Project Description

FeNiCo Glass Sealing Alloy

Kovar (4J29) is an iron-nickel-cobalt alloy with a coefficient of thermal expansion similar to that of hard (borosilicate) glass. This makes it especially suitable for uses which require a matched-expansion seal between metal and glass parts. Thus kovar finds wide usage in the electronics industry for metal parts bonded to hard glass envelopes for such devices as power tubes, x-ray tubes, etc., and other applications requiring glass-to-metal seals.


4J29, 4J44

Similar Grade

4J29KovarK94610/F151.398129НКNILO K


YB/T5235-93, Q/SYY 7-1999, ГОСТ 10994-1974, ГОСТ 14080-78

Product Forms

Sheet, Strip, Plate, Round Bar, Flat Bar, Forgings, Hexagon, Wire

Chemical Composition, %


Average Linear Expansion Coefficient

AlloyHeat treatmentAverage linear expansion coefficients, a/10-6*k-1ConditionTensile strength, MPa
4J29Heating to (900±20)℃ in hydrogen holding for one hour reheating to(1100±20)℃ holding for 15 min cooling at a rate less than 5℃per minute to 200℃ then air cooling4.6~5.25.1~5.5Soft (R)<570
1/4 hard (1/4)520~630
4J444.3~5.14.6~5.21/2 hard (1/2)590~700
3/4 hard (3/4)600~770


FeNiCo Glass Sealing Alloys take on a constant coefficient under -60℃~+40O℃. They can firmly match seal in hard glass.

Kovar was invented to meet the need for a reliable glass-to-metal seal, which is required in electronic devices such as light bulbs, vacuum tubes, cathode ray tubes, and in vacuum systems in chemistry and other scientific research. Most metals cannot seal to glass because their coefficient of thermal expansion is not the same as glass, so as the joint cools after fabrication the stresses due to the differential expansion rates of the glass and metal cause the joint to crack.

Kovar not only has thermal expansion similar to glass, but its nonlinear thermal expansion curve can often be made to match a glass, thus allowing the joint to tolerate a wide temperature range. Chemically, it bonds to glass via the intermediate oxide layer of nickel(II) oxide and cobalt(II) oxide; the proportion of iron oxide is low due to its reduction with cobalt. The bond strength is highly dependent on the oxide layer thickness and character. The presence of cobalt makes the oxide layer easier to melt and dissolve in the molten glass. A grey, grey-blue or grey-brown color indicates a good seal. A metallic color indicates lack of oxide, while black color indicates overly oxidized metal, in both cases leading to a weak joint.


Launching tube oscillation tube ignitron, transistor as well as electro-vacuum component etc. The alloy is a typical international Fe-Ni-Co hard glass sealing alloy. The performance is stable after long term use by the aeronautical plant. It is mainly used for the glass sealing of the electric vacuum components, such as the emitter tube, the oscillating tube, the ignition tube, the magnetron, the transistor, the sealing plug, the relay, the elicited line of the integrated circuit, the chassis, the shell and the bracket. The expansion coefficient of the selected glass should be matched with the alloy in application. The stability of low temperature tissue was examined strictly according to the temperature. Proper heat treatment should be carried out during the processing to ensure that the material has good deep drawing performance. The tightness of the forged material should be strictly inspected when using the forged material. FeNiCo Glass Sealing Alloys are fit for manufacturing electronic vacuum apparatus such as emitting tubes, oscillators, ignition rectifiers, transistors, pipe-closers and outer casings of relay.