Machining Kovar for Glass to Metal Seals: What the Seal Needs
Kovar glass to metal seal parts are machined much like Invar 36, but the seal only works if the shop and the sealing house respect two extra requirements: a surface free of carbon and cold work, and a controlled oxide for the glass to bond to. Kovar, the iron nickel cobalt alloy covered by ASTM F15 (UNS K94610), expands a typical 5.0 to 5.5 ppm/°C from 30 to 200 °C, which is why it seals to borosilicate glass and alumina and why Invar 36 does not.
Why Kovar matches glass and Invar 36 does not
A glass to metal seal is made hot. The glass flows onto the metal at several hundred degrees, wets the oxide on the surface and then the whole assembly cools to room temperature. From the glass set point down, metal and glass are locked together. If the metal shrinks faster than the glass, the glass ends up in tension and cracks. If the metal shrinks much slower, the glass may be pushed into compression at the interface and the bond can shear. Kovar’s expansion curve tracks borosilicate glass through that whole cooling path.
The table lists typical mean CTE values near room temperature. They vary by heat lot, glass type and interval; the mill cert and the glass supplier’s data govern.
| Material | Typical mean CTE, ppm/°C | Interval |
|---|---|---|
| Kovar, ASTM F15 | 5.0 to 5.5 | 30 to 200 °C |
| Borosilicate sealing glass | about 4.6 to 5.2 | 0 to 300 °C |
| Alumina ceramic, 94 to 96 % | about 6.5 to 7.5 | 25 to 300 °C |
| Alloy 42, ASTM F30 | 4.0 to 6.0 | 30 to 300 °C |
| Invar 36, ASTM F1684 | 1.2 to 1.6 | 20 to 100 °C |
| 304 stainless | 16 to 17 | 20 to 100 °C |
Invar 36 is the wrong alloy for a matched seal because it barely moves while the glass shrinks around it. Kovar sits within a fraction of a ppm/°C of the glass, and stays close to that line up to its Curie temperature of about 435 °C, well above the glass set point. The Wikipedia entry on Kovar describes the alloy’s origin and composition, 29 % nickel and 17 % cobalt with the balance iron. For a side by side view of the three low expansion alloys see Invar 36 vs Kovar vs Super Invar.
Typical Kovar seal parts
Most Kovar parts that reach a machine shop fall into a few families:
- Lids and covers for hermetic packages, flat plates with a step or a seal ring, often later gold or nickel plated.
- Feedthrough pins and headers, turned pins sealed through glass beads into a Kovar or stainless body.
- Seal rings, eyelets and sleeves, short turned rings that a glass preform is fused into.
- Housings and bases for sensors, laser diodes, photodetectors and RF windows, where a window or a feedthrough seals into the machined body.
The common feature is a bore, a pin or a face that becomes the seal interface. That feature carries the tight tolerance and the surface finish requirement; the rest of the part is ordinary machining.
How Kovar machines compared with Invar 36
If a shop cuts Invar 36 well it will cut Kovar. Both alloys are soft in the annealed state, roughly 70 to 85 HRB typical, both are gummy, both work harden when a tool rubs and both produce long stringy chips. Shops use the same recipe: sharp positive rake carbide or HSS, steady feed so the edge always takes a chip, flood coolant and climb milling. The general guidance in Invar 36 machining applies almost unchanged.
The differences are small but worth knowing. The table lists typical shop observations; the shop’s own experience with its tooling governs.
| Property | Invar 36 | Kovar | Effect on machining |
|---|---|---|---|
| Density, g/cm3 | 8.05 | 8.36 | Slightly heavier stock, no practical effect |
| Annealed hardness, HRB | about 70 to 80 | about 75 to 85 | Kovar marginally tougher because of cobalt |
| Machinability, % of B1112 | roughly 40 to 50 | roughly 40 to 50 | Same band, same speeds and tool life |
| Curie temperature | about 230 °C | about 435 °C | No effect on cutting; matters in service |
| Surface after cutting | Cold worked layer raises CTE | Cold worked layer and carbon pickup harm the seal | Kovar needs an anneal before sealing |
| Coolant residue | Cleaned before heat treat | Must be fully cleaned before anneal | Sulfur or chlorine residues can spoil the seal |
The last two rows are the whole story. On an Invar part the shop worries about cold work because it raises the CTE and lets the part drift. On a Kovar seal part the shop worries about it because a strained, contaminated surface will not take a uniform oxide and the glass will not wet it evenly.
What has to happen before sealing
A machined Kovar part is not ready to seal. Three steps sit between the finished part and the glass, and the drawing or the sealing house specification should state each of them.
Decarburizing anneal
Carbon near the surface reacts with the glass during sealing and forms carbon monoxide bubbles at the interface, which are leak paths. The surface also carries machining stress and a cold worked layer. Both are removed by an anneal in wet hydrogen, typically at high temperature in the region of 900 to 1100 °C, that burns carbon out of the surface layer and recrystallizes the metal. The exact cycle is the sealing house’s process and the drawing should reference it rather than invent one.
Cleaning
Cutting fluids, especially those with sulfur or chlorine additives, leave residues that survive a rinse and contaminate the anneal furnace and the oxide. Parts are degreased, often ultrasonically, before the anneal, and the shop should know that a seal part is coming so it can pick a coolant that cleans off. Burrs are removed before the anneal, because a burr at the seal bore leaves a void under the glass. Our guide to Invar drilling and tapping covers burr control in small holes, which is the usual trouble spot on headers.
Controlled oxidation
Glass does not bond to bare metal. It bonds to a thin adherent oxide that dissolves partly into the glass during sealing. After the anneal, parts are oxidized in air or a controlled atmosphere, typically at several hundred degrees for a set time, to grow an oxide of the right thickness and color. Too thin and the glass does not wet; too thick and the oxide flakes off and the seal leaks. The sealing house usually owns this step because it is tied to its glass and its furnace. If a machine shop is asked to deliver parts “ready to seal”, it needs the oxidation specification in writing.
Tolerances and surface finish on seal features
Seal quality depends on the gap between the metal and the glass preform being consistent. Typical tolerances a shop holds on a stable Kovar part are ±0.01 to ±0.025 mm on seal bores and pin diameters, with ±0.005 mm possible on small pins with extra passes and inspection at 20 °C. Surface finish on sealing faces and bores is typically Ra 0.8 µm or better; a rougher surface traps gas and leaves voids under the glass, while a polished surface can reduce oxide adhesion, so the drawing should state the finish rather than asking for “as smooth as possible”.
Three details help:
- Edge breaks at the seal bore. A small, consistent chamfer or radius lets the glass meniscus form without a stress riser. Specify it.
- Concentricity of pins to the bore. Off center pins give uneven glass thickness, which cracks on cooling. A concentricity or position callout to ASME Y14.5 is more useful than tightening the diameters.
- Flatness of lids. Lids that are later seam welded or soldered need flatness on the seal ring, typically 0.02 to 0.05 mm depending on size; state it and state the inspection temperature.
For general tolerance guidance the numbers in Invar machining tolerances carry over to Kovar.
Worked example: a Kovar seal ring versus a stainless one
Take a hypothetical Kovar seal ring, 12 mm bore, into which a borosilicate glass preform with a 10 mm pin at the center is fused. Sealing sets the glass at roughly 500 °C and the assembly cools to 20 °C, a drop of 480 K. The question is how far the metal bore and the glass want to shrink relative to each other, which is what stresses the seal.
Shrinkage of a 12 mm diameter over 480 K = 12 mm × CTE × 480.
| Ring material | Typical CTE, ppm/°C | Bore shrinkage, mm | Mismatch to glass at about 5.0, mm |
|---|---|---|---|
| Kovar | 5.3 | 0.031 | 0.002 |
| Alloy 42 | 5.0 | 0.029 | 0.000 |
| Invar 36 | 1.4 | 0.008 | 0.021 |
| 304 stainless | 16.5 | 0.095 | 0.066 |
Values are typical and the calculation is illustrative; the real seal depends on the glass’s expansion curve, not just its room temperature mean. The Kovar ring and the glass shrink within about 0.002 mm of each other over the whole cooling path. The stainless ring shrinks 0.066 mm more than the glass and would crush it. The Invar 36 ring shrinks 0.021 mm less than the glass, pulling the glass away from the metal and opening the seal. This is why a designer who has standardized on Invar 36 for its stability still specifies Kovar for the sealed feedthrough on the same instrument.
When to specify Kovar, Alloy 42 or Invar 36
A short decision guide, with the alloy comparison in Invar 36 vs Kovar vs Super Invar and Alloy 42 machining for detail:
- Kovar when sealing to borosilicate glass or brazing to alumina, when the part sees several hundred degrees in processing and when the sealing house specifies it.
- Alloy 42 when sealing to softer glasses at lower temperatures, for stamped lead frames and for lower cost matched expansion parts where Kovar’s cobalt is not needed.
- Invar 36 when the requirement is lowest expansion near room temperature and there is no glass seal on the part. Its passivation and corrosion behavior also differs from Kovar’s, which matters for bare parts.
Kovar is magnetic, like Invar 36. Both alloys corrode slowly in humid air, so sealed assemblies are usually plated after sealing. The reference page of Invar specifications lists the standards and UNS numbers for all three alloys.
What to put on the RFQ
A Kovar seal part quote is only accurate if the shop knows where its responsibility ends. Include:
- Alloy and spec. Kovar to ASTM F15, UNS K94610, with the supply condition.
- Seal features. Which bores, pins and faces are seal interfaces, with tolerances, finish, edge breaks and position callouts.
- Delivery condition. Machined and cleaned only; or annealed and cleaned; or annealed, cleaned and oxidized to a referenced specification.- Coolant and cleanliness. State that the parts are for glass sealing so the shop can choose a coolant that cleans off and can plan degreasing and burr removal.
- Inspection. Whether you need a dimensional report on seal features and at what temperature.
- Quantity and repeat volume. Headers and rings are often repeat parts.
When you send an RFQ with the drawing, we match it with up to three shops that cut Kovar and Invar and understand what a seal part needs, and each shop quotes you directly with the same delivery condition in front of it.
Frequently asked questions
Why is Kovar used for glass to metal seals instead of Invar 36?
A seal survives cooling from the glass set point only if the metal and the glass shrink at nearly the same rate. Kovar expands a typical 5.0 to 5.5 ppm/°C, close to borosilicate sealing glasses and to alumina. Invar 36 at 1.2 to 1.6 ppm/°C shrinks far less than the glass and the seal would crack or leak.
Is Kovar harder to machine than Invar 36?
Not in a meaningful way. Both are soft, gummy, work hardening nickel iron alloys that cut like austenitic stainless. Kovar is slightly denser and a little tougher because of its cobalt, but the same tools, speeds and coolant work. The real difference is the surface preparation Kovar needs before it can be sealed.
What heat treatment does a machined Kovar part need before sealing?
Typically a decarburizing anneal in wet hydrogen at high temperature to remove carbon from the surface and relieve machining stress, then cleaning and a controlled oxidation that produces the thin adherent oxide the glass bonds to. The sealing house often does these steps; the RFQ should say who is responsible.
Can a machine shop supply Kovar parts ready to seal?
Some can, through a heat treater or a sealing partner. Others deliver machined and cleaned parts for the sealing house to anneal and oxidize. Both arrangements work if the drawing states the delivery condition. Problems come from parts that arrive with cutting fluid residue, burrs or a cold worked surface nobody annealed.
Reviewed against
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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