The effect of intrinsic casting defects on the mechanical properties of austempered, alloyed ductile iron

Measurements of the solute distribution between nodules and of the tensile properties at two different levels in a 25 mm keel block of a ductile iron of composition 3.39%C, 2.56%Si, 0.25%Mo, 0.29%Cu and 0.37%Mn after hipping and austempering and after austempering at 375 °C are reported. A significant drop in tensile properties sufficient to cause the iron to fail the standard was found at the level nearest to the top of the block. This is attributed to intrinsic casting defects affecting microporosity and nodule count at the two levels. The fall in tensile properties is attributed mainly to the lower nodule count and its associated increased carbide level, increased solute segregation and lower nodularity. Although at present it is not possible to measure quantitatively the relative contributions of these three factors, the present results show that the increased solute segregation associated with the lower nodule count causes a significant drop in tensile properties.