Iron is an allotropic material. The temperature at which the allotropic changes take place in iron is influenced by alloying elements, the most important of which is carbon. Information about the portion of

The part of iron-carbon alloy system diagram between pure iron and an interstitial compound, iron carbide (Fe3C), containing 6.67 percent carbon by weight is called iron-iron carbide equilibrium diagram. It may be noted that though it is called as equilibrium diagram, it is not a true equilibrium diagram, since equilibrium implies no change of phase with time. In fact, the compound iron carbide decomposes into iron and carbon (graphite). This decomposition takes a very long time at room temperature, even at 1300°F, it takes several years to form graphite. The iron carbide is called meta stable phase. Therefore, iron-iron carbide diagram even though technically represents meta stable conditions, can be considered as representing equilibrium changes, under conditions of relatively slow heating and cooling.

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