Metallurgy fundamentals: A plain carbon steel is termed as hypo-eutectoid steel when its carbon content is __________ relative to the eutectoid composition (approximately 0.8% C).

Introduction / Context:
This question tests core steel metallurgy terminology related to the iron–carbon phase diagram. Correctly identifying hypo-eutectoid steels is essential for predicting phase transformations (ferrite plus pearlite formation) and for choosing heat-treatment schedules in manufacturing and design.

Given Data / Assumptions:

• Eutectoid composition in plain carbon steel is taken as about 0.8% carbon.
• Only the carbon percentage varies; other alloying effects are ignored for the definition.
• Room-temperature microstructures are considered after slow cooling.

Concept / Approach:
By definition, steels are classified relative to the eutectoid point: hypo-eutectoid (less carbon than eutectoid), eutectoid (at the eutectoid carbon), and hyper-eutectoid (greater than eutectoid). Hypo-eutectoid steels transform on cooling to a mixture of proeutectoid ferrite plus pearlite, because the ferrite field extends to the left of the eutectoid point.

Step-by-Step Solution:
Identify eutectoid carbon content reference: approximately 0.8% C.Define classes: hypo-eutectoid < 0.8% C; eutectoid ≈ 0.8% C; hyper-eutectoid > 0.8% C.Apply definition: a steel is hypo-eutectoid if its carbon content is below 0.8%.Therefore, the correct completion is “below 0.8%”.

Verification / Alternative check:
Indicator microstructure: presence of proeutectoid ferrite along grain boundaries with pearlite colonies confirms hypo-eutectoid character.

Why Other Options Are Wrong:
Above 0.8% indicates hyper-eutectoid steel (proeutectoid cementite + pearlite).

Exactly 0.8% describes eutectoid steel (fully pearlitic after slow cooling).

Between 1.0% and 1.5% is plainly hyper-eutectoid.

Indeterminate is incorrect because composition alone is sufficient for the classification.

Common Pitfalls:
Confusing 0.77% vs 0.8% values; for exam purposes, 0.8% is the standard rounded reference for the eutectoid composition.

Final Answer:
below 0.8%