Toshiro Kobayashi1 and Hiroshi Tachibana2
1Department of Metallurgical Engineering, Faculty of Engineering, Nagoya University, Nagoya
In cast irons, ductile cast iron has good ductility and toughness, for the graphite morphology is spheroidal in this iron. It seems that the possibility of strengthening and toughening the ductile cast iron lies in the modification of the matrix structure by heat treatment and the addition of alloying elements. In this study, Ni was added to the ductile cast iron and various matrix structures were obtained by heat treatment. The effects of these treatments on the toughness were then examined.
When the Ni added ductile cast iron was transformed isothermally from the (α +γ ) eutectoid temperature range after ferritic annealing, the ferrite plus bainite duplex structure was obtained and this structure showed a good combination of strength and toughness. The most suitable condition for this treatment to give good toughness to lower temperatures was investigated. It was also shown that the improvement of low temperature toughness can be achieved by a special heat treatment. Then the Ni added duplex structure ductile cast iron was compared with the ferritic ductile cast iron and the austenitic ductile cast iron by the U-notched Charpy test, fracture toughness test, and tensile test. The Ni added duplex structure ductile cast iron exhibited a good combination of high strength, toughness, and fracture toughness.
It was shown, however, that the transition temperature of the Ni added duplex structure does slightly decrease in the U-notched Charpy test but largely decrease in the unnotched Charpy test. This phenomenon may be due to the transformation induced plasticity effect in the retained austenite phase. It is assumed, therefore, that the main cause in the improvement of low temperature toughness in the Ni added duplex structure results from the stabilizing effect in retained austenite by the Ni concentration into the austenite phase during holding in the (α +γ ) range.
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