Abstract: ZG0Cr14Ni5Mo2Cu stainless steel cone skirt lock castings are used for connecting the aircraft tail cover with the body. During the magnetic powder detection, there were a number of suspected linear magnetic cracks on the surface, and then the fluorescence penetration detection method was used for auxiliary detection. There was no abnormal fluorescence display at the original magnetic trace display. The surface morphology, microstructure characteristics, composition distribution and magnetic changes were studied, and the experiments were carried out, such as surface morphology observation by electron microscopy, metallographic examination of the section of the display area, energy spectrum analysis of the components at the microstructure area, hardness test of materials, XRD phase analysis and simulated heat treatment. The results show that there are no surface defects at the magnetic trace display area of the cone skirt lock, and there are residual austenite structures of thick and large strips between the dendrites of the cast steel. This difference in structure destroys the magnetic continuity of the material, resulting in the appearance of magnetic traces on its surface. The macro and microscopic structures of the cone skirt lock are not uniform, and there are austenitic rich areas, which are related to the segregation of C, S and P elements. This study provides a basis and reference for the determination of magnetic trace during magnetic powder detection.