Alloy
Kovar Machining: Parts That Seal to Glass and Ceramic
Kovar is not chosen for the lowest expansion. It is chosen because its expansion matches borosilicate glass and alumina, so a seal survives heating and cooling. We match your drawing with shops that machine Kovar and understand the anneal and cleanliness a seal needs.
Why Kovar and why the sequence matters
A glass-to-metal or ceramic-to-metal seal only survives if the metal and the glass expand together from the sealing temperature down to room temperature. Kovar was designed for that: its expansion tracks borosilicate glass and alumina over the range that matters. Feedthroughs, lids, headers and window frames in electronic packages are made from it for that reason.
Machining Kovar is easier than machining Invar 36 in one sense, it is tougher and less gummy, and harder in another, because the surface condition and cleanliness after machining decide whether the seal works. Oil, embedded grit and cold worked surfaces all cause seal defects.
Most Kovar parts are machined, then annealed in a controlled atmosphere and decarburized, then cleaned and oxidized to the right color before sealing. A shop that has made sealing parts knows that sequence. We match your RFQ with those shops.
Kovar 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 | 29 % Ni, 17 % Co, balance Fe. Low C, Mn and Si (ASTM F15 limits) |
| Typical CTE | 5.0 to 5.5 ppm/°C from 30 to 200 °C, close to borosilicate sealing glasses. Rises above about 400 °C |
| Curie temperature | About 435 °C |
| Density | 8.36 g/cm3 |
| Hardness, annealed | About 80 HRB. Tougher than Invar 36, chips a little better |
| Magnetic | Ferromagnetic |
| Specs and names | ASTM F15, UNS K94610. Trade names include Kovar, NILO K, Pernifer 29/18, Dilver P, 1.3981 |
| Stock forms | Round bar, sheet, strip and tube. Deep drawn and stamped parts are common alongside machined ones |
How shops machine Kovar
The cutting is ordinary. The order of operations and what happens between them is what protects the property you are paying for.
Machine
Sharp tools, moderate speeds, flood coolant. Avoid burnishing the surface. Kovar chips more like a tough steel than like Invar 36.
Anneal and decarburize
Typical practice anneals in wet hydrogen near 900 to 1100 °C to remove carbon from the surface and relieve stress. The drawing or your sealing process spec sets the cycle.
Clean and oxidize
Parts are degreased and then oxidized to a controlled oxide layer that the glass wets. Grey to grey-green oxide is what most sealing processes want.
Seal and inspect
Sealing is your process or your seal vendor. Leak test and visual inspection of the meniscus tell you whether the machining and cleaning were right.
| Operation | Tool | Speed | Feed | Note |
|---|---|---|---|---|
| Milling, carbide | Coated, sharp | 40 to 70 m/min | 0.05 to 0.12 mm per tooth | Tougher than Invar, similar feeds |
| Turning, carbide | Positive insert with chip breaker | 45 to 80 m/min | 0.1 to 0.25 mm/rev | Good chip control on tube and ring parts |
| Drilling | HSS-Co or carbide | 12 to 22 m/min | 0.05 to 0.15 mm/rev | Peck, flood coolant |
| Deburring | Manual or tumble | shop practice | light | No embedded abrasive on sealing surfaces |
Where Kovar parts are used
Electronic packages
Lids, seal rings, headers and bases for hermetic packages that mate to glass or alumina.
Feedthroughs and connectors
Pins, bodies and flanges for vacuum and hermetic feedthroughs sealed with borosilicate glass.
Optical windows and detectors
Frames that hold a glass window or a detector die where the seal must survive thermal cycling.
Vacuum tubes and sensors
Any part that glass is sealed onto and that will be baked or cycled in service.
Typical tolerances on Kovar
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 |
|---|---|---|
| Sealing bores and pin holes | ±0.01 to ±0.025 mm | Position and roundness matter more than size for the seal |
| Flatness on lids and flanges | 0.01 to 0.03 mm | After anneal. Anneal can move thin parts |
| Surface finish on sealing areas | Ra 0.4 to 0.8 µm | Smooth, free of burnish and embedded grit |
| Non-sealing features | ±0.05 to ±0.1 mm | Keep them loose |
What to put on a Kovar RFQ
Shops quote what they can read. These six lines get you comparable quotes. The full list is in the RFQ drawing checklist.
Alloy: "Kovar per ASTM F15, UNS K94610" and whether the part will be glass sealed, ceramic brazed or both.
Condition required at delivery: as machined, annealed, or annealed and oxidized to your sealing spec.
Which surfaces seal and the finish they need. Mark them on the drawing.
Cleanliness and packaging requirements, because contamination shows up as seal failures.
Whether you need a decarburization or anneal certificate with the parts.
STEP file, PDF drawing, quantity and the sealing process the parts go into, so the shop can advise.
Kovar guides
- Materials
Machining Kovar for Glass to Metal Seals: What the Seal Needs
Kovar glass to metal seal machining: why ASTM F15 Kovar matches borosilicate glass, how it machines, the anneal and oxide steps before sealing and the RFQ.
- 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
Machining Alloy 42 (ASTM F30): Thin Parts, Flatness and Alloy Choice
Alloy 42 machining: ASTM F30, typical CTE 4 to 6 ppm/°C, used in lead frames and ceramic packages. Thin part fixturing, stamping vs machining and alloy choice.
- Process
Invar Passivation: What It Does and When It's Worth Specifying
What passivation actually does to a machined Invar or Kovar surface, how it differs from plating, and when it is worth specifying over leaving the part bare.
- Process
Surface Finish and Coating Options for Machined Invar Parts
Typical achievable surface finish on machined Invar 36, why work hardening affects it, and which coatings are used for corrosion protection or emissivity control.
- Choosing a Supplier
Invar Mill Certifications: What a Certified Material Report Should Show
What a standard Invar 36 or Kovar mill cert includes, when to ask for measured CTE data, and how to read a certified material test report before it reaches your shop.
- Materials
Invar is ferromagnetic: what that changes in the shop and in service
Is Invar magnetic? Yes, Invar 36 and Kovar are ferromagnetic below their Curie points. What it means for magnetic chucks, demagnetizing, sensors and the RFQ.
- Materials
Invar 36 by Other Names: Regional Trade Names and Specs
Why the same 36 percent nickel-iron alloy is called Invar 36, Pernifer 36, Nilo 36 or FeNi36 depending on the supplier and region, and how to match specs across them.
Have a Kovar 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 Kovar machining
Something else? Email .
Why not use Invar 36 for a glass seal?
Because the expansion would not match. Borosilicate sealing glasses expand around 5 ppm/°C. Kovar tracks that curve from the sealing temperature down. Invar 36 at about 1.3 ppm/°C would put the glass in tension and crack it.
Is Kovar harder to machine than Invar 36?
Not really. It is tougher and less gummy, so chips break better. The difficulty is downstream: the surface has to be clean, free of cold work and embedded grit, then annealed and oxidized correctly before sealing. Shops that have made sealing parts know this.
Does a Kovar part need heat treatment after machining?
For sealing parts, almost always. A wet hydrogen anneal decarburizes the surface and relieves stress, then a controlled oxidation prepares it for glass. Your sealing process spec defines the cycle. Structural Kovar parts that are not sealed may only need a stress relief.
Can I get Kovar parts already oxidized for sealing?
Some shops can arrange it through the same heat treaters that do the anneal. Say so on the RFQ with the oxide spec you need, and we match shops that can deliver that condition.
Do you machine Kovar parts yourselves?
No. We are a matching service. We connect your RFQ with up to three shops that machine Kovar and understand what a sealing part needs after the cutting is done.
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