Lower Critical Point

  • Occurs at 723°c.
  • Temperature change occurs.
  • The metal contracts dimensionally, glows less brightly and loses its magnetism.
  • Known as Decalescence.

Upper Critical Point

  • Varies depending on amount of carbon in the steel.
  • Above this point, the metal exists as FCC austenite.

Eutectic Point

  • Liquid to solid
  • Occurs at 1140°C at 4.3% carbon
  • Liquid steel changes to solid austenite and cementite.

Eutectoid Point

  • Solid austenite changes to solid pearlite.
  • Occurs at 723°C at .83% Carbon.


  • Almost Pure Iron.
  • Above a temperature of 1390°C or below 910°C, iron has a BCC lattice.
  • Also known as alpha iron.
  • Contains less than .2% Carbon.


  • Cementite is an inter-metallic compound of Iron and Carbon.
  • It is a hard and brittle material.


  • Exists Between 1390°C and 910°C.
  • Has a BCC structure.
  • Also known as Gamma iron.


  • Pearlite is a mixture of alternate layers of ferrite and cementite. It gives off a sheen similar to that of Mother of Pearl.
  • Occurs at .83% Carbon. (Eutectoid Point)


  • Martensite has a body centred tetragonal shape.
  • It occurs when austenite is quenched rapidly.
  • It has a stretched axis due to its retention of carbon.
  • It is a hard needle like structure.
  • Strong but brittle.


Allotropy of Iron

  • Between 1390°C and 910°C, Iron has a FCC structure known as austenite.
  • Above 1390°C and below 910°C iron exists in BCC structure known as ferrite.
  • When iron changes from BCC to FCC at 1390°C, a slight shrinkage in volume occurs as the atoms are more tightly packed together.
  • The reverse reaction, changing from FCC to BCC at 910°C causes an expansion in volume.