The main reason why grey cast iron is widely used in cylinder block is that it has better thermal conductivity than vermicular cast iron and nodular cast iron. The thermal diffusion coefficient represents the internal thermal diffusion capacity of an object, which is determined by the formula α=λ/ （ ρ CP) means, where: α Is the thermal diffusivity or thermal diffusivity, m2/s; λ Is the thermal conductivity or thermal conductivity, w/ (m · K); ρ Is the density, kg/m3; CP is the specific heat capacity, j/ (kg · K). It is studied that the specific heat capacity and density of gray cast iron are basically unchanged at different temperatures. It can be concluded that when the density and specific heat capacity of the material are unchanged, the greater the thermal conductivity, the greater the thermal diffusivity of the material. Therefore, the thermal diffusivity (thermal diffusivity) can be used to reflect the thermal conductivity of gray cast iron. This test analyzes the effect of carbon equivalent on the thermal conductivity of gray cast iron by comparing the thermal diffusivity of materials at different temperatures.
It can be seen from the figure that when the carbon equivalent is 3.8%, the thermal conductivity of cast iron is the best. With the increase of temperature, the thermal diffusivity of gray cast iron with different carbon equivalent gradually decreases. It can be seen from the formula that with the increase of carbon equivalent, the dependence of gray cast iron on temperature is higher; When the carbon equivalent is 3.2%, the thermal diffusivity decreases by 0.97cm2/s when the temperature rises by 100 ℃; When the carbon equivalent is 3.4%, the thermal diffusivity decreases by 0.94cm2/s when the temperature rises by 100 ℃; When the carbon equivalent is 3.6%, the thermal diffusivity decreases by 1.07cm2/s when the temperature rises by 100 ℃; When the carbon equivalent is 3.8%, the thermal diffusivity decreases by 1.35cm2/s when the temperature rises by 100 ℃. When the carbon equivalent increases from 3.2% to 3.8%, the temperature dependent coefficient of thermal diffusivity of gray cast iron increases by 39.2%. The reason for this result is that the microstructure of gray cast iron obtained in this test is mainly graphite and pearlite.
The thermal conductivity of graphite in cast iron is much higher than that of other phases, but the thermal conductivity of pearlite is also the most affected by temperature. The thermal conductivity of pearlite is less affected by temperature and remains unchanged with the increase of temperature. Therefore, with the increase of carbon equivalent, the graphite content in gray cast iron increases gradually, which leads to the improvement of thermal conductivity of the material, and the dependence on temperature also increases gradually.
Variation of thermal diffusivity of gray cast iron with different carbon equivalent with temperature:
Where: α Is the thermal diffusivity, α Subscript of is carbon equivalent; T is the temperature.