Crystal Structure of Cementite and Martensite
Iron Carbide or Cementite (Fe3C): It is chemical compound of iron and carbon of composition Fe3C. It is of orthorhombic structure. Its unit cell has 12 iron atoms and 4 carbon atoms. It has carbon content of 6.67%. Its melting point is approximately 1550 degree C Cementite is magnetic upto 210 degree C. It is non-ductile and brittle, interstitial compound of low tensile strength but high compressive strength.
It iron-iron carbide, the melting point of iron is shown by point (1538 degree C) and melting point of cementite is (1500 degree C). The structure changes from bcc ferrite to fcc austenite at point G (912 degree C) and from y-fcc to a-fcc at point N (1394 degree C).
When cooling rate is increased above critical value i.e. upon water quenching, the time is too less for carbon to diffuse out of solution. Therefore the fcc structure does not change to bcc structure while carbon is trapped in solution. The resulting structure is body center tetragonal which is called martensite. It is super saturated solid solution and non-cubic structure. The two dimensions of body center tetragonal unit cell are equal while the third side is slightly expandd because of trapped carbon. Because carbon is present in the lattice, slip does not occur easily. That is why, martensite is hard, strong and brittle. It has needle like structure.
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