INFLUENCE OF CASTING SIZE AND GRAPHITE NODULE REFINEMENT ON FRACTURE-TOUGHNESS OF AUSTEMPERED DUCTILE IRON

Casting size affects the solidification cooling rate and microstructur e of casting materials. Graphite nodules existing in the structure of ductile iron are an inherent and inert second phase that cannot be mod ified in subsequent heat-treatment processing. The matrix and the fine ness of the second phase undoubtedly have some impact on the fracture toughness of the as-cast material, as does the subsequent heat treatme nt, as it alters the microstructure. This research applied austemperin g heat treatment to ductile iron of different section sizes and graphi te nodule finenesses. The influence of these variables on the plane st rain fracture toughness (K-IC) of the castings so treated was compared to that of the as-cast state. Metallography, scanning electron micros copy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.