Kovar
An iron-nickel-cobalt alloy for glass-to-metal seals
Typical values for the temper in the title
Kovar contains about 29% nickel, 17% cobalt and the balance iron. The composition was designed so that its expansion (about 5×10⁻⁶ per °C between 30 and 200 °C) matches hard borosilicate glass and ceramics. This allows metal to be sealed to glass or ceramic without the joint cracking on cooling and temperature changes.
It is used for hermetic seals in electronic packages, tubes and glass-to-metal joints in vacuum systems.
Mechanical and physical properties
| Property | Value | Note |
|---|---|---|
| Tensile strength Rm | 517 MPa | |
| Yield strength Rp0.2 | 310 MPa | |
| Elongation A | 30% | |
| Hardness | 180 HB | |
| Modulus of elasticity E | 138 GPa | |
| Density ρ | 8.36 g/cm³ | |
| Thermal expansion coefficient | about 5×10⁻⁶ per °C (30–200 °C) | matched to borosilicate glass |
| Machinability | 45% | relative rating in the site database |
Chemical composition (wt %)
| Element | Range | Role |
|---|---|---|
| Ni nickel | about 29 | expansion coefficient |
| Co cobalt | about 17 | expansion coefficient |
| Fe iron | balance (about 54) |
Nominal composition. Exact ranges are in ASTM F15.
Conditions and heat treatment
In these alloys what matters is the expansion coefficient, and it depends on the material condition. So annealing and dimensional stabilization follow the specification.
| Condition | Properties | When to choose it |
|---|---|---|
| Annealed | machinable and deep-drawable | delivery condition |
| Controlled oxidation before sealing | an oxide layer that bonds to glass | the sealing process |
Applications
- Hermetic electronic packages
- Glass-to-metal seals
- Vacuum system feedthroughs
- Microwave and laser packages
- Joints to ceramics
Fabrication
Welding
Welds well by TIG and MIG. Careful cleaning before welding is especially important: oil, paint or sulfur on the surface cause weld cracking.
Machining
Nickel alloys work harden, stay strong when hot, and wear tools. Use low speed, a steady feed that does not "rub", sharp rigid tools and plenty of coolant.
Sealing
Sealing to glass relies on a controlled oxide layer. The process is sensitive to cleanliness and atmosphere.
Compared with similar materials
| Material | Rm / Rp0.2 | Welding | When it is the better choice |
|---|---|---|---|
| Invar 36 | 485 / 275 MPa | Good | much lower expansion · material page |
| 304 / 304L | 515–620 / 205–310 MPa | Good — ER308L | stainless: much higher expansion · material page |
| Monel 400 | 480–550 / 170–345 MPa | Good | nickel-copper, for corrosion · material page |
Common mistakes
Substituting Invar because "both are low expansion"
Invar expands much less than the glass, and the seal will crack.
Sealing to soft glass
Kovar suits hard borosilicate glass. Other glasses need other alloys.
Skipping oxidation control
Without the right oxide layer the seal will not be hermetic.
Equivalents, standards and supply
| EN | 1.3981 |
|---|---|
| ASTM | F15 |
| Standards | ASTM F15 / MIL-I-23011 Class 1 |
| Forms | plate · rod · wire |
| Producers | Carpenter · VDM |
Learn more
Values are typical and for comparison only. For design, rely on the standard and the mill certificate. Sources: ASTM (B127, B164, B168, B443, B575, B619 and others), Special Metals and Haynes data sheets, and the METAL HUB materials database. Inconel and Monel are trademarks of Special Metals, and Hastelloy of Haynes International.
Updated: October 31, 2026