Overview

1.3981 is an iron–nickel–cobalt controlled-expansion alloy (nominally Fe-29Ni-17Co) designed to have substantially the same thermal-expansion characteristics as borosilicate glass, so that a tight, hermetic mechanical joint can be maintained between metal and glass over a wide temperature range. This datasheet presents the material within the European (DIN / EN / Werkstoff-Nummer) standard system.

Most metals cannot be sealed directly to glass because their coefficient of thermal expansion differs from glass; as a joint cools, the mismatch cracks the seal. This alloy has a low expansion coefficient — about 5 ppm/K between 30 and 200 °C, rising toward ~10 ppm/K near 800 °C — that closely matches borosilicate (e.g. Pyrex) glasses and alumina ceramics. Its non-linear expansion curve can be tailored to a given glass so the joint tolerates a wide temperature range. Chemically it bonds to glass through an intermediate nickel-oxide/cobalt-oxide layer; the cobalt makes the oxide easier to melt and dissolve into the molten glass, and a grey to grey-blue surface indicates a good seal.

It is used wherever a reliable hermetic glass-to-metal or ceramic-to-metal seal is required: electronic-package bodies and bases, transistor and integrated-circuit headers and lead frames, vacuum-tube and microwave-device components, submounts in microelectronics, and material transitions in vacuum chambers. The alloy is readily fabricated and welded, and forms its sealing oxide on controlled oxidation prior to glass sealing.

1. Physical Properties

Values per manufacturer / reference data, annealed condition.

2. Chemical Composition (Limiting, wt %)

Composition per DIN 17745 / SEW 385 (W.Nr. 1.3981, NiCo29-18 / ).

Nominal: Fe-29Ni-17Co. A controlled-expansion Fe-Ni-Co alloy ( / FeNi29Co18) whose thermal expansion matches borosilicate glass and alumina ceramics over 80-450 °C — used for glass-to-metal hermetic seals in electronic packaging.

3. Mechanical Properties

Annealed condition (strip), per DIN 17745 for W.Nr. 1.3981.

Values per EN/DIN; confirm against the inspection certificate (EN 10204).

Controlled thermal expansion ~5.1–5.3 ×10⁻⁶/°C (25–450 °C); Curie temperature ~435 °C; density 8.36 g/cm³.

4. Thermal-Expansion and Sealing Behaviour

5. Heat Treatment

A controlled-expansion Fe-Ni-Co alloy; not age-hardenable. Heat treatment is for annealing / stress relief and for oxide conditioning prior to glass sealing.

Anneal / stress relief Bright (hydrogen) anneal or vacuum anneal per manufacturer guidance to stabilise dimensions and expansion behaviour and remove cold-work stresses.

Oxidation before sealing A controlled oxidation is applied to develop the correct NiO/CoO oxide layer for glass bonding; oxide thickness and character strongly affect seal strength. A grey to grey-blue sealed surface indicates a sound seal.

6. Weldability and Joining

Readily welded (resistance, laser and arc methods), brazed and soldered, and routinely sealed to glass and ceramic. For solder and corrosion service it is usually plated (e.g. nickel and/or gold).

Cleanliness and correct oxide conditioning are critical to hermetic-seal quality.

7. Machinability and Fabrication

Machining / Fabrication Guidelines

Forming Processes

8. Applications

9. Available Product Forms and Standards (EN / DIN System)

Iron-nickel-cobalt controlled-expansion (glass-sealing) alloy. W.Nr. 1.3981 (NiCo29-18); VDM trade name Pernifer 2918.

10. Comparison with Related Alloys (Werkstoff-Nummer System)