Ductile and Compacted Graphite Iron Casting Skin - Evaluation, Effect on Fatigue Strength and Elimination
Compacted graphite (CG) iron features a good combination of tensile strength, impact resistance, thermal conductivity and damping capacity. This combination makes CG iron a material of choice for various applications, especially for the automobile industry. The mechanical properties of CG iron listed in the standards (i.e. ASTM) are for machined specimens. However, since most iron castings retain the original casting surface (a.k.a. casting skin), the actual performance of the part could be significantly different from that of the machined specimens. Recent studies have shown the negative effect of the casting skin, but little quantification of its effect on mechanical properties is available. Further, the understanding of its mechanism of formation is at best incomplete. In this research, the effect of the casting skin on mechanical properties in CG and ductile irons (DI) is explored. The differences in tensile and fatigue properties between as-cast and machined samples were quantified and correlated to the casting skin features. It was found that the presence of the casting skin was accountable for 9% reduction of tensile strength and up to 32% reduction of fatigue strength (for CG iron with 40% nodularity).Several mechanisms of the casting skin formation are proposed in this research. The formation of ferritic and pearlitic rims is explained by decarburizing/carburizing reactions at the mold/metal interface. Mg depletion and solidification kinetics effect were identified as the formation mechanisms of the graphite degradation. A 2-D thermal diffusion model was formulated based on Mg depletion theory. The model can be used to predict the casting skin thickness when Mg depletion is the dominant mechanism. Furthermore, using the asymmetric Fe-Gr phase diagram, some instances of casting skin formation were explained based on solidification kinetics theory.The experimental microstructural evidence and the theoretical progress were conducive to the development of methods for the minimization of the casting skin formation. A serie of experiments was conducted to evaluate the effect of mold coatings on the casting skin formation. It was found that the thermal conductivity of the inactive coatings played an important role. The results from experiments with FeSi and graphite coatings supported the proposed mechanisms. FeSiMg was found effective in suppressing the casting skin formation. Furthermore, key findings are highlighted including the effect of carbon equivalent (CE) and the effect of section thickness.Shot blasting is suggested as an effective means for the elimination of the casting skin effect. The removal of the casting skin by shot blasting was observed. In addition, the work hardening created by the shot blasting can be seen through the plastic deformation on the sample surface. Both contributed to the improvement of the tensile and fatigue properties.
Materials Science, Metallurgy, Compacted Graphite Iron, Casting Skin, Ductile Iron, Mechanism of Formation, Tensile Strength, Fatigue Strength, Shot Blasting, Mold Coating
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