First subareas of a layer of powder applied on a substrate are heated locally with a process beam so as to heated, but not yet interconnect the grains of the powder in the first subareas. A temperature profile of the applied layer is evaluated proximate to the first subarea as a function of the distance from the respective first subarea and/or the time. The local thermal conductivity and/or local diffusivity are determined for the respective first subarea based on the temperature profile. Second subareas of the applied layer which at least partially overlap the first subareas are then heated locally with the process beam so as to interconnect the grains of the powder in the second subareas. A process variable of the process beam influencing heating of the second subareas is determined based on the local thermal conductivity or diffusivity previously determined for the at least partially overlapping first subarea.