Improving the fracture toughness of dualphase austempered ductile iron

Dual Phase Austempered Ductile Irons, DPADI alloys have a microstructure with a combination of ausferrite and pro-eutectoid ferrite along with graphite and residual/retained austenite. To reach this particular microstructure ductile iron is austenized in the austenitepro- eutectoid ferrite region before being carried though an isothermal heat treatment somewhere between the bainite-start and martensite-start temperature. The special austenization procedure makes DPADI prime candidates for applications that require a high fracture toughness resistance like suspension components. The aim of this study is to identify features in the microstructure of dual-phase ADI that directly affect the fracture toughness of the investigated alloys. To do this alloys subjected to various ADI heat treatments have been impact tested and thereafter analysed by light optical microscopy, X-ray diffraction and hardness testing. From Thermo Calc Modelling and experimental investigation of the austenite to pro-eutectoid transformation zone, it was found that an austenitization temperature of 790°C resulted in a microstructure, which avoided perlite formation. In the investigation of industrial ADI heat treatment it was found that the last and first to cool regions of the sample geometry had an impact toughness of around 70 and 100 J, respectively. Furthermore a comparison of conventional ADI and DPADI heat treatments showed that the DPADI heat treatment had an impact toughness of 90 J with a scatter of ±15 J, whereas the conventional ADI heat had the same impact strength with >±20 J scatter. This reduced scatter was attributed to the encapsulation of the detrimental Mo-rich carbides by pro-eutectoid ferrite, after light optical and scanning electron microscopy. Moreover, it was observed that detrimental reductions in impact toughness resulted in areas where slag inclusion, carbides or voids were present. Consequently, it is recommended that more attention is directed to the ductile iron casting process to safeguard the success of the ADI heat treatmentIngeniería Industria