Alloy 42

Alloy 42 exists for one reason: its coefficient of expansion of roughly 5.3 µm/m·°C is a close match to silicon, to alumina ceramics and to the softer sealing glasses, and it holds that value nominally constant from room temperature to around 300°C. That is why it dominates semiconductor lead frames, where a mismatch against the silicon die would crack the package on thermal cycling, and why it is used for glass-to-metal seals in electronic tubes, lamps and transformer and capacitor bushings. It is also the standard core for copper-clad sealing wire.

Note that Alloy 42 does not compete with Alloy 36 or Alloy 32-5 on absolute expansion — at 5.3 µm/m·°C it is roughly four times Alloy 36 and eight times Alloy 32-5. It is chosen for the match, not the minimum, and it holds that match over a wider temperature span than either. Where the lowest possible movement is wanted, use Alloy 32-5 at room temperature or Alloy 36 across a wider band.

Machining and forming stresses change the expansion coefficient. This is the single most important processing fact about this family, and Carpenter states it explicitly for Super Invar: machining, forming and deep drawing introduce stresses that promote variation in thermal expansion behaviour, so parts should be heat treated at or as close to finish size as possible. For a threaded fastener that matters a great deal, because thread rolling is heavy local cold work. Exotic Alloy therefore machines and threads first and performs the stabilising heat treatment on the finished part — if a component is machined after its final stabilising treatment, the expansion coefficient it was bought for is no longer guaranteed.

These alloys are not corrosion resistant. Chromium is an incidental residual rather than an alloying addition — 0.5% maximum in Invar 36, 0.25% maximum in Alloy 42, and not specified at all in Super Invar — so there is no passive film and these materials rust in damp air much like plain carbon steel. Nickel, cadmium, tin or dry-film coating is required for any exposed or humid service, and components are commonly supplied plated. Passivation is not an option because there is nothing to passivate.

These are property alloys, not strength alloys. All three grades are soft iron-nickel austenites with room-temperature tensile strengths around 480–490 MPa and yield strengths of 240–280 MPa — roughly comparable to annealed 304 stainless and well below any structural bolting material. Nobody specifies Invar for load. They are specified because they barely move when the temperature changes, and a fastener in one of these alloys is there to hold a dimensionally stable assembly together without introducing thermal mismatch of its own.

Invar is a registered trademark originating with Imphy Alloys, and Nilo is a trademark of the Special Metals Corporation group. Exotic Alloy is an independent manufacturer and is not affiliated with or endorsed by either; the designations are used here solely to identify the alloy chemistry, which is defined by the UNS number and ASTM specification given above.