ferrite (alpha-iron)

Ferrite is plain body-centered-cubic iron, the soft, ductile, magnetic iron of everyday steel at room temperature. It is iron in its low-temperature crystal form, and it can barely hold any carbon at all, like a nearly full sponge that just will not soak up any more.

More precisely, ferrite (alpha-iron) is the BCC solid solution of carbon in iron, stable below 912 degrees C. Its carbon solubility is tiny: only about 0.022 wt% at most (reached at 727 C) and far less at room temperature, because the interstitial holes in the BCC lattice are small. Ferrite is soft (around 80 Brinell), ductile, and ferromagnetic below its Curie temperature of 770 C. A separate high-temperature BCC form, delta-ferrite, exists above 1394 C.

Ferrite gives steel its ductility and toughness; a low-carbon steel is mostly ferrite with a little pearlite, which is why it bends rather than snaps. Inside pearlite, soft ferrite lamellae alternate with hard cementite. And because ferrite holds so little carbon, cooling austenite (which had dissolved plenty) forces the excess carbon out as cementite, which is exactly the basis of the eutectoid reaction.