Influence of Silicon Content, Strain Rate and Temperature on Toughness and Strength of Solid Solution Strengthened Ferritic Ductile Cast Iron

抄録

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-pearlite 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 3～4%Si ferritic ductile cast iron at stroke speed of 10⁻³～10² mm/s, and at −20～22℃. 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 σ_b,max to the strain rate show abrupt dropping of these characteristic values at critical strain rate, depending on silicon content and test temperature. σ_b,max keeps increasing with increasing strain rate $\dot{\varepsilon}$ as far as the fracture origin is ductile, it slightly decreases over $\dot{\varepsilon}_\sigma = \dot{\varepsilon}_{fD}$ where the dimple fracture completely disappears. σ_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/$\dot{\varepsilon}$). The critical R values for E_t, E_i, and σ_b,max (R_t, R_i and R_σ) decrease linearly with decreasing the silicon content of iron. The critical value for σ_b,max (R_σ) is lowest, indicating R_σ gives a wider design tolerance.