CI · cast iron
Compacted Graphite Iron (CGI)
The graphite is in worm-like compacted form between grey and ductile, which gives roughly 70% more strength and 45% more stiffness than grey iron - and makes it noticeably harder to machine, especially after the surface has chilled.
Quoting this material
- Typical lead time
- 20–30 working days
- Condition
- As cast, with a chilled or hardened skin in some applications
- Achievable finish
- Ra 1.6 µm typical · Ra 0.8 µm best
Machined in-house to your drawing: see the precision CNC machining service and the machine and inspection capability it runs on.
Machinability
Our own 1–10 shop ranking, not a published index: 10 is free-cutting brass C360, 1 is Inconel 718. It ranks how a material behaves on the floor - cycle time, chip control and tool life together. Where a published index exists it is quoted in the note.
The graphite is in worm-like compacted form between grey and ductile, which gives roughly 70% more strength and 45% more stiffness than grey iron - and makes it noticeably harder to machine, especially after the surface has chilled.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength | 350–450 | MPa |
| Stiffness gain vs grey iron | ~45 | % |
| Hardness | 200–250 | HB |
| Damping | Better than ductile, close to grey | - |
| Thermal conductivity | 39 | W/m·K |
| Modulus of elasticity | 145 | GPa |
Hardness, tensile and thermal figures are typical published values for the condition stated. Actual mill certificates govern; send the specification with the drawing and we confirm against the certificate before the material is cut.
Condition
As cast, with a chilled or hardened skin in some applications
Heat treatment
No general hardening treatment; the structure comes from the foundry chemistry and the magnesium treatment that produces the compacted graphite. Where a wear face is needed it is induction hardened after rough machining.
Achievable surface finish
Ra 1.6 µm
Ra 0.8 µm
The compacted graphite means very good damping and a finish that holds up in a sliding application.
Tooling and cutting parameters
Cutting speed
- 80–180 m/min (carbide)
Feed
- 0.12–0.30 mm/rev turning · 0.08–0.20 mm/tooth milling
Tooling
- Coated carbide with a tough substrate; ceramic for the chilled skin only
Coolant
- Flood or high-pressure mist - the skin is abrasive and the heat control matters
Starting points for carbide on a rigid setup, not a guarantee. The right numbers depend on the machine, the toolholder and the fixturing - ask and we will say what we would actually run.
Typical applications
- Diesel engine blocks and heads
- Brake discs and drums
- High-performance machine tool structures
- Cylinder liners
- Marine and heavy-duty engine components
DFM notes for this grade
- The strength/stiffness combination is why modern diesel blocks are CGI: lighter walls for the same rigidity than grey iron, with enough strength for the firing loads
- It machines badly compared with grey iron, and the chilled skin on a casting is genuinely abrasive. Where CGI appears, the quote has to reflect the tool consumption honestly
- Damping is close to grey iron, which is why it is also used for machine structures where the stiffness needs to be higher without losing vibration absorption
Questions buyers ask
- Why is CGI used for diesel engine blocks?
- Stiffness for weight. A CGI block can be walled thinner than grey iron for the same rigidity, which cuts engine weight, and the compacted graphite keeps enough damping and thermal conductivity. It also has the strength to take higher cylinder pressures.
- Why is CGI harder to machine than grey iron?
- The compacted graphite does not break chips the way flakes do, and the matrix is stronger and more pearlitic. Add the chilled skin that forms on a casting where the metal meets the mould, and the first few millimetres are genuinely abrasive.
Grades we would quote instead
GJL300 (Grey Iron 300)
6/10The strongest of the standard grey irons: harder matrix, slightly tougher on the tool and still cutting clean and dry like its family.
GGG60 / Ductile Iron 600-3
5/10A high-strength ductile iron: 600 MPa with 3% elongation.
ADI (Austempered Ductile Iron)
3/10Ductile iron put through an austempering heat treatment to reach 800–1,100 MPa.