Ductile or Grey Iron: Choosing the Casting Grade

Why grey iron damps and ductile iron does not, what EN-GJL and EN-GJS grades actually guarantee, and which one belongs under a machine base.

When a machine base is going to be cast, the next question is which iron. The honest answer is that grey iron is the default for a reason most people can state loosely and few can state precisely.

The difference is the shape of the graphite

Both are iron with a few percent carbon, and in both the carbon comes out of solution as graphite rather than staying combined. What differs is the shape it takes.

In grey iron, graphite forms flakes. Those flakes are essentially internal cracks: they interrupt the metal matrix, which is why the material is comparatively weak in tension and why a grey iron casting breaks with a grey, granular fracture face rather than bending.

In ductile iron, magnesium treatment makes the graphite form spheres instead. Spheres interrupt the matrix far less, so the material keeps much more of steel’s toughness and will yield and stretch before it breaks.

Everything else follows from that one difference.

Why the weaker material is usually correct

Those flakes that ruin the tensile strength are also enormous internal interface area, and interface area is where vibration turns into heat. Grey iron damps, and it damps because it is full of flat graphite discontinuities. Ductile iron, having traded those away for toughness, damps noticeably less.

Under a machine base, you rarely need tensile strength. The structure works mostly in compression and bending at modest stress. What you need is for the machine to stop ringing. So the material engineers describe as weak and brittle is the one that belongs there, and the tougher grade is the compromise.

This is also why substituting one for the other is a real engineering change. A machine that was marginal on chatter with grey iron can be unacceptable with ductile.

Reading the grade designations

European practice under EN 1561 and EN 1563:

• EN-GJL-xxx is grey iron with Lamellar (flake) graphite. EN-GJL-200, 250, 300.
• EN-GJS-xxx-y is ductile iron with Spheroidal graphite. EN-GJS-400-15, 500-7, 600-3.

The number is minimum tensile strength in MPa. For ductile iron the second number is minimum elongation as a percentage, which is the whole point of the grade. The old German designations, GG25 for EN-GJL-250 and GGG40 for EN-GJS-400-15, are still in daily use and you will see them on drawings.

Roughly, EN-GJL-250 / GG25 is the workhorse machine base grade: a pearlitic matrix with fine flake graphite, giving usable strength and hardness while keeping the damping and the excellent machinability that a base needs.

The trap: strength is section-dependent

Here is what catches people out on a heavy casting.

The number in the grade is measured on a separately cast test bar, at a defined size, under EN 1561. Your machine base is not a test bar. A thick section cools slowly, coarsens the graphite and produces lower strength and hardness than the bar suggests.

On a base with 60 mm walls, the strength in the wall will sit meaningfully below the grade minimum. This is normal, expected and covered in the standard, and it is not a defect. But it means:

• Specify the grade and the governing section thickness.
• Agree where any test material comes from: a separately cast bar, a cast-on lug, or excised from the casting itself. These give different numbers and the difference is not small.
• If a hardness range matters for machining or for a wear face, specify it separately and say where it is measured.

Skip this and you will have an argument at inspection where both parties are reading the same standard correctly.

Where ductile iron does belong

Not every part of a machine is a bed. Ductile iron is right when:

• The part sees impact or shock, or a cyclic tensile load.
• A failure would be dangerous and you want yielding rather than a brittle break.
• The section is thin and highly stressed.
• It is a bracket, arm, lever, housing or pedestal rather than a damping structure.

Presses, crushers and anything with a sudden load reversal frequently use ductile for exactly these reasons.

What to put on the casting drawing

• Grade to EN 1561 or EN 1563, with the equivalent legacy designation if your shop uses it.
• Governing section thickness.
• Where test material is taken and which properties are pass or fail.
• Hardness range and measurement location, if it matters.
• Required non-destructive testing and the acceptance class.
• Whether a casting may be repaired, and by what method.
• Ageing or stress relief before machining, and whether you want the chart.

That last one is not optional on a precision base. A casting cools unevenly in the mould and carries residual stress; rough machining releases it and the part moves. Read casting defects and what to reject and the stress relief and flatness guide alongside this one.

Once grey iron is chosen, the order is specified in our cast iron machine base guide.

Common questions

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