Based on the national standard GB / T 9441-2009 metallographic examination of nodular cast iron, the following four parameters are selected to express the relevant parameters of ductile iron:

**(1) Number of graphite balls**

The image J software is used to segment the spheroidal graphite cast iron in the polished state. Three different fields of view of each sample are selected for measurement and statistics. N1 is the total number of graphite spheres in the field of view, N2 is the number of graphite spheres separated by the perimeter of the field of view. The area of the field of view is a constant value, which is recorded as a. Then, the number of graphite balls per unit area and the number of graphite balls per unit area under the same test area are calculated by the national standard calculation formula. Finally, the average value of the data obtained from three different parts can be obtained, and the number of graphite balls per unit area of each part can be obtained.

The number of graphite spheres in the field of view area a is n:

The number of graphite balls per unit area n f (number / mm2) is as follows:

Where:

N1 — the total number of graphite spheres in the field of view (PCs.);

N2 — the number of graphite spheres cut off by the perimeter of the field of view (PCs.);

N — the number of graphite spheres under the field of view area a (PCs.);

A — field area a (mm2).

**(2) Average area and distribution of graphite spheres**

The overall particle size of graphite is reflected by the average area of graphite ball, and the proportion of graphite with different particle size is reflected by the area distribution of graphite, which is called the uniformity of graphite. The distribution data of graphite spheres in three parts can be obtained by Image J statistical analysis. The average area and distribution of graphite balls in each part can be obtained by Excel data processing and origin drawing.

**(3) Shape factor F and its distribution of graphite spheres**

One of the factors affecting the performance of nodular cast iron is the shape of graphite ball. The rounder the shape is, the smaller the splitting effect is, and it is difficult to produce stress concentration. The shape factor F is used as a parameter to reflect the roundness of graphite balls. The value of F is in the range of 0 to 1. The closer to 1, the rounder the graphite particles are. The calculation formula is as follows:

Where:

F — shape factor, value: 0 ~ 1;

S — area of each graphite ball (μ m2);

L — perimeter of each graphite ball (μ m).

**(4) Equivalent diameter of graphite ball**

The number of graphite balls and its equivalent diameter have great influence on the mechanical properties of ductile iron. The more the number of graphite balls, the smaller the equivalent diameter, the better the mechanical properties.