For two types of steel, the heat-resistant 12% Cr–steel X 20 CrMoV 12 1 and the fine-grained pressure vessel steel 20 MnMoNi 5 5 (A 508 cl.3), a wide range of toughness values in the upper shelf was realized by different sulfur contents and the inclusion of similar weld metals. Additionally, the pressure vessel steel 22 NiMoCr 3 7 (A 508 cl.2) was also investigated. Tensile, fracture mechanics and Charpy V-notch impact tests, as well as detailed microstructural investigations with respect to the size distribution and density of non-metallic inclusions and precipitates, were carried out. In order to ensure ductile behaviour, a test temperature of 150 °C was chosen. The relevance of two quantitative relations available for the calculation of the J-integral, Ji,phys, at physical crack initiation using tensile test data and microstructural parameters, were examined by comparison with the corresponding experimental Ji,phys-values. Only one quantitative relation was able to give good agreement between calculated and experimental Ji,phys-values. This holds not only for the base materials but also for the weld metals. The importance of the size and density of the non-metallic inclusions became quantitatively obvious with the consequence that their size times density is a decisive parameter for toughness. Observations of void initiation, growth and coalescence illustrate the fracture process.