It can be seen from the table that when the sample wall thickness increases from 20mm to 40mm, the graphite spheroidization rate in nodular cast iron decreases, and further increases the sample wall thickness to 60mm. There is little difference between the graphite spheroidization rate and the sample with wall thickness of 40mm. This is because when the wall thickness of the sample increases, the solidification time of nodular cast iron is prolonged, the residual magnesium content and residual rare earth content in nodular cast iron are reduced, and the spheroidization recession is serious; At the same time, due to the increase of solidification time of the sample, the thermodynamic undercooling of the metal flame body decreases, the carbon escape impairment near the graphite ball decreases, and the austenite halo cannot be formed rapidly. Therefore, the graphite ball will contact with the metal flame for a long time, causing the uneven diffusion of some trace active elements (such as oxygen, sulfur, etc.) in the liquid metal to the graphite crystal core, resulting in the distortion of the graphite in the eutectic growth stage and the formation of an incomplete graphite ball. Therefore, with the increase of the sample wall thickness, the spheroidization rate of the graphite ball decreases.
| Specimen thickness /mm | Number of graphite balls per unit area | Spheroidization rate |
| 20 | 180 | 0.96 |
| 40 | 147 | 0.92 |
| 60 | 127 | 0.93 |
The figure shows the matrix structure of nodular cast iron with different wall thickness. Among them, figures (a) – (c) are the matrix structure diagrams of samples with wall thickness of 20mm, 40mm and 60mm respectively.
It can be seen from the figure that the structure of nodular cast iron is composed of pearlitic carbide with “ox eye” structure. In the nodular cast iron with wall thickness of 20mm, there are more “ox eye” structures. This is mainly related to the diffusion of carbon atoms in the cast iron during solidification. In addition, there are more carbides in the three samples with different wall thickness.
Whether graphite or carbide precipitates in the solidification process of cast iron mainly depends on the thermodynamic and kinetic conditions that affect the crystallization of graphite and cementite. According to the comprehensive analysis of the conditions for obtaining nodular cast iron, the main factors affecting the carbide precipitation of nodular cast iron include the cooling rate of the metal melt and the core state of the solution.
When the metal solidifies, if the cooling rate increases, although the metal melt has the crystallization temperature according to the stable system or metastable system, the dynamic conditions for transformation into cementite are more sufficient than those for transformation into graphite, because the composition and structure of the parent phase are less different from that of cementite. In addition, the temperature at which eutectic reaction occurs decreases, and it is difficult for carbon and iron atoms to diffuse and migrate, so it is too late to move for a long distance. Therefore, it is more difficult to form w (c) =100% graphite than to form w (c) =6.67% cementite.
In addition, the nature, quantity and thickness of the crystal nuclei in the metal melt will play an important role in the solidification of the metal melt. Untested graphite, oxide, carbide, nitride, intermetallic compound and gas in metal melt
Body can be used as the base for graphite precipitation under certain conditions, which is conducive to the graphitization of cast iron. The magnesium element in the added spheroidizing agent is a strong whitening element. When the magnesium element content is small, it is easy to combine with sulfur, oxygen and other elements to form compounds as the graphite core, increasing the nucleation of graphite. If the content of residual magnesium is high, the tendency of iron whitening will increase.
Therefore, in the lost foam casting process, due to the small size of the casting, the rapid solidification rate of the metal solution, and the short residence time after spheroidization and inoculation, more magnesium and rare earth elements remain in the cast iron, which is the main reason for the occurrence of free carbides in the nodular cast iron.