<p>The design of heavy-section ductile iron (HSDI) castings relies on international standards that limit the maximum wall thickness to 200 mm. This study investigates the influence of solidification time (3–20 h) and casting position (central zone vs. transition zone) on the microstructure and tensile properties of specimens extracted from cubes (side length of 350 mm, 700 mm, and 1000 mm) made of ferritic ductile iron (EN GJS 400-15). The results indicate a decrease in mechanical properties with increasing solidification time, specifically 11% for yield strength, 27% for ultimate tensile strength, and 78% for elongation, which is attributed to grain and nodule coarsening, solidification defects, and degenerate graphite. Regarding the casting position, at a fixed solidification time (10 h), the 1000 × 1000 × 1000 mm³ cube transition zone samples show a coarser microstructure, leading to a 27% decrease in elongation and an 11% decrease in ultimate tensile strength compared to the 700 × 700 × 700 mm³ cube thermal centre samples. However, the results also reveal that international standards tend to underestimate the actual mechanical properties of the material. To facilitate practical applications, empirical equations are proposed, enabling reliable prediction of strength properties of HSDI based on key microstructural parameters, independent of solidification time and casting position.</p> Graphical Abstract <p></p>