For the application of heat treatable steels such as 42CrMo4, especially in large scale components, a uniform microstructure after quenching and tempering is necessary to obtain the required properties. However, in large scale components undesired microstructural components like bainite/perlite/ferrite may appear. To increase the hardenability without generating additional costs, the investigated steel was alloyed with boron. Boron has the tendency to segregate to austenite grain boundaries and delay the transformation to perlite/ferrite. However, in industrial components precipitates were found at prior austenite grain boundaries which decreased the ductility. This diploma thesis was aimed at clarifying what kind of precipitates are present and how their formation can be avoided by optimizing the heat treatment by applying defined cooling conditions. The precipitates, which were examined by atom probe, are most likely M23(BC)6. For maintaining sufficient impact strength a very fast cooling from austenitization temperature is necessary. The occurrence of precipitates along grain boundaries can be observed already at a slightly slower cooling rate.