Incomplete austenitization
Incomplete austenitization occurs when the annealing temperature or holding time during heating is insufficient to completely transform the initial microstructure into the face-centered cubic phase. Remaining undissolved carbides or ferrite islands impair the subsequent heat-treatment result.
If you want to normalize the structure, you have to bring the workpiece completely to austenitizing temperature. If you stay too cold or hold too short, the carbides won't dissolve properly and you'll keep a hard, brittle ferrite portion in the core. That will come back to bite you during welding with cracks in the heat-affected zone. So: heat it up properly until it's fully soaked, then give it enough time for the structure to transform homogeneously. With carbide-rich steels, you need to be especially patient.