Gray iron or SG (ductile) iron: which does your part need?

The difference is the graphite

Cast iron holds more carbon than steel can dissolve, so the surplus appears as graphite inside the metal. In gray iron that graphite forms flakes. In SG iron, magnesium is added to the ladle before pouring and the graphite forms spheres instead, which is why it is called spheroidal graphite, ductile or nodular iron.

Those flakes behave like tiny internal cracks. They soak up vibration and let the metal machine easily, but under a sudden load a flake tip is where a crack starts. Spheres have no sharp ends, so the metal around them stretches instead of splitting. That single change takes the iron from brittle to tough.

What that means in practice

Typical published values for common grades, to give a sense of the gap:

Choose gray iron when

The part is loaded steadily rather than suddenly, and you want heat and noise taken out of the machine.

• Brake drums, where heat has to leave quickly and the surface must wear evenly against a lining
• Machine bases, housings and covers, where damping stops chatter
• Pump housings and brackets under steady load
• Anything you will machine heavily, since gray iron cuts easily and spares your tooling

Choose SG (ductile) iron when

The part carries the machine, a shock, or a load that reverses millions of times.

• Axle housings and tubes, which take the weight of the machine and whatever it pulls
• Wheel hubs, where a crack is a safety problem rather than a repair
• Crankshafts, which bend and unbend with every firing stroke
• Clutch and drivetrain parts that take sudden engagement loads

How to tell which one you already have

If you have a broken part in your hand, the fracture tells you most of it. Gray iron breaks with a dull, dark grey face, almost sooty, and the break is flat and sudden. SG iron shows a brighter, silvery, more crystalline face, and the metal around the break is often slightly deformed where it stretched first.

Two more clues: gray iron is noticeably easier to file, and a ring test helps, since a gray iron part gives a dull thud when tapped while an SG iron part rings. None of this replaces a proper check. Any foundry worth using will confirm the material before making your pattern.

What to tell the foundry

You do not need a grade number to get a sensible quote. If you can describe the part and what it does, a foundry can tell you which iron it should be.

• What machine or vehicle it belongs to, and what the part holds or carries
• Whether the original failed, and how: cracked, worn, bent or seized
• Any grade your drawing specifies, if there is one
• Whether the part will be machined after casting