Effect of normalizing temperature on properties of nodular cast iron

Nodular cast iron is a spheroidal graphite cast iron obtained by spheroidization and inoculation. From the aspect of production, the mechanical properties of nodular cast iron are improved mainly through the optimization of alloy composition, casting process and heat treatment process. Choosing a reasonable heat treatment process can greatly improve the properties of nodular cast iron, and normalizing treatment is a common heat treatment process to improve the mechanical properties of nodular cast iron. Nodular cast iron has high carbon content, most of which appear in the form of graphite balls, while the rest of carbon exists in its matrix. Therefore, when the nodular graphite casting is heated to a certain temperature, the carbon atoms diffuse, and the carbon atoms on the surface of the graphite ball diffuse into the matrix austenite. When the temperature decreases, the carbon atoms in the supersaturated solid solution desolvate and adhere to the graphite ball.

Gao et al. Found that the tensile strength of nodular cast iron increased with the increase of annealing time, and the diffusion ion coefficient of carbon atom was calculated to be 0.56 μ M2 / s, which belongs to short-range diffusion. It is found that the carbon atoms in graphite diffuse incompletely and can only diffuse around the graphite to combine the graphite with the matrix. Controlling the heating temperature, holding time and cooling rate of nodular cast iron can change the microstructure and properties of nodular cast iron. The eutectoid transformation temperature of nodular cast iron is in a certain range, in which the volume fraction ratio of ferrite and pearlite is related to the rise and fall of temperature. Therefore, in order to improve the tensile strength of nodular cast iron by increasing the pearlite content, normalizing treatment is generally adopted, and the normalizing temperature is 30 ~ 50 ℃ above the upper limit temperature of Eutectic Transformation. Na Jianhong et al. Studied the effect of normalizing treatment on different depths of nodular graphite castings, and found that the hardness deviation at different depths of different layers at the same position after normalizing treatment was small. In practical production, there are often a small amount of free cementite in the microstructure of nodular cast iron, which is easy to decompose at high temperature. Therefore, the heating temperatures of 870 ℃, 900 ℃ and 930 ℃ were designed to study the changes of microstructure and mechanical properties of nodular cast iron after normalizing.

Compared with as cast nodular cast iron, the tensile strength increases to 759 MPa and the elongation decreases to 5.4% at the normalizing temperature of 870 ℃. When the normalizing temperature is increased to 930 ℃, the tensile strength does not change significantly, which is 763mpa, and the elongation increases significantly to 9.5%. Compared with as cast nodular cast iron, the proportion of pearlite in the matrix of normalized nodular cast iron increases, and the pearlite content in nodular cast iron further increases when the temperature increases from 870 ℃ to 930 ℃. When normalizing nodular cast iron, normalizing temperature is an important factor affecting the decomposition of cementite in nodular cast iron matrix. At the normalizing temperature of 930 ℃, the cementite in the sphere decomposes, which significantly improves its elongation.