Analysis of graphite morphology and distribution and mechanical properties of nodular cast iron

Figure 1 (a) and figure 1 (b) show the hardness and strength changes of three groups of different samples respectively. It can be seen from the figure that the change trend of both hardness and strength (including yield strength and tensile strength) is qt-cfm > qt-tcm > qt-cwm. With the increase of the number of graphite balls, the diameter of graphite balls decreased, and the hardness and strength of the three groups of samples decreased gradually. Qt-cwm has the lowest hardness, yield strength and tensile strength, which are 12.2%, 18.76% and 12.45% lower than qt-cfm respectively.

The impact energy value reflects the impact toughness of the material. The higher the impact energy value, the better the impact toughness of the material. Figure 2 shows the low temperature impact energy values of three groups of materials at – 25 ℃. It can be seen from the figure that the impact properties of the three groups of samples are quite different, which is just opposite to the change trend of hardness and strength. The lowest impact energy value of qt-cfm is 5.3 J, and the impact energy values of qt-tcm and qt-cwm are 13.7 J and 17.3 J respectively, which are 158.4% and 203.5% higher than that of qt-cfm, indicating that refining the graphite diameter and improving the spherical regularity of graphite balls can greatly improve the impact toughness at low temperature.

In order to reveal the impact fracture mechanism from the perspective of microstructure, the fracture morphology of different samples is analyzed, and the corresponding SEM diagram is shown in Fig. 3. It can be seen from 3 (a): there are few dimples in the ferrite matrix structure of qt-cfm fracture, mainly showing continuous and discontinuous River cleavage fracture, with obvious cleavage steps, and a small number of tear pits on the cleavage fracture. Part of graphite falls off during impact deformation, forming corresponding holes, and there are obvious entanglement and tear edge structures near the holes. Part of the graphite remains on the original fracture, adheres to the matrix ferrite during impact deformation, and there is no obvious tear mark around, indicating that the resistance to crack initiation and propagation is small. Therefore, qt-cfm is mainly brittle fracture.

As can be seen from figures 3 (b) and 3 (c), qt-tcm and QT  During the impact deformation of CWM, the gap between the residual graphite ball and the matrix ferrite wall is large, and the matrix ferrite has obvious plastic deformation, which hinders the crack propagation. Part of the graphite falls off and forms corresponding holes, and obvious entanglement and tear edge structures and dimple structures of different sizes are produced near the holes. There are a few river like cleavage fractures and obvious cleavage steps on the fracture surface of the sample. Therefore, the fracture mode of qt-tcm and qt-cwm is a mixed fracture mode of plastic fracture (dimple structure) + brittle fracture (River cleavage fracture), and the plastic fracture mode is the main mode. Compared with qt-tcm, the number of dimples around the cavity of qt-cwm sample is more, and the gap between the graphite ball retained at the source port and the matrix is larger, so the low-temperature toughness of qt-cwm is higher than that of qt-tcm.