<p>Defects and metallurgical discontinuities significantly affect the magnitude and the variability of the mechanical properties of materials; this can be very significant in castings because of their sensitivity to chemical compositions and section thicknesses. A new procedure for assessing the integrity of Ductile Irons (DIs) that is based on tensile-strain-hardening analysis has been proposed <i>via</i> the modelling of experimental tensile-flow curves with the constitutive equation of Voce. The goodness of this approach is based on the unexpected regular strain-hardening behaviour of defective materials that has been called “Defects-Driven Plasticity” (DDP), which gives rise to the definition of a new Material Integrity Index (MII). The tensile-flow behaviour and microstructural analysis of 4.2 wt pct Si DI that was produced in different geometries and Y-blocks were reported on in order to study the sensitivity of the microstructural and mechanical properties to section thickness in this innovative grade. The tensile mechanical properties seemed to indicate that the mechanical behaviour of the section with the slowest cooling rate was better than the thinner section (which was in contrast with the microstructural results). The new Material Integrity Index that was based on the strain-hardening analysis rationalised the mechanical behaviours of all of the sections with different cooling rates differently, thus resulting in integrity assessments that were consistent with the microstructural findings.</p>