cast iron

If steel is iron with a pinch of carbon, cast iron is iron with a lot of it, more than about 2 percent carbon, usually 2 to 4 percent, plus a good dose of silicon. That much carbon melts at a lower temperature and flows beautifully into a mold, which is why cast iron has made engine blocks, cookware, pipes, manhole covers, and machine bases for centuries. It is cheap, easy to cast into complex shapes, and easy to machine.

With so much carbon, most of it cannot stay dissolved and comes out as a separate carbon-rich phase, and the FORM that carbon takes is what defines each cast iron. In gray iron, silicon pushes the carbon out as flakes of soft graphite; the flakes act like internal cracks, so gray iron is weak in tension and brittle but superb at damping vibration and easy to machine (that gray fracture surface gives it its name). In white iron, fast cooling instead locks the carbon into hard, brittle cementite, giving a glass-hard, wear-resistant but un-machinable material. Malleable iron is white iron heat-treated until the carbide breaks down into rounded graphite clumps, restoring some ductility.

So cast iron is a family, not one material, and the trade is castability and cheapness against the brittleness that flake graphite brings. That brittleness is exactly the problem ductile (nodular) iron was invented to fix, by rounding the graphite into spheres, which is why so much modern cast iron is actually ductile iron. Gray iron is still everywhere it does not need to survive a sharp impact: brake rotors, engine blocks, and heavy machine frames that benefit from its vibration damping.