Alloy
Invar 36 Machining: Parts That Hold Size When the Temperature Moves
Invar 36 (Fe-36Ni, ASTM F1684) grows about one tenth as much as steel when it warms. Machining it well is about the sequence, not the machine. Send your drawing and we match it with shops that cut Invar 36 and can run the stress relief your part needs.
Why Invar 36 and why the sequence matters
Invar 36 is bought for one property: near-zero thermal expansion between roughly minus 100 and plus 100 °C. A 1 m bar of Invar 36 grows about 0.012 mm when it warms by 10 °C. The same bar in aluminum grows 0.23 mm. That is why optical benches, layup tools and metrology frames are made from it.
The property is fragile. Cold work from cutting and residual stress from roughing raise the expansion near the surface and let the part creep for days or weeks after it leaves the machine. A shop that machines Invar 36 like stainless can deliver a part that measures perfectly at the shop and no longer fits in your instrument.
That is why precision Invar 36 parts are machined in stages: rough with an even allowance, stress relieve, finish with light cuts, then stabilize and inspect at 20 °C when the drawing asks for it. We match your RFQ with shops that already work this way.
Invar 36 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 | 36 % Ni, balance Fe. C max 0.10 %, Mn max 0.60 %, Si max 0.35 % (typical F1684 limits) |
| Typical CTE | 1.2 to 1.6 ppm/°C from 20 to 100 °C. About 2.5 ppm/°C mean to 150 °C. Rises steeply toward the Curie temperature |
| Curie temperature | About 230 °C. Above it the alloy expands like a normal nickel steel |
| Density | 8.05 g/cm3 |
| Hardness, annealed | 70 to 80 HRB (about 130 to 160 HB) |
| Machinability | Roughly 40 to 50 % of B1112, similar band to 304 stainless. Gummy, work hardens |
| Magnetic | Ferromagnetic at room temperature |
| Specs and names | ASTM F1684, UNS K93600 (standard), K93601 (free machining), K93603. Trade names include Invar 36, NILO 36, Pernifer 36, 1.3912 in the EN system |
| Stock forms | Round bar, flat bar, plate, sheet, strip and some forgings. Thick plate has the longest lead time |
How shops machine Invar 36
The cutting is ordinary. The order of operations and what happens between them is what protects the property you are paying for.
Rough
Remove most of the stock with sharp positive tools and steady feeds. Leave an even 0.5 to 1 mm on faces that matter so the part can move during stress relief without running out of material.
Stress relieve
A typical cycle holds the part at 315 to 345 °C for one to two hours and cools it slowly. Drawings for optics and space parts often call for a fuller anneal, quench, temper and stabilize sequence. The drawing or spec governs.
Finish
Light passes with sharp tools and flood coolant bring the part to size without cold working the surface again. Fixturing is soft and even so the part is not sprung into shape.
Stabilize and inspect
Parts soak to 20 °C before they are measured. Critical features are checked on a CMM in a temperature controlled room. A stabilization soak at about 95 °C for 24 to 48 h follows for the most demanding parts.
| Operation | Tool | Speed | Feed | Note |
|---|---|---|---|---|
| Milling, carbide | Sharp positive rake, coated | 30 to 60 m/min | 0.05 to 0.12 mm per tooth | Keep the feed up. Rubbing work hardens the surface |
| Turning, carbide | Positive insert, small nose radius for finishing | 40 to 70 m/min | 0.1 to 0.25 mm/rev | Chip breakers matter, chips are long and stringy |
| Drilling | HSS-Co or carbide, split point | 10 to 20 m/min | 0.05 to 0.15 mm/rev | Peck, flood coolant, do not dwell at the bottom |
| Tapping | Spiral flute, coated | 3 to 8 m/min | pitch | Roll form taps work well on small threads. Consider thread inserts |
Where Invar 36 parts are used
Optics and photonics
Lens cells, mirror mounts, laser cavity spacers and optical benches that must keep alignment as the room warms.
Composite tooling
Layup molds and mandrels for carbon fiber parts. Invar 36 grows about as little as the laminate does in the autoclave.
Metrology and semiconductor equipment
Reference frames, stage parts and gauge bodies where a few microns of drift is already a defect.
Space and cryogenics
Instrument benches and fittings that see large temperature swings in orbit or go down to liquid nitrogen temperature.
Typical tolerances on Invar 36
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 |
|---|---|---|
| Bores and mounting faces | ±0.01 to ±0.025 mm | After stress relief, finished in a second setup, measured at 20 °C |
| Flatness on a plate up to 300 mm | 0.01 to 0.02 mm | Needs even stock removal from both faces and a soak before inspection |
| Position of a hole pattern | 0.02 to 0.05 mm | Datum scheme on the drawing decides the fixture |
| Small features, ground or lapped | ±0.005 mm | Possible with extra passes and a stabilization cycle. Costs real time |
| Non-critical outer profile | ±0.1 mm | Loosen it. Every tight number you do not need is money |
What to put on a Invar 36 RFQ
Shops quote what they can read. These six lines get you comparable quotes. The full list is in the RFQ drawing checklist.
Alloy and spec: "Invar 36 per ASTM F1684, annealed" or the UNS number. Say if the free machining grade is acceptable.
Heat treatment: "stress relieve after roughing" or the full cycle from your spec. If you are not sure, write "shop to recommend" and we match shops that can advise.
Working temperature range and which dimensions must hold, so the shop can plan the sequence around them.
Tolerances only on features that matter, with the inspection temperature (20 °C per ISO 1) called out.
Certs you need: mill cert with chemistry is normal. Measured CTE for your heat lot or a heat treatment chart costs extra, so ask up front.
A STEP file plus a PDF drawing. Quantity and a realistic yearly volume.
Invar 36 guides
- Materials
Invar Machining: What Buyers Need to Know Before Sending a Drawing
How Invar machining differs from stainless: why shops rough, stress relieve then finish, which tolerances are realistic and what to put on your Invar 36 RFQ.
- 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.
- Process
Speeds, Feeds and Tool Life for Machining Invar 36
Typical Invar 36 machining parameters for milling, turning, drilling and tapping: tool material, speed, feed, coolant and a worked 12 mm end mill example.
- Process
Invar Machining Tolerances: What a Shop Can Hold and What It Costs
Typical Invar machining tolerances by feature, why 20 °C inspection matters, which GD&T callouts count on Invar and how to turn a temperature band into a CTE.
- Cost
Why Invar 36 Parts Are Costly and How Shops Price Them
Where Invar machining cost comes from: material, slow cutting, stress relief, inspection at 20 °C and certs, with a worked quote breakdown and ways to cut it.
- Process
Heat Treating Invar 36: What Each Step Does and How to Specify It
Invar 36 heat treatment: anneal, quench, stress relief and stabilize, with typical temperatures and times, where it fits in machining and the drawing note.
- 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.
- Materials
Why Invar Work-Hardens and How Shops Control It During Machining
The mechanism behind work hardening in Invar 36, why it shows up as tool wear and poor finish, and the cutting practices shops use to keep it from taking over a job.
Have a Invar 36 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 Invar 36 machining
Something else? Email .
Is Invar 36 hard to machine?
It is not hard in the hardness sense. It is gummy and work hardens like austenitic stainless. Shops use sharp positive tools, steady feeds and flood coolant, and they machine it in stages so the cutting does not raise the expansion of the finished part.
What tolerance can a shop hold on Invar 36?
On a stable, well supported part, ±0.01 to ±0.025 mm is a common range. Tighter is possible on small features with extra passes and inspection at 20 °C, but it costs more and the shop should confirm it on your geometry before you commit.
Does Invar 36 always need stress relief?
Not always. Loose tolerance parts that are not thermally critical can skip it. Optical, metrology and tooling parts almost always need at least one stress relief between roughing and finishing, because that step is what keeps the part stable after it ships.
Which spec should I put on the drawing?
ASTM F1684 is the usual bar and plate spec for Invar 36 in North America, with UNS K93600 for the standard grade and K93601 for the free machining grade. European drawings often use 1.3912. Say which condition you want, normally annealed.
Do you machine Invar 36 parts yourselves?
No. We are a matching service. We read your RFQ and connect you with up to three shops that have cut Invar 36 and can arrange the stress relief your drawing calls for. You place the order with the shop.