Kovar Material Properties: Composition, Physical & Mechanical Data

If you work in hermetic packaging, glass-to-metal sealing, or high-reliability electronics, you already know the name Kovar. What you may not have on hand is a single reference that pulls together its composition, physical behavior, mechanical values, and practical fabrication guidance all in one place. This article is that reference.

At KELTRYN, we machine Kovar components every day — housings, lids, sealing rings, flanges, and feedthrough structures for customers who cannot afford a seal failure. Over the years, we've learned that engineers don't just need a datasheet; they need to understand why Kovar behaves the way it does, and what that means for their part design, sourcing, and manufacturing decisions. Let's get into it.

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What Is Kovar?

Kovar is a nickel-cobalt-iron controlled-expansion alloy, standardized under ASTM F15 and registered as UNS K94610. It was developed in the 1930s specifically to solve one of the most stubborn problems in electronics manufacturing: how to seal metal components to glass or ceramic without cracking either material during thermal cycling.

The signature characteristic of Kovar is its thermal expansion curve. Over the temperature range commonly used for glass and ceramic sealing processes, Kovar's coefficient of thermal expansion (CTE) closely matches that of borosilicate glass and alumina ceramics. When a metal and a glass expand and contract at nearly the same rate, the joint between them survives temperature swings without developing destructive stresses. That matching is not accidental — it is engineered through precise control of the nickel and cobalt content.

Kovar is sold under a number of trade names around the world. If you source material globally, you will encounter NILO K (Special Metals), Pernifer 2918 (Vacuumschmelze), Rodar, and Dilvar P1. These are not substitutes for Kovar; they are Kovar-equivalent grades, and for most designs they are interchangeable provided the mill cert matches your requirements.

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Kovar Chemical Composition (ASTM F15)

The controlled-expansion behavior of Kovar comes down to chemistry discipline. The nominal composition ranges under ASTM F15 are tight — intentionally so.

| Element | Nominal Composition (wt %) |

|---|---|

| Nickel | 28.5 – 29.5 |

| Cobalt | 16.5 – 17.5 |

| Iron | Balance |

| Manganese | ≤ 0.50 |

| Silicon | ≤ 0.20 |

| Carbon | ≤ 0.04 |

| Aluminum | ≤ 0.10 |

| Zirconium | ≤ 0.10 |

| Titanium | ≤ 0.10 |

Why so tight on nickel and cobalt? Because those two elements control the thermal expansion curve. The Fe-Ni-Co system exhibits a low-expansion "invariant" region around this composition. If nickel drifts high or low by even a fraction of a percent, the CTE shifts, and your glass seal may no longer be stress-free. Cobalt is particularly important for maintaining the flatness of the expansion curve up to the sealing temperature.

The trace elements matter more than most engineers realize. Manganese and silicon are added as deoxidizers and help control grain size during melting. Carbon must be kept low — elevated carbon can form carbides at grain boundaries, which hurt ductility and can degrade the surface finish after etching. Aluminum, zirconium, and titanium are controlled because they form stable oxides that can interfere with the glass-to-metal bond during sealing. In practice, the quality of the melt and the cleanliness of the alloy are what separate a Kovar lot that seals reliably from one that gives your process team headaches.

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Kovar Physical Properties

These are the numbers engineers reference most often when evaluating a material for a new package design.

Density

• 8.36 g/cm³ (0.302 lb/in³)

Kovar is noticeably denser than aluminum or titanium, and slightly denser than typical stainless steels. For weight-sensitive aerospace or portable device applications, that matters — though for most hermetic packages, the absolute mass of a Kovar housing is small.

Mean Coefficient of Thermal Expansion (CTE)

The CTE of Kovar is not a single number; it is a function of temperature. The values below are mean CTE values measured from 20°C to the indicated temperature:

| Temperature Range | Mean CTE (µm/m·°C) |

|---|---|

| 20 – 100°C | 4.9 – 5.4 |

| 20 – 300°C | 4.6 – 5.2 |

| 20 – 400°C | 5.0 – 5.5 |

| 20 – 450°C | 5.5 – 6.0 |

| 20 – 500°C | 6.5 – 7.0 |

The expansion curve is not perfectly linear. Up to roughly 400°C, the CTE stays low and flat — this is the range that matches borosilicate glass and alumina. Above 400°C, the curve begins to rise more steeply, which is why sealing processes must be carefully controlled and why post-seal cooling rates matter.

Thermal Conductivity and Specific Heat

• Thermal conductivity: ~17 W/m·K at 20°C
• Specific heat capacity: ~460 J/kg·K at 20°C

Kovar is a poor conductor of heat compared to copper or aluminum. From a machining standpoint, this means heat generated at the cutting edge does not dissipate quickly — one reason Kovar machining demands sharp tooling and controlled parameters. From a design standpoint, it means thermal management in high-power packages requires careful thought.

Electrical Resistivity

• ~49 µΩ·cm at 20°C

Resistivity increases with temperature. Kovar is not a conductor in the copper sense; it is used in electronic packages primarily for its sealing and structural properties, not its electrical performance. When current-carrying capability is needed, Kovar leads are often copper-cored or plated with high-conductivity materials.

Magnetic Properties

Kovar is ferromagnetic below its Curie temperature of approximately 435°C. It has high magnetic permeability at low field strengths, which can be either a benefit or a problem depending on the application. For switching devices and magnetic components, the magnetic behavior is sometimes exploited. For sensitive sensor packages — especially those near electron beams or magnetic detectors — the permeability of Kovar can interfere with operation, and alternative materials may be required.

Melting Range

• Approximately 1450°C

Kovar does not have a sharp melting point; it solidifies over a range. The practical implication is that all brazing and sealing operations occur well below the melting range, typically in the 800–1100°C zone depending on the braze alloy.

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Kovar Mechanical Properties

In the annealed condition — which is how you should machine it for most applications — Kovar offers a balance of strength and ductility that makes it forgiving in fabrication yet robust in service.

Typical Annealed Values (Bar / Rod)

| Property | Typical Value |

|---|---|

| Tensile Strength | 520 – 580 MPa (75 – 84 ksi) |

| Yield Strength (0.2% offset) | 340 – 400 MPa (49 – 58 ksi) |

| Elongation | 30 – 35% |

| Hardness | 80 – 95 HRB (~150 – 170 HV) |

| Modulus of Elasticity | ~138 GPa (20.0 Msi) |

| Poisson's Ratio | ~0.30 |

Typical Values (Sheet / Strip)

Sheet and strip in the annealed condition are slightly softer and more ductile than bar stock, with tensile strength around 490–560 MPa and elongation often exceeding 35%. The exact values depend on final temper and thickness.

Effects of Cold Work and Heat Treatment

Kovar responds to cold work much like other metals. Rolling, drawing, or forming increases strength and hardness while reducing ductility. A heavily cold-worked Kovar strip can reach tensile strengths of 700 MPa or more, but at the cost of elongation. Critically for sealing applications, cold work also raises the CTE — the very property you selected Kovar for. If a part is formed cold and then subjected to a glass-sealing cycle, the internal strain energy will be relieved, and the expansion behavior will shift. For critical hermetic parts, always re-anneal after significant cold forming and before any precision machining or sealing operations.

The standard anneal is 850–900°C in dry hydrogen for roughly 15–45 minutes depending on section thickness. Dry hydrogen acts as a reducing atmosphere, preventing oxidation during the anneal and leaving a clean, bright surface that is ready for plating or sealing. If you do not have hydrogen capability, vacuum annealing is an acceptable alternative, though the surface finish will not be as bright.

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Corrosion Resistance and Surface Stability

Let me answer the question we hear constantly: Will Kovar rust?

The honest answer is: yes, under the right conditions, it can.

Kovar has moderate corrosion resistance in clean, ambient indoor atmospheres. In a dry, controlled environment, it is stable for years. But it is not a stainless steel. It contains no chromium, so it does not form the passive, self-healing oxide layer that gives stainless steels their corrosion resistance. In humid atmospheres, in the presence of salts, or in chemically aggressive environments, Kovar will oxidize and can develop surface rust.

What does that mean in practice?

• If you store bare Kovar parts in a humid warehouse, you may see surface discoloration — a light brown or gray film. That is surface oxidation, and it is usually removable with light cleaning or etching.
• If the part is exposed to repeated condensation or salt spray, pitting and red rust can develop. That is a real problem for a sealing surface.
• During glass-to-metal sealing, the Kovar surface is deliberately oxidized — a controlled oxide layer is what actually bonds to the glass. But that oxide is thin, tightly adhered, and part of a controlled process. It is not the same as uncontrolled atmospheric rust.

For critical applications — particularly medical implants, aerospace electronics, or anything with a 20-year service life — Kovar components should be plated or coated. Nickel plating (often sulfamate) and gold plating are the two most common finishes. Nickel provides a diffusion barrier and a base for soldering or brazing. Gold provides corrosion protection and a wire-bondable surface. If you are designing a Kovar package, design the plating finish at the drawing stage, not after the fact.

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Kovar Equivalents and Cross-References

If you have searched for Kovar material in a global supply chain, you may have asked: What is Kovar equivalent to?

The good news is that Kovar is produced and sold under many names, and the equivalents are direct and well-documented.

| Trade Name | Producer / Region |

|---|---|

| NILO K | Special Metals (USA / UK) |

| Pernifer 2918 | Vacuumschmelze (Germany) |

| Rodar | Historically, various US suppliers |

| Dilvar P1 | Various European suppliers |

| K94610 | UNS designation |

The corresponding standards are:

• ASTM F15 — the primary specification for flat products, rod, and wire
• UNS K94610 — unified numbering system designation
• AMS 7726 — aerospace material specification covering bars, forgings, and wire
• DIN 17745 — German standard, where the grade is designated NiCo29/18
• MIL-I-23011 — US military specification for insulated materials and Kovar

One practical caution: equivalent grades are not always identical in their trace-element limits. A mill cert from one producer may show carbon at 0.02%, while another shows 0.03%. For most sealing applications, either is fine. But for high-reliability work where every seal counts, require material certification and review the actual chemistry against your application requirements. Do not assume "equivalent" means "identical."

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Is Kovar a Stainless Steel?

No. This is one of the most common misconceptions in material selection, so let's settle it clearly.

Kovar is a controlled-expansion alloy based on iron, nickel, and cobalt. Stainless steel is a corrosion-resistant alloy based on iron and chromium, with a minimum chromium content of about 10.5%. The two material families were developed for entirely different purposes, and they are not interchangeable.

Here are the key differences:

| Property | Kovar (ASTM F15) | Stainless Steel (e.g., 304) |

|---|---|---|

| Primary Alloying Elements | Ni 29%, Co 17% | Cr 18%, Ni 8% |

| Chromium Content | None | ~18% |

| Corrosion Resistance | Moderate | High (passive film) |

| CTE (20–100°C) | ~5.1 µm/m·°C | ~17 µm/m·°C |

| Magnetic | Ferromagnetic (below 435°C) | Austenitic grades are non-magnetic |

| Primary Purpose | Glass/ceramic sealing | Structural, corrosion-resistant parts |

The magnetic behavior is another point of confusion. Because Kovar is ferromagnetic, it behaves much like a "magnetic stainless steel" to the uninitiated. But a magnet tells you nothing about corrosion resistance. If you need a hermetically sealed package with stainless steel walls and a glass feedthrough, the industry standard solution is a Kovar-to-stainless transition piece — not a single alloy that does both jobs.

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Kovar vs. Alloy 42 — Key Differences

If you design electronic packages, you have likely compared Kovar against Alloy 42 (42% nickel, balance iron, no cobalt). Both are controlled-expansion alloys, but they are not interchangeable, and choosing the wrong one can cause seal failures that are expensive to diagnose.

Composition and CTE

Alloy 42 contains roughly 41–42% nickel, balance iron, with no cobalt. Its mean CTE is approximately 4.0–4.7 µm/m·°C over 30–300°C — lower than Kovar's. That lower expansion is attractive for matching certain glass families, but the expansion curve behaves differently at higher temperatures. Kovar's curve is engineered to stay flat across a broader temperature range, which is why it remains the default choice for alumina ceramic seals and borosilicate glass seals.

Which Should You Choose?

Choose Alloy 42 when:

• Cost is a primary driver (no cobalt means significantly lower raw material cost)
• The sealing temperature is modest and well-controlled
• The glass or ceramic has a lower CTE that matches Alloy 42 more closely
• The application does not demand the highest hermeticity reliability

Choose Kovar when:

• The seal must survive wide thermal cycling
• You are sealing to alumina ceramics or borosilicate glass
• Reliability and hermeticity are non-negotiable (medical, aerospace, defense)
• You need a proven material with decades of process data behind it

The cost difference is real. Cobalt is expensive and volatile in price. But in a medical implant or a satellite transponder, the cost of the raw material is trivial compared to the cost of a field failure.

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Fabrication and Heat Treatment

Kovar is a machinable alloy, but it does not machine like free-cutting steel or aluminum. If you are sending Kovar parts to a general-purpose machine shop, expect problems. If you are working with a shop that understands the material, you will get good parts.

Machinability

Kovar is sometimes described as "free-machining" relative to nickel superalloys, and that is fair. It does not have the gummy, work-hardening behavior of pure nickel. But it does work-harden, and it does not conduct heat well. The practical rules we follow at KELTRYN:

• Use carbide tooling. High-speed steel can work for soft, interrupted cuts, but carbide holds an edge and manages heat better.
• Keep speeds moderate and feeds aggressive enough to cut below the work-hardened layer. If you let the tool rub, the surface hardens and the next pass will struggle.
• Use coolant. Flood coolant, not mist, for any extended machining operation.
• Watch thin-wall sections. Kovar's modulus (~138 GPa) is lower than steel's, so thin-wall housings deflect more. Fixturing and cut strategy matter.
• Control burrs. Because Kovar is ductile, it forms burrs readily. A burr on a sealing surface is a leak path. Deburring is not an afterthought; it is part of the process.

Forming

In the annealed condition, Kovar has excellent ductility. It can be stamped, deep drawn, bent, and roll-formed. The typical elongation of 30%+ in sheet form allows aggressive forming operations without cracking. Just remember the rule from earlier: cold work raises the CTE. If the part is formed significantly, re-anneal before sealing.

Welding and Brazing

Kovar can be welded by most conventional processes. TIG welding is the most common for thicker sections. Electron beam welding is preferred for high-integrity, deep-penetration welds in hermetic packages. Resistance welding works for thin sections and lead attachments.

Brazing is the workhorse joining method for Kovar-to-Kovar and Kovar-to-ceramic assemblies. Common braze alloys include:

• Copper — used in hydrogen furnace brazing
• Gold-copper (AuCu) — for high-reliability, high-temperature service
• Silver and silver-copper eutectic alloys — for lower-temperature sealing

The key precaution: Kovar's oxide layer must be clean and controlled before brazing. Furnace brazing in hydrogen or vacuum is standard practice. Torch brazing is possible for low-volume work, but it requires careful flux management.

Annealing and Heat Treatment

The typical full anneal is 850–900°C in dry hydrogen (or vacuum) for 15–45 minutes, depending on mass. This produces a fully recrystallized, soft microstructure and resets the CTE to its specified values. For cold-worked material, this anneal is mandatory before precision machining.

For machined parts that will not be severely cold worked, a stress-relief anneal at 600–650°C for 30–60 minutes is often sufficient. It relieves machining-induced residual stress without significantly coarsening the grain structure.

Surface Preparation

Surface condition is everything in glass-to-metal sealing. The Kovar surface must be clean, free of oils, and have the correct oxide state before sealing. Typical preparation steps:

1. Degrease in solvent or aqueous alkaline cleaner
2. Etch in a mild acid solution (e.g., HCl) to remove any heat-treat scale or native oxide
3. Controlled pre-oxidation — a deliberate, time-temperature-controlled oxidation step that forms a thin, adherent oxide layer for glass bonding
4. Plating (nickel or gold) if the application calls for it

A part that has been contaminated by chlorinated cutting fluids, or one that has been overheated during machining, may not seal properly even if it meets dimensional tolerances.

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Applications of Kovar Alloy

Kovar's combination of expansion control, mechanical strength, and fabricability has made it the default material for a broad range of high-reliability applications.

Semiconductors and Integrated Circuit Packaging

Kovar lead frames, package bases, and lids are found throughout the semiconductor industry, particularly in hermetic ceramic packages. For decades, the standard IC package — the dual-inline package — relied on Kovar for both leads and sealing rings. Today, Kovar remains essential for high-reliability military, aerospace, and industrial-grade components where a plastic package cannot deliver the required hermeticity.

Power and Microwave Tubes

High-power vacuum tubes, traveling-wave tubes, and microwave tubes operate at extreme temperatures and voltages. Their glass-to-metal seals must hold vacuum integrity for decades. Kovar's expansion match to borosilicate glass makes it the material of choice for tube envelopes, internal electrodes, and feedthroughs.

Medical Devices and Implantables

Pacemakers, neurostimulators, and implantable sensors carry some of the most demanding hermeticity requirements in any industry. A pacemaker housing must keep body fluids out for 10–20 years. Kovar housings — typically gold-plated — are brazed or laser-welded to form a titanium or stainless feedthrough assembly. The medical device industry has validated Kovar over millions of implants, and it remains the standard for many hybrid packages.

Aerospace and Defense Electronics

Missile guidance systems, satellite transponders, and avionics all operate in extreme thermal environments — from non-operational storage at -55°C to operational temperatures near 150°C or higher. Kovar's matched expansion and mechanical stability make it a trusted choice for connectors, feedthroughs, and housings that must survive thousands of thermal cycles.

Photonics and Optoelectronics

Laser diode packages require precise optical alignment and hermetic sealing. Kovar's expansion match to the glass lenses and ceramic submounts used in optical assemblies helps maintain critical alignment over temperature. Fiber-optic component housings and transceiver packages use Kovar lids and bodies for the same reason.

Sensors and Vacuum Equipment

Pressure sensors, vacuum feedthroughs, and cryogenic instrumentation rely on Kovar for its sealed conductivity path and vacuum integrity. The alloy's low outgassing characteristics — a natural result of its dense, stable oxide layer — make it well-suited to ultra-high-vacuum applications.

Emerging Trends

As 5G and 6G RF systems push to higher frequencies and smaller form factors, Kovar packages are being machined to ever-tighter tolerances with thinner walls and more complex geometry. Medical device miniaturization likewise drives demand for smaller Kovar components — a 3 mm housing with a 0.2 mm wall is challenging to machine but routine for a shop that knows the material.

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Kovar Material Specifications and Compliance

If you are sourcing Kovar components, you need to understand the specification landscape. The key documents are:

• ASTM F15 — the primary material specification covering composition, physical properties, and quality requirements
• AMS 7726 — aerospace specification for bars, forgings, stock, and wire
• MIL-I-23011 — military specification covering insulation materials, including Kovar
• DIN 17745 — German standard designating the alloy as NiCo29/18

Kovar is available in a variety of forms, and your manufacturing partner should be able to source any of them:

| Form | Typical Use |

|---|---|

| Bar and rod | Machined housings, feedthroughs, connectors |

| Sheet and strip | Lids, sealing rings, stamped parts |

| Wire | Lead frames, glass-sealed leads |

| Forgings | Large flanges, specialized feedthrough bodies |

When you request a quote for Kovar machined parts, insist on:

• Material certification — a mill cert showing actual chemistry, not just the grade name
• Lot traceability — the ability to trace a finished part back to its heat and mill
• Lot consistency — if you order 1,000 parts, you do not want the first 500 from one heat and the next 500 from a different heat with a meaningfully different CTE

At KELTRYN, we treat material traceability as part of the machining contract. If we cannot provide a cert for the exact material we machined, we do not ship it as certified material.

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Frequently Asked Questions About Kovar Material Properties

What is Kovar equivalent to?

Kovar (ASTM F15 / UNS K94610) is equivalent to NILO K (Special Metals), Pernifer 2918 (Vacuumschmelze), Rodar, and Dilvar P1. These are all commercial grades of the same Fe-Ni-Co controlled-expansion alloy. Always compare mill cert chemistry rather than relying on trade name alone.

Is Kovar a stainless steel?

No. Kovar is a nickel-cobalt-iron controlled-expansion alloy. It contains no chromium and is not designed for corrosion resistance. Stainless steel, by definition, contains at least 10.5% chromium and forms a passive oxide film that resists corrosion.

Will Kovar rust?

Under humid or aggressive conditions, yes. Kovar has moderate corrosion resistance in dry indoor atmospheres, but it will discolor and can form rust when exposed to moisture, salts, or industrial pollutants. For long-term reliability, specify nickel or gold plating.

What is the difference between Kovar and Alloy 42?

Kovar contains approximately 29% nickel and 17% cobalt; Alloy 42 contains approximately 42% nickel and no cobalt. Kovar has a flatter expansion curve over a wider temperature range and matches borosilicate glass and alumina ceramics better. Alloy 42 is less expensive but has a lower CTE and is more sensitive to process variation. For critical hermetic seals, Kovar is the safer choice.

Can Kovar be welded or brazed?

Yes. Kovar can be TIG welded, electron-beam welded, resistance welded, and brazed with copper, gold-copper, or silver-based braze alloys. The most important precaution is surface preparation: clean, oxide-controlled surfaces are essential for both welding and brazing success.

What heat treatment is required after machining?

For stress relief, a 600–650°C anneal for 30–60 minutes is typical. For cold-worked material, or when the CTE must be reset to specification values, use a full anneal at 850–900°C in dry hydrogen or vacuum. Always consult with your material supplier or sealing engineer before selecting a heat-treat schedule for a critical application.

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Conclusion

Kovar has been the backbone of hermetic electronic packaging for more than eighty years, and it is not going anywhere. Its controlled thermal expansion, balanced mechanical strength, and proven fabricability make it the default choice for glass-to-metal and ceramic-to-metal sealing in semiconductor, aerospace, medical, and vacuum applications.

If you take one thing away from this article, let it be this: Kovar's properties are only as good as the discipline with which it is processed. Chemistry matters. Heat treatment matters. Machining parameters matter. Surface preparation matters. A Kovar part that is dimensionally perfect but metallurgically compromised will fail at the seal — and you will never see the failure coming until it is in the field.

At KELTRYN, we built our entire machining practice around the realities of Kovar and Fe-Ni-Co alloys. We run carbide tooling with controlled parameters to prevent work hardening. We fixture thin-wall housings so they do not deflect. We deburr sealing surfaces with the same rigor we apply to the final dimension. And we document every step so your quality team has the traceability it needs.

If you are designing a hermetic package, a glass-to-metal feedthrough, or any precision Kovar component — or if you have a drawing that just needs to be machined right — we would welcome the conversation. Send us your print, your model, or your rough sketch. We will give you honest manufacturability feedback, a transparent quote, and the kind of engineering communication that keeps high-reliability projects on schedule.

Contact KELTRYN today to discuss your Kovar machining project.