Influence of the Microstructure of Eutectoid Steel on the Cyclic Crack Propagation: Pearlite and Spheroidite

In this paper the influence of microstructure on fatigue crack growth was analysed in steel with slightly hypereutectoid composition. A material constituted of pearlite colonies and a thin layer of proeutectoid cementite (pearlitic steel) was studied in its initial condition (as received). In addition, the same material was analyzed after undergoing a spheroidization treatment obtained by an isothermal treatment on pearlitic steel at 700A(0)C and heating time of 10h. Results indicate that fatigue crack propagation curve in the Paris region is not modified by the spheroidization process. Fractometallographic analysis showed a change in the micromechanism of fatigue, evolving from transcollonial (trying to break pearlite lamellae) in the pearlitic steel to intergranular in the spheroidized steel, where cracking takes place through the layer of proeutectoid cementite.