Effect of two-step austempering process on micrstructure and properties of austempered ductile iron

The microstructure and mechanical properties of austempered ductile iron(ADI) treated by two-step austempering process and conventional single-step austempering process were investigated by means of hardness test,impact,toughness test and optical microscope.The results show that the microstructure of ADI processed by two-step austempering process,for which the samples austenitized at 900℃ for 1h were initially quenched to 285℃ in a salt bath for 10min and then the temperature of the salt bath was raised to 340℃ or 370℃ and kept at the temperatures for different time,is finer than that of ADI processed by single-step austempering process at the temperatures of 340 or 370℃ for same time.A significant improvement in hardness and impact toughness of the ADI is obtained as a result of the two-step austempering process.