Nodularising additives in cast iron Adding magnesium to molten cast iron primarily increases which property after solidification due to the formation of spheroidal graphite?

Introduction / Context:
Magnesium (and sometimes cerium) is used to treat molten cast iron to convert flake graphite into spheroidal (nodular) graphite, producing ductile iron. Recognizing the property changes stemming from this microstructural transformation is key in foundry practice and mechanical design.

Given Data / Assumptions:

• Small additions of Mg are made to liquid iron prior to casting.
• Resulting material is ductile (nodular) iron, not grey (flake) iron.
• Matrix conditions (ferritic, pearlitic) can be adjusted by heat treatment.

Concept / Approach:
In grey iron, flake graphite acts as sharp stress concentrators, drastically reducing tensile strength and ductility. Mg treatment rounds the graphite into nodules, minimizing stress concentration and enabling significant increases in elongation and tensile strength. Hardness may vary depending on the matrix but does not inherently increase simply because of Mg; the hallmark change is much-improved ductility with useful strength in tension.

Step-by-Step Solution:
Add Mg to molten iron to modify graphite morphology.Graphite becomes spheroidal; stress concentration is reduced.Measured properties show higher tensile strength and much greater elongation compared with grey iron.Hence, “ductility and strength in tension” increases.

Verification / Alternative check:
Tensile test data for ferritic ductile iron commonly show elongations of 10%–20%+ versus near-zero for grey iron; ultimate strength also rises markedly.

Why Other Options Are Wrong:
Hardness is matrix-dependent; corrosion resistance and creep strength are not the principal improvements from Mg modification; thermal conductivity tends to decrease compared with grey iron due to nodular graphite morphology.

Common Pitfalls:
Confusing nodular iron with white iron; Mg does not drive carbide formation but controls graphite shape.

Final Answer:
ductility and strength in tension