Flake graphite

Lamellar graphite is what gives gray cast iron (GJL) its typical gray fracture surface and good damping. But the flakes break the metal matrix, making the material brittle and inelastic. When welding, that's the catch: heat is conducted extremely well, so the melt cools quickly, leading to hardening and cracks. You have to preheat slowly and cool even slower, or the part will crack. And don't forget that you won't get far with regular stick electrode steel – you need special filler, otherwise the weld will be porous and cracked.