Microstructure Evolution Of Grey Cast Iron At High Temperatures And Its Effect On Tensile Strength

The brake hub is one of the most important components of a car.Its quality not only affects the safety and handling of the car,but also has an important impact on the energy saving and environmental protection of the car.At present,most domestic truck brake hubs are cast with gray cast iron.After the car is continuously braked,the temperature of the brake hub can reach 700～800 ℃,the surface oxidation and wear failure of the brake hub,as well as the significant decrease in the mechanical properties and thermal conductivity of the material caused by the decomposition of pearlite,resulted in cracks in the wheel hub during the exercise,until it was scrapped.In order to improve the safety survival rate of grey cast iron wheels during service life,the microstructure evolution of grey cast iron under high temperature and its influence on tensile strength are studied in this paper,which provides a certain theoretical guidance for improving the mechanical properties of the wheels in future actual production.In this study,the high temperature tensile strength and room temperature tensile strength of the gray cast iron samples were tested by high temperature tensile testing machine and normal temperature tensile testing machine,the thermal conductivity of the samples were tested by laser thermal conductivity tester,and the microstructure of the samples before and after heat treatment at different temperatures were observed by optical microscope and scanning electron microscope.The main research findings are listed as follows:(1)When the temperature is below 500 ℃,the thermal conductivity of the gray cast iron sample decreases rapidly,and the thermal conductivity decreases by 1.1 W/(m·K)for each 100 ℃ increase in temperature,and after the temperature is greater than 500 ℃,The thermal conductivity decreases slowly.(2)From normal temperature to 200 °C,there is little change in graphite morphology.When the temperature rises to 500 °C and 700 °C,the cementite is graphitized,the graphite sheet tends to become thicker,and the number of graphite aggregates tends to increase.(3)From normal temperature to 200 °C,the diffusion ability of the carbon atom is enhanced,the diffusion speed is accelerated,the diffusion distance is long,and the pearlite layer spacing is getting larger and larger;when the temperature continues to rise to 500 °C and 700 °C,the diffusion ability of the carbon atom is greatly enhanced,and the lamellar pearlite becomes unstable due to the higher surface energy.At the same time when the pearlite layer spacing becomes larger,the cementite layer breaks,dissolves,and becomes fine-grained cementite to reduce the surface energy.(4)The evolution of the microstructure of gray cast iron at high temperatures causes the tensile strength of gray cast iron to decrease after the temperature is higher than 200 ℃.The tensile strength decreases significantly when the temperature exceeds 300 ℃.Especially when the temperature is higher than 500 ℃,the tensile strength decreases rapidly.When the temperature is 800 ℃,the tensile strength is only 17.2% of that at room temperature.