Invar and Kovar: machining metal that refuses to move
Most engineering materials expand about 10 to 24 parts per million per degree. Invar 36 expands about 1.2. That near-immunity to thermal movement is the entire reason it exists, and it is why it turns up in metrology frames, optical benches, composite layup tooling, cryogenic tanks and satellite structure.
Kovar solves a related problem: its expansion curve is deliberately matched to borosilicate glass and alumina, so a glass-to-metal seal survives thermal cycling without cracking. It is the standard material for hermetic electronic packages and feedthroughs.
Both are nickel-iron alloys. Both are unpleasant to machine.
What you are working with
| Invar 36 | Kovar | |
|---|---|---|
| Composition | 64 % Fe, 36 % Ni | 54 % Fe, 29 % Ni, 17 % Co |
| CTE, 20–100 °C | ~1.2 ppm/K | ~5.5 ppm/K |
| Useful range | below ~230 °C | matched to glass to ~450 °C |
| Hardness, annealed | ~140 HB | ~150 HB |
| Tensile strength | ~490 MPa | ~520 MPa |
| Why you would use it | dimensional stability | glass and ceramic sealing |
Invar’s low expansion disappears above its Curie temperature, around 230 °C. Heat an Invar part past that in service and it behaves like ordinary nickel-iron.
Why they machine badly
Gummy and ductile. Like Monel, these alloys smear rather than shear. Built-up edge forms fast, finishes tear, and a dull tool makes it worse immediately. The same rules apply as in Monel 400.
Work hardening. Moderate but real. Light passes and dwelling create a hardened skin.
Poor thermal conductivity, around 13 W/m·K for Invar. Heat stays at the edge.
And the irony: the part barely expands, but the cutting zone still gets hot, and the heat gradient through a large part during machining is enough to distort it while it is being cut — even though it will be perfectly stable once uniform.
Parameters
Coated carbide, sharp positive geometry, generous lubricating coolant:
| Operation | Surface speed | Feed |
|---|---|---|
| Turning, roughing | 50–80 m/min | 0.15–0.30 mm/rev |
| Turning, finishing | 70–110 m/min | 0.08–0.18 mm/rev |
| Milling, roughing | 45–70 m/min | 0.08–0.15 mm/tooth |
| Milling, finishing | 60–90 m/min | 0.05–0.10 mm/tooth |
| Drilling | 12–20 m/min | 0.05–0.12 mm/rev |
Keep the feed up. Keep the tool sharp. Change tools on schedule rather than on failure. None of that is new if you have read the other alloy guides here — it is the same underlying behaviour.
The part that matters: dimensional stability
Buying Invar and then machining it carelessly wastes the reason you bought it. Residual stress will move an Invar part just as it moves a steel one — and on a part specified for micron stability, that is the whole ballgame.
The sequence for a stability-critical Invar part:
- Stress relieve the raw material, typically around 830 °C, then slow cool.
- Rough with 1–2 mm stock, symmetrically.
- Stress relieve again, typically 315–370 °C for one to four hours.
- Semi-finish, leaving 0.2–0.5 mm.
- Age or stabilise at around 95 °C for 48 hours for the most demanding metrology work.
- Finish machine, light and balanced.
Steps three and five are what separate an Invar part that holds its dimensions for years from one that relaxes over the first few months. They are also the steps most often cut to save cost, and the ones you cannot add afterwards. The general principle is in distortion and stress relief.
Kovar-specific notes
Kovar parts almost always get plated — typically nickel, sometimes with gold over it — because bare Kovar corrodes and because the seal and braze processes need a wettable surface. Plating thickness matters dimensionally on small hermetic parts, so say whether your tolerances apply before or after plating. This is a surprisingly common source of rejected parts.
Surface condition matters too. A glass-to-metal seal needs a controlled oxide layer; machining marks, embedded contaminants and the wrong surface finish all affect seal integrity. If the surface will be sealed, specify it rather than leaving it as a general callout — surface finish on exotic alloys.
Availability
Neither alloy is a stock item in most forms. Invar is available as plate and bar from specialist distributors; unusual sections are mill orders with long lead times. Kovar is generally available in small bar and sheet, since the electronics industry consumes it in those forms, and rarely in anything large.
Design to available stock sizes if you possibly can — the lead time is the metal.
On the drawing
- Name the specification. Invar 36 is UNS K93600, ASTM F1684. Kovar is UNS K94610, ASTM F15.
- State the required thermal expansion if it is a functional requirement, with the temperature range it applies over. The alloy name alone does not guarantee a tested CTE.
- Specify the stabilisation treatment if dimensional stability over time matters. Do not assume it is included.
- For Kovar, state whether dimensions are before or after plating, and name the plating specification.
Background: Invar.
Send the drawing and the stability requirement — on these alloys the heat treatment sequence is most of what you are buying.