With the development of industrialization in China, ductile iron has become the main metal material in industrial production due to its good comprehensive performance, mainly composed of ferrite matrix, with a typical grade of QT450-10. Due to its excellent mechanical properties, it has been widely used in metallurgical machinery equipment, petrochemical industry, pipeline industry, transportation and other industries. Due to the different service environments of castings, the performance requirements for ductile iron are becoming increasingly high.
Compared with mixed matrix ferritic ductile iron, silicon solid solution strengthened ferritic ductile iron has higher strength, toughness, and processability. This excellent performance has led to increasing research on high silicon ferrite ductile iron today. When the silicon content (mass fraction) of ductile iron with license plate number QT500-7 is increased to 3.5%, its base The body tissue is transformed into all ferrite, with a tensile strength of 500 MPa, elongation twice as high as before, and a smaller range of hardness fluctuations; By increasing the silicon content to 3.2%~3.4%, automobile wheels were developed using high silicon ferrite ductile iron, reducing the hardness fluctuation at different parts of the same casting from the original 30~40 HBW to 15 HBW. The tensile strength of the produced castings can reach 590 MPa, and the elongation rate can reach 16%; The material of the produced vehicle brake lever housing is ductile iron, with a silicon content of 3.0%~4.5%, a tensile strength of 400~650 MPa, a yield strength of 70%~90% of the tensile strength, and an elongation of 10%~12%. The matrix structure is mainly ferrite. Due to the high content of silicon element, ductile iron can undergo brittle fracture. Therefore, it is difficult to apply high silicon ductile iron to thin-walled large pipe fittings, and research in this area is also relatively scarce. The difference between EPC process and traditional sand casting is that the gasification of the foam white mold will take away part of the heat, so it is particularly important to control the pouring temperature. If the pouring temperature is too low, non-metallic inclusions will easily occur, while if the pouring temperature is too high, the spheroidizing effect will be reduced and carbonization will affect the performance; The silicon content also affects the eutectic temperature of ductile iron and thus affects the matrix structure. Therefore, the study of using lost foam casting to obtain high silicon ductile iron with a fully ferritic structure is of great significance. High silicon nodular cast iron with silicon content of 2.9%~4.6% was prepared by lost foam casting process, and its microstructure and mechanical properties were studied. The mechanism of silicon element on the matrix structure and mechanical properties of high silicon nodular cast iron was analyzed, providing a theoretical basis for the process of lost foam casting large high silicon nodular cast iron pipe fittings.
While keeping the carbon equivalent basically unchanged, the silicon content increases while the carbon content relatively decreases. 1 t medium frequency induction furnace is used for smelting, and EPC process is used for casting. EPS material is used for foam white mold, the main component of coating is bauxite, the molding sand is dry silica sand, the pouring temperature is 1496 ℃, the negative pressure is 0.05 MPa, and the pouring time is 30 s. Wire feeding spheroidizing method is used for spheroidizing, the addition amount of spheroidizing agent is 1.3%, and the addition amount of inoculant is 75 ferrosilicon, 1.5%~2.5%. The wall thickness of the foam white mold Y-shaped sample is 24 mm, and the size is determined according to the national standard GB/T 1348-2019 Nodular Iron Castings. The matrix structures of ductile iron and high silicon ductile iron are both composed of ferrite. From a thermodynamic perspective, as the silicon content increases, the eutectic transition temperature rises, and the eutectic temperature range of stable and metastable systems increases, providing favorable conditions for the formation of ferrite. From a dynamic perspective, as the solidification process of ductile iron occurs, the austenite generated at high temperatures undergoes solid phase transformation when it decreases to the eutectoid temperature. Due to the dense distribution of graphite in high silicon ductile iron, the distance between austenite and graphite is shortened, and the carbon in austenite is easily dissolved and diffused onto eutectic graphite. After the carbon in austenite diffuses out, it is easy to The core of ferrite precipitates at the austenite interface, which facilitates the formation of ferrite. With the increase of silicon content, the strength and hardness of high silicon ferrite ductile iron increase, and the yield strength ratio also increases. The yield strength ratio refers to the ratio of the yield strength to the tensile strength of the material. After yielding under stress, the material changes from elastic deformation to plastic deformation. Materials with a high yield strength ratio have stronger resistance to deformation, are less prone to plastic deformation, and can save materials and reduce costs; The size of spherical graphite gradually decreases, thus reducing its cutting effect on the metal matrix. The increase in silicon content leads to a gradual increase in the eutectic temperature of high silicon ductile iron, which improves the undercooling of the solidification front of the molten iron, increases the nucleation rate of ferrite, refines the grain size, increases the number of grain boundaries, and increases the hindrance to dislocation movement. Its strength, hardness, and elongation increase; But with the increase of silicon element, solute atoms will form Coriolis gas clusters near dislocations, reducing the plastic deformation ability of the material, resulting in a decrease in elongation. Under the combined effect of the two, the elongation of high silicon ductile iron samples remains basically unchanged.
Ductile iron samples with silicon content ranging from 2.9% to 4.6% were prepared using the lost foam casting process. The matrix structure is ferrite. When the silicon content is 2.92%, the average diameter of spherical graphite in QT450-10 ductile iron samples is about 35.6 μ m, the average number of graphite spheres per unit area is about 151/mm2, and the average size of ferrite grains is about 43.766 2 μm; When the silicon content is 4.59%, the average diameter of spherical graphite in the high silicon ductile iron sample is about 26 μ m, the average number of graphite spheres per unit area is about 223/mm2, and the average size of ferrite grains is about
It is 35.381 1 μ m. QT450-10 ductile iron with a silicon content of 2.92%, tensile strength of 441 MPa, Brinell hardness of 128 HBW, and elongation of 17%; High silicon ductile iron with a silicon content of 4.59%, tensile strength The hardness is 683 MPa, Brinell hardness is 186.1 HBW, elongation is 17.5%, and the comprehensive mechanical properties are high.
(3) Samples of high silicon ductile iron with silicon content ranging from 3.5% to 4.6% show that an increase in silicon content results in finer and more uniform graphite spheres, while the cutting effect of spherical graphite on the metal matrix is reduced; The reduction of ferrite grain size allows silicon to affect ferrite The fine grain strengthening effect of the body is enhanced; Silicon element can replace solid solution in ferrite matrix to form solid solution strengthening, but with the increase of silicon content, its segregation index value at ferrite grain boundaries becomes larger and the segregation phenomenon becomes more severe, resulting in a decrease in the lattice constant of ferrite and an increase in the degree of lattice distortion. Therefore, with the increase of silicon content, the mechanical properties of high silicon ductile iron improve.