Three kinds of gray cast iron samples annealed after heating to 700 ℃, 800 ℃ and 900 ℃ were tested by XRD. The results are shown in Figure 1. It can be seen from the figure that in the gray cast iron sample annealed after heating to 700 ℃, the number of diffraction peaks of cementite and martensite is significantly reduced, but there are still cementite peaks and graphite diffraction peaks. In the gray cast iron samples annealed at 800 ℃, the diffraction peak of cementite almost disappears. After high-temperature graphitization annealing, the decomposition of cementite is significant, α The intensity of phase diffraction peak is obviously strengthened, compared with that after annealing at 900 ℃, α The half peak width of the phase diffraction peak narrows, indicating that the ferrite grain increases with the increase of temperature. Compared with critical temperature annealing, cementite decomposes more fully, precipitates more carbon atoms and graphitization more thoroughly.
Fig. 2 shows the microstructure of graphitization annealing of gray cast iron samples heated to different temperatures. As shown in Fig. 2 (a), according to the microstructure of gray cast iron sample heated to 700 ℃ below AC1 temperature for 3 hours, it can be seen that the coarse dendritic cementite in the modified layer is refined, but the cementite is not dissolved, the number of cementite distributed among cementite dendrites is reduced, a large amount of secondary cementite precipitates in austenite, and graphite begins to nucleate at the austenite grain boundary. As shown in Figure 2 (b), the microstructure of gray cast iron sample heated to 750 ℃ for 3 hours. From the figure, it can be seen that there is a large amount of decomposition of cementite structure, there is still undissolved massive cementite structure, the austenite matrix is transformed into ferrite, layered pearlite is precipitated on the matrix, and a small amount of graphite is precipitated. The graphite is distributed between pearlites in the form of dendritic dots.
Fig. 2 (c) shows the microstructure of gray cast iron sample when heated to 800 ℃ and kept for 3 hours. As the heating temperature increases, the decomposition of carbides accelerates. At high temperature, cementite basically dissolves back into austenite. It can be seen from the figure that there is no large undissolved cementite structure in the structure, and its matrix structure is austenite and pearlite. Pearlite is divided into small pieces and distributed at the ferrite grain boundary. Compared with pearlite annealed at 750 ℃, the distance between lamellae is reduced. Due to the high carbon content in austenite, it is transformed into pearlite and a small amount of graphite after annealing. The precipitated graphite is distributed in flake between cementite dendrites. The growth time of graphite is short and its shape is small. Fig. 2 (d) shows the microstructure of gray cast iron sample when heated to 850 ℃ for 3 hours. In the figure, the cementite decomposes completely. In the process of long-time insulation and slow cooling, the secondary cementite structure does not form a lamellar structure, but is refined into short strip or granular pearlite particles, which are evenly distributed in the ferrite, The distance between granular pearlite is much larger than that of flake pearlite. The matrix structure is fine ferrite grains, and the graphite size between ferrite grains becomes larger and tends to be spherical.
Fig. 2 (E) shows the microstructure of gray cast iron sample when heated to 900 ℃ for 3 hours. Due to the high heating temperature and slow cooling rate, the matrix of the modified layer is completely ferritized, and the ferrite grains are 25 μ M or so. Due to the increase of annealing temperature, carbon atoms fully diffuse from austenite interface to ferrite interface, so the number of precipitated graphite increases sharply, the shape of graphite becomes larger and the number of granular pearlite decreases. Fig. 2 (f) shows the microstructure of gray cast iron sample when heated to 950 ℃ for 3 hours. The ferrite grains grow fully and become larger in shape. The average size of ferrite grains is 30 μ M, the pearlite structure gradually disappears, the shape of precipitated graphite increases, the graphite grows into large flocculent, and there is a trend of gradual connection between graphite.