The main features of magnetic traces of point defects detected by MT. The magnetic traces are scattered and isolated small spots, stripes and cloud like discontinuous distribution. In the ASME standard, the size of the recorded defect is defined as the indication of more than 1.5mm. In normal production, it is customary to regard the approximate circular display with the aspect ratio ≤ 3 as the point defect.

After cleaning the magnetic mark of the spot defect, the color is similar to that of the casting body, and the defect shape is not visible. Mechanical sampling method was used to sample the position, and then the defect morphology was observed under scanning electron microscope (SEM), as shown in Fig. 1. It can be seen from the micrograph that most of the spot defects are closely embedded in the casting body structure, the defects are irregular floc, a small part of the defects are in the shape of falling holes, the boundary of the falling position is burr like, and there is a large color deviation between the defect location and the good position without defects.

In order to further analyze the defect qualitatively, the energy spectrum analysis (SEM) was carried out on the micro region composition at this position. The analysis results are shown in Fig. 2.

According to the analysis of the test data, the main elements that account for more than wt% of the defect position are o, Mg, Si, Zr, etc. According to the main physical and chemical reactions such as melting process, spheroidizing reaction and pouring process of nodular cast iron and thermodynamic principle, it is theoretically analyzed that the main metal oxides (SiO2, MgO, etc.) are located in this position. Due to the different magnetic properties of the elements and their oxides, the magnetic trace stacking display is formed.