Special carbide
Highly stable chemical compounds of carbon with alloying elements such as vanadium, tungsten, molybdenum, or chromium. They significantly increase the wear resistance and hot hardness of tool steels.
Special carbides are hard compounds of carbon and alloying elements such as chromium, tungsten, or titanium. They form in steel when you put in too much heat during welding or the cooling isn't right. These things make the structure brittle and can cause cracks, especially in tool steels. When welding such steel, you have to watch the heat input and cooling rate. Too fast cooling or too hot welding promotes carbide formation. Keep the interpass temperature low and let the part cool slowly, then the carbides stay finely distributed and the steel keeps its toughness.