1991 年 40 巻 453 号 p. 669-674
The impact fracture patterns of two kinds, ferritic and pearlitic matrix, of unalloyed spheroidal graphite cast irons austempered at either 573K or 673K for 5min or 2h have been correlated to Charpy impact values at room temperature. The results of this study are summarized as follows:
(1) Austempering at 573K for both matrixes induced a sharp drop in Charpy impact strength as compared with that without austempering. The coarse upper bainite structure obtained by austempering at 673K was found to improve the impact strength. This improvement of the impact strength was attributed to the retained austenite in the structure.
(2) The impact fracture surfaces with high impact strength, ferritic matrix and upper bainite structure austempered at 673K, showed a ductile dimple pattern accompanying localized plasticity in the vicinity of graphite.
(3) Brittle quasi-cleavage modes with less plasticity were observed on lower bainitic structure austempered at 573K similar to pearlitic matrix. The number of graphites observed per unit area on the brittle fracture surface was less than that on the ductile fracture one.
(4) A series of local cleavage facets were observed at the root of the notch coaxial to the fracture direction. It suggests that the impact values are associated with the plastic deformation ahead of these facets.