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In Fe-Fe3C phase diagram, the eutectoid composition is 0.8 weight % of carbon at 725 °C. The maximum solubility of carbon in α-ferrite phase is 0.025 weight % of carbon. A steel sample, having no other alloying element except 0.5 weight % of carbon, is slowly cooled from 1000 °C to room temperature. The fraction of pro-eutectoid α-ferrite in the above steel sample at room temperature is

1. 0.387
2. 0.864
3. 0.475
4. 0.775

Correct answer: 0.387

Solution

The fraction of pro-eutectoid α-ferrite can be calculated using the lever rule, which considers the composition of the steel sample and the eutectoid point. Since the steel has 0.5 weight % carbon, which is less than the eutectoid composition of 0.8 weight %, it will form pro-eutectoid α-ferrite before reaching the eutectoid transformation, resulting in a calculated fraction of approximately 0.387.

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