Grey iron and SG (spheroidal graphite, also called ductile or nodular) iron are both continuous cast iron bars — but the shape of the graphite inside them makes them behave like two different materials. Flakes give grey iron its famous damping and machinability; spheres give SG iron steel-like strength and ductility. Here is how to choose.
| Property | Grey Iron (EN-GJL-250) | SG Iron (EN-GJS-400C / 500C / 600C) |
|---|---|---|
| Graphite form | Flakes | Spheroids (nodules) |
| Tensile strength | ~250 N/mm² min | 400 / 500 / 600 N/mm² min by grade |
| Yield strength | Not meaningful — brittle failure | Defined — e.g. ≥320 N/mm² (GJS-500C) |
| Elongation / ductility | ~Nil | Up to ~15% (GJS-400C), ≥7% (500C) |
| Compressive strength | ~900 N/mm² — outstanding | High |
| Shock / impact resistance | Poor — brittle | Good — especially ferritic GJS-400C |
| Vibration damping | Excellent — flakes absorb vibration | Moderate |
| Machinability | Excellent — fastest of any iron | Very good |
| Self-lubricating bearing properties | Excellent — graphite flakes | Good |
| Pressure tightness under load | Limited by brittleness | Good — hydraulic components, valves |
| Wear resistance | Good | Better in GJS-600C |
| Relative cost | Lower | Higher |
| Standard | BS EN 1561 | BS EN 1563 |
| Stocked size range | 25–400mm ∅ | 25–400mm ∅ |
In grey iron, graphite flakes thread through the structure like tiny internal notches. Under compression they are harmless — which is why grey iron's compressive strength (~900 N/mm²) embarrasses its tensile figure — and they make the material easy to machine and superb at damping vibration. Under tension, though, each flake tip concentrates stress, so the material fails early and without warning.
The three SG grades we stock trade ductility against strength: EN-GJS-400C is ferritic, the most ductile and impact-tough; EN-GJS-500C is the general-purpose balance; EN-GJS-600C is the strongest and most wear-resistant. All grades — grey and SG — are stocked 25–400mm diameter on the cast iron bar page.
Grey iron EN-GJL-250 and ductile EN-GJS-400C, 500C and 600C — continuous cast bar, 25–400mm diameter, cut to length from one piece.