The surface layer of hot working tools is subject to alternating thermo-mechanical loads during forging. It experiences a fast increase in temperature on contact with the heated billets, followed by a steep temperature decline during application of the spray-coolant. This can lead to a cyclic surface rehardening of the tool surface layer due to the formation of a martensitic structure, which can either delay or accelerate tool failure. The microstructural changes in the tool surface layer mainly depend on the thermal, mechanical and tribological loads during forging. The influence of these loads is of particular interest to understand the effect of cyclic surface rehardening. The goal of this research is to investigate the influence of cooling parameters on microstructural changes in the tool surface layer during die forging. Mechanical and tribological loads are kept constant while cooling parameters are varied. Three internal thermocouples are applied to forging tools to measure the base tool temperature. To keep the amount of lubricant in each forging cycle at constant levels, cooling and lubrication are separated by use of boron nitride as lubricant, which is applied by electrostatic adherence. For cooling, the duration of water application is varied while maintaining pressure and spray pattern. Fourtools with differenttool base temperatures are investigated and the influence of the thermal loads on the wear behaviour displayed.