After cementite decomposition, carbon atoms diffuse from cementite austenite interface to graphite austenite interface. During annealing, carbon atoms in austenite will preferentially form graphite crystal nucleus on austenite grain boundary. Because the growth of gray cast iron graphite is hindered by long strip cementite, gray cast iron graphite grows along austenite dendrite. Therefore, after annealing at a lower temperature, The decomposition of cementite is not complete. The precipitated gray cast iron graphite is densely distributed between the cementite dendrites in a fine oval shape, and the more undigested cementite, the smaller the space left for the growth of gray cast iron graphite. The amount of precipitated gray cast iron graphite is less, and the morphology of gray cast iron graphite is also poor, as shown in Figure 1 (a). In the heat affected zone, carbon atoms will preferentially diffuse to the adjacent graphite. As can be seen in Fig. 1 (b), carbon atoms are attached to and precipitated around the flake graphite, which gradually coarsens the size of the refined flake graphite.
When heated to 700 ℃ for annealing, as shown in Fig. 2 (a). Due to the low annealing temperature, the decomposition of cementite is not very complete, the number of carbon atoms dissolved in austenite is small, and the low temperature leads to the short diffusion distance of carbon atoms. Therefore, the gray cast iron graphite nucleates in situ during the decomposition of cementite, and the number of precipitated graphite is small. The growth of gray cast iron graphite is limited by insoluble cementite dendrite. The formed gray cast iron graphite is elliptical, and the diameter of gray cast iron graphite is about 50 μ m。 A small amount of carbide precipitates and inclusions are formed in the periphery of gray cast iron graphite, forming an obvious “bright white” outer ring.
When the annealing temperature is 800 ℃, as shown in Fig. 2 (b). The amount of precipitated gray cast iron graphite increases significantly, the shape of gray cast iron graphite becomes fine spherical, and the diameter of gray cast iron graphite is 53 μ M, the precipitated graphite of gray cast iron is mostly distributed in ferrite grains, which has little effect on the grain boundary properties of ferrite. At the same time, many irregular small graphite particles and many fragments of graphite were precipitated on the ferrite matrix. Because the growth process of gray cast iron graphite balls is limited by the surrounding austenite grain boundary, when the expansion of gray cast iron graphite growth exceeds the austenite strength limit, the austenite grain breaks and the carbon atoms nucleate and grow rapidly here.
When the annealing temperature rises to 900 ℃, as shown in Figure 2 (c), compared with annealing within the eutectoid temperature line, the driving force of high-temperature graphitization annealing is stronger, the graphite of gray cast iron is more fully graphitized, the number of gray cast iron graphite is increased, the size of gray cast iron graphite is also increased, and the average size of gray cast iron graphite is increased to 55 μ m. The larger graphite of gray cast iron destroys the integrity of ferrite grain boundary. If the holding time at high temperature is prolonged, the flocculent graphite near the will gradually grow and become larger, and there will be a trend of gradual connection, resulting in poor graphite morphology of gray cast iron.