Castings sections of 12mm and 7 mm with the composition of 3.2%Si,0.5%Mn and 3.0%Si,0.1%Mn accordingly represent the ferritic structure of about 80% or more (Fig.1,b).The results obtained in further investigation show that mechanical properties (especially elongation)of these compositions of as-cast nodular cast irons are maximums. The amount of ferritic structure according to the wall-thicknesses in different sections of castings is tabulated in Table 2.The effect of cooling rate on the formation of ferritic structure is illustrated in Fig.2. In nodular cast iron with 0.5%Mn (Fig.2),by the increase of silicon content and thicknesses of castings the amount of ferritic structure is increased also. But for the cast iron with 3.4%Si a slight decreasing ferritic structure within 30-60 mm of casting thicknesses is observed.When silicon contents are 2.8% and 3.2%and thicknesses of castings section is more than 30mm such a reduction of the amount of ferritic structure is observed also for nodular cast iron with low manganese content (Fig.3).This phenomenon is probably connected with reverse segregation of chemical elements present in nodular cast iron. Fig.1 MIcrostructure of step-shape castings section of 12mm a,2.8%Si;b.3.2%Si,c,3.4%Si(Mn=0.5%) 100 90 70 0,5%Mn 50 40 121824303642485460 Thickness of casting,mm Fig.2 Effect of cooling rate on formation of ferritic structure(0.5%Mn) (1)2.8%Si,(2)3.2%Si,(3)3.4%Si 546· ， · ， · ， · 一 一 · · · · ， · 。 。 写 一 · · · 主 · 声 一 ， 尸 承 洲二 口 户口叫 叫 尸 二 二二二翻 一 ， 口甲已碑口‘ 二二石 砂己竺尸尸一夕尸 口，甲卜 叫 口 兰一一 声 厂 叹 ﹃俩白翻 ， 。 ， 呈， ， ， 笼