CI · cast iron
Grey Cast Iron GJL250 (Grade 250)
Grey iron cuts cleanly and dry: the graphite flakes act as a built-in chip breaker and lubricant, so it machines fast, throws dust instead of chips, and wears the ways if the dust is not contained.
Quoting this material
- Typical lead time
- 15–25 working days
- Condition
- As cast, stress relieved before machining where stability matters
- 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.
Grey iron cuts cleanly and dry: the graphite flakes act as a built-in chip breaker and lubricant, so it machines fast, throws dust instead of chips, and wears the ways if the dust is not contained.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength | 250 | MPa |
| Compressive strength | ~800 | MPa |
| Hardness | 180–240 | HB |
| Damping capacity | Highest of the common metals | - |
| Thermal conductivity | 46 | W/m·K |
| Modulus of elasticity | 105 | 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, stress relieved before machining where stability matters
Heat treatment
Not hardened by heat treatment. Stress relief at 550–600 °C before machining is standard practice on a large casting, because otherwise the casting moves as metal is removed and the geometry is lost after weeks rather than hours.
Achievable surface finish
Ra 1.6 µm
Ra 0.8 µm
The graphite smears slightly at the surface, so a cast iron finish looks duller than the same Ra on steel. Ground or lapped surfaces are used where flatness matters.
Tooling and cutting parameters
Cutting speed
- 150–300 m/min (carbide)
Feed
- 0.15–0.35 mm/rev turning · 0.10–0.25 mm/tooth milling
Tooling
- Coated carbide, or ceramic for high-speed roughing on cast skin
Coolant
- Dry or mist; the cast skin is abrasive and hard, so expect the first cut to consume the insert
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
- Machine tool beds and columns
- Pump and motor housings
- Brake drums and discs
- Gearbox and engine castings
- Large bases and frames
DFM notes for this grade
- It is bought for damping and stiffness: a machine bed in grey iron absorbs vibration that a steel weldment would transmit, which is why the machine is more accurate as well as quieter
- The casting has to be stress relieved before machining. A 500 kg casting that is machined straight from the foundry will move 0.1–0.3 mm as the metal is removed
- The graphite dust from machining is a housekeeping problem: keep it out of the ways and slideways, and dry-cut with extraction rather than flooding with coolant
Questions buyers ask
- Why is grey iron still used for machine beds when steel is stronger?
- Damping. Grey iron absorbs vibration at roughly ten times the rate of steel, and a machine tool that does not vibrate holds tolerance and produces a better surface. Strength is rarely the constraint on a bed; vibration is.
- Is grey iron brittle?
- Yes - it has essentially no elongation, which is by design. It is strong in compression and weak in tension, and it will crack rather than bend where the section changes sharply. That is why a bolt hole in a grey iron casting has a boss around it.
Grades we would quote instead
GGG40 / Ductile Iron 400-15
6/10Ductile iron with the graphite in nodules rather than flakes: it has real elongation, it takes impact, and it machines like a soft steel with better chip control than grey iron.
GGG60 / Ductile Iron 600-3
5/10A high-strength ductile iron: 600 MPa with 3% elongation.
CGI (Compacted Graphite Iron)
4/10The 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.