By applying a heat treatment, such as hardening and tempering, the properties of a heat treatable steel, like 42CrMo4, can be adjusted over a wide range. In addition to the heat treatment, an appropriate alloying concept is crucial to be able to guarantee an outstanding performance over a large variety of applications. In the present study, the effects of boron as a microalloying element on a heat treatable steel were examined. It is well known that boron, if added in a proper amount, can significantly retard the transformation of austenite to bainite or ferrite/pearlite in low alloyed steels. During the heat treatment, it diffuses to the austenite grain boundaries, where it reduces the driving force for the nucleation of new phases. In order to prove this theory in a heat treatable steel, a prior austenite grain boundary was prepared for atom probe tomography (APT). The specimen preparation was carried out by electro-polishing, etching, and sharpening by focused ion beam (FIB) in combination with transmission Kikuchi diffraction (TKD). In the reconstruction of the corresponding APT measurement, boron could be found at the same prior austenite grain boundary.