After the spheroidizing annealing treatment of four processes, the SEM analysis results of each structure are shown in Figure 1. Through comparison, it is found that under the condition of process 1, the spheroidizing rate of lamellar cementite in the material matrix reaches more than 95%, which is equivalent to that of process 2. The SEM structures of process 3 and process 4 show that some lamellar cementite does not spheroidize, And there are some small spherical particles that have a strengthening effect on the matrix, resulting in the strength of the material higher than the first two processes, indicating that when the heating temperature exceeds AC1 temperature, it is conducive to the spheroidization of cementite in the matrix. The main reason is that when the heating temperature exceeds the AC1 temperature, some structures in the matrix are transformed into austenite, which speeds up the diffusion of carbon in the matrix into the austenite matrix. According to the colloidal principle (the solid solubility of the second phase particles in the matrix is related to its radius of curvature, and the smaller the radius of curvature, the greater the solid solubility of the particles in the matrix). At the tip of the smaller radius of curvature, the faster the dissolution process of cementite into austenite, The faster the fracture speed of the original lamellar cementite, the positions where some carbides have edges and corners and some smaller carbides will be fully dissolved after fracture, while some larger cementites form spherical cementite after this stage. In the process of subsequent cooling, a new nucleation point is formed, and the cementite dissolved in austenite gradually precipitates at the nucleation point, making the carbides spheroidize gradually.
When the heating temperature does not exceed AC1, the matrix structure of the material is still ferrite + pearlite structure. When the material is heated to a certain temperature below AC1 temperature, the carbides at the prominent position need to diffuse for a long time in the process of heat preservation, so that the lamellar carbides can be fully broken, and more fine carbide particles will be produced after fracture. If the heat preservation time is not sufficient, As a result, some lamellar carbides cannot be completely broken, as shown in the cementite morphology of process 3 in Figure 2. Because the cementite cannot be completely broken, more rod-shaped structures can be found in the SEM morphology, which proves that the diffusion time is not enough, so that the cementite cannot be fully spheroidized.
The high strength of the material under process 4 conditions is mainly due to the small size of cementite, which produces a certain degree of dispersion strengthening effect in the matrix. In the process of plastic deformation, the fine dispersed cementite plays a pinning effect at the grain boundary. In addition, compared with the tissue spheroidization rate under the same heating temperature, it can be found that the heat preservation at 710 ℃ and 680 ℃ is increased in the cooling process of processes 1 and 4, which is conducive to improve the material spheroidization rate, and the spheroidization rate is increased by 5% respectively.
