Tool costs represent an important share of the overall cost per die casting part. Therefore, the reduction of cycle times and the increasing of service time of die casting moulds offer a great potential to reduce costs. There are basically two ways to increase lifetime: changing the material or changing the heat treatment. The aim of the present study was to gain information about the applied heat treatment, through the comparison of the mechanical properties and the microstructure of currently used die casting moulds, with regard to an optimization of material and heat treatment. Therefore, the mechanical properties of some already rejected moulds were determined by tensile test, impact test and hardness measurement. Furthermore, samples were observed by light microscope, scanning electron microscope and transmission electron microscope in order to determine the grain size and the microstructure. Subsequently, the results were compared with a heat treatment simulation and with a heat treatment recommended by the materials supplier, which should show the potential of the investigated materials. All investigated moulds showed properties outside specifications. The present mechanical properties and microstructure point to a less-than-ideal heat treatment. The common practice of converting hardness values to equivalent values in other hardness scales and to estimates of tensile strength turned out to be a possible source of errors during heat treatment. Thus, the conversion of hardness values has to be regarded critically.