Cast irons — quick identification by shop observation When filing or machining cast iron turns your hands black, what does this most directly indicate about the microstructure of the cast iron?

Introduction / Context:
Practical shop tests can quickly indicate the type of cast iron. The black residue that soils hands and tools during machining points to the presence of free graphite, a hallmark of grey and ductile irons, and contrasts with white iron where carbon is tied up in cementite (carbides).

Given Data / Assumptions:

• Machining or filing produces dark, sooty debris.
• Observation is qualitative, not a full metallographic analysis.
• Cast iron family includes grey, ductile (nodular), malleable, and white irons.

Concept / Approach:
Free graphite is soft and easily smears, leaving a black residue. In grey iron, graphite appears as flakes; in ductile iron, as nodules. White iron lacks free graphite; carbon is combined as iron carbide (Fe3C), producing hard, light-colored chips and abrasive wear on tools rather than a dark stain.

Step-by-Step Solution:
Observe machining residue: black discoloration indicates graphite smearing.Map to microstructure: presence of free graphite in grey or ductile iron.Exclude white iron, which produces hard, bright chips and minimal smearing.Therefore, the correct interpretation is “presence of free graphite”.

Verification / Alternative check:
Spark testing and fracture appearance offer corroboration: grey iron shows grey, granular fracture due to flake graphite, while white iron exhibits bright, crystalline fracture with carbides.

Why Other Options Are Wrong:
Cementite-rich white iron does not smear black; pearlite/ferrite alone without graphite will not cause the sooty residue; martensitic cases arise after special heat treatments, not typical bulk cast irons.

Common Pitfalls:
Assuming all dark chips mean graphite; contamination or lubricant can also darken chips, but persistent blackening on dry filing strongly suggests free graphite.

Final Answer:
presence of free graphite (graphite flakes or nodules)