KRAWCZYK Janusz AGH University of Science and Technology

The development of fatigue cracks is often analyzed on the basis of their geometry and is explained merely by the state of tensions, without the consideration of the microstructure. The crack resistance is, however, strictly connected with the microstructure. An analysis of the course of the crack development on the working surface of cast steel metallurgical rolls was performed. It was stated that the precipitations of ledeburitic and secondary cementite are the ways of an easy propagation of the above mentioned cracks. We can observe an especially strong dependence between the lattice of ledeburitic cementite precipitations and the lattice of cracks which forms on the surface of the metallurgical rolls. In the case of a continuous lattice of ledeburitic cementite precipitations, there is a possibility of chipping of large fragments of the material which correspond to the volume of the primary austenite grain. It was stated that the graphite precipitations have an ambiguous role in the formation and development of thermal fatigue cracks. The graphite precipitations present on the surface of the roll can provoke the nucleation of cracks, whereas the spheroidal graphite precipitations can also block the development of the crack.