The injection molding process is the most important discontinuous molding process for thermoplastics. The advantages are free form design, low post-processing effort and high reproduction precision with large quantities at the same time. However, this precision is linked to the dimensional accuracy of the melt-carrying machine parts and is therefore dependent on the wear on the surfaces. In order to increase the efficiency of the injection molding process, ever higher injection volume rates are required, which - due to friction and high shear stress on the melt - lead to large heat inputs into the mold surface. Tool steels for use in injection molding machines are therefore required not only to be resistant to wear but also to be resistant to tempering. For applications under particularly abrasive wear conditions, steels are used whose structure consists of a martensitic matrix, hard primary carbides and secondary hardening carbides precipitated during tempering. Since the structure set during production is in the metastable state, high temperatures that occur during operation can lead to changes in the microstructure. The aim of this thesis was to simulate cyclic thermal loads, as they occur during the injection molding process, using dilatometer tests and to determine their influence on the hardness and microstructure of different tool steels. In the quenching dilatometer, samples were repeatedly inductively heated to varying temperatures of up to 650°C and continuously cooled to 300°C. The hardness was determined on the surface of the samples. Furthermore, the change in the secondary hardening carbides of a powder-metallurgical steel with increasing thermal stress was examined using SEM imaging. The results showed that cyclic short-term heating to temperatures above the tempering temperature caused significant reductions in hardness in all steels examined. Depending on the steel, these were up to 250 HV1. The examination of the secondary hardening carbides showed that these were already found in small numbers in the tempered condition. After cyclic thermal loading, their proportion in the matrix increased by up to 400% compared to the tempered condition, with the mean diameter and the size distribution remaining almost unchanged.