Comparative study of fracture toughness of austempered ductile irons with upper and lower ausferrite microstructures

A ductile iron was austempered at 302 and 385 degrees C for various times t o get lower and upper ausferrite microstructures respectively. The microstr uctures were characterised by optical microscopy and X-ray diffraction. Pla ne strain fracture toughness was determined under all heat treatment condit ions. While the austempered ductile iron with lower ausferrite microstructu re showed higher fracture toughness, the one with upper ausferrite microstr ucture exhibited higher tensile toughness and strain hardening coefficient. A model was developed relating fracture toughness to the yield strength (s igma(y)) volume fraction of retained austenite (X-gamma) and the carbon con tent of the retained austenite (C-gamma). Experimental results showed excel lent agreement with the prediction of the model that K-1C(2) is proportiona l to sigma(y) (X-gamma C-gamma)(1/2).