Plain carbon steels are regarded as being those alloys of iron and carbon which contain up to 1.7 % C.
In practice, most ordinary steel also contain amounts of Mn, residual from the extraction process.
Fig -15- Part of the Iron-Carbon thermal equilibrium diagram
Pure metal iron, at temperature below 910?C, has a BCC structure, and if we heat it to above this temperature, the structure will change to FCC. The importance of this reversible transformation lies in the fact that up to 1.7 % carbon can dissolve in FCC iron.
Forming what is known as a "solid solution" whilst in BCC iron no more than 0.03 % carbon can be dissolve.
The solid solution formed when carbon dissolves in FCC iron is called Austenite, and the very weak solid solution formed in the BCC structure is called Ferrite. For most practical purposes we can regard ferrite as having the same properties as pure iron.
When carbon is precipitated from austenite it is not in the form of elemental carbon (graphite), but as the compound iron carbide, Fe3C, usually called Cementite. This substance is very hard, as the amount of carbon increases, the hardness of the steel will also increase.
Transformation will begin at the appropriate upper critical temperature (CE) and end at the lower critical temperature of 7232. The relative amount of ferrite and pearlite will depend upon the carbon content of steel, but in every case ferrite will be almost pure iron and the pearlite will contain exactly 0.83 % carbon so far we have been dealing only with structure in plain carbon steel by cooling them slowly from an austenite condition. By very rapid cooling from the austenite condition, such as would be obtained by water quenching, another structure called Martensite is produced. This does not appear on equilibrium diagram because it is not equilibrium structure.. Rapid cooling has prevented structural equilibrium from being reach.
Martensite is very hard and brittle, and the steel is used in this condition only when extreme hardness is required
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