Alloy 36

Alloy 36 holds a coefficient of expansion around 1.4 µm/m·°C between 20 and 100°C — roughly a tenth that of carbon steel — and keeps that behaviour from cryogenic temperatures upward. The low expansion is a magnetic effect and it disappears as the alloy passes its magnetic transition. Published data shows the mean coefficient climbing from 1.4 µm/m·°C at 100°C to about 2.5 at 200°C, 5.6 at 300°C and 8.5 at 400°C, by which point there is no low-expansion advantage left at all. Approximately 260°C (500°F) is the practical ceiling; above it, specify Alloy 42 if a moderately low and more nearly constant coefficient is wanted to 300°C, or accept a conventional alloy.

Unlike Alloy 32-5, Alloy 36 remains stable and tough at cryogenic temperatures, which makes it the correct choice of the two for LNG, liquid-gas and deep-cryogenic hardware. It is also widely used for composite forming tooling because its expansion is close to that of carbon fibre laminates, so a tool and the part cured on it move together.

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.