Invar 36 is a controlled-expansion nickel-iron alloy holding roughly 35-37% nickel on an iron balance, registered as UNS K93600 and covered by ASTM F1684. Its defining property is right in the name: near-zero coefficient of thermal expansion through roughly room temperature to about 100°C (212°F), a behavior discovered by the French physicist Charles Guillaume in 1896 (work that later earned him the Nobel Prize in Physics). This near-cancellation of thermal expansion comes from a magnetic effect (the Invar effect) that offsets the normal expansion of the iron-nickel lattice as temperature rises.

That dimensional stability makes Invar 36 the standard choice wherever a part cannot afford to grow or shrink with temperature: precision measuring instruments and gauges, composite tooling and molds (where the mold must track the low expansion of the carbon-fiber part being cured), cryogenic storage tank membranes and LNG carrier containment systems, shadow masks in older CRT displays, optical and laser benches, waveguide components, and seismic or metrology reference structures. Unlike the stainless steels covered elsewhere in this reference library, Invar 36 is chosen almost entirely for its physical (thermal) behavior rather than for corrosion resistance or strength.

A trace cobalt figure on the prior version of this page could not be verified against ASTM F1684 / UNS K93600 reference data and has been omitted rather than carried forward.

Invar 36 does not machine like the stainless steels elsewhere in this reference library, and treating it as "just another 36-nickel stainless" is the fastest way to burn through tooling. It machines much closer to a nickel-base alloy: tough, ductile chips that resist breaking, a strong tendency toward built-up edge, and rapid work hardening the moment the cutting edge rubs instead of shears. Despite a relatively modest hardness (typically well under 110 HB in the annealed condition), Invar is notorious for being gummy and difficult to hold tolerance on, precisely because the material deforms plastically ahead of the tool rather than shearing cleanly.

Invar 36 should be machined in the fully annealed condition whenever possible, since cold work or prior machining passes leave a hardened skin that compounds on subsequent passes. Sharp, positive-rake carbide is essential — a dull or neutral-rake edge invites smearing and built-up edge almost immediately. Feeds need to be light but the depth of cut must still be sufficient to get underneath any work-hardened layer from the previous pass; timid, rubbing cuts are worse than a slightly heavier, decisive one. Cutting speeds run well below what the modest hardness alone would suggest — this is a material where dialing speed back, not up, is usually the fix for poor tool life.

Turning speeds trend toward the lower end of the range for heavier roughing passes and toward the upper end for light finishing cuts, where a smaller work-hardened layer needs to be cleared. Values assume a rigid setup, sharp positive-rake carbide, and the material in the annealed condition.

Turning

Parting / Grooving

Milling