Cooling of 3% C cast iron

Cooling path

1. Above the liquidus, the alloy is liquid.
2. Between the liquidus and 1147 °C, primary austenite dendrites separate; the liquid becomes richer in carbon, reaching 4.3% C at 1147 °C.
3. At 1147 °C the remaining liquid undergoes the eutectic reaction $L \rightarrow \gamma + \text{Fe}_3\text{C}$, giving ledeburite.
4. Below 1147 °C, the austenite loses carbon as secondary cementite and at 727 °C reaches 0.76% C and transforms to pearlite.
5. At room temperature: pearlite (dendrites) + cementite + transformed ledeburite. The structure is hard and brittle white cast iron.

Fractions just above 1147 °C

Primary austenite (tie line 2.14 to 4.3):

$$W_{\gamma'} = \frac{4.3 - 3.0}{4.3 - 2.14} = \frac{1.3}{2.16} = 0.602$$

Liquid, which becomes ledeburite:

$$W_{L} = \frac{3.0 - 2.14}{4.3 - 2.14} = \frac{0.86}{2.16} = 0.398$$

Phases just below 1147 °C

Total cementite (tie line 2.14 to 6.67):

$$W_{Fe_3C} = \frac{3.0 - 2.14}{6.67 - 2.14} = \frac{0.86}{4.53} = 0.190$$ $$W_{\gamma} = 1 - 0.190 = 0.810$$

Answer: primary austenite 60.2% and ledeburite 39.8%; just below 1147 °C austenite 81.0% and cementite 19.0%.