This page includes notes on iron and what happens when iron /carbon mixtures are cooled from liquid to solid.   The notes are based on the Iron Phase Diagram (equilibrium diagram ).  A "Phase" is a form of material having characteristic structure and properties. It is a form of the material which has identifiable composition, structure and boundaries separating it from other phases in the material volume.  The diagram below shows shows the phases present when when Fe-C alloys (C up to 7%) are cooled from liquid to solid.

The left side of the diagram represents pure iron and the right hand of the diagram represents an alloy with 6,67% C. which result on cooling in the formation of Cementite.    This is a intermetallic compound (iron carbide-Fe₃ C) which although not 100% stable is but is to all practical purposes a stable phase.  The phase diagram shown is therefore a meta-stable phase.

Note: Different reference sources indicate the Eutectoid point at 0,8% C and 0,77% C.

Iron can exist in three forms:

α... BCC crystal with crystal dimension a = 2,86 Angstrom exists at temperatures up to 910^oC                 
γ... FCC crystal with crystal dimension a = 3,65 Angstrom exists at temperature range 910^oC to 1403^oC 
δ... BCC crystal with crystal dimension a = 2,93 Angstrom exists at temperature range 1403^oC to 1535^oC

The phased diagram includes four solid phases

α Ferrite
..The solid solution of carbon in iron. At 0% C this is pure iron. BCC crystal structure.
The maximum solubility of carbon in iron is 0,02% at 723^oC. At 0 ^oC temperature the solubility falls to 0,008%. The carbon atoms are located in the crystal interstices.

Austenite
The solid solution of carbon in γ iron is called austenite . This has a FCC crystal structure with a high solubility for carbon compared with α ferrite.  The solubility reaches a maximum of 2,08% at 1148^oC . The solubility decreases to 0,8% at 723 ^oC The carbon atoms are dissolved interstitially.  The difference in solubility between the austenite and α Ferrite is the basis for the hardening of steels

Cementite
This is an intermetallic compound which contains 6,67% C and 93,3% Fe.  Cementite is a hard brittle compound with and orthorhombic crystal structure each unit cell has 12 Fe atoms and 4 C atoms

δ Ferrite..
This is a solid solution of carbon in iron and has a BCC crystal structure.  The maximum solubility or C in Fe is 0,09% at 1495^oC. This has no real practical significance in engineering.

The lever rule can be applied to any phase region an provides an indication of the proportions of the constituent parts at any point on the phase diagram.

Applying the lever rule to the eutectoid point (0,80% C at 723^oC )
Wt% Ferrite = 100 (6,67 -0,8)/ 6,67- 0,02) = 88%
Wt% Cementite = 100 (0,8- 0,02) /6,67- 0,02) = 12%

If the carbon content of the cooled solid is less than Eutectoid (about 0,8% C) the solid is identified as a hypoeutectoid steel: most steels are this form.   If the carbon content is more then 0,8% then the solid is a hypereutectoid steel.   Hypereutectoid steels with carbon content over 1,2% C are very brittle.  Few steels are made with carbon contents over 1,2%.

Generally in order to increase the strength of steel other alloying elements are added which increase the strength while retaining toughness and ductility

A eutectoid solid raised to a temperature just above 723^oC and held at this temperature for some time will become a homogeneous austenite phase.  If this solid is then slowly cooled the entire structure will change to a lamellar structure of alternate plates of ferrite and cementite.   This structure is called pearlite because of its mother-of-pearl appearance.  As calculated above pearlite consists of 88% (mass) ferrite and 12% (mass) cementite

Transformation of a hypereutectoid steel (> 0,8% C) will result in pearlite in a matrix of cementite.  Transformation of a hypoeutectoid steel (< 0,8% C ) will result in pearlite in a matrix of α Ferrite.

Below is an Isothermal Transformation (IT) diagram, also called a TTT (Time, Temperature, Transformation) curve for a eutectoid steel test piece which has been rapidly cooled in a bath at a set temperature, held for a time and then water quenched.

It can be seen that if the transformation is allowed to take place at a higher temperature then, as above coarse pearlite is formed.

If the test piece is cooled to a lower temperature in a bath a finer pearlitic structure results.

If the test piece is rapidly cooled to a temperature below a value M_s (which varies with the carbon content then a new metastable phase is produced called Martensite.  Martensite is a supersaturated solid solution of carbon in ferrite.

If the test piece is cooled rapidly at a temperature between 220^oC and 525^oC a phase structure between pearlite and martensite is formed. This is called Bainite

Bainite is a constituent which forms from austenite in a tempertures range below 530 ^oC and above M_s . Bainite forms together with pearlite in steel which are cooled somewhat too fast to form a complete pearlite structure. Bainite is like pearlite a mixture of ferrite and iron carbide but in a different form. The bainite structure varies from a featherlike pattern to pattern of lens shaped particles depending on the temperature range of formation. (Featherlike constituent in upper temperature range and lens like in the lower temperature range). Bainite is harder, stronger and tougher than ferrite-pearlite structures at lower temperatures.

Martensite is the hardest structure formed from austenite.    It is a distorted BCC (tetrogonal) it is a body centred tetrogonal structure.  The distortion is caused by trapped carbon atoms which have not been able to nucleate into cementite

Some features of martensite are listed below