Abstract
Results of previously published analyses of the parameters controlling the plasticity and fracture mode of ferritic steels are summarized and generalized as follows: Fracture mode is controlled by the mechanism of plastic deformation. Fracture mode transitions from transcrystalline ductile to transcrystalline cleavage at low temperatures and to intercrystalline at elevated temperatures are the consequence of deformation twinning and grain boundary sliding, respectively. The actual temperature and strain rate of the fracture mode transitions are affected by the grain size and by the parameters controlling the deformation stress of individual grains. The actual extent and combination of metallurgical strengthening parameters is morphologically manifested as the microstructure of the steel. However, the dominant role of the microstructure as a complex qualitative characteristic of the steel cannot be replaced by quantitative material parameters (strength and grain size), since the relative contribution of different strengthening mechanisms to the strength of the matrix and to the probability of activation of deformation modes, controlling the fracture mode transitions, is different.

Key words
fracture mode, transcrystalline cleavage, intercrystalline fracture, grain size, strength, ferritic steel

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