Heat treatable steels, such as 42CrMo4, show excellent mechanical properties due to their martensitic microstructure after a suitable heat treatment. Especially for large components it is of utmost importance to retard the transformation of austenite to undesired microstructure constituents. It is well known that boron, if added in a proper amount, can significantly retard the transformation of austenite to bainite or ferrite/perlite. During the heat treatment it diffuses to the austenite grain boundaries, where it reduces the driving force for the nucleation of new phases. The aim of this work was to determine whether the expected transformation retardation occurs in 42CrMo4 and whether boron is actually situated at prior austenite grain boundaries. Therefore, TTT-diagrams were created. A prior austenite grain boundary was prepared for atom probe tomography by electro-polishing, etching and sharpening by focused ion beam in combination with transmission electron backscatter diffraction measurements. Additionally, simulations with Thermo-Calc were carried out. A slight transformation retardation of bainite could be seen in the TTT-diagrams. Furthermore boron could be found at a prior austenite grain boundary.