The difference is graphite shape
In gray cast iron the graphite is distributed in flakes. Flakes act like internal notches, so stress concentrates at their tips. That is why gray iron has low strength, low plasticity and low toughness compared with ductile iron.
In ductile iron the graphite is nodular. A sphere does not concentrate stress the way a flake tip does, so cracks do not run along the graphite. The result is a cast iron with tensile strength and elongation that approach cast steel while keeping the castability and machinability of iron.
What gray iron is good at
Vibration damping. The same graphite flakes that weaken the metal also absorb vibration, which is why machine tool beds, columns and saddles are almost always gray iron. A ductile iron bed of the same geometry would ring.
Machinability. Gray iron cuts easily and the chips break cleanly, so machining cost per part is low.
Cost. It is the cheapest of the common cast irons per kilogram and it needs no nodularising treatment.
Wear under sliding contact. Graphite flakes act as a solid lubricant on sliding surfaces such as machine ways.
What ductile iron is good at
Load carrying. QT450-10 gives 450 MPa minimum tensile strength with 10 percent elongation. Gray iron has effectively no elongation.
Impact and fatigue. Parts that are bolted under preload, dropped, or cycled need elongation. This is why hubs, valve bodies, rope drums and link arms are ductile iron.
Pressure containment. Ductile iron valve and pump bodies pass hydro test reliably where gray iron is a risk.
Replacing weldments. A single ductile iron casting often replaces a fabricated assembly and removes the fatigue risk at the weld toe.
Grade cross reference at a glance
| Rough Grade Equivalence | Detail |
|---|---|
| Gray iron HT200 | EN-GJL-200, GG-20, FC200, ASTM A48 No.30 |
| Gray iron HT250 | EN-GJL-250, GG-25, FC250, ASTM A48 No.35 to No.40 |
| Ductile iron QT450-10 | JS/450-10, GGG-40 to GGG-50, FCD 450-10, ASTM A536 65-45-12 |
| Ductile iron QT600-3 | JS/600-3, GGG-60, FCD 600-3, ASTM A536 80-55-06 to 100-70-03 |
The selection rule
If the part is static, needs to be rigid and needs to damp vibration, use gray iron. If the part carries load, sees impact, contains pressure or is cycled, use ductile iron.
Where a part sits on the boundary, the deciding question is usually elongation. If your drawing calls for any minimum elongation at all, gray iron is out.
One caution: do not upgrade to ductile iron and keep the same wall thickness. Ductile iron solidifies differently, so a section that was sound in gray iron may need feeding changes. Send us the drawing and we will tell you what has to change.