Gray Iron vs. Ductile Iron: Differences, Properties and Applications
Gray iron and ductile iron share almost the same chemistry. Both are iron with roughly 3–4% carbon plus silicon. Yet one is brittle and the other can stretch before it breaks. The choice affects how the part performs, how the mold and risers are designed, how the part machines, and what goes on the material certificate.
This guide covers the difference, the grades we work with, typical applications, and the checks that confirm you received the grade you specified.
It comes down to the shape of the graphite
In both irons, part of the carbon separates out as graphite while the metal solidifies. The shape of that graphite drives almost every difference between the two.
Gray iron: graphite forms as interconnected flakes. On a fractured surface the flakes give the gray color that names the material. Flakes interrupt vibration and conduct heat well. But each flake tip concentrates stress like a small crack, so gray iron breaks with almost no plastic deformation.
Ductile iron (also called nodular or spheroidal graphite iron): before pouring, the iron is treated with magnesium and then inoculated. The graphite then grows as separate spheres. A sphere has no sharp edge to start a crack, so the surrounding metal can carry tension, bend and absorb impact.
The metal around the graphite, called the matrix, matters too. A more ferritic matrix gives softer, more ductile iron. A more pearlitic matrix gives higher strength and hardness. That is why FCD450 and FCD550 behave differently even though both have nodular graphite.
Gray iron vs. ductile iron at a glance
Gray iron wins on damping, heat conduction, castability and cost. Ductile iron wins whenever the part must survive tension, bending or impact.
| Property | Gray iron | Ductile iron |
|---|---|---|
| Graphite shape | Flakes | Spheres (nodules) |
| Minimum tensile strength, our grades | 250–300 MPa | 450–550 MPa |
| Elongation | Close to zero | Measurable, depending on grade (see grade table) |
| Impact and shock resistance | Low | High |
| Vibration damping | Excellent | Moderate |
| Thermal conductivity | Higher | Lower |
| Machinability | Very good, short broken chips | Good, longer chips |
| Shrinkage tendency during casting | Lower, easier to feed | Higher, needs careful riser design |
| Relative material cost | Usually lower | Usually higher (extra treatment and process control) |
Grades we cast and machine
| Grade (JIS) | Type | Min. tensile strength | Min. elongation | Nearest EN grade |
|---|---|---|---|---|
| FC250 | Gray | 250 MPa | — | EN-GJL-250 |
| FC300 | Gray | 300 MPa | — | EN-GJL-300 |
| FCD450 | Ductile | 450 MPa | 10% | EN-GJS-450-10 |
| FCD500 | Ductile | 500 MPa | 7% | EN-GJS-500-7 |
| FCD550 | Ductile | 550 MPa |
Not sure which grade fits your part?
The right iron is the one that matches the load case, not the one with the highest number on the spec sheet. Over-specifying ductile iron adds cost. Under-specifying gray iron adds risk.
Send us your drawing or 3D file along with:
- What the part does and the loads it sees
- The material standard you follow (JIS, EN or ASTM)
- Estimated annual volume (we work from 100 pcs per order)
Our foundry and machining engineers will review it together and recommend a starting grade, with the reasons behind it.