In order to better analyze and study the types of secondary carbides obtained after quenching of high chromium cast iron, the chemical composition of carbides of high chromium cast iron was analyzed by EPMA point scan, and the diffraction pattern of high chromium cast iron was calibrated by saed. The chemical composition of carbides in M31 is higher than that of carbides in EPA 1 in the figure. It can be seen from the table that the atomic ratio of M / C of secondary carbide at point a is 2.9, close to 3. Therefore, the secondary carbide at point a is m3c carbide.
As shown in Table 2, the chemical composition of EPMA point scan eutectic carbide M7C3 in Figure 2 (the atomic ratio m / C in eutectic carbide M7C3 is 7 / 3, i.e. 2.3). It can be seen from table 2 that when the carbon content in the eutectic carbide of high chromium cast iron is 7.9 wt.%, that is, the eutectic carbide in Figure 2 (a), the atomic fraction is converted to 27.5 at.%, Then M / C is 2.6. When the carbon content in the eutectic carbide is 8.2 wt.%, that is, the eutectic carbide in Figure 2 (b), the atomic fraction is 28.6 at.%, M / C is 2.6. Are greater than 2.3. Therefore, through comparison, it can be found that the carbon content of eutectic carbide M7C3 in high chromium cast iron should be slightly greater than 8.2 wt.%, and the Cr content should be about 50 wt.%.
Taking the content of C, Cr and Mn in the eutectic carbide of high chromium cast iron scanned by EPMA (Table 2) as the reference value, the EPMA line scanning diagram of the atomic chemical composition content of C, Cr and Mn under the quenching condition of 950 ℃ as shown in Fig. 3 is obtained. As can be seen from Fig. 3, C and Cr atoms are unevenly distributed, and C and Cr atoms are mainly dissolved in eutectic carbides, which is quite different from the matrix composition. The contents of C and Cr in the matrix are 0.7 wt.% and 10 wt.% respectively. The contents of Mn atoms in the matrix structure and eutectic carbide of high chromium cast iron are 1.5 wt.% and 1.8 wt.% respectively, that is, Mn atoms are evenly distributed in the structure. The atomic contents of C, Cr and Mn in the region of hole a surrounded by eutectic carbides in high chromium cast iron are the same as those in the matrix. The atomic contents of carbon and chromium at the boundary B between eutectic carbide and matrix of high chromium cast iron are 0.7 wt.% and 10 wt.% respectively, which are the same as the matrix structure.
As shown in Figure 4, two kinds of selected area electron diffraction patterns (SAED) of rod carbide and their calibration are shown. The experimental results show that the included angles of (111 ത) and (124 ത) crystal directions, (111 ത) and (013 ത) crystal directions in Fig. 4 (b) are 28.9 ° and 56.9 ° respectively, and the theoretical values are 29.9 ° and 58.3 ° respectively. (1 ത 26) and (035) crystal directions, the included angles between (1 ത 26) and (111 ത) crystal directions are 26.5 ° and 106.9 ° respectively, and the theoretical values are 26.1 ° and 106.5 ° respectively. By calculating the difference between the experimental and theoretical values of the two calibration results, it can be found that the difference is within the error range. Therefore, the rod-shaped carbide shown in Fig. 4 (a) is M7C3, and its crystal clusters are [2 ത 31 ത] and [85 ത 3] respectively, with a size of about 1 μ m。 The rod carbide as shown in Fig. 5 was also observed by TEM, but the carbide type of high chromium cast iron was not calibrated due to experimental factors.
