Dependence of fracture toughness of austempered ductile iron on austempering temperature

Ductile cast iron samples were austenitized at 927 degrees C and subsequent ly austempered for 30 minutes, 1 hour, and 2 hours at 260 degrees C, 288 de grees C, 316 degrees C, 343 degrees C, 371 degrees C, and 399 degrees C. Th ese were subjected to a plane strain fracture toughness test. Fracture toug hness 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 correlat ed with microstructural features such as bainite morphology, the volume fra ction of retained austenite, and its carbon content. It was found that frac ture toughness was maximized when the microstructure consisted of lower bai nite with about 30 vol pet retained austenite containing more than 1.8 wt p et carbon. A theoretical model was developed, which could explain the obser ved variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and ret ained austenite content. A plot of K-IC(2) against sigma(y)(XgammaCgamma)(1 /2) resulted in a straight line, as predicted by the model.