Fatigue crack propagation in a ferritic-pearlitic DCI. Overload effects on damaging mechanisms

Ductile iron discovery in 1948 renewed the interest on the cast iron family. These cast irons (DCIs) combine the good castability of gray irons and high toughness values of steels; they are also characterized by an interesting fatigue crack propagation resistance. Considering their interesting mechanical properties, DCIs are widely used in the critical automotive parts (e.g., crankshafts, truck axles, etc.), and in many other application, like pumps, pipes or turbine components. DCIs can be considered as special-type composites, with graphite nodules embedded in a metal matrix. DCIs performances are strongly affected by the graphite elements morphological peculiarities (e.g., graphite elements nodularity, volume fraction, density, distribution, dimension). Different combinations of the mechanical properties can be obtained depending on the matrix microstructure: focusing on ferritic-pearlitic DCIs, different combination of ductility or tensile strength values can be obtained depending on the phases volume fractions and distribution. In this work, a ferritic-pearlitic DCI was considered. Compact Type (CT) specimens were metallographically prepared, chemical etched and, then, fatigue precracked and overloaded. According to a step by step procedure, lateral surfaces were observed by means of a Scanning Electron Microscope (SEM) and by means of a Digital Microscope (DM), in order to investigate the influence of the matrix microstructure and of the graphite nodules on damaging micromechanisms.