Gray iron and ductile iron look similar and cost about the same per kilogram. Yet they behave very differently under load. Choosing the wrong one causes field failures that no heat treatment can fix. This guide explains the difference in plain language and shows where each grade belongs.
The one difference that matters: graphite shape
Both irons are iron with 3 to 4 percent carbon. In gray iron, the carbon forms flat graphite flakes. In ductile iron, magnesium treatment turns that graphite into round nodules. The flakes act as internal cracks, so gray iron is brittle. The nodules let ductile iron bend, so it behaves more like steel.
Where gray iron wins
Do not dismiss gray iron. Its graphite flakes dampen vibration extremely well. Therefore, it remains the first choice for machine tool bases, gearbox housings and brake drums. It also machines beautifully and costs less than ductile. Typical grades run from ASTM A48 Class 20 to Class 40.
Where ductile iron wins
Any part that sees tension, shock or pressure belongs to ductile iron. ASTM A536 65-45-12 covers most general engineering parts. For higher strength, 80-55-06 and 100-70-03 are available. Our agricultural machinery castings use ductile iron for hubs and bearing housings that take shock loads every day.
Cost reality
Ductile iron costs only 5 to 10 percent more than gray iron at the melt. The real premium comes from process control, because magnesium treatment must be timed and verified. A foundry without nodularity testing will quote cheaper and deliver risk. Always ask for nodularity on the certificate; 85 to 95 percent is the normal target.
Typical applications side by side
| Part | Better choice | Why |
|---|---|---|
| Gearbox housing | Gray iron | Vibration damping |
| Wheel hub | Ductile iron | Shock and tension |
| Machine base | Gray iron | Damping and stability |
| Pressure housing | Ductile iron | Pressure integrity |
| Pipe and fittings | Ductile iron | Impact toughness |
Our round baler sprocket hub is a practical example: ductile iron selected because chain loads are shock loads.
Frequently asked questions
Is ductile iron always safer?
No. For static, vibration-prone parts, gray iron performs equally and costs slightly less.
Can a casting be changed from gray to ductile later?
Yes, usually without pattern changes, but wall sections may need review because ductile shrinks differently.
Not sure which grade fits your part? Send the drawing to our engineers through the contact page for a free material recommendation.