Production of machinable ferritic-ausferritic ADI by selection of proper partially austenitizing time

Austempered ductile iron (ADI) is well known as useful engineering material and can compete with forged steel in many applications. But its poor machinability have caused to be not quite welcomed by manufacturers. Therefore, it is essential to modify production process to achieve desire machinable ADI by controlling of heat treatment parameters. The present study was, therefore, performed to clarify the influence of austenitizing time on cutting forces in rough turning operations and to obtain the optimum austenitizing duration of improving the machinability. To attain this goal, the samples of ferritic ductile iron (FDI) were austenitized at temperature of 900 oC for 5 to 60 min, followed by austempering into a salt bath at 370°C for 60 min to produce dual matrix structures (DMS) with different ausferrite volume fractions. Image analysis was employed to quantitatively evaluate the microstructure. The hardness were determined via Brinell test method. As a criteria adopting for machinability, cutting forces measured by Kistler dynamometer. The results indicated that the ausferrite fraction and hardness increased by increasing austentizing time according to the Johnson-Avrami model. Increasing austenitizing time to 12 min resulted in 40-50% and 36% improvement on the resultant cutting force and Specific cutting power, respectively, when compared to ADI. The resultant cutting force was correlated with feed rate as a power model with exponents of 0.73, 0.80 and 0.85 for FDI, DMS and ADI, respectively. Obtained results indicates the selection of proper duration of isothermal austenitizing play the key role to achieve ADI with desire machinability.