Controlled-expansion alloy · Open-die forgings
1.3981 Forgings – NiCo29-18 / Kovar Seamless rolled rings, discs, shafts, sleeves, flanges and round bars, forged to DIN 17745 and ASTM F15
W.-Nr. 1.3981 EN NiCo29-18 UNS K94610 ASTM F15 GB 4J29 AMS 7727 (bars and forgings)
In short
1.3981 is the German material number for NiCo29-18, an iron-nickel-cobalt controlled-expansion alloy containing about 29 % nickel and 17 % cobalt, balance iron. Its mean coefficient of thermal expansion stays near 5 × 10⁻⁶/K from room temperature up to roughly 450 °C. That is close enough to borosilicate glass and alumina ceramic to allow hermetic glass-to-metal and ceramic-to-metal seals. The same alloy is sold as Kovar, Nilo K, Pernifer 2918, Rodar and Dilver P1, and is covered by DIN 17745 / SEW 385, ASTM F15 (UNS K94610), AMS 7726, 7727 and 7728, and Chinese standard YB/T 5231 (grade 4J29).
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No. 1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. We forge 1.3981 into seamless rolled rings, discs, shafts, sleeves, bushings, tube sheets, flanges and round bars. Material is melted by EAF+VOD or ESR, supplied annealed, ultrasonically tested to EN 10228-3, SEP 1921 or ASTM A388, and certified to EN 10204 3.1 or 3.2. For a quotation write to sales@steelforgepieces.com or call +86 189 2135 9659.
Last reviewed Jiangyin Jiangnan Metal Co., Ltd.
Key data at a glance
Values below are typical for wrought and forged 1.3981 in the annealed condition. The guaranteed values for any order are the ones printed on the certificate that ships with the material.
- Nominal composition
- 29Ni-17Cobalance iron
- Density
- 8.36g/cm³
- Mean CTE 20–450 °C
- 5.1–5.5× 10⁻⁶/K
- Inflection point
- 450°C, expansion rises sharply above
- Curie temperature
- 435°C, ferromagnetic below
- Melting point
- 1450°C
- Tensile strength
- 530MPa min, annealed
- Seals to
- Borosilicateglass and Al₂O₃ ceramic
1.3981 forged products we manufacture
We produce every item below to customer drawing or specification. Open-die forging and ring rolling give a grain flow that follows the contour of the part. For seal housings and vacuum components this matters, because porosity or centreline segregation carried over from cast or welded stock will eventually show up as a leak.
| Product form | Description |
|---|---|
| Seamless rolled rings | Radial-axial rolled, supplied black or rough machined |
| Forged discs and blanks | Upset forged from billet, grain flow running radially |
| Round bars | Forged then peeled, turned or centreless ground |
| Shafts and spindles | Stepped shafts forged near net shape to cut machining time |
| Sleeves, bushings, hollow bars | Trepanned or mandrel forged. See our forged bushings |
| Flanges and tube sheets | Weld neck, blind, slip-on, or to drawing |
| Forged tubes and pipe sections | Bored from solid forged blanks |
| Valve bodies and seat rings | To drawing. See our forged valve seat rings |
Dimensions, machining allowance and tolerance follow the customer drawing. Send the drawing or the required size range with your enquiry and we will confirm what we can forge in one piece.
Chemical composition of 1.3981
1.3981 is a low-carbon Fe-Ni-Co alloy. The nickel and cobalt bands are narrow because the property under control is the expansion curve, not strength. Chromium, copper and molybdenum are held down to residual levels; above those levels they shift the expansion behaviour and interfere with the oxide film that carries the glass bond.
| Element | DIN 17745 · 1.3981 (NiCo29-18) | ASTM F15 · UNS K94610 | YB/T 5231 · 4J29 |
|---|---|---|---|
| Nickel (Ni) | 28.0 – 30.0 | 29.0 nominal | 28.5 – 29.5 |
| Cobalt (Co) | 17.0 – 18.0 | 17.0 nominal | 16.8 – 17.8 |
| Iron (Fe) | balance | balance | balance |
| Carbon (C) | 0.05 max | 0.04 max | 0.03 max |
| Silicon (Si) | 0.30 max | 0.20 max | 0.30 max |
| Manganese (Mn) | 0.50 max | 0.50 max | 0.50 max |
| Phosphorus (P) | 0.020 max | – | 0.020 max |
| Sulphur (S) | 0.020 max | – | 0.020 max |
| Chromium (Cr) | 0.20 max | 0.20 max | 0.20 max |
| Copper (Cu) | 0.20 max | 0.20 max | 0.20 max |
| Molybdenum (Mo) | 0.20 max | 0.20 max | 0.20 max |
| Al, Mg, Zr, Ti (each) | – | 0.10 max | – |
Nickel and cobalt can be aimed inside the standard band to hit a particular expansion target. Tell us the mating glass or ceramic on the enquiry and we will specify the melt accordingly. Melting route is EAF+VOD, or ESR remelt where a tighter inclusion rating is required.
Physical properties and the expansion curve
The expansion curve is the reason this alloy exists. It runs close to flat up to roughly 450 °C, then turns upward as the material approaches its Curie temperature and loses the magnetostrictive contraction that had been holding thermal expansion down. Seals should be designed to work below the inflection.
Expansion values in table form
| To 100 °C | 200 °C | 300 °C | 400 °C | 450 °C | 500 °C | 600 °C | 700 °C | 800 °C |
|---|---|---|---|---|---|---|---|---|
| 5.9 | 5.5 | 5.1 | 4.9 | 5.1 | 6.2 | 7.5 | 8.7 | 9.4 |
Other physical properties at 20 °C
| Density | 8.36 g/cm³ (0.302 lb/in³) |
|---|---|
| Melting point | 1450 °C (2640 °F) |
| Curie temperature | 435 °C, ferromagnetic below, paramagnetic above |
| Specific heat capacity | 500 J/(kg·K) |
| Thermal conductivity | 17.5 W/(m·K) |
| Electrical resistivity | 0.49 µΩ·m (49 µΩ·cm) |
| Modulus of elasticity | 138 to 160 GPa, condition dependent |
Mechanical properties of forged 1.3981
1.3981 is not a structural alloy. It is bought for its expansion behaviour, and its room-temperature strength is modest. The figures below are typical for forged material in the annealed condition, tested transverse to the principal grain flow.
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength Rm | 530 MPa min | 77 ksi min |
| Yield strength Rp0.2 | 370 MPa min | 54 ksi min |
| Elongation A₅ | 30 % min | 30 % min |
| Reduction of area | 55 % | 55 % |
| Hardness, annealed | 150 HV | 80 HRB |
1.3981 work hardens readily. Where a cold-worked temper such as quarter hard, half hard or hard is called for, that is normally supplied as strip or bar rather than as a forging. State the required temper and test direction at enquiry stage.
Forging, heat treatment and fabrication
Hot forging
We heat billets to about 1150 to 1200 °C for open-die work and ring rolling, and finish forging above roughly 950 °C. Reheat rather than carry on forging into the lower range, because the alloy loses ductility and can tear. Long soaks above 1200 °C should be avoided; the grain coarsens and both machinability and seal reliability suffer.
Annealing
The standard delivery condition is annealed at 850 to 1000 °C (1560 to 1830 °F), held for 30 minutes per 25 mm of section, in dry or wet hydrogen or cracked ammonia. The temperature itself is not critical, but long holds above 900 °C promote grain growth. Cool in the furnace under protective atmosphere to about 175 °C before exposing the parts to air, otherwise they oxidise and distort.
Oxidising treatment for glass sealing
Before glass sealing, parts are deliberately oxidised in air between 600 and 1000 °C. Film thickness is chosen to suit the sealing process. Too thin and the bond is weak; too thick and the oxide layer becomes the failure plane. The bond itself is chemical, carried by nickel and cobalt oxides dissolving into the glass.
Machining and welding
1.3981 machines like a gummy austenitic. Use sharp carbide tooling, positive rake, rigid setups, plenty of coolant and a steady feed so the surface does not work harden and glaze. The alloy joins readily by TIG, electron beam and resistance welding. Keep carbon, sulphur and phosphorus pickup low during welding and cleaning, since contamination degrades both the expansion match and the oxide that carries the seal.
Where 1.3981 forgings are used
Most applications come back to the same requirement: a metal part that has to expand and contract in step with a glass or ceramic part over a wide temperature range, without breaking the seal.
-
Vacuum and electron tubes
Power tubes, microwave and travelling wave tubes, X-ray tube envelopes and high-power transmitting valves, where a hermetic glass-to-metal seal has to survive repeated thermal cycling.
Rings · sleeves · forged tubes · flanges
-
Semiconductor and microelectronic packaging
Transistor headers and leads, diode bases, integrated circuit lead frames, hybrid circuit housings and lids, and optoelectronic component cases.
Bars · discs · forged blanks
-
Sensors and instrumentation
Pressure and temperature sensor bodies, laser and measurement housings, metrology fixtures, and medical device feedthroughs that need a leak-tight ceramic-to-metal joint.
Bushings · shafts · machined forgings
-
Microwave and RF hardware
Cavity resonators, echo boxes and filters, magnetic shielding cans, and precision condenser blades where dimensional drift with temperature would detune the assembly.
Rolled rings · sleeves · plates
-
Lighting and sealed devices
Electric lamp bulb components, photography flash bulbs, and sealed switchgear where the feedthrough has to stay hermetic for the life of the product.
Pins · rods · small forgings
-
Precision mechanisms
Bimetal thermostat elements, temperature regulators, circuit breaker parts, clock balance wheels and positioning devices that rely on a predictable, repeatable expansion rate.
Discs · rings · bar stock
1.3981 compared with Invar 36, Alloy 42 and Alloy 52
Customers often reach 1.3981 after looking at the other Fe-Ni sealing and low-expansion alloys. They are not interchangeable. The choice depends on what the metal has to seal against, or on how little it is allowed to move.
| Alloy | W.-Nr. / UNS | Nominal | Mean CTE ×10⁻⁶/K | Curie pt. | Chosen for |
|---|---|---|---|---|---|
| 1.3981 / Kovar | 1.3981 · K94610 | 29Ni-17Co-Fe | 5.1 (20–450 °C) | 435 °C | Sealing to hard borosilicate glass and alumina ceramic |
| Invar 36 | 1.3912 · K93600 | 36Ni-Fe | 1.2–1.5 (20–100 °C) | 279 °C | Minimum movement: LNG containment, composite moulds, optical benches |
| Alloy 42 / Invar 42 | 1.3917 · K94100 | 42Ni-Fe | 4.4–5.3 (20–300 °C) | 350 °C | Sealing to soft and lead glasses, semiconductor lead frames |
| Alloy 52 | – · N14052 | 51Ni-Fe | 10.2 (20–400 °C) | 500 °C | Sealing to soft soda-lime type glasses |
Match the metal to the glass rather than the other way round. Give us the glass or ceramic designation on the enquiry and we will confirm the grade before any material is cut.
1.3981 equivalents, standards and trade names
| German material number | 1.3981 (Werkstoffnummer) |
|---|---|
| EN / DIN designation | NiCo29-18, X3NiCo29-18, DIN 17745, SEW 385 |
| American | ASTM F15, UNS K94610, AMS 7726 (wire), AMS 7727 (bars and forgings), AMS 7728 (sheet, strip, plate), MIL-I-23011 Class 1 |
| Chinese | 4J29 (YB/T 5231) |
| French | AFNOR NF A54-301 |
| International | BS ISO 19960 |
| Trade names | Kovar, Nilo K, Pernifer 2918, Rodar, Dilver P1, Nicoseal, Nicosel, Telcoseal, Sealvar, Fenicoloy, Ulbravar, Dilaton 29/18, Alloy K, Alloy 29-17 |
Trade names belong to their respective owners and are listed here for cross-reference only. If your drawing calls out one of these names, see our Kovar forgings page, which covers the same alloy from the trade-name side.
Testing, inspection and certification
1.3981 parts usually end up inside something that has to hold vacuum, so subsurface soundness matters more than tensile numbers. We agree the test scope before production and record it on the certificate.
| Ultrasonic testing | EN 10228-3, SEP 1921 or ASTM A388, acceptance class to customer specification |
|---|---|
| Chemical analysis | Product analysis by spectrometry, reported element by element against the applicable standard |
| Mechanical testing | Tensile and hardness on integral or separately forged test blocks |
| Thermal expansion | Dilatometry over the agreed temperature interval, on request |
| Surface examination | Visual and dimensional, with dye penetrant or magnetic particle on request |
| Grain size and microstructure | ASTM E112, on request |
| Certification | EN 10204 3.1 as standard. EN 10204 3.2 with third-party witness by TUV, Bureau Veritas, SGS or Lloyd's Register on request |
Traceability runs from heat number through forging, heat treatment and NDT, and appears on the certificate.
Request a quotation for 1.3981 forgings
Send a drawing or a description covering grade, product form, dimensions, quantity, delivery condition and the test scope you need. If you are still choosing between grades, tell us what the part has to seal against and the service temperature, and we will confirm whether 1.3981 is the right alloy before quoting.
- Manufacturer
- Jiangyin Jiangnan Metal Co., Ltd.
- Works address
- No. 1 Chengxiqiao Road, Zhouzhuang Town,
Jiangyin City, Jiangsu Province, China - Telephone
- +86 189 2135 9659
- Delivery condition
- Annealed, with EN 10204 3.1 or 3.2 certification
- Please include
- Drawing or dimensions, quantity, standard and test scope
Frequently asked questions about 1.3981
What is 1.3981 material?
1.3981 is the German Werkstoffnummer for NiCo29-18, an iron-nickel-cobalt controlled-expansion alloy containing about 29 % nickel and 17 % cobalt with the balance iron. It is specified in DIN 17745 and SEW 385, and it is used to make hermetic glass-to-metal and ceramic-to-metal seals because its thermal expansion closely matches borosilicate glass and alumina ceramic up to about 450 °C.
Is 1.3981 the same as Kovar?
Yes. Kovar is a registered trade name for the alloy that DIN 17745 designates NiCo29-18 with material number 1.3981, and that ASTM F15 covers as UNS K94610. Nilo K, Pernifer 2918, Rodar, Dilver P1 and the Chinese grade 4J29 are the same material under different names. The composition bands of the individual standards differ slightly, so the applicable standard should still be stated on the order.
What is the thermal expansion coefficient of 1.3981?
The mean coefficient of linear thermal expansion measured from 20 °C is about 5.5 × 10⁻⁶/K to 200 °C, 5.1 to 300 °C, 4.9 to 400 °C and 5.1 to 450 °C. Above the inflection point near 450 °C the expansion rate climbs steeply and reaches roughly 9.4 × 10⁻⁶/K at 800 °C. Seals should be designed to operate below the inflection.
Is 1.3981 magnetic?
Yes. 1.3981 is ferromagnetic at room temperature and stays ferromagnetic up to its Curie temperature of about 435 °C. The magnetostrictive contraction that goes with the ferromagnetic state is what holds the expansion curve flat, which is also why the curve turns upward close to the Curie point.
Can 1.3981 be forged, and what forged shapes are available?
Yes. 1.3981 is hot forged from about 1150 to 1200 °C and finished above roughly 950 °C. Jiangyin Jiangnan Metal Co., Ltd. produces 1.3981 as seamless rolled rings, forged discs and blanks, round bars, shafts, sleeves, bushings, hollow bars, flanges, tube sheets, forged tubes and valve components, all to customer drawing or specification.
What heat treatment is applied to 1.3981 forgings?
Forgings are supplied annealed between 850 and 1000 °C in dry or wet hydrogen or cracked ammonia, then furnace cooled under protective atmosphere to about 175 °C before air exposure. Long holds above 900 °C are avoided to limit grain growth. Where the part will be glass sealed, a separate oxidising treatment in air between 600 and 1000 °C builds the oxide film the seal depends on.
What is the difference between 1.3981, Invar 36 and Alloy 42?
All three are iron-nickel alloys but they target different expansion rates. Invar 36 (1.3912, 36 % Ni) has the lowest expansion at around 1.2 to 1.5 × 10⁻⁶/K, and is chosen when a part must barely move at all. Alloy 42 (1.3917, 42 % Ni) sits near 4.4 to 5.3 and seals to softer and lead glasses. 1.3981 adds 17 % cobalt to reach roughly 5.1 with a higher Curie point, which is what lets it seal to hard borosilicate glass and alumina ceramic.
Where can I buy 1.3981 forged rings and bars?
Jiangyin Jiangnan Metal Co., Ltd. manufactures 1.3981 open-die forgings at No. 1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, and exports worldwide. Send enquiries to sales@steelforgepieces.com or call +86 189 2135 9659. Include the drawing or dimensions, quantity, delivery condition and required test scope so the quotation can be issued directly.
What certificates are supplied with 1.3981 forgings?
EN 10204 3.1 inspection certificates are supplied as standard and report the heat number, chemical analysis, mechanical test results and the applicable NDT report. EN 10204 3.2 certificates with third-party witness by TUV, Bureau Veritas, SGS or Lloyd's Register are available on request. Ultrasonic testing is carried out to EN 10228-3, SEP 1921 or ASTM A388 as specified.
What information do you need to quote 1.3981 forgings?
A drawing is the fastest route. Without one, send the product form, the finished or rough dimensions, the quantity, the standard the material has to meet, the delivery condition and the test scope. If the part will be glass or ceramic sealed, tell us which glass or ceramic, since that determines where in the nickel and cobalt band the melt should be aimed.
Standards and reference sources
The data on this page is consolidated from the governing standards and from published producer datasheets. Where sources differ inside the permitted composition band, the wider range is shown.
- DIN 17745 and SEW 385, wrought alloys with defined physical properties: nickel-iron and nickel-cobalt-iron alloys. Material number 1.3981, designation NiCo29-18.
- ASTM F15, standard specification for iron-nickel-cobalt sealing alloy, UNS K94610.
- SAE AMS 7726, AMS 7727 and AMS 7728, covering wire, bars and forgings, and sheet, strip and plate.
- YB/T 5231, Chinese specification for expansion alloy 4J29.
- EN 10204, metallic products, types of inspection documents, clauses 3.1 and 3.2.
- EN 10228-3, SEP 1921 and ASTM A388, ultrasonic examination of forgings.
- Producer datasheets for Alloy K and 1.3981 published by METALCOR GmbH and Deutsche Nickel GmbH.
Values are typical and given for guidance during design and enquiry. They do not replace the material certificate, which states the guaranteed values for the heat actually supplied.