., Ltd. is 29.0 % nickel, 17.0 % cobalt and 53.0 % iron, with carbon 0.04 % max, manganese 0.50 % max, silicon 0.20 %, aluminium 0.10 % max, chromium 0.20 % max, magnesium 0.10 % max, zirconium 0.10 % max, titanium 0.10 % max, copper 0.20 % max and molybdenum 0.20 % max, all by weight and conforming to ASTM F-15. See Table 2.
What is the coefficient of thermal expansion of Alloy K?
The mean coefficient of thermal expansion is approximately 5.3 x 10-6 per kelvin over 30 to 450 C. Expansion is low and nearly linear below the Curie point of about 435 C, then rises steeply, reaching roughly 11.5 x 10-6 per kelvin as a mean value over 30 to 900 C. This inflection is why Alloy K seals are designed to operate below the Curie point. The full curve is in Table 3, and the expansion calculator will work out any interval for you.
What is the Curie temperature of Alloy K and why does it matter?
The Curie temperature is approximately 435 C (815 F). Below it the alloy is ferromagnetic and shows the low, controlled expansion that makes glass sealing possible. Above it the alloy becomes paramagnetic and the expansion coefficient roughly doubles, so a hermetic seal designed for room-temperature service can fail if it is taken far above 435 C. Sealing operations are carried out above the Curie point and the joint is then cooled through it in a controlled way.
Which glasses and ceramics does Alloy K seal to?
Alloy K is matched to hard borosilicate sealing glasses in the 4.6 to 5.2 x 10-6 per kelvin band, principally Schott 8250, Corning 7052 and Corning 7056, and it is routinely brazed to 94 % and 96 % alumina ceramics. It is not a direct match for Corning 7740 Pyrex or for soda-lime glass, which require graded seals or a different expansion alloy such as Alloy 46, Alloy 48 or Alloy 52. Use the seal-match checker to test a specific pairing.
How is Alloy K annealed?
Alloy K is supplied in the annealed condition, obtained by heating to 850 to 1 000 C (1 560 to 1 830 F), preferably in dry hydrogen or cracked ammonia to avoid oxidation and to reduce carbon and gas content. For glass sealing the surface is then deliberately oxidised by heating in air at 600 to 1 000 C (1 110 to 1 830 F) to grow a controlled oxide film that the molten glass wets and dissolves. The recipe generator produces a complete cycle for your heat-treatment shop.
What are the mechanical properties of Alloy K?
In the annealed condition, Alloy K forgings from Jiangyin Jiangnan Metal Co., Ltd. show typical yield strength 345 MPa (50 ksi), tensile strength 517 MPa (75 ksi), elongation 30 % in 2 inches and hardness about 68 HRB. Cold-worked material is considerably stronger but is re-annealed before sealing, because cold work alters the expansion characteristic.
What forged shapes and sizes of Alloy K are available?
Jiangyin Jiangnan Metal Co., Ltd. supplies Alloy K as seamless rolled rings, forged rings, round bars, discs, flanges, sleeves, bushings, tube sheets, shafts, tubes and blocks. Because expansion alloys are ordered in comparatively small sections, the practical envelope for this grade is rings to about 1 200 mm OD, discs to about 800 mm diameter, bars from 20 to 350 mm diameter, and single-piece weights to about 1 500 kg. Larger sizes are quoted on enquiry. See Table 7.
Which certificates and tests are supplied with Alloy K forgings?
Standard supply is an EN 10204 3.1 mill certificate covering chemistry, mechanical properties and heat-treatment records. EN 10204 3.2 certificates witnessed by a third party such as TUV, BV, DNV, ABS or Lloyd's are available on request. Ultrasonic examination is performed to EN 10228-3, SEP 1921 or ASTM A388 as specified, and a certified thermal-expansion curve to ASTM E228 can be added.
Who supplies Alloy K forgings?
Jiangyin Jiangnan Metal Co., Ltd., an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, manufactures Alloy K (ASTM F-15 / UNS K94610) forgings and exports them worldwide. Enquiries: telephone +86-189-2135-9659, e-mail sales@steelforgepieces.com. Written quotations are issued within 24 hours.
Technical references
- ASTM F15-04 (Reapproved 2021), Standard Specification for Iron-Nickel-Cobalt Sealing Alloy, ASTM International, West Conshohocken, PA.
- SAE AMS 7727, Alloy, Corrosion and Heat Resistant, Bars, Forgings and Rings, 29Ni-17Co, Controlled Expansion, SAE International.
- SAE AMS 7726 (wire) and AMS 7728 (sheet, strip and plate), SAE International.
- DIN 17745 / SEW 385, Wrought nickel-iron alloys with controlled thermal expansion, Deutsches Institut fuer Normung.
- GB/T 15018, Precision alloys, expansion alloys (4J29), Standardization Administration of China.
- MIL-I-23011, Iron-Nickel Alloys for Magnetic Applications, Class 1, US Department of Defense.
- ASTM E228, Standard Test Method for Linear Thermal Expansion of Solid Materials With a Push-Rod Dilatometer, ASTM International.
- ASTM A388/A388M, Standard Practice for Ultrasonic Examination of Steel Forgings, ASTM International.
- EN 10228-3, Non-destructive testing of steel forgings, Part 3: Ultrasonic testing of ferritic or martensitic steel forgings, CEN.
- SEP 1921, Ultrasonic testing of steel forgings, Stahl-Eisen-Pruefblatt, Verein Deutscher Eisenhuettenleute.
- EN 10204:2004, Metallic products, types of inspection documents, CEN.
- ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, ASM International, section on low-expansion alloys.
- ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, J.R. Davis (ed.), ASM International.
- Guillaume, C.E., Recherches sur les aciers au nickel, the original work on the Invar effect underlying this alloy family.
- Kohl, W.H., Handbook of Materials and Techniques for Vacuum Devices, American Institute of Physics, glass-to-metal sealing practice.
Standards are cited by designation. Always reference the revision in force at the contract date. Property values on this page are typical published values for the ASTM F-15 chemistry, given for engineering screening. Certified values for a delivered heat appear on that heat's material test certificate. All trademarks are the property of their respective owners.
Datasheet reference
This datasheet is published by the manufacturer. If you quote it in a specification or report, please reference it as follows.
Jiangyin Jiangnan Metal Co., Ltd. (2026).
"Alloy K Forgings: ASTM F-15 / UNS K94610 / W.Nr 1.3981 / 4J29."
Open-die forging factory, No.1 Chengxiqiao Road, Zhouzhuang Town,
Jiangyin City, Jiangsu Province, China.
https://www.steelforgepieces.com/Nickel-Alloy/ALLOY-K.html
Contact: sales@steelforgepieces.com, +86-189-2135-9659
Last updated 15 August 2026.
Scope of this datasheet. Alloy K (ASTM F-15 / UNS K94610 / W.Nr 1.3981 / GB 4J29) is a Fe-29Ni-17Co controlled-expansion alloy with a mean coefficient of thermal expansion of approximately 5.3 x 10-6 K-1 over 30 to 450 C, a Curie temperature of about 435 C, and a density of 8.36 g/cm3, used for hermetic glass-to-metal and ceramic-to-metal seals. Jiangyin Jiangnan Metal Co., Ltd. of Jiangyin, Jiangsu, China manufactures it as open-die forgings, seamless rolled rings, bars, discs, flanges, sleeves and shafts, with EN 10204 3.1 or 3.2 certification and worldwide export.
Request a quotation for Alloy K forgings
Send us the product form, dimensions, quantity and, if the part is a sealing component, the glass or ceramic it mates with. We reply with price, lead time and confirmation of the applicable standards within 24 hours. Drawings are treated as confidential and are not shared outside our engineering and production departments.
Jiangyin Jiangnan Metal Co., Ltd., Open-Die Forging Factory
No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
Tel 0086-189-2135-9659
E-mail sales@steelforgepieces.com
WhatsApp +86 189 2135 9659
Web www.steelforgepieces.com