Assessment of the austempering process evolution through tensile testing

The capability of a constituent equation based on physical parameters to relate the plastic behavior with the microstructure was investigated in Austempered Ductile cast Iron 1050 (ADI 1050) to assess the stability of the residual austenite produced during austempering to obtain the optimum ausferritic microstructure. The critical time-window to interrupt austempering is that when the austempering transformation has come to saturation and an austenitic fraction characterized by maximum stability is produced (Stage I of austempering), before the precipitation of detrimental carbides e' takes place (Stage II of austempering). The ADI 1050 was quenched after different times during austempering, and the quenched samples were deformed through tensile testing at room temperature. The trends of ultimate tensile strengths Rm and elongations to rupture eR were consistent with the observed microstructure, and for very long austempering times a significant reduction in ductility was found, which could be rationalized as the occurring of e' precipitation. However, the Rm and eR data trends showed a large dispersion, so that it was impossible to identify unambiguously the optimum timewindow of austempering. For this reason, tensile flow curves were modelled with a constituent equation based on physical parameters related to the microstructure of materials. These parameters showed a lower dispersion with smooth trends against the austempering times, so they allowed to determine the critical time-window to stop the austempering industrial process for the best ausferrite.