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How does carbon make Iron Strong ?

When Carbon is dissolved in Liquid Iron, and if the solution is solidified slowly the carbon tends to separate out giving a structure which is a mixture of pure iron and graphite. This gives a fairly tough iron which breaks with a dark glistering fracture. It is easily machined. If the same iron is cast and cooled quickly, it is hard , has a higher tensile strength , is difficult to machine and breaks with a close, white fracture. The white constituent is Iron Carbide , called cementite.   





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