CGI (Compacted Graphite Iron)
Cast iron with compacted (vermicular) graphite
Typical values for the temper in the title
In CGI the graphite is in short, thick "worms", a shape between the flakes of gray iron and the spheres of ductile iron. This gives a combination: about 1.5โ2 times stronger and stiffer than gray iron, while keeping better thermal conductivity and damping than ductile iron.
This combination suits modern diesel engine blocks and cylinder heads, which run at high combustion pressures: walls can be thinner and the engine lighter. Producing CGI requires very tight control of magnesium in casting.
Mechanical and physical properties
| Property | Value | Note |
|---|---|---|
| Tensile strength Rm | 300โ450 MPa | |
| Yield strength Rp0.2 | 250โ350 MPa | |
| Elongation A | 1.5% | |
| Hardness | 179โ255 HB | |
| Modulus of elasticity E | 130โ145 GPa | |
| Density ฯ | 7.10 g/cmยณ | |
| Machinability | 40% โ significantly harder than GCI | relative rating in the site database |
Chemical composition (wt %)
| Element | Range | Role |
|---|---|---|
| C carbon | about 3.5โ3.8 (typical) | as compacted graphite |
| Si silicon | about 2โ2.5 (typical) | |
| Mg magnesium | in a narrow, controlled window | sets the graphite shape |
| Fe iron | balance |
Typical composition. The standard specifies properties and a maximum share of spheroidal graphite.
Grades and conditions
In cast iron, the grade is chosen by minimum strength, tested on a cast test bar. In a thick section properties are lower than the test bar, because it cools more slowly.
| Grade / condition | Properties | When to choose it |
|---|---|---|
| EN-GJV-300 to 500 | min Rm 300 to 500 MPa | by grade |
| ASTM A842 | grades 250 to 450 | the American equivalent |
Applications
- Diesel engine blocks
- Cylinder heads
- Exhaust manifolds
- Heavy brake discs
- Thermally loaded parts
Fabrication
Machining
Much harder than gray iron (database: "significantly harder than GCI"), requiring adapted tools and process.
Welding
Difficult: the high carbon forms hard, brittle zones near the weld. Repairs use nickel electrodes, preheat and slow cooling, in short runs.
Production
Tight magnesium control: too little gives gray iron, too much gives ductile iron.
Compared with similar materials
| Material | Rm / Rp0.2 | Welding | When it is the better choice |
|---|---|---|---|
| Gray Cast Iron | 150โ400 / 98โ250 MPa | Difficult โ Ni Rod | flake graphite: cheap, easy to machine, weaker ยท material page |
| Ductile Iron | 414โ825 / 276โ621 MPa | Medium โ Ni Rod | spheroidal graphite: strong and ductile, conducts heat less ยท material page |
| 319.0 | 186 / 124 MPa | Medium | aluminum for engine blocks, much lighter ยท material page |
Common mistakes
Machining CGI with gray-iron parameters
Tool wear is much faster.
Choosing CGI without suitable casting control
A small deviation in magnesium changes the material completely.
Comparing only strength with ductile iron
The CGI advantage is the combination with thermal conductivity.
Equivalents, standards and supply
| EN | EN-GJV-450 |
|---|---|
| ASTM | A842 |
| Standards | ASTM A842 / EN 16079 |
| Forms | casting only |
| Producers | Georg Fischer ยท Nemak ยท Tupy |
Learn more
Values are typical and for comparison only. In cast iron, properties depend strongly on wall thickness and cooling rate in casting. For design, rely on the standard and test-bar results. Sources: ASTM A48, A536, A897, A532, A439 and EN 1561/1563/1564, producer data sheets, and the METAL HUB materials database.
Updated: October 31, 2026