Qualification of the mechanical properties of Wire Arc Additive Manufacturing (WAAM) products is an essential step for a successful introduction of this process in industry. To realise this, the relationship between processing conditions, the thermal history and the resulting mechanical properties needs to be established. Hardness is an easy-to-check property that correlates well with such tensile properties. This thesis describes the prediction of the relationship between thermal cycling during deposition of a WAAM produced multilayer block and the resulting hardness. A 3D transient heat model was built to investigate the effect of the thermal behaviour. The weld process was simulated using temperature boundary conditions instead of a direct heat source to calculate the thermal field. The material deposition was simulated using the element birth-death method. The calculated thermal data was used for hardness estimation based on the t8/5 time, i.e. the time to cool from 800 to 500 °C, to determine the hardness distribution throughout the deposited blocks. Temperature was measured during the experiments with thermocouples to validate the thermal model, while hardness was measured to verify the hardness results. The thermal boundary condition based model was found to be in good agreement with experimental results. The modelled results show a correct prediction of the decrease in hardness with increased layer height.