Influence of austenitizing temperature on fracture toughness of a low manganese austempered ductile iron (ADI) with ferritic as cast structure

An investigation was carried out to examine the influence of austenitizing temperature on the resultant microstructure and mechanical properties of an unalloyed and low manganese ADI and with an as cast (solidified) ferritic structure. The investigation also examined the influence of austenitizing temperature on the fracture toughness of this material. Compact tension and round cylindrical tensile specimens were prepared from a nodular cast iron without any alloying elements (e.g. nickel, molybdenum or copper) and with very low manganese content and with an as cast (solidified) ferritic structure. These were then austenitized at several temperatures ranging from 871 degrees C (1600 degrees F) to 982 degrees C (1800 degrees F) and then austempered at a constant austempering temperature of 302 degrees C (575 degrees F) for a fixed time period of 2 h. Microstructure was characterized through optical microscopy and X-ray diffraction. Tensile properties and plane strain fracture toughness of all these materials were determined and correlated with the microstructure. Fracture surfaces were examined under scanning electron microscope to determine the fracture mode. The results of this investigation indicate that the austenitizing temperature above 982 degrees C (1800 degrees F) has a detrimental effect on the fracture toughness of this material. Both volume fraction of austenite and its carbon content increased with austenitizing temperature. The strain hardening exponent of this material was found to increase with increase in the austenitic carbon content i.e. (XgammaCgamma)(1/2) where X-gamma is the volume fraction of austenite and C-gamma is the carbon content of austenite. A Hall-Fetch type relationship was found to exist between yield strength and mean free path of dislocation motion, d in ferrite. A model for fracture toughness of ADI has been developed. Present test results indicate good agreement with the model. (C) 1999 Published by Elsevier Science S.A. All rights reserved.