Cast Irons — Hyper-Eutectic Definition at the Eutectic Composition of 4.3% C Iron–carbon alloys in the cast-iron range are termed hyper-eutectic when their carbon content is __________ 4.3%.

Introduction / Context:Cast irons solidify through a eutectic reaction near 4.3% carbon (with silicon affecting the equivalent carbon). Classifying compositions as hypo- or hyper-eutectic helps predict primary phases formed during solidification and the resulting microstructure and properties.

Given Data / Assumptions:

• Eutectic composition for Fe–C (no modifiers) is about 4.3% C.
• Terminology refers to composition relative to the eutectic point.
• Silicon can shift “carbon equivalent,” but the definition here addresses nominal carbon.

Concept / Approach:Hyper-eutectic means the composition lies to the right of the eutectic point on the Fe–C diagram, i.e., carbon content higher than 4.3%. Such melts tend to precipitate primary graphite (in grey irons) prior to eutectic solidification, influencing shrinkage, fluidity, and final properties. Hypo-eutectic (below 4.3%) favors primary austenite formation before the eutectic reaction.

Step-by-Step Solution:Identify reference: eutectic at 4.3% C.Hyper-eutectic → composition greater than eutectic.Therefore, fill blank with “more than.”Implication: expect primary graphite/austenite differences in as-cast structures.

Verification / Alternative check:Foundry practice uses “carbon equivalent” (C + Si/3 + P/3) to estimate solidification path; hyper-eutectic behavior corresponds to higher CE values leading to graphite flotation risk.

Why Other Options Are Wrong:

• Equal to: exactly eutectic, not hyper-eutectic.
• Less than: defines hypo-eutectic, the opposite region.

Common Pitfalls:Confusing eutectic (liquid→two solids) with eutectoid (solid→two solids at ~0.8% C in steels). The 4.3% figure pertains to the cast-iron eutectic, not steel.

Final Answer:more than