Issue34

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 34 (2015) 406-414; DOI: 10.3221/IGF-ESIS.34.45 407  Carbides, due to different causes, with a key role played by the presence of carbide promoting elements such as Mn, Cr, V, Mo, and by a rapid cooling rate, Fig. 4;  Irregular graphite, due to high holding and/or long holding temperature or to a poor inoculation;  Slag inclusions, that can be due to different causes (e.g., inadequate slag control from pouring system);  Shrinkage, due to inadequate feed of available metal, excess of magnesium, under or over inoculation;  Gas holes, that can be due to many causes (e.g., melting procedures). Figure 1 : Defects in DCIs. Exploded graphite (x200). Courtesy of Zanardi Fonderie S.p.A. Figure 2 : Defects in DCIs. Chunky graphite (x200). Courtesy of Zanardi Fonderie S.p.A. Figure 3 : Defects in DCIs. Compacted graphite (x100). Courtesy of Zanardi Fonderie S.p.A. Figure 4 : Defects in DCIs. Carbides (x200). Courtesy of Zanardi Fonderie S.p.A. All these defects can strongly affect the DCI mechanical behaviour, considering both static or quasi static or cyclic loading conditions [4]. Considering the ferritic DCI, corresponding to lower  K, very short secondary cracks are observed (Fig. 5) and the main damaging micromechanism due to the graphite nodules presence is the graphite elements – ferritic matrix debonding, with or without residual graphite (Fig. 5 and 6), respectively). This residual graphite seems to be due to a mechanical properties gradient inside the graphite nodules, with the nodule core (obtained directly from the melted DCI) that is characterized by lower wearing properties and lower hardness values with respect to the outer graphite nodule shield, obtained by means of a carbon atoms solid diffusion during the cooling process [5, 6]. This internal gradient implies the nucleation and propagation of secondary cracks inside the graphite nodules: this mechanism depends on the matrix microstructure and is more evident with static or quasi-static loadings, but can be also observed during fatigue crack propagation (as in Fig. 4 and 6).