Alloy
Super Invar Machining: When Invar 36 Is Still Too Much Movement
Super Invar swaps some nickel for cobalt and drops the expansion to about 0.5 ppm/°C, but only over a narrow temperature band and only if the heat treatment is right. We match your drawing with shops that have cut it and know the stabilization cycle it needs.
Why Super Invar and why the sequence matters
Super Invar is the answer when a calculation with Invar 36 still shows more growth than the tolerance allows. A 500 mm bar over a 10 °C swing grows about 0.0065 mm in Invar 36 and about 0.0025 mm in Super Invar. Telescope structures, laser resonators and lithography reference parts are where that difference matters.
The trade-off is range and stability. The near-zero expansion holds only from roughly room temperature to about 90 °C. Cool it well below zero and it can transform and never come back. Skip the stabilization cycle and it can drift over months. It is a more demanding material to specify than to cut.
On the machine it behaves like Invar 36: sharp tools, steady feeds, flood coolant and a staged sequence. What changes is the heat treatment, which the drawing must define, and the inspection, which is nearly always at 20 °C after a soak.
Super Invar at a glance
Typical handbook values for planning. The mill certificate for your heat lot is the number to design to. Full cross-reference on the Invar specs reference.
| Property | Typical value |
|---|---|
| Composition | About 32 % Ni, 5 % Co, balance Fe. Low carbon |
| Typical CTE | 0.3 to 0.6 ppm/°C from roughly 20 to 90 °C. Rises faster than Invar 36 above 100 °C |
| Low temperature limit | Below about minus 60 °C the alloy can transform to martensite and lose its low expansion permanently. Not for cryogenic use |
| Density | About 8.1 g/cm3 |
| Hardness and machinability | Close to Invar 36. Gummy, work hardens, same tooling approach |
| Magnetic | Ferromagnetic |
| Specs and names | Covered by ASTM F1684 as the iron-nickel-cobalt grade. Trade names include Super Invar 32-5 |
| Stock forms | Round bar, flat bar and plate from fewer mills than Invar 36. Expect longer lead times and minimum quantities |
How shops machine Super Invar
The cutting is ordinary. The order of operations and what happens between them is what protects the property you are paying for.
Rough
Same approach as Invar 36. Even allowance, no rubbing, no heavy clamping that springs the part.
Anneal and quench
A typical sequence anneals near 830 °C and water quenches, which sets the low expansion. This is usually done on the rough blank, not the finished part.
Temper and finish
A temper near 315 °C relieves quench stress. Finishing follows with light cuts.
Stabilize and inspect
A long soak near 95 °C, often 24 to 48 h, settles the structure. Measure at 20 °C afterward. This step is what makes Super Invar stay Super Invar.
| Operation | Tool | Speed | Feed | Note |
|---|---|---|---|---|
| Milling, carbide | Sharp positive rake, coated | 30 to 55 m/min | 0.05 to 0.10 mm per tooth | Slightly more conservative than Invar 36 |
| Turning, carbide | Positive insert | 35 to 65 m/min | 0.1 to 0.2 mm/rev | Chip control as for Invar 36 |
| Grinding and lapping | Friable wheel, light passes | shop practice | light | Common for the final flat faces on optical parts |
| Drilling and tapping | HSS-Co or carbide | 10 to 18 m/min | 0.05 to 0.12 mm/rev | Peck, flood coolant, thread inserts where threads are loaded |
Where Super Invar parts are used
Telescope and laser structures
Mirror cells, spacer tubes and resonator frames where a micron of length change detunes the instrument.
Lithography and metrology
Reference frames and stage parts held in a temperature controlled enclosure, exactly the band where Super Invar works.
Space optics
Benches and mounts, as long as the operating range stays above the transformation limit.
Not for
Cryogenic fixtures, autoclave tooling above 100 °C or anything that sees a wide swing. Use Invar 36 there.
Typical tolerances on Super Invar
What a stable, well fixtured part can hold after the right sequence. Tighter is possible on small features at real cost. See the tolerance guide.
| Feature | Typical | Note |
|---|---|---|
| Optical mounting faces | ±0.005 to ±0.01 mm | Ground or lapped after the stabilization cycle |
| Bores and locating features | ±0.01 to ±0.025 mm | Finished after temper, measured at 20 °C |
| Flatness on faces up to 300 mm | 0.005 to 0.015 mm | Requires the full cycle and even stock removal |
| Non-critical features | ±0.1 mm | Keep them loose |
What to put on a Super Invar RFQ
Shops quote what they can read. These six lines get you comparable quotes. The full list is in the RFQ drawing checklist.
Alloy: "Super Invar, Fe-32Ni-5Co per ASTM F1684" and the condition.
The full operating temperature range, including storage and shipping. If it goes below about minus 60 °C, say so, because Super Invar is probably the wrong choice.
Heat treatment cycle from your spec, or "shop to recommend, stabilized condition required".
Which dimensions must stay within what over that range, so the shop plans the sequence and the inspection around them.
Whether you need measured CTE data for the heat lot. Fewer mills make Super Invar, so allow lead time for material.
STEP file and PDF drawing with datums and the inspection temperature.
Super Invar guides
- Materials
Super Invar: The Lower CTE Option, Its Limits and How Shops Machine It
Super Invar machining: typical CTE 0.3 to 0.6 ppm/°C over a narrow band, the transformation risk below about -60 °C, the stabilization cycle and an example.
- Materials
Invar 36 vs Kovar vs Super Invar: Which Alloy for Your Part
Invar 36 vs Kovar vs Super Invar: typical CTE bands, Curie point, specs and machining notes, with a decision table for mounts, glass seals and space parts.
- Materials
Invar for Optical Benches and Telescope Structures: Why CTE Stability Matters
Why optical and telescope designers specify Invar 36 or Super Invar for mounts and structural benches, and what that means for how the parts are machined.
- Materials
Invar and Super Invar parts for wafer stages and reference frames
Invar semiconductor lithography parts explained: stage bodies, metrology frames, why sub-micron drift matters, cleanliness, magnetics near motors and RFQ notes.
- Materials
Long-Term Dimensional Stability: Does Invar Age After Machining?
Whether machined Invar parts keep moving over months or years after stress relief, and what shops do to minimize long-term dimensional drift beyond the initial process.
- Process
Stress Relief for Invar Machined Parts: When and How Shops Do It
Why Invar parts move after roughing, typical stress relief and stabilization cycles, where they sit in the machining sequence and how to write the drawing note.
- Materials
Invar vs Zerodur and ULE Glass: Choosing a Low-Expansion Material
How machined Invar compares with Zerodur and ULE glass ceramics for low-expansion optical structures, and which applications favor a metal over a glass ceramic.
- Materials
How the CTE of Invar 36 Changes With Temperature and Condition
Invar 36 CTE vs temperature: typical 1.2 to 1.6 ppm/°C from 20 to 100 °C, rising past 150 °C and near plain steel above the Curie point. Tables and an example.
Have a Super Invar drawing ready?
Send the short version now. We will ask for anything missing. For full details up front use the full RFQ form. Reply within 24 business hours.
Your files go only to the shops we match. Ask for an NDA and nothing is shared until it is signed.
Questions about Super Invar machining
Something else? Email .
How is Super Invar different from Invar 36 on the machine?
Very little. Same tooling, feeds and coolant, same staged sequence. The differences are in the heat treatment, which sets and stabilizes its lower expansion, and in the material supply, which comes from fewer mills at longer lead times.
When should I choose Super Invar over Invar 36?
When the growth check with Invar 36 still exceeds your tolerance and the part lives between about 0 and 90 °C. Outside that band Super Invar loses its advantage, and below about minus 60 °C it can transform permanently.
Does Super Invar need a stabilization cycle?
For any part where its low expansion is the reason you chose it, yes. A typical sequence is anneal, water quench, temper near 315 °C and a long soak near 95 °C. Without it the part can drift over months.
Can Super Invar go to cryogenic temperature?
It should not. Below roughly minus 60 °C some of the structure can transform to martensite, which raises the expansion for good. Invar 36 keeps its low expansion down to liquid nitrogen temperature and is the usual choice there.
Do you machine Super Invar parts yourselves?
No. We are a matching service. We connect your RFQ with up to three shops that have machined Super Invar and can run or arrange the stabilization cycle it needs.