Factors influencing microstructure and mechanical properties of as cast and heat treated 400-18 grade ductile cast iron

摘要

As cast and heat treated 400-18 grade ductile cast iron was obtained in different foundry conditions, using various metallic charges, Mg treatment with and without rare earth (RE) additions and various inoculation parameters. Pearlitic influence factor (P{sub}x) and antinodularising action factor (K{sub}1) were found to have an important influence on ultimate and tensile yield strength and ductility, as did Mn and P content, the metallurgical quality of the melt, RE addition and inoculation power. The influence of Mn was in turn influenced by the phosphorus and residual element contents. To obtain an as cast ferritic structure <0.03%P, <0.2%Mn and P{sub}x<2.0 is required. At the same low levels of Mn and P, increasing residual content (P{sub}x>2.0) leads to the appearance of pearlite in the as cast structure, but a ferritic structure is obtained after a short annealing treatment. Lower level of phosphorus (<0.025%) and residual elements (P{sub}x<2.0) allow as cast ferritic structures to be obtained at relatively high Mn content (0.32-0.38%). Higher P (0.04-0.045%) and Mn (0.25-0.35%) contents stabilised pearlite at lower residual levels (P{sub}x<2.0). The action of antinodularising elements could be counteracted up to K{sub}1=2.0, by including REs in the Mg treatment alloy: RE additions are beneficial for K{sub}1<1.2 and compulsory for K{sub}1>1.2. Silicon additions were found to significantly decrease elongation and moderately increase yield and tensile strength at 150-170 HB hardness. The recommended final chemical composition for as cast 400-18 grade is 3.5-3.7%C, 2.4-2.5%Si, ≤0.18%Mn, ≤0.025%P, ≤0.01%S, 0.04-0.05%Mgres (P{sub}x<1.5, K{sub}1<1.1). Use of high purity pig iron, RE bearing FeSiMg and a powerful inoculant is also recommended.