The lamellar spacing of pearlite will also affect the tensile strength of ductile. Generally, the smaller the lamellar spacing is, the higher the tensile strength is. In order to further observe the morphology of pearlite lamellae and explore the effect of alloy elements Mn and Cu on the spacing of pearlite lamellae, the microstructure morphology of No. 1 ~ 3 casting samples magnified by 8000 times was further observed by scanning electron microscope. The results are shown in Fig. (a) ~ (c).
It can be seen from the figure that the pearlite morphology of the three groups of ductile iron casting samples is mainly lamellar structure, which is closely related to the concentration fluctuation in the solution. When one phase forms and grows, it will cause the change of element concentration near it, resulting in the nucleation and growth of the other phase. In this way, the layered structure is formed.
Because the lamellar spacing of pearlite tissue is not a specific value, but an average value in an interval distributed along the median value. Therefore, according to the method proposed by brown and Ridley, select a circular area with diameter D, and then count the number of layers n perpendicular to the circle in the area, using the formula:
Where S0 is the pearlite lamellar spacing and M is the magnification.
According to the formula, the pearlite lamellar spacing of No. 1 ~ 3 ductile iron casting samples are 495 nm, 444 nm and 306 nm respectively. It can be seen that with the increase of the content of alloy elements Mn and Cu, the pearlite structure becomes more uniform and the pearlite lamella is refined, indicating that both elements can significantly refine and stabilize the pearlite structure. At the same time, because the refining effect of Cu on Pearlite lamella is stronger than that of Mn, the lamellar spacing of No. 3 ductile iron casting sample (Fig. (c)) is lower than that of No. 2 ductile iron casting sample.