Influencing factors on as-cast and heat treated 400-18 ductile iron grade characteristics

As-cast and heat-treated 400-18 ductile iron (DI) grade was obtained in different foundry conditions, as metallic charge, Mg-treatment alloy and inoculation. It was found that the Pearlitic Influence Factor (Px) and Antinodulizing Complex Factor (K1) have an important influence on property of DI, depending on the Mn and P level, the metallurgical quality of iron melt, rare earth (RE) and inoculation. It was also found that the influence of Mn is depended on the phosphorus and residual elements level in ductile iron. Less than 0.03％P and 0.2％Mn and Px＜2.0 are the basic conditions to obtain as-cast ferritic structure. At the same lower level of Mn and P, the increasing of residual elements (Px＞2.0) determines presence of pearlite in as-cast structure, while ferrite structure is obtained after a short annealing heat treatment. Lower level of phosphorus (P＜0.025％) and residual elements (Px＜2.0) allow to use relative high Mn content (0.32％-0.38％), in condition of ferritic structure, including in as-cast state. High P (0.04％-0.045％) and Mn (0.25％-0.35％) content stabilized pearlite, especially at lower level of residual elements (Px ＜2.0). Antinodulizing action of elements was counteracted up to K1=2.0 level, by RE included in Mg-treatment alloy, which are beneficial for K1＜1.2 and compulsory for K1＞1.2. Si has a significant influence on the mechanical properties of heat treated ductile irons: an important decreasing of elongation level and a moderate increasing of yield and tensile strength and their ratio in 150-170 HB typical hardness field. A typical final chemical composition for as-cast 400-18 ductile iron could include 3.5％-3.7％C, 2.4％-2.5％Si, max.0.18％Mn, max.0.025％P, max.0.01％S, 0.04％-0.05％Mgres.for Px＜1.5 and K1＜1.1. High purity pig iron, RE-bearing FeSiMg and powerful inoculant are also recommended.