Rolled Steel Surface Inspection using Microwave Methods

Abstract

Rolled steel is used in a variety of products such as soup, paint and spray cans as well as automobile body panels (doors, hoods), ship structures, etc. For most of these applications it is important that the rolled steel be devoid of any defects, in particular surface defects. For example surface defects can leave an automobile door with unacceptable spot discoloration and container cans with holes. These defects can be separated into two broad categories of steel induced defects such as holes, laminations and slivers and roll mark defects such as pock marks, pick ups, bruises and wire marks. In general, roll mark defects are much more subtle that the steel induced type, and consequently harder to detect using any available detection method. Since microwave signals do not penetrate inside conductors such as steel and are only sensitive to surface features and perturbations, they are a good candidate for steel surface inspection. In particular, near-field microwave methods allow for optimization of the standoff distance for increased detection sensitivity. In recent years, microwave near-field inspection techniques have been used to study surface properties of metals and graphite composite materials for defects such as surface cracks and impact damage^1–5.