In the course of this thesis, the influence of sulfur and the hardening process was studied on the formation of the mixed grain in high speed steel HS 6-5-2-5. The mixed grain is the result of abnormal grown grains, which have a negative effect on the mechanical properties. With the dilatometer and the differential scanning calorimeter (DSC), the transformation behavior of the annealed materials was characterized and compared with values obtained by thermodynamic simulation. The annealed and tempered microstructures were examined by light and scanning electron microscopy. Furthermore, different time-temperature programs were defined and quenching experiments performed in the dilatometer. In the quenching experiments the heating rate, austenitising and preheating temperatures were varied. The development of the austenitic grain size, and the carbide size and fraction were determined from light and scanning electron micrographs of the quenching experiments. The studies with a lower preheat temperature and a short holding time showed a positive effect on the grain growth. The obtained information was included in a simulation model, on which an optimized temperature control for the hardening process was developed. These were subsequently traced in a small scale experiment in the dilatometer. In the microstructures the abnormal grain growth could be suppressed by the adjustment of the hardening process.