Effect Of Trace Element Bi On Microstructure And Properties Of Ductile Cast Iron With Different Wall Thickness

Ductile cast iron has excellent properties,so advanced ductile iron castings are widely used in transportation,energy,metallurgy,national defense and other key application fields.In particular,thin-walled ductile cast iron and thick cross-section ductile cast iron are highly favored by the advanced equipment industry.However,the cooling rate of thin-walled ductile cast iron and heavy section ductile cast iron is too fast or too slow,which leads to the defect of graphite in the microstructure of ductile cast iron.of graphite,Trace element Bi can increase the number of graphite spheres,refine the size of graphite spheres,reduce graphite distortion,and significantly improve the quality and properties of thin-walled and heavy section ductile cast iron castings as the surface active element.Using the five-finger sample method for reference,through the design and casting of cylindrical samples with different diameters and different Bi content in the form of inoculant,the effect of Bi content on the microstructure of ductile cast iron samples with different cooling rates were studied,and verifies the effect of Bi element in different types of ductile cast iron samples.The trace element Bi has obvious influence on the graphite morphology and matrix microstructure of ductile cast iron with different wall thickness.When the wall thickness is constant,with the increase of Bi,the size of graphite spheres decreases,the number of graphite spheres increases,and the spheroidization rate increases at first and then decreases when the wall thickness is 10mm,30mm,70mm and 150mm,respectively.For the sample with the wall thickness of 10mm,when the addition of Bi is 0.004%,the shape of graphite sphere is the best,the spheroidization rate is 84.9%,the size of graphite sphere is 23.2μm,the number of graphite sphere is 277/mm~2.The content of cementite decreases from 27%to 3.4%.For the samples with the wall thickness of 30mm,70mm and 150mm,when the addition of Bi is 0.012%,the shape of graphite sphere is the best,the spheroidization rate are 84.5%,82.3%and 76.4%,respectively,the size of graphite sphere is 38μm,54μm and 59μm,the number of graphite sphere are 160/mm~2,91/mm~2 and 51/mm~2,respectively,and the content of cementite are 0%,0.40%and 0.67%,respectively.Selecting three kinds of ductile cast iron materials,and preparation of Y-shaped ductile cast iron samples with different chemical composition and different Wall thicknesses by gravity resin sand mold method and adding 0.012%Bi for inoculation treatment.The experimental results show that when 0.012%Bi is added to the ductile cast iron samples of A,B and C with different chemical composition,Bi can have a good inoculation effect and the morphology of graphite can be improved.When the Bi content is the same,with the increase of wall thickness,the Brinell hardness of the sample variation little,the tensile strength decreases,and the elongation increases gradually.When the wall thickness is constant,when 0.012%Bi is added,the hardness of ductile cast iron samples with wall thickness of 12.5mm,25mm and 75mm in group A are 210HBW,201HBW and 198HBW,respectively,and the tensile strength are650MPa,562MPa and 518MPa,respectively,the elongation are 9.3%,10.6%and11.3%,respectively,the addition of 0.012%Bi eliminated most of the broken graphite,improved the graphite morphology and counteracted the deterioration of graphite morphology caused by high silicon in nodular cast iron samples in group A samples.In group B,the hardness of ductile cast iron samples with wall thickness of 12.5mm,25mm and 75mm are 248HBW,242HBW and 232HBW,respectively,the tensile strength are 831MPa,776MPa and 645MPa,respectively,and the elongation are 3.6%,4.7%and 5.2%respectively,the 0.07%Sn element in group B samples has little effect on the graphite morphology of nodular cast iron samples,and does not interfere with graphitization elements,so the effect of Bi in group B samples is the same as that in group A samples.In group C,the hardness of ductile cast iron samples with wall thickness of 12.5mm,25mm and 75mm are 200HBW,193HBW and 184HBW,respectively,the tensile strength are 520MPa,513MPa and 489MPa,respectively,and the elongation are 10.8%,11.1%and 12.6%,respectively.the addition of 0.012%Bi not only counteracts the deterioration of graphite morphology caused by high silicon,but also greatly improves the graphite morphology in group C samples.