Grey iron and SG iron are made from similar raw materials and poured in the same foundry, but they behave differently in service because the graphite inside them takes a different shape.
The difference is the graphite
In grey iron, graphite forms as interconnected flakes. Each flake acts as a tiny internal notch, so grey iron has modest tensile strength and almost no elongation. The same flakes absorb vibration and conduct heat well, and they make chips break cleanly during machining.
In SG iron (spheroidal graphite, also called ductile or nodular iron), a small, controlled magnesium treatment makes the graphite form as round nodules. Without sharp flake edges, the metal matrix carries load much more effectively, giving tensile strength from about 400 MPa upwards and real elongation.
Side-by-side comparison
| Property | Grey iron (IS 210) | SG iron (IS 1865) |
|---|---|---|
| Graphite form | Flakes | Spheroids (nodules) |
| Tensile strength | 150–400 MPa by grade | 350–900 MPa by grade |
| Elongation | Below 1% | About 2–22% by grade |
| Impact resistance | Low | Good to high |
| Vibration damping | Excellent | Moderate |
| Thermal conductivity | High | Lower |
| Machinability | Excellent | Good |
| Relative cost | Lower | Higher |
When grey iron is the better choice
- When vibration matters: machine bases, housings and frames benefit from grey iron's damping.
- When heat must move: brake drums and discs use grey iron's thermal conductivity and resistance to thermal cracking.
- When loads are mainly compressive: grey iron's compressive strength is several times its tensile strength.
- When machining cost is significant: grey iron machines quickly, with good tool life.
When SG iron is the better choice
- When parts see tension, bending or shock: brackets, hubs, insulator caps and suspension parts need ductility.
- When failure must not be sudden: SG iron deforms before it fractures, giving a safety margin.
- When you are replacing steel: SG iron can replace fabrications and forgings with a near-net-shape casting.
- When weight reduction is a goal: higher strength allows thinner sections.
Rule of thumb: start with grey iron when stiffness, damping or heat transfer dominate, and move to SG iron when the part must carry tensile or impact loads.
Cost and lead time
SG iron costs more per kilogram. It needs magnesium treatment, tighter chemistry control, more careful feeding design and microstructure checks on every heat. A part that fails in grey iron is still far cheaper to make in SG iron than to fabricate in steel.
How we help you decide
Send your drawing, the loads the part sees and the operating environment. Our team will recommend a material and grade, and we can cast both grey iron and SG iron from 0.5 to 200 kg in the same plant.