Which statement about cast iron is correct in standard materials practice? (Consider composition, manufacture, and typical remelting route.)
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ACast iron is obtained by purifying pig iron to remove most impurities.
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BCast iron is manufactured directly into required shapes by casting.
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CCast iron generally contains about 2 to 4.5 percent carbon along with other impurities.
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DCast iron is never remelted in a cupola furnace.
Answer
Correct Answer: Cast iron generally contains about 2 to 4.5 percent carbon along with other impurities.
Explanation
Introduction:Cast iron is an iron–carbon alloy with relatively high carbon content and accompanying impurities (Si, Mn, P, S). The question checks recognition of a compositionally accurate and contextually correct statement.
Given Data / Assumptions:
- Cast irons solidify with graphite or cementite depending on composition and cooling.
- Manufacturing practice commonly involves remelting pig iron with scrap and flux in a cupola or induction furnace.
- Carbon range for cast iron exceeds that of steels.
Concept / Approach:Cast irons are not “purified” pig iron; instead, they retain higher carbon and are cast into shape. Typical total carbon is about 2–4.5 percent, with silicon often 1–3 percent to promote graphitization.
Step-by-Step Solution:
1) Review A: “purifying pig iron” is misleading; cast iron keeps high C and impurities; reject.2) Review B: Casting into shape is true but incomplete alone and could be ambiguous versus steels; we seek the most precise, unambiguous statement.3) Review C: The composition range 2–4.5% C accurately characterizes cast iron; accept.4) Review D: Cupola remelting is common; the statement saying “never remelted” is false; reject.Verification / Alternative check:Textbook data tables list typical carbon ranges and standard melting routes (cupola/induction). Casting shops routinely remelt charge mixes to pour into moulds.
Why Other Options Are Wrong:
- A: Suggests purification, which contradicts the retained high carbon nature.
- B: Partly true but not as defining as composition; also many alloys are cast, so this is not unique.
- D: Opposite of common practice; cupola remelting is traditional.
Common Pitfalls:Confusing cast iron with steelmaking; equating “purifying” with standard remelting; ignoring that composition definition is the clearest differentiator.
Final Answer:Cast iron generally contains about 2 to 4.5 percent carbon along with other impurities.