Research Progress on fatigue properties of vermicular graphite cast iron

It is found that the fatigue properties of cast iron are not directly related to the conventional mechanical properties of gray cast iron. A large number of test data show that the fatigue strength / tensile strength of gray cast iron is ≈ 0.25-0.4. Grain refinement can improve the fatigue strength limit of cast iron; Element P is a harmful element in cast iron, which will reduce the tensile properties of gray cast iron, but an appropriate amount of element P is beneficial to the fatigue properties. With the development of high strength gray cast iron, there are many studies on cast iron technology, including improving cast iron strength and reducing production cost. At present, the research on cast iron fatigue is roughly divided into thermal fatigue, mechanical fatigue and high temperature fatigue. Thermal fatigue resistance refers to the resistance of materials to fatigue cracks during repeated heating and cooling. It is a comprehensive function of high temperature toughness, high temperature yield strength, thermal stability and oxidation resistance of materials.

The graphite morphology in vermicular cast iron is vermicular and a small amount of flocculent, which is between flake graphite and spherical graphite. Because the performance of cast iron mainly depends on the graphite morphology, the comprehensive performance of vermicular cast iron is also between gray cast iron and nodular cast iron. Vermicular graphite cast iron has excellent thermal conductivity, strong wear resistance, high friction coefficient and good cost performance. It is used to make high-speed train brake disc, engine cylinder block and cylinder head. Because many parts made of vermicular cast iron need to operate at high temperature, Zhu Zhengyu and others studied the thermal fatigue properties of vermicular cast iron with different matrix structure. Molten iron was melted in 500kg medium frequency induction furnace and prepared by wire feeding vermicular treatment process. The vermicular rate of vermicular cast iron ranged from 40% to 90%, The upper limit temperature of thermal fatigue set in the thermal fatigue test is 600 ℃ and 900 ℃ respectively. After thermal cycling, the two sides of the sample are slightly polished, and the length of the main crack is measured and analyzed under the vision of 30x microscope.

The results show that the main thermal fatigue crack length of vermicular graphite cast iron decreases in the range of 20 ℃ – 600 ℃. It is concluded that the initiation and development of fatigue crack in vermicular graphite cast iron can be effectively prevented with the increase of pearlite content in matrix structure, and the thermal fatigue properties of vermicular graphite cast iron will increase with the increase of pearlite content in matrix structure; When the cycle temperature is between 20 ℃ – 900 ℃, because the upper temperature limit is above the eutectoid transformation temperature of 727 ℃, the matrix structure of vermicular graphite cast iron will change from pearlite to austenite. With the increase of temperature, vermicular graphite begins to dissolve in austenite. After the temperature decreases, graphite will precipitate in austenite. The process of graphite dissolution and precipitation changes the position of graphite. Each temperature cycle will produce many voids due to the change of graphite position, which will change the volume of vermicular graphite cast iron, resulting in great phase transformation stress. The change of phase transformation stress will increase the internal stress of the material and reduce the thermal fatigue performance of the material, On the other hand, the appearance of voids will also oxidize vermicular graphite cast iron. Therefore, when the cycle temperature range is 20 ° – 900 °, the thermal fatigue performance of vermicular graphite cast iron decreases with the increase of pearlite content in its matrix structure.

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