Steel Definition: Carbon ≤ ~2.0–2.1%
Classification based on carbon %, NOT microstructure
Cast Iron Definition: Carbon > ~2.1%
Graphite morphology determines type (gray/white/ductile)
Gray Cast Iron: Graphite as flakes
Good compression, brittle in tension, excellent damping
White Cast Iron: Cementite dominant (Fe₃C)
Hard & brittle, forms with rapid cooling
Ductility Source: Slip systems dependent on carbon positioning
Distributed carbon → free dislocation movement
Damping Capacity: Graphite flakes absorb vibration
Why cast iron preferred for machine tool beds
Heat Treatment Limit: Cannot change graphite morphology
Can only alter matrix structure (stress relief, hardness)
Forging Suitability: Cast iron fails before plastic deformation
Requires absence of stress concentrators (graphite flakes)
Strength vs Toughness: High compression ≠ impact resistance
Steel excels under tensile & impact loading
Golden Rule: Know the loading condition!
Compression→cast iron; Tension/Impact→steel