In recent years, there has been a significant interest in the processing and development of austempered ductile cast irons (ADI). Heat-treated ADI have been reported to have excellent mechanical properties and it appears that this material can be processed into major engineering components with a wide range of versatile properties.
ADI is a heat-treated alloyed nodular cast iron with a microstructure that consists of high carbon austenite and acicular bainitic ferrite with graphite nodules dispersed in the matrix. When discussing the microstructure of ADI, it is necessary to distinguish the residual austenite that exists at the isothermal transformation temperature from the retained austenite that remains untransformed at ambient temperature. During the isothermal transformation, carbon partitions between ferrite laths and austenite. Undesirable phases, such as martensite and iron carbides, may also be present in smaller quantities, but it is understood that a maximum retained austenite volume fraction is important if good mechanical properties are to be achieved. The base iron chemistry and alloy additions to ductile iron play important roles in ADI technology. The addition of alloying elements during the production of ADI is often considerably higher than the levels used in the production of conventional grades of ductile irons. The alloy in the austenitized condition (obtained by holding at a temperature in the range 820?C950 ??C long enough to dissolve the austenite with up to 2.1 wt.% carbon) is quenched to the austempering temperature and subjected to isothermal transformation at temperatures between 250 and 450 ??C before cooling to ambient temperature. The microstructure can be controlled through the variation of alloying elements, temperature and the time of austempering. The choice of austempering parameters is critical. The isothermal heat treatment should be terminated at the point that allows an optimum balance between bainitic ferrite and retained austenite, but before carbide precipitation. The formation of carbides may cause the structure to become embrittled and thus degrade the mechanical properties of ADI.
Since the majority of ADI components should possess satisfactory austemperability, alloying elements serve to delay the transformation of austenite. Due to a severe restriction on the maximum thickness of bulk unalloyed ADI that can be heat-treated successfully, alloying with Si, Mn, Mo, Ni and Cu is aimed at providing a desired hardening ability. The importance of alloying additions depends on their relative effectiveness on the austempering process. Si has a very important role because the addition of more than 2 wt.% Si may retard carbide precipitation, producing carbide-free austenite in a bainitic microstructure. The effect of Si and of individual or combined additions of Cu, Mo, Ni, and Mn on the transformation characteristics of ADI have been reported earlier. Mn and Mo delay the austempering process, but Cu does not affect the carbon diffusion in austenite or the stability of austenite. However, it has been reported that Cu suppresses carbide formation in lower bainite, whereas Ni, which acts in a similar way to Cu if present in excess of 0.5%, slows down the bainitic reaction, causing the formation of martensite at austenite cell boundaries on cooling.
The focus of this paper is the influence of additions of Cu and Cu + Ni on the properties of ADI. The specific items of study are the effects of austempering time and alloy composition on the microstructure, the impact properties and the fracture mechanism after austenitizing at 900 ??C and subsequent austempering at 350 ??C.
Addition of Cu + Ni delays the transformation kinetics of residual austenite (isothermal) resulting in a shift of the maximum of volume fraction of retained austenite to 3 h of austempering, compared to 2 h in ADI alloyed with Cu. In the same time, higher maximum value of the volume fraction of retained austenite was observed in ADI alloyed with Cu + Ni. In the same ADI a substantial plastic deformation at the peak of impact energy is associated with the highest volume fraction of retained austenite. It has been demonstrated that the volume fraction of retained austenite strongly affects impact energy of both irons, i.e. with increasing content of retained austenite up to a maximum value impact energy increases, then a decrease occurs with the decrease of retained austenite.