Nickel Alloy Kovar
UNS K94610 · ASTM F15 · EN 1.3981
Kovar is a nominally Fe-29Ni-17Co low-expansion alloy (ASTM F15) whose thermal expansion from 20 to 450 °C closely matches borosilicate hard glass and alumina ceramic.
Values for the O condition
Kovar tempers and conditions
Kovar is supplied in 6 conditions, soft to hard. O is the default FabDigit quotes; the values on this page are for it.
Compare all 6 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Wet-hydrogen (decarburizing) annealed sealing grade | Pick when parts must be glass-sealed or vacuum-fired: hydrogen anneal removes carbon and residual gas for bubble-free seals. | 310 | 490 | 33 | 135 HB | 3.5 % IACS |
ODefault Annealed | Standard stocked condition for stamping, machining, welding and glass/ceramic sealing. | 345 | 520 | 30 | 145 HB | — |
Quarter hard | Use when stamped parts need extra stiffness but still some bend ductility. | 480 | 620 | 15 | 10 HRC | 3.4 % IACS |
Cold drawn | Standard as-supplied bar/wire condition for machined pins, headers and feedthrough leads. | 540 | 660 | 14 | 16 HRC | 3.4 % IACS |
Half hard | For flat springs, contacts and lids where moderate strength and flatness matter more than deep forming. | 620 | 720 | 8 | 22 HRC | 3.4 % IACS |
Hardened | Highest strength condition, chosen for spring contacts and thin stiff wire; almost no cold-bend reserve and CTE is best re-annealed before sealing. | 760 | 860 | 3 | 30 HRC | 3.4 % IACS |
What is Kovar?
Kovar is a nominally Fe-29Ni-17Co low-expansion alloy (ASTM F15) whose thermal expansion from 20 to 450 °C closely matches borosilicate hard glass and alumina ceramic. It is the standard material for glass-to-metal seals, vacuum tube envelopes and hermetic microwave and optoelectronic packages; annealed material gives 345 MPa yield, 520 MPa tensile, 30% elongation and 145 HB, and it cold-works, welds and brazes easily. Corrosion resistance is poor, so nickel or gold plating is usual, thermal and electrical conductivity are low, and expansion behaviour changes above the Curie point near 435 °C, limiting precision sealing to the 450 °C ceiling. Kovar has a density of 8.36 g/cm³ (0.302 lb/in³), heavier than 54% of nickel alloy grades. It melts at 1,450°C (2,642 °F). Kovar has a yield strength of 345 MPa (50 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 62% of nickel alloy grades. Elongation at break is 30% and the elastic modulus is 138 GPa (20 Msi).
What is Kovar used for?
- Glass-to-metal seal headers
- Vacuum tube components
- Hermetic package bases and lids
- Microwave and RF device housings
- TO-can optoelectronic packages
- Ceramic-to-metal feedthroughs
- Lead frames and sealing pins
- X-ray tube components
Advantages
- Easy to machine — 32/100, better than 95% of nickel alloy grades
- Low embodied carbon — 8 kg CO₂/kg, better than 93% of nickel alloy grades
- High electrical conductivity — 3.5 % IACS, better than 91% of nickel alloy grades
- Conducts heat well — 17 W/m·K (9.82 BTU/hr·ft·°F), better than 84% of nickel alloy grades
- Forms and bends easily — 75/100, better than 82% of nickel alloy grades
Limitations
- Needs corrosion protection — 28/100, worse than 99% of nickel alloy grades
- Low stiffness — 138 GPa (20 Msi), worse than 98% of nickel alloy grades
- Limited service temperature — 450°C (842 °F), worse than 86% of nickel alloy grades
Kovar properties
Typical room-temperature values for Kovar-O; bars are the position among nickel alloy grades.
Physical3
Kovar has a density of 8.36 g/cm³ (0.302 lb/in³), heavier than 54% of nickel alloy grades. It melts at 1,450°C (2,642 °F).
Mechanical13
Kovar has a yield strength of 345 MPa (50 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 62% of nickel alloy grades. Elongation at break is 30% and the elastic modulus is 138 GPa (20 Msi).
Thermal5
Kovar is rated for continuous service to 450°C (842 °F). It conducts heat at 17 W/m·K (9.82 BTU/hr·ft·°F), better than 84% of nickel alloy grades. Thermal expansion is 5.5 µm/m·K (3.06 µin/in·°F).
Electrical3
Kovar conducts electricity at 3.5 % IACS, better than 91% of nickel alloy grades.
Chemical & Environmental3
Corrosion resistance is poor (28/100), worse than 99% of nickel alloy grades.
Sustainability3
Producing a kilogram of Kovar takes about 100 MJ of energy and emits 8 kg of CO₂ — less than 95% of nickel alloy grades. Typical recycled content is 25%.
Manufacturability7
Kovar's machinability is poor (32/100, better than 95% of nickel alloy grades); weldability very good (80/100); formability very good (75/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the nickel alloy family
Yield strength against density for every nickel alloy grade FabDigit runs. Kovar is highlighted; hover a dot for its name, or open the interactive family chart.
Working with Kovar
Is Kovar easy to machine?
Machines like an austenitic Ni-Fe alloy — gummy and work-hardening: use sharp carbide, positive rake, heavy feed, low-to-moderate speed and flood coolant to avoid glazing.
Can Kovar be welded?
Readily GTAW, resistance, electron-beam and laser welded to itself; keep the weld and heat-affected zone clean and low in carbon, and use matching Kovar filler for sealing-critical joints.
Can Kovar be formed, bent or molded?
Annealed strip stamps, spins and deep-draws well; interstage anneals at 800–900 °C in dry hydrogen restore ductility and reset the expansion curve.
Manufacturability
The process limits FabDigit's quote engine applies to Kovar. Design inside these and the part quotes instantly; outside them it routes to an engineer.
CNC machining
| Machinability | Fair32 / 100 |
|---|---|
| ISO material group | S.1 |
| Cutting speed (rough / finish) | 40 / 55 m/min |
| Minimum wall | 1.5 mm |
| Minimum internal radius | 2 mm |
| Drill depth limit | 3× diameter |
| Best surface (machined / polished) | Ra 1.6 / — |
| Hardenable after machining | No |
Sheet metal
| Formability | Better75 / 100 |
|---|---|
| Weldability | Good80 / 100 |
| Bendable | Yes |
Kovar chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| Ni | 28 – 30 | 29 |
| Co | 16.8 – 17.8 | 17 |
| Clow carbon required for gas-free glass seals | ≤ 0.04 | 0.02 |
| Mn | ≤ 0.5 | 0.3 |
| Si | ≤ 0.2 | 0.1 |
| Al | ≤ 0.1 | |
| Mg | ≤ 0.1 | |
| Zr | ≤ 0.1 | |
| Ti | ≤ 0.1 | |
| Cr | ≤ 0.2 | |
| Cu | ≤ 0.2 | |
| Mo | ≤ 0.2 | |
| P | ≤ 0.03 | |
| S | ≤ 0.03 | |
| Febalance, ≈53–54 % | balance |
What surface finishes work on Kovar?
No anodizing — form a thin controlled Fe/Ni/Co oxide for glass sealing, or electroplate Ni (1–5 µm) with optional gold for solderability, brazing and corrosion protection.
Surface treatments FabDigit runs on nickel alloy parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Equivalent designations
Standards and trade names that resolve to Nickel Alloy Kovar in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- K94610
- ASTM
- F15
- EN / DIN / Werkstoff-Nr.
- 1.3981
- Also called
- Alloy K
- Other
- FeNi29Co174J29
How much does Kovar cost?
Kovar is a premium-cost nickel alloy ($$$), pricier than about 57% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Kovar — frequently asked
What is Kovar used for?
Kovar is typically used for glass-to-metal seal headers, vacuum tube components, hermetic package bases and lids, microwave and RF device housings, TO-can optoelectronic packages and ceramic-to-metal feedthroughs. In short: glass-sealing low-expansion alloy.
What is the yield strength of Kovar?
Kovar has a typical yield strength of 345 MPa (50 ksi) and a tensile strength of 520 MPa (75.4 ksi) — weaker than 62% of nickel alloy grades. Strength varies by condition: see the 6 listed tempers.
Is Kovar easy to machine?
Not especially — machinability is rated poor (32/100, better than 95% of nickel alloy grades). Machines like an austenitic Ni-Fe alloy — gummy and work-hardening: use sharp carbide, positive rake, heavy feed, low-to-moderate speed and flood coolant to avoid glazing.
Can Kovar be welded?
Yes — weldability is rated very good (80/100). Readily GTAW, resistance, electron-beam and laser welded to itself; keep the weld and heat-affected zone clean and low in carbon, and use matching Kovar filler for sealing-critical joints.
What surface finishes work on Kovar?
No anodizing — form a thin controlled Fe/Ni/Co oxide for glass sealing, or electroplate Ni (1–5 µm) with optional gold for solderability, brazing and corrosion protection.
Can Kovar be formed, bent or molded?
Annealed strip stamps, spins and deep-draws well; interstage anneals at 800–900 °C in dry hydrogen restore ductility and reset the expansion curve.
What is the maximum service temperature of Kovar?
Kovar is rated for continuous use to about 450°C (842 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between Kovar-O and Kovar-H?
O is the default condition — standard stocked condition for stamping, machining, welding and glass/ceramic sealing. H: highest strength condition, chosen for spring contacts and thin stiff wire; almost no cold-bend reserve and CTE is best re-annealed before sealing. Yield strength is 345 MPa in O versus 760 MPa in H.
How much does Kovar cost?
Kovar is a premium-cost nickel alloy ($$$), pricier than about 57% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Can FabDigit make parts in Kovar?
Yes — Kovar is available for CNC machining and sheet metal with instant online pricing. Upload a STEP file to get a price and a DFM check.
Sources
- ASTM F15 — Standard Specification for Iron-Nickel-Cobalt Sealing Alloy — ASTM International (2023)
- ASM Handbook Vol. 2 — Properties and Selection: Nonferrous Alloys and Special-Purpose Materials (controlled-expansion alloys) — ASM International (1990)
- Kovar / ASTM F15 alloy technical datasheet — specialty melter (CRS Holdings / Ni-Fe alloy producers) (2021)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.