Dependence of fracture toughness of austempered ductile iron on austempering temperature

Ductile cast iron samples were austenitized at 927 degrees C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at 260 degrees C, 288 degrees C, 316 degrees C, 343 degrees C, 371 degrees C, and 399 degrees C. These were subjected to a plane strain fracture toughness test. Fracture toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pet retained austenite containing more than 1.8 wt pet carbon. A theoretical model was developed, which could explain the observed variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained austenite content. A plot of K-IC(2) against sigma(y)(XgammaCgamma)(1/2) resulted in a straight line, as predicted by the model.