| Austenite |
This phase is only possible in carbon steel at high temperature.
It has a Face Centre Cubic (F.C.C) atomic structure which can contain
up to 2% carbon in solution.
|
| Ferrite |
This phase has a Body Centre Cubic structure (B.C.C) which can hold
very little carbon; typically 0.0001% at room temperature. It can
exist as either: alpha or delta ferrite.
|
| Cementite |
Unlike ferrite and austenite, cementite is a very hard intermetallic
compound consisting of 6.7% carbon and the remainder iron, its chemical
symbol is Fe3C. Cementite is very hard, but when mixed with soft ferrite
layers its averidge hardness is reduced considerably. Slow cooling gives
course perlite; soft easy to machine but poor toughness. Faster cooling
gives very fine layers of ferrite and cementite; harder and tougher
|
| Pearlite |
A mixture of alternate strips of ferrite and cementite in a single grain.
The distance between the plates and their thickness is dependant on the
cooling rate of the material; fast cooling creates thin plates that
are close together and slow cooling creates a much coarser structure possessing
less toughness. The name for this structure is derived from its mother
of pearl appearance under a microscope. A fully pearlitic structure
occurs at 0.8% Carbon. Further increases in carbon will create cementite
at the grain boundaries, which will start to weaken the steel.
|
| Martensite |
If steel is cooled rapidly from austenite, the F.C.C structure rapidly
changes to B.C.C leaving insufficient time for the carbon to form pearlite.
This results in a distorted structure that has the appearance of fine needles.
There is no partial transformation associated with martensite, it either
forms or it doesn't. However, only the parts of a section that cool
fast enough will form martensite; in a thick section it will only form to
a certain depth, and if the shape is complex it may only form in small
pockets. The hardness of martensite is solely dependant on carbon
content, it is normally very high, unless the carbon content is exceptionally
low.
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