Correlation between microstructure and plastic behaviour in low alloyed ductile iron through a deterministic model

Ductile irons are attractive materials because of their low cost and wide range of applications. The low cost comes from easy route of production in near-net-shape components through foundry, and from the low content of alloying elements, which is also topical for the critical raw materials issue. Ductile irons are Fe-C-Si alloys with hypereutectic compositions with an appropriate Mg content to produce graphite in nodular shape, in a matrix of bcc ferrite and eutectoid pearlite in different volume fractions. The control of microstructure produced during solidification is a key issue to obtain the optimal plastic behaviour of ductile irons. So nodule counts, nodularity of graphite nodules, ferritic grain size and pearlite volume fractions have to be measured for foundry feedback. Therefore, a deterministic model for the correlation between the measured microstructure and the plastic behaviour is fundamental to understand the microstructure features that actually affect the mechanical behaviour and to produce a helpful feedback to the industrial producer. Samples of ductile iron GJS 400 have been produced through four different moulds with increasing thicknesses to test the effect of the solidification rate on the final microstructures that have been investigated and quantified through scanning electron microscopy. The samples were also tensile-tested to produce the plastic flow curves that are fundamental for the foundry feedback and also for the component designers. The model used to describe the tensile plastic flow curves are dislocation-density-related constitutive equations and incorporates physical parameters correlated to the microstructural features. The model has been tuned on the ferritic materials produced in the fastest cooling rate and can well predict the tensile plastic flow curves of the samples from the different moulds.