Research Progress on fatigue properties of nodular cast iron

Nodular cast iron obtains spherical graphite through inoculation and spheroidization. Because the graphite is spherical, it is not easy to produce stress concentration, which can effectively improve the mechanical properties of cast iron [35], especially plasticity and toughness. Blazsamec et al. Studied the low cycle fatigue properties of nodular cast iron used to manufacture high-speed iron brake discs, mainly studied the tensile and fatigue properties at room temperature, 300 ℃ and 400 ℃, and observed and analyzed the matrix structure at room temperature and 400 ℃.

The test results show that the matrix structure of nodular cast iron is ferrite and pearlite, the distribution of spherical graphite is uniform, and there is no obvious change in the matrix structure and graphite morphology at room temperature to 400 ℃. The test results of mechanical properties show that the elastic modulus of nodular cast iron does not change with temperature, and the yield strength and tensile strength decrease with the increase of temperature. The fatigue life decreases by about 50% at 300 ℃, but the fatigue life decreases obviously at 400 ℃, which is only about 10% at room temperature. The stress response curve shows that nodular cast iron shows slight cyclic hardening at room temperature and obvious cyclic hardening trend at 300 ℃, but it shows initial cyclic hardening at 400 ℃, followed by obvious cyclic softening until fracture. Yu Sirong et al. Studied the effect of alloy elements on the thermal fatigue resistance of nodular cast iron for die material. The test results show that Si has a significant effect on the thermal fatigue resistance of nodular cast iron. The content of Si directly determines the overall thermal fatigue resistance of nodular cast iron for die material. Mo element can promote grain refinement. Within a reasonable range, the higher the amount of Mo element is, The more obvious the grain refinement effect is. Grain refinement makes the structure more uniform, and the thermal fatigue resistance of the material is higher.

The main material of automobile exhaust manifold is nodular cast iron, which is connected with the engine through flange. It is the component with the highest working temperature in the exhaust system. The temperature changes periodically during operation, resulting in thermal stress and thermal strain in the material, that is, thermal fatigue damage. Lian Zhensong et al. Studied the thermal fatigue behavior of high silicon molybdenum nodular cast iron and high nickel nodular cast iron under different temperature cycles, observed and analyzed the main crack and metallographic structure. The results show that the main thermal fatigue crack is mainly located at the largest surface of the sample, and the main crack is formed by the expansion and connection of a large number of microcracks,. Fatigue microcracks generally form at the grain boundary and near the carbide. The existence of these “impurities” weakens the grain boundary strength. Microcracks nucleate and initiate here, and are easy to produce “self cracks” in the thermal cycle. At the same time, due to the serious oxidation at the crack tip, the increase of oxidation products increases the tension at the crack tip and accelerates the crack propagation. At high temperature (800 ℃ – 1000 ℃), the spherical graphite disappears due to high temperature oxidation and forms countless small “grooves”, resulting in a significant reduction in the strength of nodular cast iron and accelerating the generation of microcracks. It is found that the thermal fatigue property of high nickel nodular cast iron is better than that of high silicon molybdenum nodular cast iron.