To improve the mechanical properties and consequently the service life of die casting tools, high-pressure gas quenching is usually carried out followed by several annealing steps. The finite element method (FEM) is used to optimize the heat treatment process with the goal to improve material properties, reduce residual stresses and lower the risk of crack formation. The thermomechanical model accounts for martensitic and bainitic phase changes, the formulation of transformation-induced plasticity and an inverse optimization routine for adapting the thermal boundary conditions of the vacuum furnace. The quenching simulation is calibrated by laboratory tests and validated by industrial heat treatments of test geometries, with residual stress measurements being performed. Sufficiently high and evenly distributed hardness values are desirable in order to improve the performance of die casting tools over their service life. For this purpose, an extensive test program is created that includes many combinations of annealing times and temperatures. The measured hardness values after annealing form the basis for a hardness prediction model for a hot-work steel grade, which can be implemented into the finite element model.