Technical guide
Ductile Iron vs Grey Cast Iron
Both are cast iron. The difference is the shape the graphite takes inside the metal — and everything a specifier cares about follows from it.
The difference
It comes down to graphite
Ductile iron is a cast iron. The family divides not by the base metal but by the form the graphite takes as the casting solidifies, and that single difference sets everything else.
In grey iron the graphite forms flakes. Each flake is a discontinuity in the metal around it and behaves as an internal stress raiser, so the material is stiff and brittle: strong in compression, weak in tension, and with essentially no ability to stretch before it breaks.
In ductile iron a magnesium treatment during melting makes the graphite form spheres instead. A sphere does not concentrate stress the way a flake does, so the surrounding metal keeps its strength — roughly double the tensile strength of grey iron, and, more importantly, real elongation where grey iron has almost none.
Side by side
The numbers behind the two
Typical structural grades of each. The elongation row is the one that matters most on a road.
| Material | Graphite form | Min tensile(MPa) | Min elongation(%) | Under overload |
|---|---|---|---|---|
| EN-GJS-500-7 | Spheroidal (ductile) | 500 | 7 | Deforms first |
| EN-GJL-250 | Lamellar (grey) | 250 | — | Cracks without warning |
Elongation is not specified for the grey grades — in practice it is well under one per cent, which is why the cell is empty rather than carrying a number.
The consequence is a failure mode, not a datasheet entry. An overloaded grey iron cover cracks, and it does so suddenly — a cracked cover in a carriageway is an open hole. Ductile iron bends first, which turns what would have been a sudden failure into a visible defect that somebody notices and reports.
The strength has a second effect worth having on site: a load class can be reached in less metal. Lighter covers are easier and safer to lift, and cheaper to freight — which on a container of covers is not a small line.
Reading the code
EN-GJS-500-7, one part at a time
The designation is not a product code. Every segment of it specifies something.
| Part of the code | What it specifies |
|---|---|
| EN | Designated to the European standard — EN 1563 for this material |
| G | Cast iron |
| JS | Spheroidal graphite — ductile iron. JL in this position means lamellar graphite, which is grey iron |
| 500 | Minimum tensile strength, 500 MPa |
| 7 | Minimum elongation, 7 per cent |
Two letters carry the whole distinction. JS is spheroidal graphite — ductile iron. JL in the same position is lamellar graphite — grey iron. A specification that reads EN-GJL where it meant EN-GJS has changed material without changing shape, and nothing about the finished casting will look different.
GGG50 is the older DIN designation for the same material as EN-GJS-500-7. It remains in common use across the Gulf, which is why our specifications carry both.
On a schedule
“Cast iron” is not a specification
EN 124-2 covers gully tops and manhole tops made of cast iron — and that scope includes grey iron as well as ductile. A line item calling for “cast iron covers to EN 124” is therefore satisfied by a material the design may never have intended, and it will pass a paper check without anyone querying it.
Write the material designation instead: ductile iron to EN-GJS-500-7, or GGG50 if that is the convention on the project. It costs one line and closes the gap.
Every cover, channel and grating we publish is cast in EN-GJS-500-7. Dimensions, load classes and weights for all of them are in the product tables, and test certificates are available on request.
Common questions
Materials and designations
- What is the difference between ductile iron and cast iron?
- Ductile iron is a cast iron. The family splits by the shape the graphite takes: in grey iron it forms flakes, which act as internal stress raisers and make the metal brittle, and in ductile iron a magnesium treatment makes it form spheres, which do not. Same base metal, very different behaviour — roughly double the tensile strength and, crucially, measurable elongation instead of effectively none.
- What does EN-GJS-500-7 mean?
- GJS is cast iron with spheroidal graphite — ductile iron. 500 is the minimum tensile strength in megapascals and 7 is the minimum elongation as a percentage. GGG50 is the older DIN designation for the same material, still widely used in the Gulf, and the two appear together on our specifications for that reason.
- Does specifying “cast iron to EN 124” guarantee ductile iron?
- No, and this is worth checking on a schedule. EN 124-2 covers gully tops and manhole tops made of cast iron, which includes both grey and ductile grades. If ductile iron is what the design assumes, write the material designation — EN-GJS-500-7, or GGG50 — rather than the family name, or the specification permits something it did not intend to.
- Why does it matter for a manhole cover specifically?
- Because of how each fails. A grey iron cover that is overloaded cracks suddenly, and a cracked cover in a carriageway is an open hole. Ductile iron deforms before it fails, which turns a sudden failure into a visible defect somebody can report. The strength also lets a load class be met in less metal, so covers are lighter to lift and safer to handle.
- Is ductile iron the same as SG iron or nodular iron?
- Yes — spheroidal graphite iron, SG iron, nodular iron and ductile iron are all the same material under different names. On a European specification it will appear as EN-GJS, and on older or German-derived documents as GGG.