04.3 · Materials & Machinability
Cast Iron
Cast iron is an alloy of iron with 2–4% carbon and 1–3% silicon. The carbon is not fully dissolved — it appears as graphite flakes or nodules in the microstructure. Castings are poured into molds near net shape, then CNC-machined to final dimensions. Cast iron machines differently from steel — it produces short chips, is abrasive, and doesn't need coolant.
Major Cast Iron Types
| Type | Microstructure | Properties | Typical use |
|---|---|---|---|
| Gray cast iron (GG20/GG25, ASTM A48) | Flake graphite | Excellent vibration damping, good castability, brittle | Engine blocks, machine bases, brake discs |
| Ductile iron (GGG40/GGG50, ASTM A536) | Nodular (spheroidal) graphite | Tougher and stronger than gray iron, still casts well | Gears, crankshafts, hydraulic housings |
| Compacted graphite iron (CGI) | Vermicular graphite | Between gray and ductile — higher strength | Modern diesel engine blocks |
| Malleable iron | Tempered carbon nodules | Heat-treated white iron, shock-resistant | Pipe fittings, hand tools |
Machining Cast Iron
What's easy
- Short chips: gray cast iron fractures into small chips — no chip breaker needed, no tangling around the tool.
- No coolant needed: cast iron machines dry. The graphite acts as a lubricant, and coolant causes the fine iron dust to form a sticky paste that clogs machines.
- Good surface finish: cuts cleanly at moderate feeds.
What's hard
- Abrasiveness: the graphite flakes and hard silicon particles cause flank wear on the tool. Use uncoated or TiCN-coated carbide (K-type grade).
- Scale/skin: castings have a hard outer scale (the mold skin). Always take a cut deep enough to break through the scale on the first pass — cutting on the scale kills the tool edge.
- Dust: cast iron generates fine, dark dust. Use a vacuum/extraction. The dust is conductive and can damage electrical components if it builds up in the machine.
Cutting parameters
| Material | Vc (carbide, m/min) | Feed (mm/rev) | Coolant |
|---|---|---|---|
| Gray cast iron (GG25) | 80–150 | 0.15–0.3 | Dry (or air blast) |
| Ductile iron (GGG40) | 70–120 | 0.1–0.25 | Dry (or light flood) |
| Hardened cast iron (HRC 50+) | 50–80 (CBN) | 0.1–0.2 | Dry |
Cast iron scale: the first cut on a casting must be deep enough (at least 0.5 mm) to get below the hard sand-blasted skin. Cutting on the scale causes rapid flank wear. If stock is uneven, take a facing pass first to find the real surface.