Effect of surface roughness on eddy current signal of ductile cast iron with different matrix

摘要

This research deals with nondestructive evaluation method for the microstructure of ductile cast iron castings with different surface roughness. Thin wall ductile cast iron has been developed to reduce weight of the casting parts and it is proposed to apply to automobile parts. However, difference in thickness of cast iron will result in difference in microstructure and mechanical properties. To apply it for structural parts, nondestructive evaluation method on the microstructure and mechanical properties of cast iron is required. An eddy current method is one of the techniques to detect material difference, and there are several attempts to adopt the method for evaluating cast iron matrix. In that case, effect of surface condition, including surface roughness or residual stress should be taken into consideration. This study examined the effect of surface roughness on evaluating ductile cast iron matrix with the eddy current method. Ductile cast iron specimens with two types of matrix were prepared. A shaping machine was used to cut their surface and two cutting conditions were employed to change surface roughness of each specimen. Authors made two specimens for same matrix and cutting condition. One of the specimens of each condition was annealed to reduce the effect of residual stress on the surface. Measurements of eddy current signal on the specimens were performed by using an eddy current testing instrument. The equipment displays two signals Vx and Vy with 90degree different phase. Eddy current signals Vx and Vy showed dependence on the surface roughness and the matrix of cast iron specimens. By coordinate rotation, Vx correlate with the surface roughness and Vy has correlation with the matrix. Difference in eddy current signal between annealed specimens and as-machined specimens showed same tendency with the effect of the difference in matrix. The results indicate that the effect of the surface roughness on the eddy current signal can be distinguished from the effect of cast iron matrix, and residual stress due to the surface finish should be taken into consideration.