Invar 42 (also called Alloy 42) is a controlled-expansion nickel-iron alloy holding roughly 41-42.5% nickel on an iron balance, registered as UNS K94100 and covered by ASTM F30. It belongs to the same low-expansion alloy family as Invar 36, but the higher nickel content shifts the thermal expansion coefficient upward — Invar 42 does not reach the near-zero expansion of Invar 36, but it holds a low, consistent expansion rate up to roughly 315-345°C (600-650°F) and, critically, its expansion coefficient is closely matched to silicon and to common ceramic and glass materials such as alumina, beryllia, and borosilicate glass.

That matched expansion behavior is what makes Invar 42 the default choice for glass-to-metal and ceramic-to-metal seals: semiconductor package leadframes, electronic tube and CRT electron gun components, microelectronic packaging, vacuum device housings, and thermostat components where a metal part is bonded or sealed directly to glass or ceramic and cannot be allowed to expand away from it during thermal cycling.

A trace chromium figure on the prior version of this page could not be verified against ASTM F30 / UNS K94100 reference data (standard Invar-type alloys are not chromium-bearing) and has been omitted rather than carried forward.

Invar 42 shares its close relative Invar 36's machining personality, not the machining personality of a stainless steel. It is tough and ductile with a strong tendency to smear and work harden ahead of the cutting edge rather than shear cleanly, and it produces gummy, difficult-to-break chips that resist the kind of clean fracture seen in carbon or alloy steel. Somewhat lower nickel-driven hardness than Invar 36 (typically under 100 HB annealed) does not translate into easier cutting — the alloy's plastic, adhesive behavior at the tool tip is the limiting factor, not raw hardness.

As with Invar 36, the best results come from machining in the annealed condition, using sharp positive-rake carbide, and taking a decisive depth of cut rather than a light, rubbing pass that leaves a work-hardened skin for the next tool pass to fight through. Built-up edge is a persistent risk at low-to-moderate speeds, so coolant and edge preparation both matter more here than on general stainless. Conservative feeds combined with adequate depth of cut, rather than high speed, are the primary lever for tool life on this material.

Turning speeds trend toward the lower end for heavier roughing and toward the upper end for light finishing cuts. Values assume a rigid setup, sharp positive-rake carbide, and the material in the annealed condition.

Turning

Parting / Grooving

Milling