4. Hardening and changes in the structural state of iron

In practice, the main factor in iron hardening is the precipitation of iron carbides or carbide-forming elements contained in the steel.

This is made possible by changes in the crystallographic state of iron, which are accompanied by significant variations in carbon solubility. Indeed, if we consider the Fe/C diagram (figure 1 ), we see that in the solid state :

• At high temperatures, the iron crystal lattice is face-centred cubic (γ-iron) and the solubility of carbon can reach 2% by mass;

• at low temperatures, the iron crystal lattice is cubic-centered (α-iron) and carbon solubility is very low (no more than 0.02%).

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