According to the statistics of pearlite content in the structure of nodular cast iron under corrosion state, the gray data of pearlite and graphite ball in the corrosion sample are close to each other, so the two should be regarded as a whole when using image J analysis. The method to distinguish pearlite from graphite is to calculate the total area at by the unified calculation of the two. The graphite sphere area is marked as Ag and the pearlite area is AP. There is a fixed area value a under each field of view. Therefore, the area AF of ferrite is the sum of the total area minus the area of pearlite and graphite ball, as shown in the formula.

Therefore, the calculation formula of pearlite content in different parts of ductile iron profile can be obtained as follows:

Where:

AP — pearlite area under the test field of view (μ m2);

Ag — the area of graphite ball under the test field of view (μ m2);

At — area of pearlite and graphite ball under test field of view (μ m2);

AF — ferrite area under the test field of view (μ m2);

η P — pearlite content (%);

A — test field area (μ m2).