In this work, the influence of the metallic matrix heterogeneities and the spheroidal graphite nodules distribution on both crack initiation and propagation and damage evolution during tensile loading of ferritic spheroidal graphite cast iron is examined. The experimental methodology involves specialized metallographic techniques, step by step tensile loading, microscopic observation by using optical and scanning electron microscopy and three-dimensional (3D) reconstruction of the graphite nodules distribution. The results show that the graphite nodules are the major heterogeneities responsible for inducing the development of cracks in the metallic matrix.Crack initiation is preferentially located at the irregular contour of graphite nodule cavities, ferritic grain boundaries and internodular areas highly strained. The final fracture involves cracks mainly propagating through the internodular ligaments of matrix-nodule debonded areas belonging to the firstto-freeze zones resulting from the solidification process. K E Y W O R D S fracture surface, in situ testing, micromechanisms of fracture, spheroidal graphite cast iron, tensile test
A high silicon cast steel was studied in the as-cast condition in order to characterise its solidification macrostructure and microsegregation. The steel, poured into 32 mm-keel-block-shaped moulds, has a coarse solidification structure and marked microsegregation, containing low-alloyed areas with a total alloy content (Cr + Mn + Si) of 2.3 wt-% and high-alloyed zones of 5.3 wt-%. The bainitic transformation behaviour at 300°C was studied at different austempering times. The bainitic reaction occurs at different rates within the specimen volume, because of its chemical heterogeneity. An austempering heat treatment leads to an inhomogeneous carbide-free bainitic microstructure with different phase amounts, morphologies and sizes. The heterogeneous distribution of sizes and chemical compositions of retained austenite is speculated to benefit mechanical properties.
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