It can be seen from the table that the tensile strength and impact toughness of nodular cast iron show high comprehensive mechanical properties at 260~300 ℃, while at 240 ℃, although the tensile strength of nodular cast iron is very high, the elongation and impact toughness after fracture decrease significantly.
| Group number | Isothermal temperature / ℃ | Impact toughness /j · cm-2 | Tensile strength /MPa | Elongation after fracture /% | Surface hardness /HRC |
| 1 | 300 | 75.1 | 1 469 | 3.7 | 43-45 |
| 2 | 280 | 67.9 | 1 470 | 3.2 | 44-46 |
| 3 | 260 | 60.1 | 1 513 | 2.8 | 46-48 |
| 4 | 240 | 43.7 | 1 536 | 1.2 | 50-51 |
From the view of fatigue properties of ductile iron, the castings with higher isothermal temperature are mainly composed of bainite above the structure, and no quenched martensite is found in the white zone. The material shows good toughness. The spacing of shell lines on the fracture surface of the sample is small, there are many dimples around the graphite, and its fatigue strength is also high. Although the toughness and plastic toughness of lower bainite are better than those of upper bainite, the lower bainite often coexists with the quenched martensite in the white zone in the Austempered structure of nodular cast iron. The strength and hardness of isothermal quenched nodular cast iron dominated by lower bainite are better than that of upper bainite, while the plastic toughness is slightly poor. Relatively speaking, the upper bainite often coexists with residual austenite, showing better toughness.
Therefore, the better metallographic structure is: fine upper bainite and lower bainite, a small amount of fragmented residual austenite, and no quenched martensite.