Phases in 0.80% carbon steel

Steel with 0.80% carbon is the eutectoid steel. Pearlite itself contains 0.80% C, so the steel is entirely pearlite.

Data: carbon in steel $C = 0.80\%$; carbon in pearlite $= 0.80\%$; carbon in cementite ($Fe_3C$) $= 6.67\%$. Carbon dissolved in ferrite is neglected (about 0.02%).

Pearlite

$$\text{Pearlite} = \frac{0.80}{0.80} \times 100 = 100\%$$

Cementite

$$\text{Cementite} = \frac{0.80}{6.67} \times 100 = 11.99\%$$

Ferrite

$$\text{Ferrite} = 100 - 11.99 = 88.01\%$$

Free (proeutectoid) ferrite is $100 - 100 = 0\%$, because all the ferrite and cementite are present together as lamellae in pearlite.

Answer: Pearlite = 100%; cementite = 11.99%; ferrite = 88.01% (all of the ferrite and cementite lie inside the pearlite).