From isarn, the old Saxon word for iron.
The atomsĀ in metals are arranged in a regular three-dimensional pattern called a crystal structure. In the case of iron it can be visualised as a series of cubes stacked side by side, and one on top of the other. The corners of the cube are atoms and each corner is shared by eight adjoining cubes or cells. As well as corner atoms each unit cell contains additional atoms, with one atom at the centre of the cell, it is termed a body-centred cubicĀ structureĀ (bcc), with atoms at the centre of each face of the cell it is termed a face centred cubicĀ structureĀ (fcc).
Pure iron is capable of existing in three forms, all of which are stable within different temperature ranges. Between room temperature and 911Ā°C iron has a body-centred cubic, bcc crystal structureĀ and is termed Ī¬ (Alpha) iron, (commonly known as ferrite). At 91IĀ°C a crystalline transformationĀ occurs and the bcc structureĀ changes to face- centred cubic, fcc. This form is termed Ī³ (Gamma) iron (austenite) and exists up to 1392Ā°C, at which temperature the structureĀ again changes to bcc, the high temperature, Ī“ (Delta-ferrite) form.
Other metallic elements, when added to iron, have their atoms interspersed in the gaps between the iron atoms and in this way alloysĀ are formed. The addition ofĀ carbonĀ to iron, as in the case of steel, causes alterations to the crystal structureĀ by the imposition of carbonĀ atoms into the gaps between iron atoms; e.g. in gamma-iron, austenite. Rapid cooling of steelĀ by quenchingĀ from the austeniticĀ temperature range produces crystallographic transformationĀ to the meta-stable hardĀ phase, martensite.
See also ferrous.