Constitution of iron-carbon alloys
Unalloyed steels

Iron stands out as the most commonly used metal of all. However, it is not generally used in its pure state, but serves as the basic component for a wide range of alloys. Most of these are derived from the "iron-carbon" binary system, whose composition is dictated by the corresponding equilibrium diagram. Intriguingly, this diagram shows a duality, reflecting the fact that these alloys can exist, at equilibrium, in two distinct forms: "iron-graphite" and "iron-cementite". This depends on whether the carbon in the iron is free or combined. This singular property goes a long way towards explaining the remarkable diversity of iron-based alloys and the resulting products.

The diversity is also due to the fact that some of these alloys can, in a non-equilibrium...