Invar 36 and Kovar are iron–nickel based controlled expansion alloys. Neither is chosen for strength or corrosion resistance, but for a single property: how much they expand when heated.
Two different objectives
The composition of Invar 36 is simple: 36.0% nickel, the balance iron. That ratio gives the lowest coefficient of thermal expansion among the known metallic materials. Around room temperature Invar practically does not expand at all — which is where its name comes from (invariable).
Kovar contains ~29% nickel and ~17% cobalt, the balance iron. The purpose of the cobalt addition is not to bring expansion to zero but to place it on a particular curve: the expansion behaviour of Kovar matches that of borosilicate glass and ceramics over a wide temperature range.
The difference becomes clear here. In a glass-to-metal seal, if the metal expands differently from the glass, the glass cracks during cooling. Because Kovar expands and contracts together with the glass, it provides a hermetic seal. Invar cannot do this — it expands so little that it is incompatible with glass.
Comparison in numbers
| PROPERTY | Invar 36 | Kovar |
|---|---|---|
| Nickel (Ni) | 36.0% | ~29% |
| Cobalt (Co) | — | ~17% |
| Iron (Fe) | 64.0% | Balance |
| Design objective | The lowest possible thermal expansion | An expansion curve matched to glass and ceramics |
| UNS | K93600 · K93601 | K94610 |
| W.Nr (DIN/EN) | 1.3912 | 1.3981 |
| ASTM | — | F15 |
Values are taken from our product datasheets; section thickness and the applied heat treatment regime change the result.
Which one, and when?
Invar 36 is required — precision measuring instruments and gauges, optical benches and telescope structures, composite tooling (if the mould does not expand while the part cures, the dimensions hold), LNG tank membranes, seismograph and interferometer components, satellite optical structures. Anywhere dimensional stability is the absolute priority.
Kovar is required — glass-to-metal hermetic seals, hermetic electronic package bodies, microwave and vacuum tube components, power semiconductor packages, sensor and connector bodies. Any metal part to be joined to glass or alumina ceramic.
A practical rule: if it must not expand at all, Invar; if it must expand together with glass, Kovar.
Note
Both alloys are magnetic and their corrosion resistance is low; if they are to be used outdoors, a coating is required. Their machinability is also limited — they require a low cutting speed and sharp tooling.
Technical datasheets
Invar 36 — chemical composition, mechanical values, standard equivalents
Kovar — chemical composition, mechanical values, standard equivalents
Both grades are supplied to order in round bar, flat bar, plate, sheet, pipe and forged forms. Contact us for a quotation.