Kovar Melting Point: The Complete Engineering Guide to This Nickel-Cobalt Alloy

If you work in hermetic packaging, semiconductor equipment, aerospace electronics, or glass-to-metal sealing, you've likely crossed paths with Kovar. It's one of those materials that shows up everywhere behind the scenes: inside the feedthrough on a vacuum chamber, under the lid of a hermetic RF package, in the glass-sealed pins of a medical implant header.

But here's the thing about Kovar: it's easy to specify and hard to process well. Much of that difficulty traces back to one fundamental property — the Kovar melting point — and how it interacts with everything else the alloy does. In this guide, I'll walk through the melting behavior, the full physical and mechanical property set, and the practical implications for anyone machining, brazing, or sealing this material.

I'm an applications engineer at KELTRYN, a precision CNC machining shop that specializes in Kovar and Fe-Ni-Co alloy components. We machine housings, lids, caps, sealing rings, flanges, and frames for customers who can't afford a seal failure. This article is the resource I wish I'd had when I started working with the alloy.

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What Is Kovar? (And What Kind of Metal Is It?)

Let's clear up the most common question first: What kind of metal is Kovar?

Kovar is a nickel-cobalt-iron alloy — nominally 29% nickel, 17% cobalt, with iron making up the balance. Its UNS designation is K94610, and it's governed by ASTM F15. It belongs to a family of materials called controlled-expansion alloys, sometimes called Fe-Ni-Co alloys or "glass-sealing alloys."

It is not a superalloy. It's not a stainless steel. That distinction matters because engineers sometimes assume Kovar will behave like a corrosion-resistant grade of stainless. It won't. Kovar is designed for one dominant job: matching the thermal expansion of hard borosilicate glasses and alumina ceramics so that when a metal part is sealed to a glass or ceramic part, the two materials expand and contract together — preventing cracks, leaks, and stress fractures.

The alloy was developed specifically for glass-to-metal and ceramic-to-metal sealing applications. Over the years, it has become the default material for hermetic electrical feedthroughs, vacuum tube components, semiconductor packages, and countless other high-reliability enclosures.

You'll find Kovar sold under a variety of trade names depending on the supplier:

| Trade Name | Manufacturer |

|---|---|

| Kovar® | CRS Holdings / Carpenter |

| NILO K | Special Metals |

| Pernifer 2918 | Vacuumschmelze |

| Rodar | Carpenter |

| Dilvar P1 | Nippon Yakin |

Same fundamental alloy, slightly different processing histories. The key spec to reference is always ASTM F15.

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The Melting Point of Kovar: What You Need to Know

Here's the number you came for: Kovar melts at approximately 1450°C (2642°F).

But if you're doing real engineering work, you need more precision than that. Kovar doesn't have a single melting point — it has a melting range, like most commercial alloys. The solidus (the temperature at which the alloy begins to melt) is approximately 1450°C, and the liquidus (where it's fully molten) is closer to 1510°C (2750°F).

Between those two temperatures, the material exists in a slushy, partially molten state. That's typical for multi-component alloys — the different elements melt progressively rather than all at once. For Kovar specifically, the iron-nickel-cobalt system produces a fairly narrow mushy zone, which is convenient for processing, but you should never design a process that approaches anywhere near this range.

How Kovar's Melting Point Compares to Common Metals

Putting Kovar in perspective helps when you're selecting brazing filler metals or planning heat treatment:

| Material | Melting Range (°C) |

|---|---|

| Kovar | ~1450–1510 |

| 304 Stainless Steel | ~1400–1450 |

| Copper | 1084 (single point) |

| Aluminum 6061 | ~582–652 |

| Silver | 961 (single point) |

Kovar's melting point is right in the neighborhood of stainless steel. That's useful because it means the alloy can survive many high-temperature processes — brazing, glass sealing, annealing — without risk of melting, as long as you stay below roughly 1350°C with a comfortable margin.

Why the Melting Range Matters in Practice

For brazing, welding, and heat treatment, the melting range is your upper bound. Some specific implications:

• Brazing: Most Kovar brazing operations use silver-copper or copper-based filler metals that flow well below 900°C. That leaves a huge safety margin below the solidus. However, if you're tempted to use a high-temperature braze alloy (nickel-based fillers that flow above 1000°C), you need to be careful — not about melting the Kovar itself, but about excessive grain growth and CTE drift.
• Welding: TIG and resistance welding processes create localized temperatures well above the melting point. That's expected — you're locally melting the parent material. The challenge is controlling heat input so you don't crack the weld or cause excessive oxidation.
• Annealing: Standard Kovar annealing is done at 850–900°C, far below the melting range. This is intentional: it's enough to recrystallize the material and set the correct thermal expansion behavior without risking partial melting or grain coarsening.
• Glass sealing: Glass-to-metal seals are typically made at 850–1050°C depending on glass composition. Again, comfortably below Kovar's melting range, but the sealing process changes the oxide layer on the Kovar surface, which is the whole point of a "matched" seal.

The practical takeaway: the Kovar melting point tells you what the material can survive — not what it wants. The real constraints on processing temperature come from the CTE curve, the surface oxide condition, and the grain structure.

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Chemical Composition of Kovar (ASTM F15)

The composition of Kovar is tightly controlled under ASTM F15. Small deviations in nickel and cobalt content can shift the thermal expansion curve significantly, which is why certified material with full traceability is non-negotiable for sealing applications.

The nominal composition looks like this:

| Element | Nominal Weight % | ASTM F15 Range |

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

| Nickel | 29 | 28.5 – 29.5 |

| Cobalt | 17 | 16.5 – 17.5 |

| Iron | Balance | — |

| Manganese | 0.50 max | 0.50 max |

| Silicon | 0.20 max | 0.20 max |

| Carbon | 0.04 max | 0.04 max |

| Aluminum | 0.10 max | 0.10 max |

| Zirconium | 0.10 max | 0.10 max |

| Titanium | 0.10 max | 0.10 max |

What Each Element Does

Nickel and cobalt are the stars of the show. Together, they tune the thermal expansion of the iron matrix so that the CTE stays low and stable from room temperature up to roughly 400–450°C. The ratio of nickel to cobalt is what creates the characteristic "Kovar curve."

Iron is the balance and provides the structural backbone.

The trace elements — manganese, silicon, aluminum, zirconium, titanium — are present partly as deoxidizers and partly to control grain growth and improve wetting during glass sealing. Their upper limits are specified for good reason. Excess aluminum, for example, can interfere with the formation of the copper-oxide-rich scale that's needed for a strong glass bond.

Impurity control isn't just a nicety — it's a reliability issue. In hermetic sealing, a composition that's even slightly off-spec can produce a seal that survives thermal cycling in testing but cracks in the field. That's why we always recommend purchasing Kovar with a certified mill test report confirming ASTM F15 compliance.

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Physical Properties of Kovar

Density and Melting Point

Kovar has a density of approximately 8.35 g/cm³ (0.302 lb/in³). It's a dense material — noticeably heavier than aluminum, comparable to stainless steel. For packaging engineers watching weight budgets in aerospace applications, that's a real consideration.

Recapping the thermal data:

| Property | Value |

|---|---|

| Solidus Temperature | ~1450°C (2642°F) |

| Liquidus Temperature | ~1510°C (2750°F) |

| Density | 8.35 g/cm³ |

| Boiling Point | ~4000°C (reference only) |

The boiling point is essentially irrelevant for any real manufacturing process — you'll never get Kovar anywhere near boiling. But it's sometimes asked about in materials comparisons, so we include it for completeness.

Coefficient of Thermal Expansion (CTE)

This is the property that defines Kovar. In the annealed condition, Kovar exhibits a CTE of approximately 5.5–6.5 ppm/°C over the 20–400°C range. That low, flat expansion curve is what makes it the go-to material for matched seals.

Here's how it lines up against common sealing partners:

| Material | CTE (ppm/°C) |

|---|---|

| Kovar (annealed, 20–400°C) | 5.5 – 6.5 |

| Borosilicate glass (e.g., Pyrex 7740) | ~5.0 |

| Alumina ceramic (94–96%) | ~6.5 – 7.0 |

The match is not perfect — and that's okay. In a "matched" glass-to-metal seal, the metal is chosen to run slightly higher in CTE than the glass. When the assembly cools from the sealing temperature, the metal compresses slightly around the glass. That compressive stress is actually beneficial; it puts the glass in compression, which is where glass is strong. The tensile strength of glass is poor, but its compressive strength is excellent.

The genius of Kovar is that its CTE curve is engineered to track the glass's expansion curve not at just one temperature, but across the entire cooling range from sealing temperature to room temperature. That's what separates Kovar from Invar, which has an even lower CTE but only over a narrow temperature range.

Magnetic Properties — How Magnetic Is Kovar?

This is a question we get from engineers designing high-frequency components and magnetic shielding. The answer: Kovar is strongly ferromagnetic at room temperature.

Here's the data that matters:

• Kovar is ferromagnetic below its Curie temperature of approximately 435°C (815°F).
• Above the Curie temperature, it transitions to paramagnetic — it loses its spontaneous magnetization and becomes essentially non-magnetic.
• At room temperature, its magnetic behavior is broadly similar to that of low-carbon steel or other iron-based soft magnetic alloys.

The magnetic transition is directly tied to the alloy's thermal expansion behavior. The nickel and cobalt that suppress thermal expansion also dictate the magnetic ordering. It's a classic Invar-type effect: the magnetism and the expansion are coupled.

What this means in practice:

1. For magnetic shielding: Kovar can work as a shielding material at temperatures below ~400°C, but it's usually not the first choice. High-permeability alloys like MuMetal are more effective when no expansion match is needed. Kovar gets used when you need a shield and a glass seal in one assembly.

1. For high-temperature electronics: If your assembly operates above 435°C, Kovar will become paramagnetic. That can be an advantage — no magnetic interference from the package itself — or a headache if you were relying on the ferromagnetic response for a magnetic circuit.

1. For machining and handling: Small machined Kovar chips can cling to tooling and magnets. It's a minor annoyance, but worth knowing if you're running automated inspection or part handling.

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Mechanical Properties of Kovar

Kovar's mechanical properties are respectable, though not exotic. In the annealed condition, you can expect:

| Property | Annealed Value |

|---|---|

| Yield Strength | ~345 MPa (50 ksi) |

| Tensile Strength | ~515 MPa (75 ksi) |

| Elongation | ~30% |

| Hardness | ~75–85 HRB |

These values are for annealed material, which is how Kovar is typically supplied for forming and sealing. In the cold-worked condition, strength rises substantially — tensile strength can exceed 800 MPa — but ductility drops, and the CTE behavior can shift. For that reason, we generally recommend that parts destined for hermetic sealing be machined in the annealed state and, if heavily cold-worked, receive a final annealing treatment before sealing.

One thing I want to emphasize: Kovar retains useful mechanical strength at elevated temperatures. At 400–500°C, it's still stiff enough to support structural loads in a sealed assembly. This is important because many glass-sealed components experience elevated temperatures during processing and operation. A material that softens too early would allow the seal to distort.

At KELTRYN, we routinely machine Kovar housings with thin walls — down to a few tenths of a millimeter in some RF packages. The material's ductility in the annealed state is forgiving, but work-hardening is a real risk. More on that in the fabrication section.

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Thermal Properties Beyond Melting Point

The melting point gets all the attention, but for most applications, the more important thermal properties are conductivity, specific heat, and the practical service temperature limit.

Thermal Conductivity

Kovar has a thermal conductivity of approximately 17 W/m·K at room temperature.

That puts it in the same range as stainless steel (~16 W/m·K) — which is to say, not great. By contrast, copper is around 400 W/m·K, and aluminum is around 200 W/m·K.

For electronics packaging, this matters in two ways:

1. Heat dissipation: A Kovar package is not a good heat spreader. If you're putting a power-hungry die in a hermetic Kovar enclosure, you'll need supplementary thermal paths — copper-tungsten heat sinks, thermal vias, or beryllia/alumina substrates inside the package.

1. Sealing processes: The low thermal conductivity means Kovar heats up unevenly if you're not careful. In brazing or glass sealing, that can lead to localized overheating near the joint while other zones remain too cold. We advise customers to use slow, controlled heating profiles and, where possible, furnace brazing rather than torch brazing for production work.

Specific Heat

Kovar's specific heat is about 460 J/kg·K. Like thermal conductivity, it's low compared to copper (385 J/kg·K is actually lower, but copper's density and conductivity more than make up for it).

Practically, this means Kovar heats and cools relatively quickly per unit mass, but the combination of moderate density, moderate specific heat, and low conductivity produces a material that is thermally sluggish locally. If your sealing process depends on precise temperature cycles, account for soak times — the part needs time to reach equilibrium through its thickness.

Maximum Continuous Service Temperature

For sealed assemblies, the practical upper limit is around 400–500°C. Here's the reasoning:

• The CTE match with glass is only maintained up to roughly 450°C. Above that, the expansion curves of Kovar and borosilicate glass begin to diverge.
• Extended exposure to temperatures near 900°C or above can cause CTE drift — the alloy's expansion characteristics change permanently, which can render a previously good seal design unreliable on thermal cycling.
• The oxide scale that's critical for the glass seal can continue to grow at high temperatures. A thick, loose oxide degrades the seal strength.

For non-sealed applications — say, a structural bracket or a fixture — Kovar can handle higher temperatures without catastrophic failure. But if you're using Kovar because you need a matched glass seal, respect the 400–500°C continuous service ceiling.

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Does Kovar Rust? Corrosion Resistance

Straight answer: Yes, Kovar can rust. It is not a stainless steel.

Kovar contains iron as its balance element — roughly 54% by weight. It does not have the chromium that gives stainless steels their passive, self-healing oxide layer. So in humid environments, salt-laden atmospheres, or aggressive chemical exposure, Kovar is vulnerable to corrosion.

But here's the nuance that surprises a lot of engineers: the oxide that forms on Kovar isn't just an unwanted side effect — it's a required processing step.

For a glass-to-metal seal to work, the Kovar surface must be deliberately oxidized under controlled conditions to form a thin, adherent oxide scale — primarily iron-nickel-cobalt spinels. The glass wets this oxide layer and forms a chemical bond with it. If the part is too clean, the glass won't bond. If the oxide is too thick or too loose, the seal will be weak and leak.

This means corrosion control for Kovar is application-dependent:

• For sealing: The oxide layer is controlled as part of the process. Parts must be cleaned and handled to avoid contamination before oxidation.
• For service in harsh environments: Kovar components that aren't sealed to glass are typically plated — nickel, gold, or nickel-gold plating is common. Plated Kovar is what you'll find in most semiconductor packages and medical devices.
• For storage: Raw Kovar should be kept in dry, climate-controlled conditions. Bare Kovar parts stored in a damp shop environment will develop surface rust within days or weeks.

At KELTRYN, we see this constantly. Customers send us Kovar components for machining, and we always ask about the end-use environment and whether the part will be plated. Getting ahead of corrosion concerns during design saves expensive rework later.

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Fabrication, Machining, and Heat Treatment of Kovar

This is where the material separates the experienced shop from the rest. Kovar is machinable, but it demands respect.

Machinability

Kovar machines like a soft stainless steel. It's tougher than plain carbon steel, gummy enough to produce stringy chips, and prone to work-hardening if you let tools dull.

Practical guidelines:

• Tooling: Use sharp carbide tools with positive rake angles. Dull tools cause rubbing, which work-hardens the surface and shortens tool life.
• Speeds and feeds: Moderate cutting speeds — comparable to 300-series stainless — with aggressive enough feed rates to stay under the work-hardened layer.
• Coolant: Flood coolant is strongly recommended. Heat control is critical for thin-wall parts. Uncontrolled heat can cause distortion, and on near-net-shape parts, it can shift the CTE behavior locally.
• Chip control: Kovar produces tough, stringy chips. Chip breakers or peck cycles help in drilling and milling.
• Burr control: This is the big one for sealing components. A burr on a sealing ring or lid edge is a leak path waiting to happen. We use secondary deburring operations and, for critical features, wire brushing or vibratory finishing.

Machining Kovar isn't dramatically harder than machining 304 stainless, but it rewards experience. The adjustments come in thin-wall rigidity, burr management, and knowing how to handle near-net-shape blanks without distorting them.

Forming and Cold Working

In the annealed state, Kovar is quite ductile — 30% elongation gives you decent room for forming operations like deep drawing, bending, and stamping.

However, Kovar work-hardens rapidly. Heavy deformation without intermediate annealing will cause cracking, especially in sharp corners. The typical approach:

1. Form in steps.
2. Anneal between steps (850–900°C in a controlled atmosphere).
3. Final anneal after all forming is complete.

For parts that are going to be glass-sealed, the final anneal is essential. Cold work changes the thermal expansion behavior; annealing restores the correct, stable "Kovar curve."

Welding and Brazing

Brazing is the preferred joining method for most Kovar assemblies. Silver-copper eutectic fillers (e.g., Ag-28Cu, melting at ~780°C) are the workhorses. They flow beautifully on Kovar, produce strong joints, and — critically — process well below Kovar's melting range and below the temperature where excessive grain growth occurs.

For higher-temperature service, copper brazing is possible, but the brazing temperature (~1100°C) gets close enough to Kovar's melting point that you need careful temperature control and short process times. Grain growth becomes a real concern.

Welding Kovar is also feasible. TIG, MIG, and resistance welding all work. But because Kovar is a controlled-expansion alloy, welding introduces localized thermal cycles that change the microstructure and can create residual stress. Pre-weld cleaning and post-weld stress relief are not optional extras — they're required for reliable parts.

Annealing

The standard Kovar anneal is 850–900°C in a dry hydrogen atmosphere. The hydrogen atmosphere serves two functions: it prevents unwanted oxidation, and it helps control the residual carbon content — high carbon levels suppress the thermal expansion and degrade sealing performance.

Annealing accomplishes three things:

1. Recrystallizes cold-worked material, restoring ductility.
2. Develops the correct CTE by establishing the right metallurgical structure.
3. Sets up the surface condition for subsequent oxidation and sealing.

Improper annealing — wrong temperature, wrong atmosphere, or too-rapid cooling — can leave the material with an unstable CTE or an undesirable grain size. If you're using a heat-treating vendor, confirm they have experience with controlled-expansion alloys and can certify the process.

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Kovar Specifications, Standards, and Cross-References

When you're sourcing Kovar, use the standards to remove ambiguity. Here's the quick reference:

| Standard | Description |

|---|---|

| ASTM F15 | Standard specification for iron-nickel-cobalt sealing alloy |

| AMS 7726 | Sheet, strip, and foil |

| AMS 7727 | Bar and rod |

| ISO 11222 | Iron-nickel-cobalt sealing alloy |

| UNS K94610 | Unified numbering system designation |

Global equivalents:

| Region | Designation |

|---|---|

| USA | Kovar®, Rodar (Carpenter) |

| UK | NILO K (Special Metals) |

| Germany | Pernifer 2918 (Vacuumschmelze) |

| France | Dilvar P1 |

| Japan | KV-1 |

Kovar is available in a wide range of mill forms: rod, bar, sheet, wire, strip, tubing, and custom forgings. For CNC machined components, we typically source bars and plates with mill certifications, then machine to print. For very thin-wall or large-diameter parts, forgings or near-net shapes can be cost-effective at higher volumes.

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Applications: Where Kovar Melting Point and Properties Matter Most

Glass-to-Metal and Ceramic-to-Metal Seals

This is Kovar's home turf. Vacuum tube feedthroughs, hermetic connectors, pressure sensors, and viewport assemblies all rely on Kovar because it forms a genuine metallurgical bond with glass.

The melting point matters here because the sealing process — typically 850–1050°C — must melt the glass without melting or degrading the metal. The window between the sealing temperature and Kovar's 1450°C solidus is what makes the process controllable.

Semiconductor Packaging and Integrated Circuits

Kovar lead frames and hermetic packages have been standard for decades. Even as advanced packaging evolves — with 5G devices, automotive electronics, and aerospace systems demanding higher reliability — Kovar remains the material of choice when you need a true hermetic seal around sensitive microelectronics.

We machine Kovar package bases, lids, and lead frames at KELTRYN for customers who need flatness within microns across the sealing plane. A lid that isn't flat won't seal, regardless of how well the material performs.

Aerospace and Defense Electronics

Radomes, RF connectors, hermetically sealed housings, and gyroscope components — aerospace electronics live in a world of extreme temperature swings, vibration, and humidity. Kovar's CTE match with ceramic substrates prevents solder joint fatigue over thousands of thermal cycles.

The long-term reliability requirement is why aerospace buyers demand full material traceability. If there's any doubt about the composition or heat treatment of a Kovar part, it doesn't go on the aircraft.

Medical Devices and Implantables

Implantable pulse generators, sensor housings, and diagnostic instruments increasingly use Kovar for hermetic feedthroughs. The material itself is not inherently biocompatible — it contains cobalt and nickel, which can be allergenic — so Kovar components in medical devices are almost always plated with gold or other biocompatible coatings.

The precision requirements are staggering. We machine Kovar components with features measured in hundreds of microns for implantable applications, where a single burr or machining mark can compromise the sealing surface.

Power Tubes and Microwave Tubes

Klystrons, magnetrons, and high-power vacuum tubes have been around for nearly a century, and they still depend on Kovar glass-to-metal seals. These devices operate at high temperatures and high voltages, and the vacuum inside the tube means the seal cannot be anything less than perfect.

With CTE matching across the entire operating range, and a melting point high enough to survive the tube's processing and operation, Kovar proves that sometimes the best material is the one that's been proven for decades.

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Frequently Asked Questions About Kovar

What is the exact melting point of Kovar?

There isn't a single "exact" melting point. Kovar melts over a range of approximately 1450–1510°C. The solidus is around 1450°C, where the first liquid forms, and the liquidus is around 1510°C, where the alloy is fully molten. When you see "melting point 1450°C" in datasheets, it's referring to the solidus. Design your processes to stay well below this — brazing, sealing, and annealing all occur at lower temperatures for good reason.

How magnetic is Kovar?

Kovar is ferromagnetic at room temperature — strongly magnetic, similar to carbon steel. It remains ferromagnetic up to its Curie temperature of approximately 435°C (815°F). Above that, it becomes paramagnetic and loses its spontaneous magnetization. This matters for high-temperature electronic packaging and magnetic shielding considerations.

Will Kovar rust?

Yes, Kovar can rust. It contains no chromium and doesn't have stainless steel's corrosion resistance. In humid or salt environments, bare Kovar will corrode. However, a controlled thin oxide layer is essential for glass-to-metal sealing. For harsh service environments, Kovar parts should be plated — typically nickel or gold.

Is Kovar the same as Invar?

No. Invar is a nickel-iron alloy (Fe-36Ni) that exhibits near-zero thermal expansion at room temperature but does not contain cobalt and doesn't match glass CTE over a broad temperature range. Kovar is a nickel-cobalt-iron alloy designed specifically to match the CTE of hard borosilicate glass and alumina ceramics. They're cousins in the controlled-expansion family, but they solve different problems. Don't substitute one for the other without a design review.

Can Kovar be brazed?

Yes. Kovar brazes well with silver-copper and silver-copper-palladium filler metals that melt below about 900°C. These fillers wet Kovar's surface readily, especially when the oxide layer is controlled. For higher-temperature brazing with copper fillers, you need to be cautious about grain growth and CTE drift. We recommend a controlled furnace atmosphere and a documented thermal profile for all Kovar brazing.

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Conclusion: Key Takeaways on Kovar's Melting Point and Properties

Kovar is a remarkable material — not because any single property is extraordinary, but because the combination is so well suited to high-reliability sealing applications. Let's recap what matters most:

1. Melting range: Approximately 1450–1510°C. Process temperatures for brazing, sealing, and annealing always stay well below this range. The practical service ceiling for sealed assemblies is 400–500°C.

1. CTE matching: The whole reason Kovar exists. At 5.5–6.5 ppm/°C from 20–400°C