Slow cooling of 1.2% C steel

Microstructure development

1. At high temperature the alloy is austenite of 1.2% C.
2. On cooling to the A$_{cm}$ line (about 900 °C for 1.2% C), austenite becomes saturated with carbon and pro-eutectoid cementite precipitates, mostly as a network along the austenite grain boundaries.
3. The remaining austenite loses carbon and reaches 0.76% C at 727 °C.
4. At 727 °C it transforms to pearlite.
5. Room-temperature structure: cementite network + pearlite. The network makes the steel hard and brittle, which is why such steels are spheroidise-annealed.

Lever rule (just below 727 °C)

Pro-eutectoid cementite (tie line 0.76 to 6.67):

$$W_{Fe_3C'} = \frac{1.20 - 0.76}{6.67 - 0.76} = \frac{0.44}{5.91} = 0.0745$$

Pearlite:

$$W_{P} = \frac{6.67 - 1.20}{6.67 - 0.76} = \frac{5.47}{5.91} = 0.9255$$

(b) In 1 kg

Pro-eutectoid cementite = 0.0745 kg = 74.5 g; pearlite = 0.9255 kg = 925.5 g.

(c) Total phases

$$W_{Fe_3C} = \frac{1.20 - 0.022}{6.67 - 0.022} = \frac{1.178}{6.648} = 0.1772 \quad (177.2\ \text{g})$$ $$W_{\alpha} = 1 - 0.1772 = 0.8228 \quad (822.8\ \text{g})$$

Answer: pro-eutectoid cementite 7.45%, pearlite 92.55%; per kg: 74.5 g cementite network + 925.5 g pearlite; total ferrite 822.8 g and total cementite 177.2 g.