Spherical graphite close to spherical contour appears in spheroidized cast iron. The three-dimensional morphology and internal structure of spherical graphite were studied and observed by means of scanning electron microscope, deep etching, thermal oxygen etching and ion etching. It can be observed that the surface of the sphere is uneven, with many protrusions and deep grooves and holes, and an approximately hexagonal crystal growth spiral can also be seen. The spherical outline has the characteristics of annual rings. The existence of core substances can be observed in most graphite. Graphite crystals grow radially outward from the core along the lattice direction, forming many conical crystals. The lattice base plane in spherical graphite composed of these crystals is usually perpendicular to the radiation growth direction.
There used to be a saying that spherical graphite has a crystal nucleus with a specific structure, and the growth step provided by this crystal nucleus can lead to the uniform growth of graphite crystals in three-dimensional radiation and form spherical graphite. People also put forward some related mechanisms, but later it was found that the composition and structure of spherical graphite core materials were diverse, and no specific core structure was found to make graphite spherical. Most of the identified substances are oxides and sulfides, and there are also complexes of these compounds. Such as sulfides of magnesium, cerium and rare earth elements, oxides of silicon and compounds of various oxides. Some compounds also contain elements such as nitrogen, aluminum and tellurium. Some researchers used electron probes to explore the structure of nucleation matrix materials and found that they had a double-layer structure. The inner layer is mainly composed of sulfides such as calcium and magnesium, and its size is about110 of the whole nucleation matrix. The outer layer is spine-shaped, and the main body is magnesium, aluminum, silicon, titanium and other oxides. There is a certain crystal orientation relationship between the two layers.
Other researchers observed the thermal corrosion samples with transmission electron microscope, and found that there were tiny flake graphite in the center of spherical graphite. Compared with the observation results of polarized light, it was found that each spherical graphite center had a linear or branched dark substance. This substance can be seen on 2-3 consecutive slices with a spacing of 4μm, and the triangular ribbed crystalline substance surrounds the core substance. The researchers pointed out that this observed black substance is flaky graphite. The (000 1) crystal plane of the graphite crystal near the center is perpendicular to the radiation growth direction and gradually becomes the tangent plane of the sphere outward. This kind of flake graphite located in the center of spherical graphite ball is ubiquitous. The growth of spherical graphite begins with this thin flake graphite without specific shape.
According to the exploration results of spherical graphite core materials, it can be inferred that magnesium sulfide, cerium sulfide, silicon oxide and their complexes are the main materials that constitute the nucleation matrix of spherical graphite. The crystal structure of these materials has a certain lattice matching relationship with graphite, and carbon atoms are deposited on them and further grow to form graphite nuclei. It is necessary to further study the direct relationship between the growth of central fine flake graphite and spherical graphite.
In order to explore the growth process of spherical graphite, the element distribution of spherical graphite in ductile iron was also explored and analyzed by automatic radiation image analyzer and electron probe. The results are as follows:
1, the iron content of spherical graphite is about 10 times that of flaky graphite. The ferromagnetism of spherical graphite is much higher than that of flaky graphite. In addition, silicon, manganese, titanium and other elements also exist in spherical graphite to varying degrees.
2. The spheroidizing elements cerium and magnesium are enriched in graphite lattice. Magnesium is evenly distributed on the cross section of the sphere. The concentration of cerium in the center of the sphere increased significantly, which may be related to the existence of cerium compounds. No increase in the concentration of spherical elements was found near the interface between the sphere and the metal. Because the solubility of spheroidizing elements in austenite is very low, if spheroidizing elements are transferred from the metal outside the sphere, these elements will inevitably be enriched at the interface. Therefore, it can be indirectly judged that spherical graphite is directly precipitated from the liquid phase.
3. The distribution of interfering elements such as sulfur, antimony, cadmium, tin and tellurium is similar to that of spheroidizing elements, and most of them are concentrated in spherical graphite. It is worth noting that some of these elements can be detected as atoms in flake graphite, but this phenomenon cannot be detected in spherical graphite.