The World of Cast Iron: Gray, Ductile and Compacted Graphite

In foundries, cast iron is everywhere, but gray, ductile and compacted graphite iron differ enormously. The core difference: graphite morphology.

Morphology Determines Properties

Flake graphite (gray iron): flakes cut the metal matrix like blades, causing stress concentration — low strength, brittle, but excellent damping, wear, thermal conductivity and low cost. Machine beds, housings, brake discs.

Spheroidal graphite (ductile iron): nodulizing treatment makes spherical graphite — minimal matrix cutting, tensile strength 400~800MPa, near steel. Crankshafts, gears, valves.

Vermicular graphite (CGI): between flake and spheroid — strength near ductile, thermal/damping superior, great thermal fatigue resistance. Engine blocks, heads, brake rotors.

Why Some Nodulize and Some Don't?

Nodulizing adds magnesium-based inoculant before pouring; Mg changes graphite growth into spheres. Verify by fracture (silvery, fine grain), ringing sound, and metallographic nodularity ≥80%. CGI uses vermicularizer (Mg + RE + Ti) — a tighter process window, harder than ductile.

Rapid Front-Line Judgment

Measure temperature before (nodulizing temp ~1480~1520°C) and after treatment; sample fracture, listen to sound, or use thermal analysis cups for a quick nodularity reference.

Summary

Gray, ductile and CGI are the same iron with different graphite morphology control. Front-line temperature measurement and thermal analysis are the eyes of that control.