Effects of Ni addition on the microstructure and low-temperature impact behavior of ductile iron after ferritization treatment

This study systematically investigates the influence of Ni addition on the microstructure, mechanical properties, and low-temperature impact behavior of ductile iron following ferritization treatment. Fully ferritic matrices were achieved at Ni contents below 6 wt%, wherein Ni refined ferrite grains and increased the high-angle grain boundaries (HAGBs) fraction. Further increasing Ni content hindered the carbon diffusion, suppressing the ferritization process even at relatively low cooling rates. Additionally, Ni segregation at Fe/graphite interfaces retarded ferrite precipitation kinetics, delocalizing ferrite nucleation away from graphite. Ni addition strengthened the ferritic ductile irons through substitutional solid solution strengthening and grain refinement. The ferritic ductile iron containing 6 wt% Ni exhibited ultimate tensile strength (UTS) of ∼600 MPa and total elongation (TL) of ∼15 %. Critically, Ni addition enhanced the low-temperature toughness, lowering the ductile-to-brittle transition temperature (DBTT) from −40 °C (unalloyed) to −90 °C (6 wt% Ni). This improvement was attributed to the passivation of cracks via Ni solute, elevated HAGB fraction, and reduced kernel average misorientation (KAM) value.