By controlling the size and distribution of graphite diameter and the spherical regularity of graphite, the correlation degree with the macro mechanical properties of nodular cast iron was established. From the above results, in qt-cwm spheroidizing process, the sample spheroidizing agent is evenly distributed with high absorption rate, the number of graphite balls is the largest, the graphite spherical regularity is good, the average diameter is the smallest, and the fluctuation of the whole diameter distribution is the smallest. Such graphite structure can greatly improve the low-temperature impact properties of nodular iron, but the hardness and strength decrease slightly. Qt-cfm spheroidizing agent has poor distribution and absorption, the least number of graphite balls, the worst spherical regularity of graphite, the largest difference in diameter and diameter distribution, high hardness and strength, but the lowest low-temperature impact toughness. The graphite structure parameters of qt-tcm are between qt-cfm and qt-cwm, and their properties are also between them.

In order to establish the relationship model between graphite structure parameters and properties and determine the weight relationship between graphite structure parameters and properties, the number of graphite balls, average diameter, standard deviation, maximum value and skewness of each sample are taken as covariates, and the macro properties such as hardness, yield strength, tensile strength and impact energy are taken as dependent variables for neural network analysis, A model for the correlation between graphite microstructure parameters and macro properties. The gray line is positive correlation with synaptic weight, the blue line is negative correlation with synaptic weight, and the activation function of hidden layer is hyperbolic tangent function.

As shown in the figure, the relationship model between graphite microstructure parameters and properties is established by neural network. The higher the importance ratio, the greater the impact on the properties. It can be seen from the figure that the influence weight of graphite spheroidization rate is the highest and has the greatest impact on performance, followed by the standard deviation of graphite ball diameter and the number of graphite balls, while the influence weight of skewness and graphite ball diameter is the smallest. Therefore, in order to improve the low-temperature impact performance of cast iron with the same composition in the production process, we should first control the spheroidization rate of nodular cast iron, that is, the regularity of graphite spherical shape, so as to prevent stress concentration in the impact process, and then control the overall diameter difference and the amount of graphite.