Effect of austempering time on mechanical properties of a low manganese austempered ductile iron

An investigation was carried out to examine the influence of austempering t ime on the resultant microstructure and the room-temperature mechanical pro perties of an unalloyed and low manganese ductile cast iron with initially ferritic as-cast structure. The effect of austempering time on the plane st rain fracture toughness of this material was also studied. Compact tension and round cylindrical tensile specimens were prepared from unalloyed ductil e cast iron with low manganese content and with a ferritic as-cast (solidif ied) structure. These specimens were then austempered in the upper (371 deg rees C) and lower (260 degrees C) bainitic temperature ranges for different time periods, ranging from 30 min. to 4 h, Microstructural features such a s type of bainite and the volume fraction of ferrite and austenite and its carbon content were evaluated by X-ray diffraction to examine the influence of microstructure on the mechanical properties and fracture toughness of t his material. The results of the present investigation indicate that for this low mangane se austempered ductile iron (ADI), upper ausferritic microstructures exhibi t higher fracture toughness than lower ausferritic microstructures. Yield a nd tensile strength of the material was found to increase with an increase in austempering time in a lower bainitic temperature range, whereas in the upper bainitic temperature range, time has no significant effect on the mec hanical properties. A retained austenite content between 30 to 35% was foun d to provide optimum fracture toughness. Fracture toughness was found to in crease with the parameter (X gamma C gamma/d)(1/2), where X gamma is the vo lume fraction of austenite, C gamma is the carbon content of the austenite, and d is the mean free path of dislocation motion in ferrite.