Strengthening and Toughening of Ni Added Ductile Cast Iron by Heat Treatment in the Eutectoid Temperature Range

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Among various kinds of cast iron, 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 of the ductile cast iron lies in the modification of the matrix structure by heat treatment and the addition of alloying elements. In the present study, Ni is added to the ductile cast iron, and various matrix structures are obtained by heat treatment. Effects of these treatments on the toughness are examined.<BR>When the Ni added ductile cast iron is transformed isothermally from the (α+γ) eutectoid temperature range after ferritic annealing, the ferrite plus bainite duplex structure is obtained, and this structure shows a good combination of strength and toughness. The most suitable condition for this treatment to give good toughness even at lower temperature is investigated. Then the Ni added duplex structure ductile cast iron is compared with the ferritic cast iron and the austenitic ductile cast iron by the U-notched Charpy test, fracture toughness test, and tensile test. It is shown that the Ni added ductile cast iron with duplex structure exhibits a good combination of high strength, toughness, and fracture toughness.<BR>It is shown, however, that the transition temperature of the Ni added duplex structure slightly increases compared with the one of the usual ferritic structure in the U-notched Charpy test but largely decreases in the unnotched test. This phenomenon may be due to the transformation induced plasticity effect in the retained austenite phase. It is assumed, therefore, that the main part in the improvement of low temperature toughness in the Ni added duplex structure results from the stabilizing effect in the retained austenite phase by the Ni concentration during holding in the (α+γ) range.

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