Kovar Chemical Composition Explained: The Fe-Ni-Co Alloy Behind Modern Sealing Technology

Introduction: What Is Kovar?

Every engineer who has designed a hermetic package eventually meets Kovar. It is the material that makes glass-to-metal and ceramic-to-metal sealing possible without the cracks, leaks, and CTE mismatches that plague lesser alloys. Kovar is a nickel-cobalt-ferrous alloy — roughly 52% iron, 29% nickel, and 17% cobalt — engineered for one dominant purpose: to expand and contract at nearly the same rate as borosilicate glass and alumina ceramics across a wide service temperature range.

But Kovar is not just "another alloy." Its controlled chemical composition is what separates it from generic Fe-Ni alloys. Small deviations in cobalt or nickel content, or trace impurity levels, can shift the coefficient of thermal expansion (CTE) enough to destroy a glass-to-metal seal during the cool-down cycle. That is why Kovar is governed by strict specifications like ASTM F15 and UNS K94610, and why responsible suppliers treat chemistry control as seriously as dimensional tolerance.

Kovar was developed in the early 20th century specifically to solve a problem that had frustrated vacuum tube manufacturers for decades: finding a metal that could be sealed directly to borosilicate glass without fracturing under thermal cycling. The solution came in the form of a Fe-Ni-Co alloy whose expansion curve could be "tuned" to match common sealing glasses. That tuning is entirely a function of the chemical composition — the ratio of nickel to cobalt, the purity of the iron balance, and the tight control of trace elements.

One quick clarification before we dive deeper: Is Kovar ferrous? Yes. Iron makes up roughly 52% of the alloy by weight, which means Kovar is a ferrous alloy and, consequently, magnetic. If your application requires non-magnetic hardware, Kovar is not the right starting point — but for hermetic packaging, its magnetic behavior is rarely a concern and is often irrelevant inside sealed enclosures.

The Full Kovar Chemical Composition (ASTM F15 / UNS K94610)

Kovar is not a vague "in the range of" material. Its chemistry is tightly specified under ASTM F15 and the Unified Numbering System designation UNS K94610. Every element — from the primary alloying components down to the incidental impurities — is defined with allowable ranges that must be met on a heat-by-heat basis.

Here is the full Kovar chemical composition:

| Element | Symbol | Weight % |

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

| Nickel | Ni | 28.0 – 30.0% |

| Cobalt | Co | 16.0 – 18.0% |

| Carbon | C | 0.02% max |

| Manganese | Mn | 0.30% max |

| Silicon | Si | 0.30% max |

| Phosphorus | P | 0.02% max |

| Sulfur | S | 0.02% max |

| Aluminum | Al | 0.10% max (optional) |

| Magnesium | Mg | 0.10% max (optional) |

| Zirconium | Zr | 0.10% max (optional) |

| Titanium | Ti | 0.10% max (optional) |

| Iron | Fe | Balance (~52%) |

A few notes on interpretation:

• Nickel and cobalt are the real drivers. They are not listed as "nominal" — they are specified as ranges, and the actual measured values must fall inside those ranges on the mill certificate.
• Carbon is kept extremely low (0.02% max). High carbon promotes carbide precipitation at grain boundaries, which hurts formability and can interfere with oxide formation during glass sealing.
• Manganese and silicon are controlled for deoxidation during melting, but excess levels can degrade the surface oxide layer that is essential for a strong glass-to-metal bond.
• Phosphorus and sulfur are tramp elements. They cause hot shortness and brittleness during hot working, so they are held under 0.02%.
• The optional elements (Al, Mg, Zr, Ti) appear when the melt requires additional deoxidation or grain refinement. They are not present in every heat, but when they are, they must stay within 0.10%.

Let me give a direct answer to one of the most common questions we get from engineers every week: How much cobalt is in Kovar? Kovar contains 16% to 18% cobalt by weight. Cobalt is the critical element that suppresses thermal expansion and stabilizes the alloy's body-centered cubic (BCC) structure at seal temperatures. Without that cobalt content, the alloy's expansion curve would rise too steeply and it would not match borosilicate glass.

Kovar Equivalents and Cross-Reference Alloys

Because Kovar is a trade name rather than a generic material designation, manufacturers around the world produce near-identical alloys under their own brand names. This leads to the common question: What is Kovar equivalent to?

The most widely recognized Kovar equivalents are:

| Trade Name | Country / Manufacturer | Base Specification |

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

| Kovar | USA (historically Carpenter, CRS) | ASTM F15 / UNS K94610 |

| Nilo K | UK (Special Metals) | ASTM F15 |

| Dilver P | Germany (Aperam / Imphy) | DIN 17445 |

| Vacon 12 | Germany (Vacuumschmelze) | ASTM F15 |

| Pernifer 2912 | France (Imphy Alloys) | ASTM F15 |

Each of these equivalents maintains essentially identical chemistry and CTE behavior. The French and German alloys are often produced from the same melt sources or under the same technical licensing agreements, so the differences between them are more about documentation, country of origin, and supply chain preference than actual physical properties.

From a practical substitution standpoint, here is our guidance:

• For prototype or low-volume work, equivalents like Nilo K or Vacon 12 are typically interchangeable with Kovar, provided the material cert shows conformance to ASTM F15 chemistry and the correct CTE range.
• For high-reliability aerospace, medical, or defense applications, do not substitute without explicit engineering approval. Your customer drawing may call out "Kovar per ASTM F15" — and while a listed equivalent may satisfy the chemistry, the approved manufacturer list (AML) may lock you to a specific brand.
• For glass-sealing applications, always verify that the equivalent alloy uses the same surface finishing and cleaning processes. Some manufacturers supply Kovar with a special oxidation treatment that affects seal quality.

Applicable specifications you are likely to see on drawings and purchase orders include:

• ASTM F15 — the core chemistry and CTE specification
• MIL-I-23011 — military specification for iron-nickel-cobalt sealing alloys
• SAE AMS 7726 — aerospace material specification for bars, wire, and strip
• NEMA — standards for glass-to-metal seal components
• DIN 17445 — German standard for Fe-Ni-Co sealing alloys

If a supplier cannot provide certification to at least ASTM F15, treat the material with suspicion. We see a growing number of off-spec "Kovar-type" materials in the market, particularly from sources that cannot document their melt chemistry.

How the Chemical Composition Creates Kovar's Signature Properties

The entire identity of Kovar is built on one physical property: a low, controlled coefficient of thermal expansion. Specifically, Kovar exhibits a CTE of approximately 4.5 to 5.5 × 10⁻⁶ /°C between 30°C and 450°C. Borosilicate glasses such as Corning 7052 and 7056 expand in almost exactly the same range. That match is what allows a metal pin, lead frame, or housing wall to be sealed directly to glass without generating stress fractures during cool-down.

How does chemistry control this? It comes down to the nickel-cobalt-iron ratio and the Invar effect. In certain Fe-Ni alloys, thermal expansion nearly vanishes at specific nickel concentrations. Adding cobalt shifts that anomalous expansion minimum to higher temperatures — exactly into the range where glass sealing takes place. Too much cobalt and the expansion curve drops too low; too little, and Kovar loses its match with glass. This is why the 28–30% nickel and 16–18% cobalt ranges are locked so tightly.

Iron, as the balance element, plays a more subtle role:

• It provides the bulk of the alloy's mechanical strength and stiffness.
• It contributes to ferromagnetism, which makes Kovar magnetic (a useful clue for quick material verification in the shop).
• It affects oxidation behavior — iron oxides dominate the scale formed on Kovar surfaces during pre-oxidation for glass sealing.

The trace elements also earn their place in the specification:

• Carbon at ≤0.02% keeps the alloy soft and ductile, improving deep drawing and stamping performance.
• Manganese and silicon act as deoxidizers; the right amounts ensure a clean, sound melt without porosity.
• Phosphorus and sulfur are minimized because they embrittle grain boundaries and degrade weldability.

What this means in practice: when you buy certified ASTM F15 Kovar, you are not just buying "iron with stuff in it." You are buying a precisely balanced metallurgical system designed to behave predictably through melting, hot working, cold forming, machining, heat treating, and final glass sealing.

Physical and Mechanical Properties Derived from the Composition

Because the chemical composition is fixed within tight ranges, the physical and mechanical properties of Kovar are highly consistent and predictable. Here is what you can expect.

Typical Physical Properties

| Property | Value |

|---|---|

| Density | ~8.36 g/cm³ |

| Melting range | ~1450°C |

| Thermal conductivity | ~17 W/(m·K) at 25°C |

| Specific heat | ~0.46 kJ/(kg·K) |

| Electrical resistivity | ~49 µΩ·cm |

| Coefficient of thermal expansion | 4.5 – 5.5 × 10⁻⁶ /°C (30–450°C) |

| Curie temperature | ~430°C |

| Magnetic properties | Ferromagnetic |

Typical Mechanical Properties

Kovar is generally supplied in the annealed condition, but mechanical values vary depending on the temper and the amount of cold work applied:

| Property | Annealed | Cold Worked (typical) |

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

| Tensile strength | ~520 – 620 MPa | Up to ~800 MPa |

| Yield strength | ~275 – 345 MPa | Up to ~700 MPa |

| Elongation | 30 – 42% | 5 – 15% |

| Hardness | 140 – 190 HV (~70 – 85 HRB) | Up to 280 HV |

| Modulus of elasticity | ~138 GPa | ~138 GPa |

A critical point for design engineers: Kovar's mechanical properties change significantly after hydrogen annealing. The bright anneal at ~900°C in a hydrogen atmosphere removes surface oxides, eliminates residual cold work, and softens the material to its most formable state. If your component will be annealed after machining and then sealed to glass, design your tolerances around the annealed condition.

Cold working, by contrast, raises strength and hardness but at the cost of ductility and dimensional stability. For deep-drawn Kovar lids and caps, we recommend working from annealed strip and using the minimum number of forming passes necessary to reach the final shape, with an intermediate anneal whenever the material exceeds 40–50% reduction.

Fabrication, Heat Treatment, and the Role of Composition Control

Kovar is not difficult to fabricate, but it is unforgiving of sloppy process control. At KELTRYN, we treat Kovar machining as a specialty discipline — the alloy's tendency to work-harden, its poor chip-breaking behavior, and the risk of burrs on sealing surfaces all demand attention. Let me walk through the key fabrication processes and how composition analysis drives the parameters.

Annealing and Hydrogen Treatment

The standard Kovar anneal is performed at 900°C in a hydrogen (H₂) atmosphere, typically for 15 to 30 minutes depending on section thickness. The hydrogen reacts with surface oxides to produce a bright, clean finish. This process also reduces carbon content near the surface — a side effect that improves the quality of the subsequent oxide layer for glass sealing.

For parts that must survive repeated sealing cycles, vacuum annealing is sometimes used instead. Vacuum annealing stabilizes the alloy by removing dissolved gases and producing a very consistent, thin oxide structure. We recommend vacuum-annealed Kovar for applications where multiple glass-sealing passes are expected, such as multi-pin headers and complex feedthroughs.

Machining and Forming

If you have machined 304 stainless steel, you have a rough idea of what Kovar is like. But there are important differences:

• Use carbide tooling. Kovar's hardness in the annealed state is modest, but work-hardening raises it quickly. Carbide inserts with positive rake angles help control the cut and avoid smearing.
• Keep tooling sharp. Dull tools generate heat, and heat accelerates work hardening. We run coolant continuously and keep cutting speeds moderate — around 60–80 m/min for turning operations.
• Control the burrs. Kovar tends to form stringy, tenacious burrs. Because sealing surfaces demand clean edges, deburring must be planned as a separate operation. At KELTRYN, we use a combination of automated deburring, manual inspection, and edge break specifications on the drawing.
• Watch thin-wall deflection. Kovar's modulus is about 138 GPa — stiff enough for most parts, but thin-wall housings and lids are still prone to chatter and springing. We support thin walls using custom fixtures and take finish passes with light depths of cut.

For forming, deep drawing, and spinning, the annealed alloy is remarkably ductile. Strip material with a grain size between 6 and 8 ASTM units gives the best deep-drawing performance. If you see orange-peeling or cracking on a formed part, suspect either excessive cold reduction or a grain size outside that window — both trace back to chemistry and heat-treatment history.

Welding and Brazing

Kovar welds well if you respect its quirks.

• TIG welding is the most common method. Use low current, small diameter filler (or autogenous welds for thin sections), and adequate shielding gas on both the face and root of the weld. The goal is minimal heat input — excessive heat promotes grain growth and can change local thermal expansion behavior.
• Resistance welding is workable for lap joints and small components. Use short weld times and controlled current; Kovar's resistivity (~49 µΩ·cm) is high enough that it heats quickly.
• Brazing to copper, stainless steel, or itself is standard practice in vacuum tube and microwave component manufacturing. Silver-copper eutectic braze alloys are the usual choice, but the base metal must be clean and oxide-free at the braze interface.

Because Kovar contains no chromium, it does not form the passivating oxide layer that stainless steel does. That is an advantage for welding (no chromium carbide sensitization) but it means corrosion resistance is limited. In service, surfaces that will not be sealed to glass should be plated — typically with nickel and gold — or protected by the surrounding hermetic package.

Surface Preparation

The surface chemistry of Kovar matters more than most engineers realize. A glass-to-metal seal does not bond to bare metal — it bonds to an oxide layer that is deliberately grown on the Kovar surface before sealing. The ideal oxide is a thin, adherent mixed oxide of iron, nickel, and cobalt (primarily FeO with dissolved NiO and CoO). The composition of that oxide layer is influenced by:

• The alloy chemistry — particularly the Ni/Co ratio, which determines the oxide's viscosity and adhesion.
• The pre-oxidation schedule — typically a controlled heat treatment in a wet hydrogen or air atmosphere at 700–800°C.
• The surface finish — machined surfaces with heavy cold work or smeared metal can produce non-uniform oxides.

If you machine Kovar, avoid heavy lubricants that leave carbonaceous residues. Those residues interfere with oxidation and cause blistering or poor wetting during glass sealing. Vapor degreasing or aqueous cleaning with the right detergent is usually required before the pre-oxidation step.

Applications for Kovar Across High-Tech Industries

Kovar is not a commodity alloy. It is used where failure is unacceptable — where a leak, a cracked seal, or a CTE mismatch could cost millions in equipment downtime or, in the medical or aerospace field, threaten lives. Here are the application areas where Kovar's chemistry earns its keep.

Semiconductor Packaging and ICs

Kovar remains a foundational material for hermetic integrated circuit packaging. Lead frames, flat lids, and package bases are stamped or machined from Kovar strip, then gold-plated and sealed to ceramic or glass. The alloy's CTE match to alumina packages prevents die stress and seal cracking through thermal cycling. We machine Kovar package rings and sealed lids for several semiconductor customers — the common thread is demanding flatness (often 0.02 mm or better) and burr-free sealing surfaces.

Vacuum and Microwave Tubes

Vacuum tubes, microwave tubes, and traveling wave tubes still depend on Kovar feedthroughs. The pins pass through glass or ceramic insulators, and the seal must hold vacuum for years. Kovar's expansion match to the insulating materials is the difference between a reliable tube and one that leaks after a hundred thermal cycles. Electrical feedthroughs and insulator-to-metal seals in this industry are almost always specified in ASTM F15.

Aerospace and Defense Electronics

Connectors, radar housings, and satellite communication modules use Kovar when hermeticity is non-negotiable. The alloy appears in sealed microwave packages, RF connector assemblies, and avionics enclosures. In these applications, Kovar is frequently nickel-gold plated and welded to stainless steel flanges. Mechanical memory, thread strength, and thermal cycling behavior all depend on maintaining the specified chemistry — substitute material sourced from a non-certified supplier is a real risk in this sector.

Medical Devices and Implantables

Pacemakers, neurostimulators, and implantable sensors rely on hermetic titanium or Kovar enclosures. Kovar is selected for feedthroughs, connector blocks, and internal sensor housings where glass-sealed electrical feedthroughs pass through the device wall. The biocompatible coating (typically a parylene or epoxy over the gold-plated Kovar) protects the metal from body fluids. The critical requirement is absolute seal integrity over a 10–15 year implant life — which brings us back to chemistry consistency.

Photonics and Optoelectronics

Laser diode packages, fiber-optic communication housings, and infrared sensor assemblies all use Kovar. The alloy's stability under temperature cycling makes it ideal for aligning optical components. In telecom transceivers, Kovar housings hold the laser diode, thermoelectric cooler, and monitor photodiode in a hermetic enclosure with a glass window or fiber feedthrough. As 5G infrastructure and data-center optics expand, we are seeing increased demand for precision-machined Kovar packages with tight tolerances on the optical alignment features.

How to Source and Verify Kovar: Composition Certifications

The most important advice I can give any engineer or buyer: do not accept Kovar without a mill test certificate (MTC) that shows the complete chemical analysis. The market contains material sold as "Kovar" that does not meet ASTM F15. Sometimes it is a simpler Fe-Ni alloy; sometimes it is a reclaimed or mixed melt; occasionally it is just mislabeled stock.

Here is what we require from our own material suppliers, and what you should require from yours:

1. Mill test certificate (MTC/EN 10204 3.1 or 3.2) listing the measured weight percent of every element in the ASTM F15 specification.
2. Heat number and lot traceability — the certificate must tie to the specific heat of material being supplied, not a generic "typical composition."
3. CTE verification — either measured values or a certified statement that the material meets the 4.5–5.5 × 10⁻⁶ /°C range.
4. Grain size documentation for strip and thin sheet, if you will be deep-drawing or forming.
5. Plating substrate confirmation if you will plate the parts — some off-spec heats do not accept gold plating uniformly.

Lot-to-lot chemistry consistency is the hidden driver of production yields. A solderable or glass-sealable package is the result of a process tuned to a specific alloy behavior. If the next lot of Kovar arrives with nickel at 28.1% instead of 29.2%, the sealing furnace profile may no longer produce a good oxide. This is why serious buyers include "chemistry must conform to ASTM F15" in their purchasing specification, and why they audit suppliers for material traceability.

One more point — compliance is becoming a larger part of material sourcing. For component sales into regulated markets, you may need UL-recognized material, REACH compliance, and RoHS documentation for plating and finishes. A competent Kovar supplier will provide these documents on request. If they cannot, that is a red flag.

Kovar is available in many product forms: round bar, plate, strip, wire, rod, and pre-cut flat lids. At KELTRYN, we machine custom parts from bar, plate, and near-net blanks, and we frequently receive customer-supplied ASTM F15 material for prototype or production runs.

Frequently Asked Questions (FAQ)

Is Kovar ferrous?

Yes. Iron is the balance element at approximately 52% by weight, which makes Kovar a ferrous alloy. It is also magnetic, which is useful for quick positive identification with a magnet — though this test alone cannot distinguish certified Kovar from other iron-based alloys.

How much cobalt is in Kovar?

Kovar contains 16% to 18% cobalt by weight. Cobalt is essential because it suppresses the alloy's thermal expansion and shifts the expansion curve so that it matches borosilicate glass in the sealing temperature range.

What is Kovar equivalent to?

Common equivalents include Nilo K (UK), Dilver P (Germany), Vacon 12 (Germany), and Pernifer 2912 (France). All of these are manufactured to the same ASTM F15 chemistry and can, in most applications, be substituted for one another with proper engineering approval.

Is Kovar a stainless steel?

No. Kovar contains no chromium, which is the defining element of stainless steel. Kovar has poor corrosion resistance in harsh environments and must be plated — typically with nickel and gold — or otherwise protected when exposed to moisture, salt, or chemicals.

Can Kovar be welded?

Yes. Kovar can be TIG-welded, resistance-welded, and brazed to itself and to compatible metals such as stainless steel and copper alloys. The key is minimal heat input and proper surface preparation. Brazing with silver-copper eutectic filler is the most common joining method in hermetic package manufacturing.

Conclusion: Choosing Kovar for High-Reliability Sealing Applications

Kovar is not a material you can simply "spec by name" and hope for the best. The entire performance of the alloy — its thermal expansion match to glass and ceramics, its machinability, its weldability, its oxide formation behavior — flows directly from its chemical composition. When you specify ASTM F15 / UNS K94610, you are drawing a line that every melt, every heat, and every mill certificate must meet.

As we see growing demand across 5G infrastructure, photonics, aerospace electronics, and implantable medical devices, Kovar's role as the go-to sealing alloy is secure. Alternative low-CTE alloys exist, including tungsten-copper composites and newer Fe-Ni-Cr formulations, but each brings trade-offs in cost, machinability, or CTE match. None offers the combination of proven seal performance, established specifications, and manufacturing familiarity that Kovar delivers.

At KELTRYN, we machine Kovar and Fe-Ni-Co alloy components every day — custom housings, lids, caps, sealing rings, flanges, frames, and complex precision parts for hermetic and high-reliability applications. We understand how the alloy's chemistry translates into real-world machining behavior: the heat sensitivity, the work hardening, the burrs, the thin-wall distortion, and the critical sealing features that must survive inspection with zero defects.

If you are designing a Kovar component and wondering whether it is machinable at your tolerances, or if you have a drawing ready and need a partner who will treat your material traceability and quality requirements with the seriousness they deserve — contact us. Send us your 2D drawing, 3D model, or even just your application concept. We will give you practical feedback on manufacturability, material certification requirements, and delivery planning. And if you know another engineer wrestling with glass-to-metal seal components, share this article with them — they will find the chemistry details here before they discover them the hard way in production.