Answer :
Final answer:
The eutectoid temperature is the specific temperature at which the internal grain structure of plain carbon steel changes, affecting its hardness and brittleness.
Explanation:
The particular temperature at which the internal grain structure of a plain carbon steel changes is known as the eutectoid temperature.
When heated through a sufficient temperature range, plain carbon steel undergoes a transformation where the FCC phase turns into the BCC phase, and excess carbon forms grains of Fe3C or cementite within the BCC iron matrix.
This change in structure affects the material properties, making it harder but potentially more brittle, and is crucial in processes like tempering steel or creating cast iron objects.
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