Static and dynamic crack initiation resistance toughness (J
IC, J
Id) and crack propagation resistance toughness (J-
Δa curve, T
mat) of austempered low alloy ductile iron were evaluated by direct current electrical potential method and instrumented Charpy impact test with compliance changing rate and key-curve analyses. The compliance changing rate and key-curve analyses are effective for evaluating dynamic crack initiation and propagation resistance toughness in this type of iron. QB' treatment [quenching in oil bath after holding in the
γ-phase region for a short time and austempering after holding in the (
α+
γ) region] and B' treatment [austempering after holding in the (
α+
γ) region] improve crack initiation and propagation resistance toughness of Mn and Ni added iron.
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