CI · cast iron
Grey Cast Iron GJL300 (Grade 300)
The strongest of the standard grey irons: harder matrix, slightly tougher on the tool and still cutting clean and dry like its family.
Quoting this material
- Typical lead time
- 15–25 working days
- Condition
- As cast, stress relieved for machining
- 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 strongest of the standard grey irons: harder matrix, slightly tougher on the tool and still cutting clean and dry like its family.
Properties as supplied
| Parameter | Value | Unit |
|---|---|---|
| Tensile strength | 300 | MPa |
| Compressive strength | ~900 | MPa |
| Hardness | 200–260 | HB |
| Damping capacity | High | - |
| Thermal conductivity | 45 | W/m·K |
| Modulus of elasticity | 110 | 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 for machining
Heat treatment
Stress relief before machining. The higher grade is achieved through the foundry chemistry and cooling rather than a heat treatment in the machine shop.
Achievable surface finish
Ra 1.6 µm
Ra 0.8 µm
The harder matrix takes a marginally better finish than GJL200 and holds it longer in service on a sliding face.
Tooling and cutting parameters
Cutting speed
- 120–250 m/min (carbide)
Feed
- 0.15–0.30 mm/rev turning · 0.08–0.20 mm/tooth milling
Tooling
- Coated carbide; ceramic for high-speed roughing on the cast skin
Coolant
- Dry or mist
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 slideways
- Press and forming machine frames
- Diesel engine blocks and heads
- Heavy-duty pump housings
- Brake drums for heavy vehicles
DFM notes for this grade
- The grade is chosen when the casting has to take a bearing load or hold a slideway: GJL300 wears better than GJL200 on an unhardened sliding surface
- Machinability is a step below the lower grades and the difference shows on a large casting in cycle time - budget the roughing honestly
- Chilled or induction-hardened slideways are common where the grey iron surface is the bearing, and that changes the machining sequence: the hardening comes after rough machining
Questions buyers ask
- Why is GJL300 used for slideways rather than GJL200?
- Because the matrix is harder and more pearlitic, so it resists wear against a sliding surface. On a machine tool the ways are the accuracy of the machine - a soft way wears into a geometry error, and regrinding is a rebuild.
- Can grey iron be hardened on the surface?
- Yes, by chill casting in the foundry (a metal insert in the mould freezes the surface white and hard) or by induction hardening a machined way. Both are foundry or post-machining processes, and the machining allowance follows from them.
Grades we would quote instead
GJL250 (Grey Iron 250)
7/10Grey 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.
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.