Intercrystalline Graphite Formation
The precipitation of carbon directly at the interfaces of primary crystallites, typical for grey cast iron with lamellar graphite.
When welding grey cast iron, the problem is that the carbon in the material is present as graphite in the base structure. Due to the high heat input during welding, this graphite can dissolve in the transition area to the weld and, during rapid cooling, precipitate again as hard, brittle cementite. This makes the transition zone extremely prone to cracking. Therefore, you must ensure that cooling is slow and controlled so that the carbon can precipitate again as soft graphite. Preheating is the be-all and end-all, and after welding, the part should cool slowly, for example in sand or thermal insulation. If you don't do this, you'll get hard spots that are hard to machine and crack quickly.