Super Invar: The Lower CTE Option, Its Limits and How Shops Machine It
Super Invar is the alloy to specify when Invar 36 is not stable enough over a narrow temperature band near room temperature. With about 32 % nickel and 5 % cobalt, it expands a typical 0.3 to 0.6 ppm/°C between roughly 20 and 90 °C, about a third of Invar 36. The price of that gain is a narrower usable range, a risk of transformation below about -60 °C and a dependence on the stabilization cycle that Invar 36 tolerates being skipped. Super Invar machining is the same shop work as Invar 36; what changes is how carefully the heat treatment is specified and certified.
What Super Invar is
Invar 36 reaches its low expansion because magnetic ordering in the iron nickel lattice cancels most of the normal thermal growth below the Curie temperature. Replacing about 5 % of the nickel with cobalt, and dropping the nickel to about 32 %, tunes that cancellation closer to zero at room temperature. The Wikipedia entry on Invar covers the family and the physics; the practical point is that Super Invar is Invar 36 with the expansion minimum sharpened and moved.
Sharpening the minimum has a cost. The curve is lower at 20 to 90 °C but it climbs faster above that, and the alloy sits closer to the composition where the iron nickel structure becomes unstable in the cold. Both limits are real design constraints, and both are covered below. ASTM F1684 covers iron nickel cobalt low expansion alloys alongside Invar 36, so the same specification family applies; the reference page of Invar specifications lists the grades.
How the two curves compare
The table lists typical mean CTE values by interval for annealed, quenched and stabilized material. Heat lot, condition and the test interval all move the numbers; the mill certificate or a dilatometer report to ASTM E228 governs.
| Temperature interval | Invar 36, typical ppm/°C | Super Invar, typical ppm/°C |
|---|---|---|
| -100 to 20 °C | about 1.0 to 2.0 | low, but transformation risk below about -60 °C |
| 20 to 90 °C | 1.2 to 1.6 | 0.3 to 0.6 |
| 20 to 150 °C | about 1.8 to 2.3 | about 1.0 to 1.5, rising fast |
| 20 to 200 °C | about 2.0 to 2.8 | about 2.5 to 3.5 |
| 20 to 300 °C | about 5 to 6 | about 5 to 7 |
Read the middle row for the headline and the rows around it for the trade. Between 20 and 90 °C Super Invar is clearly better. By 200 °C the two are similar and by 300 °C Super Invar may be slightly worse. A part that lives in a temperature controlled lab benefits; a part that bakes out at 150 °C or sits in an autoclave does not. The shape of the Invar 36 curve, including what happens above the Curie temperature, is covered in how the CTE of Invar 36 changes with temperature.
The cold transformation risk
The trade that catches people out is at the cold end. Below roughly -60 °C, part of the Super Invar structure can transform to a different crystal arrangement. When that happens the expansion in the transformed region jumps toward that of an ordinary steel and the part changes length. Warming the part back to room temperature does not fully undo it. Exposure to deep cold during shipping, in a cold storage test or on a cryogenic instrument can therefore leave a Super Invar part permanently different from the one that was inspected.
Invar 36 does not have this problem down to cryogenic temperatures, which is why cryogenic and vacuum applications almost always use Invar 36 rather than Super Invar. The practical rules:
- If the part will ever see temperatures below about -40 °C, including in transit, choose Invar 36 or confirm with the mill that the specific lot has been tested for cold stability.
- If the part must be Super Invar and will be shipped in winter, specify packing and a temperature indicator so an excursion is known.
- State the full temperature history you expect on the drawing, not just the operating range.
The exact transformation temperature depends on composition within the specification and on the heat treatment, which is another reason to ask for the mill certificate before committing to the alloy.
Where Super Invar is used
Super Invar earns its extra cost where the temperature is controlled and the tolerance is measured in microns or fractions of a wavelength:
- Telescope metering structures and mirror cells, where the spacing between optics must hold through the small swings of a dome or a thermal enclosure. See Invar optical mounts and telescope structures.
- Laser resonator spacers and reference cavities, where the cavity length sets the frequency and the environment is stabilized.
- Lithography and wafer stage components, where nanometer level positioning is done inside a temperature controlled chamber.
- Interferometer bases and metrology frames in laboratories held at 20 ± 1 °C.
- Precision gauge bodies compared against glass ceramic or fused silica references.
In every case the environment is already narrow, so the narrower band of Super Invar costs nothing and its lower CTE buys real stability. In an uncontrolled environment the situation reverses and Invar 36 is the safer choice.
How Super Invar machines and why the heat treatment matters more
On the machine, Super Invar behaves like Invar 36. It is soft in the annealed condition, gummy, work hardening and prone to long chips. Sharp positive rake carbide or HSS, steady feeds, flood coolant and climb milling are the recipe, and the practices in Invar 36 machining carry over. The cobalt makes it marginally tougher, but a shop that cuts Invar 36 will not need different tooling.
The difference is in the sequence around the cutting. The low expansion of Super Invar depends on the disordered state locked in by quenching from the anneal, and its long term stability depends on aging that disorder out under control. A typical full cycle, written here as a typical cycle with the drawing or specification governing:
- Anneal at about 830 °C, long enough for the section to reach temperature.
- Water quench to lock in the low expansion state.
- Stress relieve at about 315 °C for about 1 h, slow cool, after rough machining.
- Finish machine with light passes.
- Stabilize at about 95 °C for 24 to 48 h.
- Inspect after a soak at 20 °C.
On Invar 36 a shop can often skip the anneal and quench and rely on a single stress relief between roughing and finishing. On Super Invar that shortcut throws away most of the advantage: material that has not been quenched and stabilized can show a CTE closer to Invar 36 and can creep for months. The steps and their purposes are described in Invar 36 heat treatment; the same cycle structure applies to Super Invar. Insist on the heat treat certificate with the furnace chart. The drawing should also state whether stress relief is required again after any later operation such as welding, plating or a second machining pass.
Because the stabilization cycle takes days and the anneal and quench add furnace runs, a Super Invar part carries a longer lead time and a higher heat treatment line than the same part in Invar 36. The material itself also costs more per kilogram and is stocked in fewer sizes.
Worked example: a 500 mm spacer over 10 °C
Take a hypothetical 500 mm spacer between two mirrors in a laboratory interferometer. The lab is nominally 20 °C but the enclosure can drift by up to 10 K over a day. The designer wants to know how much the spacer length changes in Invar 36, Super Invar and Zerodur, and whether the difference matters against a budget of 0.005 mm.
Growth = 500 mm × CTE × 10 K.
| Spacer material | Typical CTE, ppm/°C | Growth over 500 mm and +10 K, mm |
|---|---|---|
| Invar 36, low end of range | 1.2 | 0.006 |
| Invar 36, high end of range | 1.6 | 0.008 |
| Super Invar, low end of range | 0.3 | 0.0015 |
| Super Invar, high end of range | 0.6 | 0.003 |
| Zerodur, typical | 0 ± 0.1 | up to 0.0005 |
| 316 stainless, for comparison | 16.5 | 0.083 |
Values are typical and the calculation is illustrative. Invar 36 misses the 0.005 mm budget at both ends of its range. Super Invar meets it with margin at both ends. Zerodur beats both by another factor of three to six, but a 500 mm Zerodur spacer is heavy, brittle, expensive to grind and cannot be threaded, so the designer would need bonded or clamped metal fittings, which bring their own expansion. Comparing the metal and glass ceramic options is covered in Invar vs Zerodur and ULE glass.
The example also shows why the temperature band matters. If the enclosure held 20 ± 2 °C instead, Invar 36 would grow only 0.0012 to 0.0016 mm and would meet the budget, and the extra cost of Super Invar would buy nothing. Tightening the environment is often cheaper than upgrading the alloy.
Choosing between Invar 36 and Super Invar
A short decision list. The full comparison including Kovar is in Invar 36 vs Kovar vs Super Invar.
- Choose Super Invar when the operating band sits between about 10 and 90 °C, the part never sees deep cold, the budget is in microns over hundreds of millimeters and the program will pay for the full anneal, quench and stabilization cycle with certification.
- Choose Invar 36 when the range is wider, when the part may see sub zero temperatures, when lead time matters or when a stress relief between roughing and finishing is all the drawing needs.
- Consider a glass ceramic when even Super Invar is not enough and the part can be a simple shape with bonded or clamped fittings.
If the choice is not obvious, run the growth calculation above with your own length, band and budget. It takes a minute and it usually settles the question.
What to put on the RFQ
A Super Invar RFQ needs everything an Invar 36 RFQ needs plus the heat treatment and the temperature history. Include:
- Alloy and spec. Super Invar, 32 % Ni 5 % Co, to ASTM F1684 or your program specification, with the mill cert and CTE data required.
- Full heat treatment. Anneal, quench, stress relieve after roughing, stabilize after finishing, with the certificate and furnace chart. Or “shop to recommend” with the operating range stated.
- Temperature history. Operating range, storage range and shipping conditions, especially any cold exposure.
- Critical dimensions and inspection. The features that carry the micron budget, the inspection temperature of 20 °C and the soak time.
- Quantity and lead time. With room for the stabilization cycle.
When you send an RFQ with the drawing, we match it with up to three shops that cut Super Invar and Invar 36 and can run and certify the full cycle. Each shop quotes you directly against the same heat treatment note, so the quotes and the parts can be compared.
Frequently asked questions
What is the CTE of Super Invar compared with Invar 36?
Super Invar has a typical mean CTE of 0.3 to 0.6 ppm/°C over roughly 20 to 90 °C, against 1.2 to 1.6 ppm/°C for Invar 36 over 20 to 100 °C. The gain is real but it only holds in that narrow band and only after a proper anneal, quench and stabilization cycle. The mill certificate or a dilatometer test governs the value for your lot.
Why not use Super Invar for everything?
Three reasons. Its low expansion band is narrower and rises sharply above about 100 °C. Below roughly -60 °C part of the structure can transform and the expansion jumps, which does not fully reverse on warming. And its stability depends on a full stabilization cycle that adds furnace time and cost. Invar 36 covers a wider range with less risk.
Is Super Invar harder to machine than Invar 36?
Not noticeably. It is a little tougher because of the cobalt, but it is the same soft, gummy, work hardening nickel iron alloy that cuts like austenitic stainless. Shops use the same sharp positive rake tools, feeds and flood coolant. The extra care goes into the heat treatment sequence and the inspection, not the cutting.
How does Super Invar compare with Zerodur or ULE glass?
Glass ceramics such as Zerodur have a typical CTE near 0 ± 0.1 ppm/°C, lower than Super Invar, but they are brittle, cannot be threaded or welded and are slow to machine. Super Invar is a metal: it takes threads, pins and bolted joints. Many instruments use both, glass ceramic for the optic and Super Invar or Invar 36 for the structure.
Reviewed against
- ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
- Wikipedia, Invar
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.
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