Issue34

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 34 (2015) 406-414; DOI: 10.3221/IGF-ESIS.34.45 413 Furthermore, the increase of the applied K I implies the generation of slip bands in the ferritic matric and the nucleation of secondary cracks (especially corresponding to graphite elements), with the coalescence of the secondary cracks that contributes to the stable crack propagation. Analogously to ferritic, ferritic-pearlitic and pearlitic DCIs, around the crack tip the increase of the applied K I generates a plastic/damaged zone instead of a “mere” plastic zone. Considering the fracture surface, it seems that graphite residuals are less evident with respect to the fracture surface obtained during the fatigue crack propagation (Fig. 19) and that the voids around the graphite elements are more “developed”, with an evident ductile morphology (Fig. 19 and 20) Figure 19 : SEM observation of the fracture surface (after overloads). Figure 20 : SEM observation of the fracture surface (after overloads). C ONCLUSIONS n this work, a long annealing heat treatment was performed on a pearlitic DCI in order to activate the carbon atom solid diffusion process and increase the thickness of the outer graphite shield. Fatigue crack propagation tests were performed on a long term annealed DCI and crack paths were investigated by means of scanning electron microscope observations. In addition, overloads effects on crack tip were also investigated. According to the experimental results, the following conclusions can be summarised: - The long duration high temperature annealing (170 h at 850°C) allows the activation of the carbon atoms solid diffusion mechanism: the final microstructure shows a ferritic matrix with graphite elements with an increased radius and a modified morphology (“degenerated” nodules). - Comparing the behaviour of the obtained ferritized DCI with degenerated graphite nodules with a “standard” ferritic DCI, fatigue crack propagation seems to be affected by the different nodules morphology. - Considering the interaction between the “degenerated” graphite nodules and the fatigue crack, the graphite shield obtained during the annealing treatement is characterized by a different mechanical behaviour if compared to the “original” graphite nodules; - Considering overload effects, Ferritized DCI with degenerated nodules show both the crack tip blunting (analogously to ferritic DCIs) and the stable crack propagation (as in pearlitic DCIs). Around the crack tip a plastic/damaged zone is observed instead of a “simple” plastic zone. A CNOWLEDGMENTS ranco Zanardi and Zanardi Fonderie S.p.A. are warmly acknowledged. I F