IRON

Iron is the chemical element with atomic number 26 and symbol Fe. Metal that is grey, ductile and ferromagnetic, it is rarely used in its pure state but forms the basis of countless alloys essential to industry and horology. Alloyed with carbon, it produces steels and smelting; combined with chromium, nickel, tungsten or molybdenum, it forms stainless steels and special steels with varied properties. In horology, iron-based alloys are used to make springs, pivots, numerous components of the movement and cases made of steel. The ferromagnetic character of iron also explains the sensitivity of certain parts to magnetic fields, at the origin of the magnetization phenomenon that disrupts the running of watches. Mastery of iron-based alloys — from their composition to their Heat treatments — is at the heart of horological metallurgy and the reliability of timepieces (garde-Time).

Iron is also at the origin of iron-nickel alloys with remarkable properties for precision horology. In 1895, the Swiss physicist Charles-Édouard Guillaume, son of a horologist, discovered Invar (about 36% nickel), whose coefficient of thermal expansion is nearly zero; he then developed Elinvar, whose modulus of elasticity varies little with temperature and which, being non-magnetic, is suitable for making hairsprings and compensating balances. This work, rewarded with the Nobel Prize in Physics in 1920, made it possible to reduce the so-called “mean temperature” error of marine chronometers and then wristwatches, before the appearance of alloys such as Glucydur or Nivarox. The Curie point of pure iron, around 770 °C, marks the temperature beyond which it loses its ferromagnetic properties. To learn more: biography of Charles-Édouard Guillaume (Nobel Foundation).