Nitrogen-alloyed plastic mould steels show a decrease in toughness when tempered at 510°C. The aim of this diploma thesis was to figure out the range of the reduced toughness as a function of austenitizing temperature, austenitizing time, cooling time and tempering temperature as well as to investigate the metallurgical reasons for that. In the frame work of this diploma thesis a nitrogen-alloyed plastic mould steel as well as comparative a nitrogen-free variant were examined. To establish the toughness behaviour, measurements of hardness in combination with instrumented impact bending tests were performed. The evaluation of microstructure was obtained by light microscopy and scanning electron microscopy. It was shown that pro-eutectoid precipitations in combination with a coarse grain size are responsible for the reduction in toughness in nitrogen-alloyed plastic mould steels.