Alloying of iron with carbon can produce a wide variety of properties. In general steels become stronger and less ductile with increasing carbon content. However a wide variety of combinations of properties can be obtained by altering the both the composition along with thermal and mechanical processing. Steel containing containing 0.4 per cent (by mass) of carbon is twice as strong as pure iron. Steels with 1 per cent carbon are nearly 3 times as strong. Iron-carbon alloys are classified by carbon content in the table below.
|Classifcation||Carbon content (mass or weight percent)|
|Intersitial free steel||?|
|Extra low carbon steel||?|
|Medium carbon steel||0.25-0.45|
|High carbon steel||0.45-1.50|
Examples of different carbon contents in different applications are shown in this figure.
Slowly cooling from the high temperature crystal structure (austenite) in carbon steels will develop microstructures as shown in this figure. At room temperature less than 0.01 per cent carbon exists in equilibrium in solid solution in the low temperature crystal structure of iron (ferrite). Carbon is instead found to exist in an intermetallic compound, iron carbide, of three iron atoms and one carbon atom. Greater proportions of this compound form at higher carbon contents. b = 0.3, c = 0.6, d = 0.8 in the figure below. The cementite forms as an intimate mixture with lamalae of ferrite and cementite. Above the eutectic composition of 0.8 per cent the carbide forms first at the previously existing austenite grain boundaries as shown in case d (1.1 per cent carbon) below.
Faster cooling will produce structures further away from the equilibrium structure. Fast cooling can produce ferrite supersaturated with carbon, which has a tetragonal closed packed structure called martensite. Intermediate cooling rates or isothermal transformation can form bainite.
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