Starting from the research on the low-temperature impact toughness of nodular cast iron, the chemical composition, mechanical properties, metallographic structure and fracture morphology of the sample were studied through the previous and subsequent experiments, and the following conclusions were drawn:
(1) Silicon can not only promote the formation of ferrite, but also strengthen ferrite by solid solution. The critical value of nodular cast iron is determined to be 1.95%. When WSI ≤ 1.95%, the effect of promoting ferrite formation is stronger than that of solid solution strengthening. The low-temperature impact toughness of nodular cast iron increases with the increase of silicon content. When WSI > 1.95%, the solid solution strengthening effect is obvious, and the low-temperature impact toughness decreases with the increase of silicon content.
(2) Copper promotes the formation of pearlite, improves the tensile strength and reduces the low temperature impact toughness of nodular cast iron. When WCu ≤ 0.2%, the low-temperature impact toughness is basically unchanged, and when WCu = 0.2% – 0.3%, the low-temperature impact toughness decreases gradually. When WCu > 0.3%, the low temperature impact toughness decreases significantly. Therefore, when the tensile strength of nodular cast iron can not meet the standard requirements, 0.2% – 0.3% copper can be added to strengthen it.
(3) Bismuth element has obvious function of fine fossil graphite, promotes the formation of ferrite in nodular cast iron and improves low temperature impact toughness. When WBI = 0.006%, the spheroidization grade and size grade of graphite balls can reach grade 1 and grade 8 respectively, which is an effective way to improve the low-temperature impact toughness of nodular cast iron
(4) In the production process, high-efficiency spheroidizing agent and compound inoculant are selected to reduce the inoculation in the package and increase the flow inoculation, which can strengthen the inoculation and improve the low-temperature impact toughness of nodular cast iron.
(5) The high and low temperature two-stage graphitization annealing heat treatment with 900-910 ℃ for hours and 725-735 ℃ for 3 hours can ensure that the matrix is all ferrite, improve the stability of mechanical properties of nodular cast iron, and improve the low-temperature impact toughness by about 2J.
(6) The low temperature impact toughness of nodular cast iron is closely related to the metallographic structure. The state of graphite ball and the content of matrix structure will have a significant impact on the low temperature impact toughness. The higher the spheroidization level and size level of graphite ball, the rounder and smaller the nodular graphite ball, and the higher the low-temperature impact toughness. With the increase of pearlite content in the matrix, the low-temperature impact toughness decreases. In order to ensure the low-temperature impact toughness of nodular cast iron, the maximum amount of pearlite in the matrix structure shall not exceed 25%.
(7) According to the observation of SEM, the impact fracture of ductile iron is brittle from the macro morphology, but it is obviously different from the micro morphology. When the low-temperature impact toughness of nodular cast iron is low, the micro morphology is characterized by cleavage fracture, and when the low-temperature impact toughness is high, there is quasi cleavage fracture. When there are sharp corners in nodular cast iron graphite balls, the sharp corners will promote the formation and propagation of cracks and make the matrix prone to fracture, which is the basis for improving the low-temperature impact toughness of nodular cast iron graphite balls.
(8) The optimum chemical composition of nodular cast iron for experimental stroke electrical accessories is: WC = 3.7-3.8%, WSI = 1.9-2.0%, WMN < 0.2%, WP < 0.04%, WS < 0.02%, WMG residue = 0.03-0.05%, wre residue = 0.01-0.02%, WBI = 0.006%. Its performance meets the standard requirements, and the maximum impact absorption energy at – 40 ℃ is 17.1j. The chain adding method can significantly reduce the production cost.