Chilled cast iron — identify the correct production method In foundry practice, how is “chilled cast iron” typically produced to obtain a very hard, wear-resistant white iron surface layer?

Introduction / Context:
“Chilled” surfaces on cast iron components such as rolls or wear plates are produced to resist abrasion. Controlling solidification rate and heat extraction can shift the microstructure from graphitic (gray) to cementite-rich (white) near the surface, greatly increasing hardness while the core remains tougher.

Given Data / Assumptions:

• Objective: create a hard, white iron skin on selected regions.
• Material: cast iron with carbon and silicon typical of gray iron.
• Process variable: cooling rate at the mold–metal interface.

Concept / Approach:
Rapid heat extraction at the mold face suppresses graphite formation and promotes cementite (Fe3C), yielding white cast iron in the chilled zone. This is achieved by using chill inserts (metal chills) or locally thicker, more conductive mold sections. The interior cools slower and forms a gray iron matrix for damping and machinability.

Step-by-Step Solution:

Verification / Alternative check:
Micrographs show ledeburitic/cementitic structure near chilled surfaces with hardness markedly higher than the gray core; hardness profiles drop from surface inward.

Why Other Options Are Wrong:

• Adding magnesium: used for spheroidal graphite (ductile iron), not for chilling.
• Annealing white iron: produces malleable iron, not chilled surfaces.
• High silicon: promotes graphitisation (softer), opposite of chilling.
• Nitriding: a steel surface treatment, not a casting solidification method.

Common Pitfalls:
Confusing chilled iron with malleable or ductile iron; assuming chemistry alone dictates white/gray without considering cooling rate.

Final Answer: