Discussion
During cooling from 900°C to 723°C, the proeutectoid α forms from γ. Find the volume % of proeutectoid α just below 723°C for the steel.
Answer: A
The proeutectoid ferrite is about 44.9%.
Just above 723°C the steel is ferrite plus austenite of eutectoid composition, so apply the lever rule between 0.02 and 0.8 wt.% C:
\[ \begin{aligned} W_{\alpha} &= \frac{0.8 - 0.45}{0.8 - 0.02} \\ &= \frac{0.35}{0.78} = 0.449 \end{aligned} \]
All that austenite then becomes pearlite just below 723°C, while the ferrite already formed stays as proeutectoid ferrite. Taking the densities of the phases as nearly equal, as the question intends, the volume fraction equals the weight fraction, 44.9%. 55.1% is the pearlite fraction, the complement.
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