Effect of Graphite Inclusions on Mechanical Properties of Austempered Ductile Iron
Recently, austempered ductile iron (ADI) has emerged as a new class of ferrous materials and represents a major achievement in cast iron technology . The mechanical strength and impact toughness of nodular iron are provided by the precipitation of the graphite phase as spheroids surrounded by ferrite (bull’s-eye structure) in a continuous pearlite matrix. The quality of ductile iron increases with the number of the graphite spheroids. A high spheroids volume fraction, which is mainly controlled by the inoculation process, limits the chemical segregation during solidification and ensures the structural homogeneity of the component. In this work, a lower value of Young modulus was obtained when the graphite phase was taken into account in the self-consistent modelling. For 12% of graphite the theoretical Young modulus agrees with the measured one (mechanical tensile test). The volume fraction of graphite was confirmed independently by micrographic observation (14%). It can be concluded that the macroscopic behaviour of ADI steel can be modelled by the self-consistent approach in which the austeno-ferritic aggregate is represented by an effective matrix, while instead of the graphite spherical empty spaces are introduced. Using such an approach it was shown that in the elasto-plastic range of deformation, presence of graphite phase caused stress relaxation.
Sabine Denis, Takao Hanabusa, Bob Baoping He, Eric Mittemeijer, JunMa Nan, Ismail Cevdet Noyan, Berthold Scholtes, Keisuke Tanaka, KeWei Xu
F. Serban et al., "Effect of Graphite Inclusions on Mechanical Properties of Austempered Ductile Iron", Materials Science Forum, Vols. 490-491, pp. 73-78, 2005