High Silicon Solid Solution Strengthened Ferritic Ductile Cast Iron possesses advantages, such as better combination of strength-elongation; higher fatigue strength, smaller section thickness sensitivity, higher machinability etc., over conventional ferrite-peadite type ductile cast iron. However, industrial application of high Si ductile cast iron is still very limited, because of the lower Charpy impact value at room temperature. As the toughness of iron strongly depends on the strain rate as well as temperature, dynamic three-point bending tests are conducted on 3 similar to 4%Si ferritic ductile cast iron at stroke speed of 10(-3)similar to 10(2) mm/s, and at -20 similar to 22 degrees C. The relations of the crack initiation energy E-i. the crack propagation energy E-p the total absorbed energy E-t and the maximum bending stress sigma(b,max) to the strain rate show abrupt dropping of these characteristic values at critical strain rate, depending on silicon content and test temperature. sigma(b,max) keeps increasing with increasing strain rate as far as the fracture origin is ductile, it slightly decreases over (epsilon)over dot(sigma) = (epsilon)over dot(fD) where the dimple fracture completely disappears. sigma(b,max) E-t and E-i of each silicon iron is well expressed in relation to strain rate-temperature parameter R, T ln(A/(epsilon)over dot(f)). The critical R values for E-t, E-i and sigma(b,max) (R-t, R-i and R-sigma) decrease linearly with decreasing the silicon content of iron. The critical value for sigma(b,max) (R-sigma) is lowest, indicating R-sigma gives a wider design tolerance.
