Expansion alloys
UNS Number | UNS K94610 |
Industry Name | Kovar, W.Nr.1.3981, NiFeCo29,Nilo K |
Product Forms | Rods, Bars, Plates, Wires, Strips |
General Characteristics
Alloy 4J29, widely known as Kovar, is an iron-nickel-cobalt low expansion alloy. It was developed to solve the mismatch in thermal expansion between ordinary metals and hard glass or ceramics during sealing. Traditional ferrous and non-ferrous metals expand much more when heated, which creates high thermal stress and causes cracks or leaks in glass-to-metal seals.
It is designed for vacuum hermetic sealing working conditions. The alloy closely matches the thermal expansion coefficient of borosilicate glass over a wide temperature range. Its key characteristics include low and stable thermal expansion, good ductility, excellent weldability and brazing performance, and reliable vacuum tightness after sealing.
When compared with Invar 36, Invar 36 has ultra-low expansion at room temperature but its expansion rises rapidly at elevated temperatures and cannot match glass expansion for sealing. 4J29 maintains matched expansion for glass sealing. Compared to 4J42, 4J42 contains no cobalt, lower cost, but its expansion curve does not fit borosilicate glass as well as 4J29.
4J29 delivers stable hermetic sealing for high-reliability electronic and aerospace packages.
C | S | P | Mn | Si | Ni | Cr | Co | Cu | Mo | Fe |
≤0.03 | ≤0.02 | ≤0.02 | ≤0.5 | ≤0.3 | 28.5-29.5 | ≤0.2 | 16.8-17.8 | ≤0.2 | ≤0.2 | Bal. |
Aerospace||Hermetic sensor housings|Sealed connector pins for avionics|Glass-sealed instrument feedthroughs|Pressure transducer metal sealing components
Optical & Precision Instruments||Glass lens sealing mounts|Hermetic housing for optical detectors|Sealed metal frames for precision optical devices
Power Electronics||Sealed power diode bases|Ceramic-to-metal sealing components|Feedthrough pins for high-voltage sealed modules
Technical Standard | Product Form |
YB/T 5231 | Bar, Wire, Strip, Sheet |
ASTM F15 | Bar, Wire, Strip, Sheet, Plate |
AMS 7728 | Bar, Wire, Strip, Sheet |
Product form | Size range (mm) |
Forged bar | Dia.70-500 |
Rolled bar | Dia.2.0-70 |
Flat bar | T4-50*W60-300 |
Wire | Dia.0.25-18.0 |
Strip | 0.15-3.0 * 20-200 |
Plate | 1-25 * ≤1200 * ≤1200 |
Seamless tube | OD 6.0-200 * 0.6-20 |
Q1: What is the typical working temperature range?
A: Its matched expansion range for glass sealing is roughly from room temperature up to 450°C. Beyond this temperature, the expansion mismatch gradually increases.
Q2: 4J29 vs Alloy 4J36 (Invar 36)?
A: Invar 36 has extremely low expansion at room temperature but its expansion rises quickly at elevated temperatures, so it cannot be used for glass sealing. 4J29 is designed specifically for matching glass thermal expansion for hermetic seals.
Q3: 4J29 vs Alloy 4J42?
A: 4J42 is cobalt-free and cheaper, but its thermal expansion curve does not fit borosilicate glass well. 4J29 contains cobalt and delivers more stable, leak-free glass-metal sealing for high-reliability packages.
Q4: What melting options are available?
A: VIM for general electronic parts. VIM+VAR is used for high-end aerospace and semiconductor packaging to reduce impurities and improve sealing stability.
Q5: What surface preparation is required before glass sealing?
A: The surface must be clean and oxide-free. A pre-oxidation treatment is commonly performed to form a uniform oxide layer, which bonds firmly with molten glass.
Q6: Can 4J29 be machined easily?
A: Yes. It has good machinability in the annealed state. Cold working will increase hardness, so annealing is recommended for complex forming.