According to GB/T7216-2009 standard, graphite morphology in gray iron can be divided into six categories: A, B, C, D, E and F.
A-flake graphite is uniformly distributed without direction; B-flake graphite and fine curly
flake graphite aggregate into chrysanthemum-like distribution; C-primary coarse straight flake graphite; D-fine curled flake graphite is undirected distribution between dendrites; E-flake graphite is directional distribution between secondary branches of dendrites; F-primary star (or spider) graphite.
A-type graphite is formed when the graphite of cast iron has strong nucleation ability, low cooling rate and Eutectic Transformation under the condition of very low undercooling. It requires sufficient carbon equivalent, sulfur content and suitable inoculation of molten iron. Without chilling or too small wall thickness, and with the completion of casting in as short a time as possible after inoculation, the casting is most likely to form A-type graphite. At this point, if the ratio of carbon equivalent to 4.26% is equal to or approximately equal to 0.9, the best graphite morphology will be obtained.
The nucleation condition of B-type graphite is worse than that of A-type graphite, and the degree of undercooling during eutectic transformation is also higher than that of A-type graphite. The undercooling graphite (D-type) is first produced at the center of eutectic cluster during crystallization. The latent heat of crystallization released reduces the degree of undercooling around the eutectic cluster to form A-type graphite, resulting in chrysanthemum-like
flake graphite.
C-type graphite mainly occurs in cast iron with high carbon equivalent (hypereutectic composition) and slow cooling. When hypereutectic molten iron is cooled, primary graphite precipitates at a certain supercooling degree after passing through the liquidus, and grows up gradually in the liquid phase. Because of the higher crystallization temperature and the longer growth time of graphite sheets, the coarse flake graphite with less branching is formed. When the temperature decreases to eutectic temperature, normal eutectic transformation takes place. The graphite produced at this time is normal eutectic graphite (i.e. A-type graphite). The final result is that normal eutectic graphite is distributed among the thick graphite sheets. Therefore, C-type graphite is composed of coarse, massive graphite and A-type graphite. In addition, although the carbon equivalent is not too high, the graphite will also be produced when the pouring temperature is high, the cooling rate is slow and the inoculation is excessive.
D-type graphite is formed between austenite dendrites with low carbon equivalent and high cooling rate in cast iron under conditions of high undercooling and well developed primary austenite dendrites. Graphite sheets are fine and non-directional. D-type graphite is commonly found in thin-walled gray iron castings with low carbon equivalent, also known as "undercooled graphite" or "interdendritic graphite".
E-type graphite is formed under the conditions of low carbon equivalent and low cooling rate. Graphite sheets are larger than D-type graphite because of the large number of primary austenite dendrites, small undercooling during eutectic transformation, few graphite cores and large eutectic clusters. Because of the slow cooling and the developed austenite dendrites, the liquid phase is mainly between the primary austenite dendrites when Eutectic Transformation occurs, and the graphite sheets formed grow along the direction of dendrites, which has a certain directivity. If the cooling rate of cast iron with E-type graphite is higher, D-type graphite will also be formed.
F-type graphite is also hypereutectic graphite in essence, which is formed in high carbon hot metal under large undercooling conditions. The composition of C-type graphite is similar to that of C-type graphite, which is characterized by the distribution of many small graphite sheets on large graphite (also known as star graphite). Bulk graphite can be regarded as the primary graphite in C-type graphite, on which small
flake graphite grows.
