Invar machining glossary
40 terms that appear on Invar drawings, quotes, certificates and in our guides, defined in plain language. Each one links to the guide that goes deeper where we have one.
Iron-nickel alloy with about 42 % nickel (ASTM F30, UNS K94100). CTE around 4 to 6 ppm/°C, used for lead frames, bimetal parts and some glass seals. Read the guide.
Heating to a high temperature, typically 830 °C for Invar 36, and cooling in a controlled way to soften the alloy, remove cold work and set the structure that gives low expansion. Read the guide.
Test method for linear thermal expansion using a push-rod dilatometer. The usual basis for measured CTE on a certificate.
Specification for the iron-nickel-cobalt sealing alloy known as Kovar, UNS K94610. Read the guide.
Specification for iron-nickel and iron-nickel-cobalt low expansion alloys in bar, plate, sheet and strip. The usual spec for Invar 36 and Super Invar.
Specification for iron-nickel sealing alloys such as Alloy 42.
Work material welded onto the cutting edge during machining. Invar is prone to it because the alloy is gummy. Sharp positive tools and steady feeds reduce it. Read the guide.
Coordinate measuring machine. Precision Invar parts are measured on a CMM in a temperature controlled room after a soak at 20 °C. Read the guide.
Plastic deformation of the metal below its recrystallization temperature, for example by cutting, bending or rolling. In Invar it raises the CTE near the surface and stores stress. Read the guide.
Coefficient of thermal expansion. The fractional change in length per degree of temperature, quoted in ppm/°C. Invar 36 is about 1.3 near room temperature, steel about 12. Read the guide.
A change in an Invar part's expansion from its annealed value caused by cold work, residual stress or an incomplete heat treatment.
The temperature above which a ferromagnetic alloy loses its magnetism. Invar 36 is about 230 °C. Above it the low expansion is lost as well.
A reference feature on the drawing from which other features are located. On an Invar part, datums decide how the shop fixtures it for finishing and measures it.
Removing carbon from the surface of a part, usually by annealing in wet hydrogen. Required on Kovar before glass sealing so the glass wets a clean surface.
An instrument that measures length change against temperature. The source of the CTE curves on a certificate.
The ability of a part to keep its size and shape over time and temperature. The property Invar is bought for, and the one machining can damage. Read the guide.
Electrical discharge machining. Wire EDM cuts Invar without cutting force, useful for thin sections and hard corners, but leaves a recast layer. Read the guide.
Invar 36 with added selenium or sulfur (UNS K93601) that breaks chips and cuts faster. Slightly different properties, check the spec allows it. Read the guide.
A hermetic joint between glass and a metal whose expansion matches the glass. Kovar with borosilicate glass is the common pairing. Read the guide.
Material from one melt at the mill. Chemistry and CTE on a certificate apply to one heat lot only.
Iron alloy with about 36 % nickel (ASTM F1684, UNS K93600). Typical CTE 1.2 to 1.6 ppm/°C near room temperature. The most used low expansion alloy.
Iron-nickel-cobalt alloy (29Ni-17Co, ASTM F15) with expansion matched to borosilicate glass and alumina. Used for seals, lids, feedthroughs and headers.
A percentage relative to B1112 free cutting steel at 100 %. Invar 36 sits around 40 to 50 %, similar to 304 stainless, which understates how gummy it is. Read the guide.
A hard phase that can form in Super Invar when it is cooled well below zero. It raises the expansion permanently, which is why Super Invar is kept above about −60 °C.
The mill test report for a heat lot: chemistry, sometimes hardness and grain size, CTE only if ordered. Ask for it on every Invar order. Read the guide.
Parts per million per degree Celsius, the unit of CTE. 1 ppm/°C means a 1 m bar changes by 0.001 mm per degree.
A thin resolidified layer left by EDM. Has different properties from the base metal and is usually removed on precision surfaces.
Stress locked into a part by machining, forming or uneven cooling. In Invar it can move the part days after inspection and raise the CTE. Stress relief removes it. Read the guide.
The first machining stage, removing most of the stock while leaving an even allowance on the surfaces that matter.
An intermediate pass after stress relief that brings the part close to size before the final light finishing cuts.
Holding a part at a fixed temperature until it is uniform through its thickness. Parts soak at 20 °C before measurement, and at about 95 °C during a stabilization cycle.
A low temperature soak, typically 95 °C for 24 to 48 h, after annealing and tempering. Settles the structure so the part does not drift over months.
A heat treatment, typically 315 to 345 °C for 1 to 2 h for Invar 36, between roughing and finishing. Removes machining stress so the part stays where it was finished. Read the guide.
Iron-nickel-cobalt alloy (32Ni-5Co) with CTE about 0.3 to 0.6 ppm/°C over roughly 20 to 90 °C. Lower expansion than Invar 36 over a narrower band. Read the guide.
A moderate temperature treatment after quenching, about 315 °C for Invar, that relieves quench stress without undoing the anneal.
An acceptance test that cycles a part between two temperatures and measures it before and after to prove dimensional stability. Read the guide.
Measuring a part at 20 °C, the reference temperature in ISO 1, after a soak. The only way a tight Invar tolerance means anything. Read the guide.
Unified Numbering System for metals. K93600 is Invar 36, K94610 is Kovar, K94100 is Alloy 42.
The increase in hardness and strength of a metal as it is deformed. Invar work hardens quickly, so a rubbing tool makes the next pass harder and the surface more stressed. Read the guide.
A glass ceramic with near zero expansion used for mirror blanks. Compared with Invar when a design needs the lowest possible expansion and can accept a brittle material. Read the guide.
Looking for spec numbers and property tables? See the Invar specs reference.