Abstract
Due to the peculiar graphite elements shape, obtained by means of a chemical composition control (mainly small addition of elements like Mg, Ca or Ce), Ductile Cast Irons (DCIs) are able to offer the good castability of gray irons with the high mechanical properties of irons (first of all, toughness). This interesting properties combination can be improved both by means of the chemical composition control and by means of different heat treatments(e.g. annealing, normalizing, quenching, austempering etc). In this work, fatigue crack tip damaging micromechanisms in a ferritic-pearlitic DCI were investigated by means of scanning electron microscope observations performed on a lateral surface of Compact Type (CT) specimens during the fatigue crack propagation test (step by step procedure), performed according to the “load shedding procedure”. On the basis of the experimental results, different fatigue damaging micromechanisms were identified, both in the graphite nodules and in the ferritic – pearlitic matrix.
Highlights
Ductile iron discovery in 1948 gave a new lease on life to the cast iron family
Different combinations of the mechanical properties can be obtained depending on the matrix microstructure [3]: ranging from ferritic to pearlitic Ductile Cast Irons (DCIs), different combination of ductility or tensile strength values can be obtained; martensitic DCIs are characterized by a very high strength, but low levels of toughness and ductility; bainitic grades are characterized by a high hardness; austenitic DCIs show good corrosion resistance, good strength and dimensional stability at high temperature; austempered grades (ADI) show a very high wear resistance and fatigue strength
Superposing white spots to the fatigue crack path corresponding to the intersections between the crack path and the graphite nodules (Fig. 5b), it is evident that crack path tortuosity is strongly affected by the graphite nodules distributions: graphite nodules are not mere voids embedded in the metal matrix but they influence the crack path increasing the fracture surface roughness and, the importance of the roughness induced crack closure effect [10]
Summary
Ductile iron discovery in 1948 gave a new lease on life to the cast iron family. these cast irons (DCIs) combine the good castability of gray irons and high toughness values of steels and they are characterized by an interesting fatigue crack propagation resistance. Fatigue crack tip damaging micromechanisms in a ferritic-pearlitic DCI were investigated by means of scanning electron microscope observations performed on a lateral surface of Compact Type (CT) specimens during the fatigue crack propagation test (step by step procedure), performed according to the “load shedding procedure”.
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