1999 Volume 85 Issue 10 Pages 765-770
Impact tensile tests of as-cast and ferritic annealed ductile cast irons are carried out at strain rates up to 103/s using a servo-hydraulic high speed testing machine. The fractography of the tensile specimens is carefully carried out with a scanning electron microscope. The experimental results demonstrate that the 0.2% proof stress and ultimate tensile strength of the materials used increase with increasing strain rate and lowering temperature. For the as-cast material with higher pearlite content in the matrix, the strain rate sensitivities of the strengths at room temperature are found to be higher than those of ferritic material. According to the relationships between the tensile properties and strain rate-temperature parameter, R-value, it is considered that for the deformation mechanisms of ductile cast irons at strain rates up to 103/s, the thermally activated process is predominantly. Also, the constitutive equations of the impact tensile strengths for these irons are made based on the strain rate-temperature parameter R. It is believed that the proposed equations can be appropriate for engineering applications. While both the work-hardening exponent and the tensile ductility increase by decreasing the R-value. However, the tensile ductility decreases by decreasing the R-value less than 4500K, because of the over-strengthening by deformation constraining occurs: It is supported by observations of fracture surfaces in the tensile specimens that micro-dimple fracture surface area induced from debondings at a large number of small inclusions near the eutectic cell boundaries increases with decreasing R-value.
