A metal such as iron is soft and ductile only when defects called dislocation are able to glide though the crystal lattice:while the dislocations are flowing they breed and eventually begin to tangle up to the extent that the metal becomes brittle.Now iron can only dissolve about 0.003% of carbon at room temperature and any further carbon in the metal exists as the carbide(Fe3C)called cementite either as very small particles precipitated from solution in the ferrite or in the form of pearlite;a mechanical mixture of thin plates of the carbide and and the room temperature form of iron called ferrite.This pearlite constituent(eutectoid) forms directly from the solution of carbon in the high temperature form of iron(austenite)which exists in steel at temperatures between 1400 and 725 deg.C.Austenite can dissolve a lot of carbon (1.7% at 1130 C and 0.87% at 825degC)If a steel is cooled slowly from a temperature of ,say,900 deg. C the carbon is able to diffuse in the iron and form pearlite which toughens iron but doesnt greatly embrittle it.If cooling is rapid,as in water quenching from 900 deg C,The carbon is trapped in the the iron The iron MUST transform from austenite to ferrite and so the crystal stucture of the ferrite is badly distorted(going under the name Martensite).martensite is extremely hard and brittle because the structure is so badly deformed that dislocations cannot glide.Martensite can be softened by tempering ,which means heating it to between about 300 to 650 deg C,this causes cementite to be precipitated in the ferrite and so some ductility is restored but pearlitic steels rapidly work harden as the dislocation motion is restricted by the presence of the pearlite.Structural steels are often of the pearlitic type whereas high grade engineering steels are mostly hardened and tempered.