The graphite morphology of ductile iron samples with different silicon content is shown in Figure 1. With reference to the international standard GB/T 9441-2021 Metallographic Inspection of Ductile Iron, the nodularity of graphite is more than 90%, and the roundness of graphite is basically unchanged.
| Sample No ω (Si) (%) | Number of graphite balls/piece · mm ^ – 2 | Diameter of graphite ball/μm |
| A(2.92) | 151 | 35.6 |
| B(3.68) | 165 | 30.8 |
| C(4.22) | 208 | 27.7 |
| D(4.59) | 223 | 26 |
The graphite morphology of ductile iron samples with different silicon content was statistically analyzed by ipp-6.0 image analysis software. The results are shown in Table 1; It can be seen that the average number of graphite balls in unit area of QT450-10 ductile iron with 2.92% silicon content is about 151/mm2, and the average size of spherical graphite is about 35.6 μ m; The average number of graphite balls in unit area of high silicon ductile iron with silicon content of 4.59% is about 223/mm2, and the average size of spherical graphite is about 26 μ m。 With the increase of silicon content, the graphite ball size of nodular cast iron is refined, and the number of graphite balls in unit area is increased. Silicon plays a role as the nucleation core of graphite. The increase of silicon content leads to the increase of the eutectic temperature range of stable system and metastable system, that is, the eutectic temperature range of austenite plus graphite is higher than that of austenite plus cementite, which is conducive to the acquisition of graphite and leads to the increase of the number of graphite spheroidal nucleation cores; Under the condition of keeping the carbon equivalent basically unchanged, with the increase of silicon content, the carbon content decreases, so the volume of graphite decreases. Therefore, the graphite ball of high silicon ferrite ductile iron is finer and more uniform.
It can be seen from Fig. 2 that the matrix structure of QT450-10 ductile iron and high silicon ductile iron with 2.92% silicon content are composed of ferrite. From the thermodynamic point of view, with the increase of silicon content, the eutectoid transition temperature increases, and the eutectoid temperature range of stable and metastable systems increases, providing favorable conditions for the formation of ferrite. From the perspective of dynamics, with the occurrence of the solidification process of nodular cast iron, the austenite formed at high temperature will undergo solid phase transformation when it is reduced to the eutectoid temperature. Due to the dense distribution of graphite in high silicon nodular cast iron, the distance between austenite and graphite is shortened, the carbon in austenite is easy to be desolved and diffused to the eutectic graphite, while the carbon in austenite is easy to precipitate the ferrite core on the austenite interface after diffusion, This is beneficial to the formation of ferrite.
| Sample No ω (Si) (%) | Ferrite content (%) | Ferrite grain size/μm |
| A(2.92) | 88.8 | 43.7662 |
| B(3.68) | 89.9 | 42.0642 |
| C(4.22) | 90 | 38.6304 |
| D(4.59) | 91.7 | 35.3811 |
The ferrite content and ferrite grain size of ductile iron samples with different silicon content are statistically analyzed, and the results are shown in Table 2. It can be seen that the average ferrite grain size of QT450-10 ductile iron sample with silicon content of 2.92% is about 43.766 2 μ m. When the silicon content is 4.59%, the average ferrite grain size of the high silicon ductile iron sample is about 35.3811 1 μ m; With the increase of silicon content, ferrite grains are refined. Silicon element increases the eutectic transformation temperature of nodular cast iron. The increase of eutectic temperature increases the undercooling at the solidification front of molten iron, increases the primary nucleation rate of ferrite, and reduces the grain size gradually.
