Cementite

Cementite is a hard and brittle microstructural phase in steel that can form in the weld metal or heat-affected zone if cooling is wrong. It forms not from sulfur but from carbon precipitating out of austenite during rapid cooling. If you have too much cementite in the structure, the weld becomes hard and prone to cracking, especially in higher-strength steels. To avoid this, you need to slow down the cooling, for example by preheating or welding slower. Make sure the heat-affected zone doesn't cool too fast, then the structure stays tougher and the weld holds better.