What is Invar?

Invar is a very low coefficient of thermal expansion metal alloy. When temperature varies of 1° C, an Invar rod 10 kilometres long varies its length of 0.8 - 2 centimetres, depending on how it has been worked. Indeed, a steel rod in the same conditions would vary of 11 cm., a brass rod of 19 and an aluminium rod of 25.5. 10 kilometres correspond to one million centimetres, so expansions are usually expressed in parts per million per C degree of temperature variation. There is an alloy called Super Invar with a smaller coefficient of thermal expansion, less than 0.63 parts per million (PPM) and in some cases, negative: the alloy reduces its length when temperature increases. This can happen by no more than 0.1 PPM per C degree of temperature variation (1 millimetres per 10 kilometres per C degree of temperature variation). Invar is composed of steel with about 36% nickel, and smaller quantities of other elements, added to increase machinability or remained from the production process, where its difficult to obtain absolutely pure alloys. Three kinds of Invar can be purchased, at least from our sources.
1) Invar 36, seldom used because very difficult to machine. Its coefficient of thermal expansion is about 1.6 PPM. This is its detailed chemical composition:

2) FM Invar (Free Machining Invar), containing tiny amounts of selenium and more manganese than Invar 36. Unfortunately, it contains twice the carbon contained in Invar 36 and this causes its long-term structural instability. In other words, it suffers from small spontaneous deformations during years.
3) Super Invar, containing more cobalt, with a coefficient of thermal expansion lower than 0.63 PPM. Experience indicates that it's the less stable kind of Invar, the most liable to spontaneous size changes. These can be minimized by annealing, the process of slowly increasing the temperature of metals after they have been temper worked, to relieve their internal tensions. This of course slightly softens the metal.