The graphite content and spheroidization rate reflect the degree of graphitization and spheroidization. The typical morphology of graphite balls in nodular cast iron with different Nb content is shown in Fig. 1.
The ratio of actual area of graphite to matrix area is graphite area fraction. The area fraction of graphite in nodular cast iron with different niobium content is shown in Figure 2. With the increase of Nb content, the graphite area fraction increases from 15% to 16% and then decreases to 14%, which is related to the dual effect of Nb on the graphite phase. When the Nb content is low, the role of NBC as heterogeneous nucleation core is dominant, which is conducive to the precipitation and growth of graphite, so the amount of graphite increases; With the increase of Nb content, the amount of graphite decreases due to the formation of more Nb and C, which consumes carbon in the matrix. However, compared with the carbon in nodular cast iron, the content of niobium is small, its role is small, and the graphite content changes little, so the effect of niobium on the graphitization of nodular cast iron is not obvious.
The ratio of the actual area of a single graphite to the area of the smallest circumscribed circle is called graphite area ratio. 81, 0.80-0.61, 0.60-0.41, 0.40-0.21 and 0.21, respectively.
With the increase of Nb content, the amount of spherical graphite decreases gradually, and the total amount of graphite after correction also decreases.
The calculation formula of spheroidization rate is as follows:
According to the formula, the spheroidization rate of nodular cast iron with different niobium content is calculated, as shown in Figure 3. It can be seen from Figure 3 that the spheroidization rate of nodular cast iron without niobium and with 0.20% niobium content are 92% and 91% respectively, and the spheroidization grade is grade 2. When the niobium content is 0.42%, the spheroidization rate is 89%, and the spheroidization grade is reduced to grade 3. When the Nb content increased to 0.69%, the spheroidization rate further decreased to 84%. With the increase of Nb content, more Nb and C are formed, which affects the diffusion of carbon atoms during graphitization and increases the reduction of spheroidization rate.