Invar 36 vs Kovar vs Super Invar: Which Alloy for Your Part

By Invar Machining editorial team· Updated September 26, 2026 · How we write these guides

Invar 36 vs Kovar comes down to one question: do you want the part to expand as little as possible, or to expand at the same rate as a piece of glass? Invar 36 and Super Invar do the first, with typical room temperature CTE of 1.2 to 1.6 and 0.3 to 0.6 ppm/°C. Kovar does the second, at 5.0 to 5.5 ppm/°C, tuned to borosilicate glass and alumina. They get grouped together because all are iron-nickel or iron-nickel-cobalt alloys with unusual thermal behavior, but they are not interchangeable. Picking the right one starts with knowing which job you need done.

Short answer

• Invar 36 for dimensional stability near room temperature: mounts, benches, tooling, metrology frames.
• Kovar for matching glass and alumina ceramic in electronic and vacuum seals, not for minimizing expansion on its own.
• Super Invar when Invar 36 still moves too much over a narrow operating band and the extra cost and lead time are justified.
• Alloy 42 where a Kovar-like expansion is needed without cobalt, mostly lead frames and thin sections.

Full comparison

All values are typical handbook figures near room temperature. A mill cert confirms chemistry, not measured CTE for your specific heat lot, a distinction covered in Invar material certification. The Invar specs reference lists the designations in full.

Property	Invar 36	Super Invar	Kovar	Alloy 42
Nominal composition	36 % Ni, balance Fe	32 % Ni, 5 % Co, balance Fe	29 % Ni, 17 % Co, balance Fe	42 % Ni, balance Fe
Typical CTE, ppm/°C	1.2 to 1.6	0.3 to 0.6	5.0 to 5.5	4.0 to 6.0
Range where that CTE holds	About 20 to 100 °C, rises gently above	About 20 to 90 °C, rises sharply above	About 30 to 200 °C, matched to borosilicate glass	Similar range to Kovar
Density, g/cm3	8.05	About 8.1	8.36	8.12
Curie temperature	About 230 °C	Below Invar 36, mill data governs	About 435 °C	Mill data governs
Typical use	Optical mounts, metrology frames, composite tooling	Space optics, laser structures	Glass-to-metal seals, hermetic packages	Lead frames, bimetals, thin seals
Machining note	Gummy, work hardens, rough then stress relieve then finish	As Invar 36, less stock on the shelf	As Invar 36, anneal and clean before sealing	Often thin, fixturing dominates flatness
Governing spec	ASTM F1684, UNS K93600	Composition, no single common callout	ASTM F15, UNS K94610	ASTM F30, UNS K94100

The Curie temperature matters because the low expansion is tied to the alloy’s magnetic state. As the part approaches the Curie point the CTE climbs toward that of an ordinary steel, which is why Invar 36 keeps its value near room temperature and loses it in a hot process. The full curve is discussed in Invar 36 CTE vs temperature.

Decision table by application

A typical starting point. The operating temperature range and the tolerance decide the final answer, and the shop’s quote governs.

Application	First choice	Why	Watch out for
Optical mount or bench, lab or instrument	Invar 36	Lowest expansion at reasonable cost, widest stock	Skipping stress relief undoes the material choice
Telescope or space optics with a well-defined band	Super Invar	Halves or better the residual growth of Invar 36	Band is narrow; above about 90 °C the gain fades
Glass-to-metal seal, feedthrough, hermetic package	Kovar	Tracks borosilicate glass and alumina through the sealing range	Not a stability alloy, do not use it for mounts
Composite layup tool, autoclave cycled	Invar 36	Holds shape across cure cycles, machinable in large plate	Thick plate lead time, stabilization after roughing
Semiconductor stage or metrology frame	Invar 36	Stable at 20 to 25 °C with tight tolerances	Inspection at 20 °C, cleanroom finishing
Lead frame, thin stamped or etched part	Alloy 42	Kovar-like expansion, no cobalt, thin strip available	Flatness set by fixturing, not by the cutter
Cryogenic structure below about 100 K	Invar 36, check data	CTE falls further with temperature	Verify with measured data, not room values

The four alloys in turn

Invar 36: where it fits and how it machines

Invar 36 is the default choice whenever a part has to keep its size and shape as the room, the autoclave or the orbit changes temperature. Optical mounts, composite layup tooling, metrology frames and instrument benches lean on it because nothing else in this family gets as close to zero movement per degree at a reasonable cost. It machines like a gummy, work-hardening material similar to austenitic stainless, covered in Invar 36 machining parameters, and a precision part built from it goes through the rough, stress relieve, finish sequence described in stress relief for Invar machined parts and on the Invar 36 machining page.

Kovar: why it is not a stability alloy

Kovar exists to solve a different problem: bonding metal to glass or ceramic without the joint cracking as temperature changes. Glass and alumina have their own expansion coefficients, and a metal part sealed to them needs to expand and contract at nearly the same rate through the working range, or the mismatch stresses the seal every cycle. Kovar’s composition is tuned to track borosilicate glass and 94 to 96 % alumina ceramic, which is why it shows up in hermetic electronic packages, feedthroughs and sensor housings rather than in optical benches.

Parts destined for a glass or ceramic seal are typically annealed and carefully cleaned before sealing, since surface oxides and contamination affect wetting and bond quality as much as the alloy’s bulk properties do. This is a different process concern from the stress relief sequence used for Invar 36, and a shop that seals Kovar routinely will have its own cleaning and annealing practice for it, covered in Kovar glass-to-metal seal parts and on the Kovar machining page.

Super Invar: lower expansion, narrower band

Super Invar trades a more complex composition, iron, nickel and cobalt instead of iron and nickel alone, for a lower expansion coefficient than Invar 36 achieves. The tradeoff is that Super Invar holds its lowest expansion over a narrower temperature band, roughly 20 to 90 °C, and its CTE rises sharply above that. It is also less common stock than Invar 36, which affects lead time and cost, covered from the sourcing side in choosing Invar stock.

Super Invar earns its cost on space optics and precision laser structures where the operating temperature range is well characterized and sits inside the alloy’s band, discussed in Invar for optical benches and telescope structures. Outside that band, Invar 36 can perform similarly for less money, so check your actual temperature range before committing. The Super Invar machining page and when Invar 36 is not enough go through that decision.

Alloy 42 in brief

Alloy 42, about 42 % nickel to ASTM F30, sits in a similar expansion range to Kovar without the cobalt, and shows up in lead frames, seals and bimetal parts where Kovar’s glass-matching properties are not the deciding factor. It is usually supplied as thin strip or sheet rather than heavy bar, so most Alloy 42 parts are stamped, etched or wire cut before any milling happens, and in thin sections fixturing, not the cutting process, sets achievable flatness. See the Alloy 42 machining page and Alloy 42 machining for lead frames.

What changes on the machine between the alloys

Cutting

All four alloys are face-centered cubic iron-nickel materials that cut like austenitic stainless: gummy, prone to work hardening under a rubbing edge and hard on tool life if the feed is too light. Typical carbide milling speeds of 30 to 60 m/min, positive rake and flood coolant apply across the family. Kovar and Super Invar, with cobalt in the mix, tend to be slightly tougher on the tool, so shops often start at the lower end of the band.

Heat treatment

This is the real difference. An Invar 36 or Super Invar precision part is roughed, stress relieved, finished and, on the tightest parts, stabilized at low temperature for a day or two. A Kovar part headed for a seal is annealed and cleaned to a surface condition the sealing process needs, and the stress relief that matters for a mount is secondary. Alloy 42 in thin section is usually annealed as strip before it reaches the shop.

Cleaning and handling

All are ferromagnetic at room temperature, so fine chips cling to the part and the fixture. Kovar for sealing needs an oxide-free, grease-free surface, so its final cleaning is a process step. Invar 36 for optics needs particle control more than chemical cleanliness. Say which you need on the RFQ.

Worked example: a hypothetical 200 mm spacer across a 10 °C swing

Take a hypothetical 200 mm long spacer in an instrument that sees a 10 °C swing between calibration and use, with a length tolerance of ±0.005 mm. Growth is length times CTE times temperature change. Values are typical and illustrative.

Alloy	Typical CTE, ppm/°C	Growth over 10 °C, mm	Inside ±0.005 mm?
Super Invar	0.45	0.0009	Yes, with margin
Invar 36	1.4	0.0028	Yes
Alloy 42	5.0	0.010	No
Kovar	5.2	0.010	No
304 stainless	16.5	0.033	No

Invar 36 holds the tolerance with about half of it in reserve, which is usually enough once the part has been stress relieved and measured at 20 °C. Super Invar buys another 0.002 mm of margin, and whether that is worth the longer lead time depends on the rest of the budget: fixturing, measurement uncertainty and the stability of the parts around it. Kovar and Alloy 42 are out for this job, which is the point: they are candidates for the window seal at the other end of the instrument, not for the spacer. The same arithmetic on a full bench is worked in Invar optical bench worked example.

Cost and lead time differences

Material cost roughly tracks nickel and cobalt content and how common the stock size is. Invar 36 is the most widely stocked of the three in common bar and plate sizes. Kovar is a close second given its volume in electronic packaging, and Super Invar is the least common, often with longer lead times for anything beyond standard sizes. Alloy 42 is common as strip and rare as bar. None of these differences is enormous on a small part, but they compound on larger structures or programs that need nonstandard thickness, a point covered in why Invar 36 parts are costly.

How to specify each on the drawing and the RFQ

State the alloy and its governing specification rather than relying on the word “Invar” alone: Invar 36 to ASTM F1684 with UNS K93600, Kovar to ASTM F15 with UNS K94610, Alloy 42 to ASTM F30 with UNS K94100 and Super Invar by nominal composition and the CTE band you need. If you are not certain which alloy your tolerance requires, state the function, the operating temperature range and whether the part seals to glass or ceramic, as covered in what to include on an Invar RFQ. That gets you a useful recommendation faster than guessing at the callout yourself. Send your drawing through the RFQ form, and we match you with shops experienced in the specific alloy your part needs.

See also: low-expansion alloys ranked by CTE, the full ranking with the sealing alloys and temperature limits.

Frequently asked questions

Reviewed against

• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications
• Wikipedia, Kovar
• 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.

Keep reading

• Super Invar: The Lower CTE Option, Its Limits and How Shops Machine It · Materials
• Machining Kovar for Glass to Metal Seals: What the Seal Needs · Materials
• Machining Alloy 42 (ASTM F30): Thin Parts, Flatness and Alloy Choice · Materials
• How the CTE of Invar 36 Changes With Temperature and Condition · Materials

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