Issue34

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 34 (2015) 406-414; DOI: 10.3221/IGF-ESIS.34.45 408 In order to investigate the influence of the graphite nodules mechanical properties gradient on the DCI mechanical properties and damaging micromechanisms, in 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 a scanning electron microscope. In addition, overloads effects on crack tip were also investigated. Figure 5 : Graphite core-graphite-shield debonding (with residual graphite). Figure 6 : Graphite nodule – matrix debonding and graphite nodule internal damaging. I NVESTIGATED MATERIAL AND EXPERIMENTAL PROCEDURE fully pearlitic DCI with a good graphite elements nodularization (chemical composition is shown in Tab.1; microstructure is shown in Fig. 7) was submitted to a long annealing heat treatment, according to the following procedure: - 170 hours at 850°C; - Cooling in furnace to lab temperature. C Si Mn S P Cu Mo Ni Cr Mg Sn 3.59 2.65 0.19 0.012 0.028 0.04 0.004 0.029 0.061 0.060 0.098 Table 1 : Investigated pearlitic DCI chemical composition (5%F + 95%P; before heat treatment). Heat treatment was performed on 10 mm thick Compact Type (CT) specimens. As a result of this heat treatment, a ferritic matrix with degenerated graphite nodules embedded was obtained (Fig. 8). Nodules were characterized by a surface with a higher roughness (“degenerated nodules”), if compared to the nodules embedded in the DCI before the heat treatment: the long term annealing activated a carbon solid diffusion process, with a consequent increase of the nodules diameters and an evident modification of their shape. After the heat treatment, CT specimens were submitted to a metallographic preparation procedure. Fatigue crack propagation tests were performed according to ASTM E647 standard [7], with a stress ratio of R = P min /P max = 0.1. Tests were performed using a computer controlled servohydraulic machine in constant load controlled conditions, considering a 20 Hz loading frequency, a sinusoidal loading waveform and laboratory conditions. Crack length measurements were performed by means of a compliance method using a double cantilever mouth gage and controlled using an optical microscope (x40). During the fatigue crack propagation tests, SEM crack path observations of the specimens lateral surfaces were performed with a step by step procedure. Furthermore, fracture surfaces were analysed by means of a scanning electron microscope, focusing both the graphite elements and the metal matrix (crack propagates always from left to right). Results were compared with the behaviour of a ferritic DCI with “normal” graphite nodules [8, 9] (chemical composition is shown in Tab. 2). A