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