Graphite
Graphite is pure carbon in a soft, layered form. In cast iron, excess carbon precipitates as graphite during solidification – the shape of these precipitates (flakes, spheres, worms) determines whether the casting is brittle or tough.
What is Graphite
Graphite is the pure, soft form of carbon – you know it from pencils. It consists of layers that slide easily over each other, which is why it's soft and dark.
Graphite in Cast Iron
Cast iron has a carbon content above 2%. The excess carbon that doesn't fit into the iron lattice precipitates as graphite during solidification. How this graphite grows makes all the difference: as fine flakes (gray iron GJL), as spheres (ductile iron GJS), or as worms (compacted graphite GJV).
Why Shape is So Important
Flakes act like countless tiny notches – they make the material brittle. Spheres, on the other hand, are not notches, so the material becomes tough and ductile. So the graphite shape is the real difference between brittle gray iron and tough ductile iron.
When Welding
In the arc, graphite melts, and where the mass cools quickly afterward, hard iron carbide forms instead of graphite – brittle martensite and ledeburite in the heat-affected zone. That's why cast iron must cool slowly so that graphite can form again.