## 3 Results and Discussion
### Creep Properties of Compacted Graphite Cast Iron  
Figure 5a-c are the creep curves of CGI at the test temperatures of 450 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, and 550 \({}^{\circ}\)C, respectively. Evidently, its creep deformations were related to three factors: test stress, creep time, and test temperature and all three were positively correlated. In addition, the creep curves under different test conditions were very different. At 450 \({}^{\circ}\)C and 200 MPa, 500 \({}^{\circ}\)C and 150 MPa, and 550 \({}^{\circ}\)C and 100 MPa, the deformation of the CGI sample continued for 100 h without breaking. The rest of the samples were deformed to fracture within 0-70 h. The reason for this phenomenon was related to the use of a wide range of stress and temperature test conditions. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Temperature/\({}^{\circ}\)C Level 1/MPa** & **Level 2/MPa** & **Level 3/MPa** & **Level 4/MPa** & **Level 5/MPa** \\ \hline
450 & 300 & 275 & 250 & 225 & 200 \\
500 & 275 & 250 & 225 & 200 & 150 \\
550 & 225 & 200 & 175 & 150 & 100 \\ \hline \end{tabular}
\end{table} 
Table 2: Different load levels for creep tests. Figure 4: Microstructure of compacted graphite iron. (**a**) \(\times\)100; (**b**) \(\times\)200. It can be observed in Figure 5 that CGI was quite prone to creep damage under a wide range of temperature and stress test conditions. As shown in Figure 5c, under the condition that the test temperature was 550 \({}^{\circ}\)C and the test stress was 100 MPa, when the sample reached 100 h test time, its creep deformation was 2.22%. As shown in Figure 5a, under the conditions of the test temperature of 450 \({}^{\circ}\)C and the test stress of 275 MPa, the creep deformation of the specimen when it reaches fracture was 3.21%, and the total creep time was 2.8 h. It can be seen that the creep deformation under high temperature and low stress and low temperature and high-stress conditions was smaller than that under high temperature and high-stress conditions, but with the increase in test time, it would also produce creep damage and cause a fracture. According to the change in creep rate, the creep curve will go through three stages in sequence: the first creep stage, the second creep stage, and the third creep stage [31]. The second stage of each creep curve in Figure 5 was linearly fitted to obtain the minimum creep rate under different test conditions, as shown in Table 3, and its value was related to the test temperature and test stress. At 500 \({}^{\circ}\)C, the minimum creep rate of the sample at 200 MPa was 0.618 \(\times\) 10\({}^{-2}\) h\({}^{-1}\), and the minimum creep rate of the sample at 225 MPa reached 2.097 \(\times\) 10\({}^{-2}\) h\({}^{-1}\). Similarly, under the test stress condition of 200 MPa, when the test temperature was increased from 500 \({}^{\circ}\)C to 550 \({}^{\circ}\)C, the minimum creep rate was accelerated from 0.618 \(\times\) 10\({}^{-2}\) h\({}^{-1}\) to 7.110 \(\times\) 10\({}^{-2}\) h\({}^{-1}\). It can be seen that the minimum creep rate of CGI increased with the increase in stress and temperature, which was consistent with the findings in [21]. In addition, at 500 \({}^{\circ}\)C, the stress increased by 12.5% from 200 MPa to 225 MPa, and the corresponding minimum creep rate increased by 239.3%. However, under the stress condition of 200 MPa, the temperature increased by 10.0% from 500 \({}^{\circ}\)C to 550 \({}^{\circ}\)C, and the corresponding minimum creep rate increased by 1050.5%. It can be seen that the effect of temperature on the minimum creep rate was greater than that of stress. Figure 5: Creep curves of CGI under different test stresses: (**a**) 450 \({}^{\circ}\)C, (**b**) 500 \({}^{\circ}\)C, and (**c**) 550 \({}^{\circ}\)C.  
It can be seen in Table 3 that when the temperature was 550 \({}^{\circ}\)C and the stress was 100 MPa, the minimum creep rate of the sample was 1.8968 \(\times\) 10\({}^{-4}\) h\({}^{-1}\).