Invar is ferromagnetic: what that changes in the shop and in service

By Invar Machining editorial team· Published September 26, 2026 · How we write these guides

Yes, Invar is magnetic. Invar 36 is ferromagnetic at room temperature and stays so up to its Curie temperature of about 230 °C, above which it becomes paramagnetic and loses its low expansion at the same time. Kovar is also ferromagnetic, with a Curie temperature around 435 °C, and Super Invar and Alloy 42 behave the same way at their own temperatures. The magnetism is not a side effect. It is the mechanism behind the low expansion, and it has practical consequences in the shop, in field sensitive instruments and in vacuum tools.

Why Invar is magnetic, and why that is the point

Invar 36 is a face centred cubic iron nickel alloy. Below its Curie temperature the alloy is ferromagnetic, and the magnetic ordering produces a volume effect, a spontaneous magnetostriction, that shrinks as the material warms. That shrinkage almost exactly cancels the normal thermal expansion of the lattice, which is why the net expansion near room temperature is only 1.2 to 1.6 ppm/°C. The Invar effect, as it is called, earned Charles Édouard Guillaume the 1920 Nobel Prize in Physics.

As the alloy approaches its Curie temperature the magnetic ordering fades, the cancellation fades with it and the expansion climbs toward that of an ordinary iron nickel alloy. That is why the CTE of Invar 36 is quoted for 20 to 100 °C and rises above that, as the CTE versus temperature guide shows. It is also why a non-magnetic Invar cannot exist. Take away the magnetism and you take away the low expansion.

Alloy	Composition, nominal	Curie temperature, typical	Magnetic at 20 °C	CTE near room temperature, typical
Invar 36	36 % Ni, balance Fe	about 230 °C	Yes	1.2 to 1.6 ppm/°C
Super Invar	32 % Ni, 5 % Co, balance Fe	about 230 °C, similar to Invar 36	Yes	0.3 to 0.6 ppm/°C
Kovar	29 % Ni, 17 % Co, balance Fe	about 435 °C	Yes	5.0 to 5.5 ppm/°C
Alloy 42	42 % Ni, balance Fe	roughly 380 °C	Yes	4.0 to 6.0 ppm/°C
316 stainless, annealed	16 to 18 % Cr, 10 to 14 % Ni	not ferromagnetic	No	16 to 17 ppm/°C

Typical handbook values. The mill certificate governs for a heat lot, and cold work can make annealed 316 slightly magnetic. The Invar specifications reference lists the standards behind each alloy.

Magnetic chucks and workholding

Because Invar is ferromagnetic, it can be held on a magnetic chuck, and for grinding flat plates that is the usual method. Three things differ from grinding a carbon steel plate.

Holding force is lower. Invar 36 has a lower saturation magnetization than mild steel, so a chuck holds it with noticeably less force at the same setting. Shops compensate with a fine pole chuck, a backstop against the grinding force and lighter cuts.

The chuck pulls the part flat. A plate with 0.1 mm of bow will be pulled down onto the chuck, ground flat and then spring back to its bow when the chuck is released. The ground face copies the bow. The fix is the same as for any thin plate: shim the low spots so the part is supported without being pulled, then take a light first pass, flip and repeat. Or use a vacuum chuck for the finish pass. The Invar grinding guide covers the sequence.

Chips cling. Fine Invar grinding swarf and milling chips are attracted to the magnetized part, to the chuck and to steel fixtures. On optical and vacuum parts a chip that rides along in a tapped hole is a contamination problem. Cleaning after magnetic work has to be more careful than after work on a non-magnetic alloy, and the fixturing and workholding guide has more on low force alternatives.

Milling on a magnetic chuck is possible for light cuts on plates but is rare in practice, because the holding force is marginal against milling loads and because clamping stress, of any kind, is what the stress relief sequence is trying to avoid.

Demagnetizing after grinding

Any Invar part that has been on a magnetic chuck, lifted with a magnet, held with a magnetic base indicator or ground with a magnetic separator in the coolant loop leaves the shop with residual magnetism. So does a part that has been near a strong permanent magnet on a tool setter. For a bracket that goes into a machine frame that does not matter. For a part that sits near a sensor, an electron beam or a magnetically levitated stage it does.

Demagnetizing is a standard shop step. The part passes slowly through a decaying alternating field, on a plate demagnetizer or a tunnel, and comes out with a residual field measured with a gaussmeter at the surface. Typical residual limits written on drawings are in the range of a few gauss, measured at a stated distance, and some instrument makers ask for lower with a specific procedure. Two practical notes:

• Demagnetize after the last magnetic operation and after any grinding, not before. A part demagnetized and then set on a magnetic chuck for a final check is magnetized again.
• Thick sections are harder to demagnetize than thin ones, and a residual field can remain in the core while the surface reads low. State the measurement method as well as the limit.

If the limit matters to your instrument, put it on the drawing with the gaussmeter distance, and ask for the reading on the inspection report.

Use near sensors and in vacuum systems

Ferromagnetic parts do two things to a magnetic field. They concentrate it, pulling flux lines through themselves, and they respond to it with a force. Both matter in some instruments.

Field sensitive instruments. Magnetometers, Hall sensors, atomic clocks, electron microscopes and electron beam lithography columns all care about the field at a particular point. An Invar part nearby distorts the field even when it carries no residual magnetism, because it is permeable. Whether that matters depends on distance, mass and orientation, and the instrument designer usually knows the answer. Machining does not change permeability, so the shop cannot fix this. The design has to.

Linear motors and magnet arrays. Lithography stages and precision positioners use permanent magnet arrays. An Invar frame near an array feels an attractive force that varies with position, which loads the bearings and adds a cogging term. Designers keep the Invar outside the motor envelope, shield it or substitute a non-magnetic material locally, as the lithography and wafer stage parts guide explains.

Vacuum systems. Magnetism itself is not a vacuum issue. Invar 36 outgasses as little as stainless steel once clean, and it is used in cryostats, vacuum chambers and space instruments. The cryogenic and vacuum applications guide covers cleaning and venting. The magnetic question in vacuum tools is the same one as above: is there a beam, a sensor or a magnet array near the part. In a cryogenic instrument there is one more point. Invar stays ferromagnetic down to cryogenic temperature, and its expansion behaviour below room temperature is not the same as above it, so a cryostat designer checks both.

Magnetic shielding. Invar is not a good shield material. Its permeability is modest next to purpose made high nickel shielding alloys. If a shield is needed, it is a separate part in a separate alloy.

Non-magnetic alternatives and their CTE penalty

There is no widely available non-magnetic metal with expansion near Invar. The candidates all pay in CTE, and the design has to decide whether a ferromagnetic part with a demagnetizing step, or a non-magnetic part with active compensation, costs less overall.

Material	Magnetic	CTE, typical, ppm/°C	Growth of a 200 mm span for 1 °C	Note
Invar 36	Yes	1.2 to 1.6	0.28 µm	Baseline
Super Invar	Yes	0.3 to 0.6	0.09 µm	Narrow temperature band
Titanium, grade 2 or 5	No	8.6	1.7 µm	About 6 times Invar 36
316 stainless, annealed	No, weakly if cold worked	16 to 17	3.3 µm	About 12 times
6061 aluminum	No	23 to 24	4.7 µm	About 17 times
Fused silica	No	0.5	0.1 µm	Brittle, joined and mounted differently
Zerodur	No	about 0 ± 0.1	0.02 µm or less	Glass ceramic, machined by grinding
Alumina or silicon carbide ceramics	No	roughly 4 to 8	0.8 to 1.6 µm	Stiff, ground not milled

Typical values at room temperature, computed at the midpoint of each range. The mill or glass certificate governs.

Titanium is the usual metal substitute where a part must sit in a field: non-magnetic, stiff for its weight and about six times the expansion of Invar 36. For a small bracket that ratio is often acceptable. For a reference span it is not, and the design moves to a glass ceramic, which the Invar versus Zerodur and ULE guide compares, or the Invar part is kept and the field problem is handled by distance and shielding.

Worked example, hypothetical

Take a hypothetical Invar 36 sensor mounting plate, 200 × 120 × 12 mm, that carries a fluxgate magnetometer at one end and mounts to an aluminum frame at the other. The instrument maker needs the plate flat to 0.01 mm and asks that the part not disturb the sensor.

The plate is milled, stress relieved, semi finished and then ground on both faces to reach the flatness. The grinding is done on a magnetic chuck with the plate shimmed at three points, light passes, flipped twice. After grinding the plate reads, say, 40 gauss at the surface. It goes through a plate demagnetizer twice, in two orientations, and reads under 2 gauss at 10 mm, which meets a hypothetical drawing limit of 3 gauss at 10 mm. The reading and the method go on the inspection report.

That handles the residual field. It does not handle permeability. The plate will still bend the Earth’s field a little at the sensor, and the instrument maker will either calibrate that out, move the sensor further along a titanium extension or accept it. The machining shop’s job ends at the residual limit, and the RFQ should say so, because a shop cannot quote a permeability it cannot change.

What to state on the RFQ

Say whether magnetism matters. If it does, give a residual field limit with the gaussmeter distance and ask for the reading on the report. Say whether magnetic chucks are permitted for grinding, or whether vacuum or mechanical holding is required for the finish pass. If the part sits near a beam, a sensor or a magnet array, say so in one line, because a shop that knows will keep the part away from magnetic lifts and separators. State the alloy and spec as usual, Invar 36 to ASTM F1684 or Kovar to ASTM F15, since the two have different Curie points and different expansion, and see the Kovar machining or Invar 36 machining page for the alloy in hand. When you send the RFQ, we match you with up to three shops that machine the alloy and can demagnetize and measure to the limit you set. Terms such as Curie temperature and permeability are defined in the glossary.

Frequently asked questions

Reviewed against

• Wikipedia, Invar
• Wikipedia, Kovar
• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Invar Grinding: When Milling Isn't Precise Enough · Process
• Invar Fixturing and Workholding: Avoiding Distortion During Machining · Process
• Invar and Super Invar parts for wafer stages and reference frames · Materials
• Invar vs Zerodur and ULE Glass: Choosing a Low-Expansion Material · Materials

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