Heating white cast iron (containing iron carbide, i.e. cementite, but no uncombined carbon) above  causes the formation of austenite in crystals of primary cementite. This austenisation of white iron occurs in primary cementite at the interphase boundary with ferrite. When the grains of austenite form in cementite, they occur as lamellar clusters oriented along the cementite crystal layer surface. Austenite is formed by diffusion of carbon atoms from cementite into ferrite.