Primary cementite
Cementite that precipitates directly from the liquid melt as the primary phase in hypereutectic alloys. It often appears as coarse needles or plates, imparting brittleness.
Primary cementite is what you see in the microstructure of hypereutectic cast iron – those hard, brittle carbide needles. They crack quickly during welding because they don't deform at all and shrinkage stress acts directly as a notch. Add to that the martensite formation during rapid cooling, and you get a crack before you're even done. That's why the part must be preheated properly, slowly and evenly. Use nickel electrodes and keep the heat input low. After welding, don't rush anything – put the part in an insulating blanket or in the oven so it cools down very slowly. Whatever you do, don't hammer or vibrate it while it's still warm.