Case-hardening steels are used for components, which surface layers are carburized and then hardened. This causes a combination of a wear-resistant surface and a tough core. The fabrication of case-hardened steels as semi-finished products must ensure that the material has a low hardness to exhibit a good machinability for subsequent processing steps. Therefore the necessary microstructure can be achieved with soft-annealing. Another way to produce this microstructure is a pearlitisation-annealing. This consists of an austenitisation and isothermal pearlitisation, which is directly included in the manufacturing process. A homogenous heat-treatment over the whole cross-section is even more difficult with increasing component dimensions. Especially close to the core areas segregation zones have a large distance to each other caused by the low deformation degree. The segregation in these areas may lead to an incomplete transformation of austenite into ferrite and carbide in an isothermal pearlitisation-annealing. To describe the phase transformation behavior of these zones, Buderus Edelstahl GmbH provided a sample to analyze the composition with electron-microprobe. The sample includes a critical segregation-zone which was taken out from a large bar of the case-hardening steel 18CrNiMo7-6. The collected data were used to create experimental alloys which are modeled with the same composition as the segregation zone in the base alloy. The transformation behavior of these alloys was analyzed via dilatometry. As a result of the dilatometry the test alloys showed a more sluggish conversion speed of austenite into ferrite and carbide compared to the initial alloy. Therefore, a way to speed up the phase transition by a temperature-time-variation of the consisting annealing process was investigated. A potential solution was a modification of the austenitisation-conditions to increase the interface-area per volume (interface area density). This affects the speed of diffusion-controlled transformations significantly by heterogeneous nucleation. Finally, the findings of the transformation behavior on the laboratory scale have been implemented in a small-scale test and verified for their feasibility for industry.