Martensite

Martensite is a single-phase material that is formed if
Austenite is cooled rapidly to room temperature. It has a Body
Centered Tetragonal crystal structure, and the carbon in the alloy
is retained as an interstitial solid solution. The carbon distorts the
crystal structure and causes strains which make the material brittle.
To reduce these strains, Martensite forms "twinned" crystals, and
the twin boundaries also make the material less ductile.

If Martensite is kept at temperatures high enough for carbon atoms to diffuse in the crystal lattice, it will convert to the thermodynamically stable form of the alloy, Pearlite. Pearlite is a two phase material with distinct alpha-iron and iron carbide phases, each of which contains some of the carbon in the alloy. Forming Pearlite reduces the strain in the material and increases its ductility. Annealing (or Tempering) processes control this reaction to form a material of the required hardness.