Development of wear resistant carbidic austempered ductile iron (CADI)

The abrasion wear resistance of irons may be improved by the incorporation of an extra phase to the matrix, typically consistent of carbides. Nevertheless, the improvement in wear resistance is generally accompanied by a decrease in the impact toughness. The aim of this work is to study the methodology to obtain a controlled precipitation of carbides in a ductile cast iron that is subsequently austempered, obtaining the so-called carbidic austempered ductile iron (CADI). This material is expected to offer high abrasion resistance but still retaining higher impact toughness than other reinforced irons, thanks to the particular characteristics of the matrix. A plate model with a copper chill allowed to evaluate both, the influence of a high cooling rate and the effect of alloying elements on carbide precipitation. Four different alloys with equivalent carbon close to the eutectic composition were used in order to evaluate the effect of chromium contents ranging from 1 to 2.5%. A detailed microstructural characterisation of the material was made, studying particularly carbide content and composition, besides their stability during the heat treatment. The abrasion wear resistance was evaluated by testing under the ASTM G 65 standard, obtaining relative wear resistance values ranging between 1.02 and 1.95 with respect to conventional ADI samples taken as reference material. The impact toughness decreased from about 138–101 J for ADI to about 26–7 J for CADI.